NCHRP CPT Synthesis (2007) Color

Final Report* NCHRP Project 20-05 Topic 37-14 Cone Penetration Testing State-of-Practice Prepared by Paul W. Mayne, PhD, P.E. Professor, Civil & Environmental Engineering Georgia Institute of Technology Atlanta, GA 30332-0355 Submitted to: Jon M. Williams Manager, IDEA and Synthesis Studies Transportation Research Board 500 Fifth Street, NW Washington, DC 20001 Transportation Research Board Synthesis Study TRB Panel Members: Michael Adams, FHWA Scott A. Anderson, FHWA David J. Horhota, Florida DOT G.P. Jayaprakash, TRB Alan J. Lutenegger, U.Mass-Amherst Kevin McLain, MoDOT Mark J. Morvant, LTRC Gary H. Person, MN DOT Tom Shantz, CALTRANS Recep Yilmaz, Fugro Geosciences 12 February 2007 (*Integrated Text & Figures Version) NCHRP Project 20-05; Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 1 SUMMARY Cone penetration testing (CPT) is a fast and reliable means of conducting highway site investigations for exploring soils and soft ground for support of embankments, retaining walls, pavement subgrades, and bridge foundations. The CPT soundings can be used either as a replacement (in lieu of) or complement to conventional rotary drilling and sampling methods. In CPT, an electronic steel probe is hydraulically pushed to collect continuous readings of point load, friction, and porewater pressures with typical depths up to 30 meters (100 feet) or more reached in about 1 to 1½ hours. Data are logged directly to a field computer and can be used to evaluate the geostratigraphy, soil types, water table, and engineering parameters of the ground by the geotechnical engineer on-site, thereby offering quick and preliminary conclusions for design. With proper calibration using full-scale load testing coupled with soil borings and laboratory testing, the CPT results can be used for final design parameters and analysis. In this NCHRP Synthesis on the cone penetration test, a review is presented on the current practices followed by the Departments of Transportation (DOTs) in the USA and Canada. A detailed questionnaire on the subject was distributed to 64 DOTs, with 56 total respondents (or 88 percent), as detailed in Appendix A. The survey questions were grouped into six broad categories, including: (1) Use of the cone penetrometer by each agency; (2) Maintenance and field operations of the CPT; (3) Geostratigraphic profiling by cone penetration testing; (4) CPT evaluation of soil engineering parameters and properties; (5) CPT utilization for deep foundations and pilings; and (6) Other aspects and applications related to cone penetration testing. Of the total number of DOTs responding, about 27% use the CPT on a regular basis, another 36% only use the CPT on one-tenth of their projects, and the remaining 37% do not use the CPT whatsoever. Overall, it can be concluded that the technology is underutilized at present and that many DOTs could benefit in adopting this modern device into their site investigation practices. In fact, the survey results show that 64% of the DOTs plan to increase their use of CPT in the future. In its simplest use of application, the cone penetrometer offers a quick, expedient, and economical way to profile the subsurface soil layering at a particular site. No drilling, soil samples, or spoils are generated, thus the CPT is less disruptive from an environmental standpoint. The continuous nature of CPT readings permit clear delineations of various soil strata, their depths, thicknesses, and extent, perhaps better so than conventional rotary drilling operations that use a standard drive sampler at 5-foot vertical intervals. Therefore, if it expected that the subsurface conditions contain critical layers or soft zones that need detection and identification, the use of CPT can locate and highlight these particular features. In the case of piles that must bear in established lower foundation formation soils, the CPT is ideal for locating the pile tip elevations for installation operations. A variety of cone penetrometer systems are available, ranging from small mini-pushing units to very large truck and track vehicles. The electronic penetrometers range in size from small to large probes with from one to five separate channels of measurements. The penetrometer readings can be as simple as measuring just the axial load over the tip area, giving the cone tip resistance (qc). A second load cell can provide the resistance over a side area, or sleeve friction (fs). With both, the electronic friction cone is the normal type penetrometer, termed cone penetration test (CPT). A mechanical type CPT probe is available for pushing in very hard and abrasive ground. With the addition of porous filters and transducers, the penetration porewater pressures (u) in saturated soils can be measured, thus termed a piezocone penetration test (CPTU). The seismic piezocone (SCPTU) contains geophones to permit profiling of shear wave velocity measurements and the resistivity piezocone (RCPTU) uses electrodes to obtain readings on the electrical properties of the soil. Details concerning the standard equipment, calibration, field test procedures, and interpretation and presentation of results are discussed in the report. Specialized testing procedures and equipment used to achieve penetration in very dense or cemented ground are reviewed in this report. NCHRP Project 20-05; Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 2 The evaluation of soil type by the CPT is indirect and must be inferred from the penetrometer measurements, coupled with a good understanding of the local and regional geology. Thus, it may be beneficial to cross-calibrate the CPT results with logs from adjacent soil test borings in order to best utilize the technology in a reliable manner. In necessary cases, a simple CPT sampler can be deployed for obtaining soil specimens for examination. In addition, video CPT systems are available to allow visual identification of soils and subsurface conditions in realtime. The cone penetrometer is instrumented with load cells to measure point stress and friction during a constant rate of advancement. The results can be interpreted within different theoretical frameworks or using empirical methods, or both. As the data are logged directly to the computer, additional sensors can be readily incorporated including: porewater pressure, resistivity, inclination, and shear wave velocity, as well as a number of environmental measurements (gamma, pH, salinity, temperature, etc.). The ability of the CPT to collect multiple and simultaneous readings with depth is a valuable asset in the screening of subsurface conditions and evaluation of natural foundation bearing materials. The recorded data are stored digitally and can be post-processed to interpret a number of geotechnical engineering parameters that relate to soil strength, stiffness, stress state, and permeability. These parameters are needed input in the design and analysis of the stability of embankments and slopes, bearing capacity of shallow and deep foundations, and engineering evaluations concerning displacements, deflections, and settlements of walls, abutments, fills, and foundation systems. In some circles, the cone penetrometer is considered to be a miniature pile foundation. Thus, the recorded penetrometer data can be utilized either in a direct CPT method or indirect (or rational) approach for evaluating the point end bearing and side friction resistance of deep foundation systems. In this report, both approaches are discussed, with a particular effort given towards describing and outlining some of the newer methods developed for the piezocone and seismic cone. Driven pilings and drilled deep foundations are considered. Methods are also reviewed for the evaluation of bearing capacity and displacements of footings and shallow foundations from CPT results. From an economical standpoint, CPTs offer cost savings, as well as time savings in site investigation. On a commercial testing basis in 2006 dollars, the cost of CPTs is between $20 to $30 per meter ($6 to $9 per foot), compared with soil test borings at between $40 to $80 per meter ($12 to $24 per foot). Postgrouting of CPT holes during closure can add another $10 to $15 per meter ($3 to $4.50 per foot), whereas post-hole closure of the larger size drilled boreholes may add another $15 to $30 per meter ($4.50 to $9.00 per foot). In earthquake regions, the cone penetration test offers several capabilities in the evaluation of seismic ground hazards. First, the sounding can be used to identify loose weak sands and silty sands below the groundwater table which are susceptible to liquefaction. Second, the measurements taken by the CPT can provide an assessment on the amount of soil resistance available to counter shearing during ground shaking. The penetrometer can also be fitted with geophones to allow the determination of downhole shear wave velocity (Vs) profiles. The Vs data are required for site-specific analyses of ground amplification, particularly in the revised procedures of the International Building Code (IBC 2000/2003). Ground engineering solutions to soft and/or problematic soils now include a wide range of soil improvement methods, including: surcharging, wick drains, dynamic compaction, vibroflotation, and deep soil mixing. Applications of the CPT are particularly useful for quality control during ground modification as they allow a quick contrast in comparing the before and after measured resistances with depth. The CPT also allows for quantification of time effects after completion of improvement. NCHRP Project 20-05; Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 3 reliability. Generally. sandy fills. sleeve friction (fs).CHAPTER 1 . While drilling and sampling practices can be adequate. 1992). Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 4 . most state DOTs either maintain their own in-house drill rigs with field crews. as depicted in Figure 1. the tip and sleeve readings alone can suffice to produce a basic cone sounding that serves well for delineating soil stratigraphy and testing natural sands.. embankments. Towards the optimal design.or 5-cm. long-term maintence. a modern and expedient approach is offered by cone penetration testing (CPT) which involves pushing an instrumented electronic penetrometer into the soil and recording multiple measurements continuously with depth (e. Overview of the Cone Penetration Test (CPT) Per ASTM D 5778 Procedures. With piezocone penetration testing (CPTU). the state engineer will want to consider safety. Under certain instances. although a system for mechanical cone penetration testing (MCPT) is also available which is less prone to damage but which is advanced slower and provides coarser resolutions using an incremental vertical step of 20-cm intervals. 1978. 1988). transducers obtain readings of penetration porewater pressures that are paramount when Figure 1. retaining walls. or else subcontract soil drilling and sampling services from outside consultant companies. Briaud & Miran. the work is manual and timeconsuming. with follow-up laboratory testing often taking an additional two to four weeks for completion of results.INTRODUCTION Site-specific soil investigations are required for the analysis and design of all highway bridge foundations. NCHRP Project 20-05. Per ASTM and international standards. and porewater pressure (u) are obtained with depth. 1988. this is accomplished using an electric cone penetration test (ECPT) with readings taken at 2-cm. three separate measurements of tip resistance (qc ). slopes. In order to collect geotechnical information. Schmertmann. Rotary drilling methods have been around for two millenia and are well-established in geotechnical practice as a means to study soil and rock conditions (Broms & Flodin. Campanella & Robertson. excavations. and pavements. For soil exploration.g. and economy in his/her deliberations of various solutions. and soils with deep water tables. Soils above the groundwater table All soil types. Provides fundamental soil stiffness with depth: Gmax = ρt Vs2. Nevertheless. Detect freshwater . Natural sands. silts. Cone penetration tests can be advanced into most soil types ranging from soft clays and firm silts to dense sands and hard overconsolidated clays. thus the method is less disruptive than drilling operations.g. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 5 . A 10-m (30-foot) sounding can be completed in about 15 to 20 minutes. pile side friction (fp). including layering. fs.conditions contain shallow groundwater conditions and fine-grained soils consisting of clays. drilled shafts. a large amount of high-quality in-situ digital data can be recorded directly by the CPT in a relatively short time period in the field.. including shallow footings. u = penetration porewater pressure (u1 at face. results are immediately available on the computer for assessment in real time by the field engineer or geologist. Fill control. driven pilings. The porewater pressures at the shoulder position are required for correcting the measured qc to the total cone tip resistance. with Table 1 providing a quick summary of the various types of CPT commonly available. Additional sensors can be provided to increase the numbers and types of measurements taken. CPTs are especially advantageous when investigating environmentally-sensitive areas and/or potentially contaminated sites. Index to contaminant plumes. Therefore. This is especially important in the postprocessing phase when determining soil engineering parameters. Notes: qc = measured point stress or cone tip resistance.to 5cm intervals) Same as CPTu with timed readings of u1 or u2 during decay Same as CPTu with downhole shear waves (Vs) at 1-m intervals Same as CPTu with electrical conductivity or resistivity readings Applications Stratigraphic profiling. undrained shear strength (su). soil types. but they are not well suited to gravels. preconsolidation stress (Pc'). Natural sands. e. qt = total cone resistance. and sands with fines. Basic Types of Cone Penetration Tests Available for Site Characterization Type of CPT Mechanical Cone Penetration Test Electric Friction Cone Piezocone Penetration Test Piezocone with Dissipation Seismic Piezocone Test Resistivity Piezocone Test Acronym MCPT ECPT CPTu and PCPT CPTù SCPTu RCPTu Measurements Taken qc (or qc and fs) on 20-cm intervals. designated qt. Uses inner & outer rods to convey loads uphole. cobbles. and geotechnical engineering parameters. Vs = shear wave velocity. No spoil is generated during the CPT. qc and fs (taken at 1. thus may be a disadvantage to those who rely strictly on laboratory testing for specifications and state code requirements. NCHRP Project 20-05. fs = measured sleeve friction. These data can be subsequently post-processed to provide quick delineations of the subsurface conditions. Hard ground Fill placement. as the workers are exposed to a minimal amount of hazardous materials. u2 at shoulder). Table 1. etc. With the CPT.salt water interface. in comparison with a conventional soil boring that may take between 1 to 1½ hours. and either face u1 or shoulder u2 (taken at 1. as well as both direct and indirect evaluations of foundation systems. or hard rock terrain. lateral stress ratio (K0). Note: Requires u2 for correction of qc to qt Normally conducted to 50% dissipation in silts and clays.to 5-cm intervals) qc. and ground modification. however. Soil samples are not normally obtained during routine CPT. g. Anderson. there remains considerable uncertainty in the proper correction of the N-values (Kulhawy & Mayne. as well as the necessary lengths of driven or drilled piling foundations for the project. 2004). TN. NCHRP Project 20-05. Figure 3 shows an example subsurface profile developed from CPT qc profiles. or SPT resistance. Schmertmann. As a complement to (or in some cases. drive samples. 1978. "blow counts". often in general accordance with ASTM D 1586 or AASHTO T-206 procedures for the "Standard Penetration Test (SPT)".. these recorded Nvalues require significant corrections to the field measurements before they can be used in engineering analysis (e. as well as additional influences such as borehole diameter. 1965. hammer system.5-m (5-foot) vertical intervals. and other factors (e. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 6 . Thus. During the advance of the soil boring. and undisturbed samples (e. It is well-known that this N-value can be severely affected by energy inefficiencies in the drop hammer system. thicknesses. as a replacement for) soil borings with SPT-N values. et al.. 1970). Tanaka & Tanaka. Fletcher. If geostratification at a site is the primary purpose of the site investigation. Robertson.3 meters (12 inches) is termed the "N-value". sample liner. The variations both vertically and laterally can be quickly determined using the cone tip resistance. soil layers. 1986). the normal practice is to secure small diameter drive samples (termed "splitspoons" or "split-barrel" samples) at 1. et al. Figure 2 shows a side-by-side comparison of an electric CPT point resistance (qc) profile with a boring log derived from two adjacent boreholes with SPT resistances (N-values) in downtown Memphis. Moreover. 1990) and the repeatability of SPTs using different equipment and drillers remains an issue (e.g. Companion Profile of CPT Cone Tip Resistance and Soil Boring Log with SPT-N values. The CPT resistance complements the discrete values from the SPTs at the site and helps to better define the interface between layers.. 1983. and relative consistencies of each stratum. A number of difficulties are now recognized with routine drilling practices in obtaining field test values. Skempton. The continuous nature of the CPT point resistance is evident in the profiling of the various strata and soil types. The recorded number of blows to drive the sampler 0.g.Figure 2. The thicknesses of soft compressible clay and silt layers can be mapped over the region and this information is useful in determining the settlements of embankment fills and shallow foundations. rod length.. then CPT soundings can be readily advanced to detail the strata across the highway alignment. and consistency.g. et al. 1999). the cone can provide similar information on the subsurface stratigraphy. Ireland.. In contrast. 1991) or reconsolidation phase (DeGroot & Sandven. both of which add to lab testing times and more elaborate procedures. Nevertheless. 2004). especially when compared with high-quality and more expensive methods. Sherbrooke. reliance on SPT solely can lead to significant oversimplifications in predicting true soil behavior. Ladd. Perhaps the best approach is one founded on a combination of quick CPT soundings to scan for weak layers and problematic zones. and JPN samplers (e. NCHRP Project 20-05. it is standard practice to obtain "undisturbed samples" using thin-walled (Shelby type) tubes (e. NGI. and roadway design.g.Figure 3. Subsurface Profile Developed from an Array of CPT qc Profiles Since soils are very complex and diverse materials within a natural geologic environment. a number of geotechnical firms and highway departments rely on SPTs from soil borings as their primary data source for bridge. onedimensional consolidation. the CPT obtains measurements directly on the soil while still in its natural environs.. thus offering a direct assessment of soil behavioral response to loading. ASTM D 1587 and AASHTO T 207) that will later be used to provide smaller specimens for "high-quality" laboratory testing.g. Additional sensors are available to produce up to four or five direct readings with depth in order to ascertain a more realistic evaluation of soil behavior. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 7 . permeability. wall. or resonant column tests. direct shear. One clear advantage of the CPT is its ability to provide three independent and simultaneous measurements. Yet..g. such as the Laval. such as triaxial shear. it is now wellrecognized that "undisturbed samples" are very difficult to obtain with this simple tube sampler. 1999). During routine drilling operations in North America. followed by rotary drilling operations to procure soil samples for examination and lab testing. Tanaka & Tanaka.. Methods for correcting laboratory testing for sample disturbance effects include either a consolidation-unloading phase (e. One purpose of Synthesis Topic 37-14 was to gather information from all state and provincial DOTs towards defining and sharing common experiences. Reasons for not using the CPT by the DOTs vary considerably across the USA and Canada. Much of the CPT work is conducted by outside consultants working under contract to the DOT. Towards this goal. Hopefully. Since many diverse geologic formations span the North American continent. but particularly focused towards studies involving soft to firm clays. the cone penetration test (CPT) is increasingly being valued as a productive and costefficient means of site investigation for highway projects. lack of expertise. loose sands. CA. giving an overall response rate of 88% to the questionnaire. embankments. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 8 . the state/province geotechnical engineer was offered the opportunity to engage responses from selected consultant testing firms to aid in the survey results. a total of 56 replies (out of 64 DOTs) were returned. With regards to geotechnical project type. the CPT has been utilized for an assortment of differing purposes. Where pertinent. NM. Appendix A contains a summary of the findings derived from the responses to the 59 questions posed in the survey. as summarized by Figure 6. In the top category. successes & failures. KS. a survey questionnaire was prepared as an initial first step in finding out the individual practices from highway departments and their consultants. Despite the advantages of the CPT. Fifteen DOTs (or 27% of the respondents) utilize the CPT on a fairly regular basis on their geotechnical projects (as shown in Figure 4). ON Percent Annual Geotechnical Projects Using CPT 30% 20% 10% None 0 5 56 Respondents (out of 64 requests) 10 15 20 25 Number of DOTs Responding Figure 4. 1. and nonfamiliarity with the technology. QB. as indicated by Figure 5. BC. VA. UT. The CPT is used to investigate a variety of different geomaterials. NCHRP Project 20-05. NE.SURVEY QUESTIONNAIRE ON CPT In many instances. with the primary applications towards bridge design.CHAPTER 2 . Use of CPT by State & Provincial DOTs ≥ 40% MN. In all. NB LA. and value in applying cone penetrometer technology in highway design & construction. SC. the TRB decided that a synthesis on the state-of-practice in cone penetration testing would be a helpful guide in its upcoming utilization. NM MO. and fills. this synthesis will be able to address these major issues. The questionnaire was directed at the 52 state DOT geotechnical engineers in the USA and their equivalents at 12 provincial DOTs in Canada. and deep foundations. Results from Survey Questionnaire on Annual Use of CPT by DOTs. current practices at 37% of responding state and provincial DOTs do not use any CPT technology whatsoever. Other common reasons for non-utilization included limited accessibility. their responses indicate that many geologic settings were apparently too hard and not conducive to successful penetration by standard CPT equipment. organic soils. Another 36% of DOTs use the CPT only on 10% of their site investigation studies. Hydro-Collapse (2). Types of Geotechnical Projects that the CPT is used on by State & Provincial DOTs. Reasons that the CPT is not used by State & Provincial DOTs. NCHRP Project 20-05. Specific identified issues are given within Figure 7. Only 6% of responses indicated difficulties.Question 8 0 Sinkholes Emb Fill Control Other Subgrades Excavations QC Ground Mod Seismic Ret. Landslide Mapping.Wall Fdns Slope Stability Deep Fdns Embankments Bridge Foundations 5 Number of Replies 10 15 20 25 30 Types of Projects [Peat. Wall Design Soil Liquefaction Soft soils delineation R. Too Hard Limited Access Lack Expertise Not Familiar Rubble Caprock Equip Unavail Too Expensive Req Skill Other Correlations Invalid None 0 5 10 15 20 Obstacles to Use of CPT Question 10 48 Respondents 25 30 35 Number of DOT Replies Figure 6. Environmental) 36 Respondents Figure 5. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 9 . with another 25% indicating an occasional problem. Over two-thirds of the DOTs have not had any unfavorable experiences with the CPT on their projects (see Figure 7). 4. Survey Results of Unfavorable Experience with the CPT by DOTs Not Applicable 18% 45 Respondents Increase a lot 11% Increase a bit 53% Stay Same 18% Decrease 0% Figure 8. Shallow hard layer and neg. Poor quality data (electronics) 3. Refusal . Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 10 .Based on the survey results.required preboring 6. Outdated DOS pile software 5. Met early refusal Yes 6% Not Applicable 3% Occasionally 25% 1. Projected Use of CPT on Future DOT Geotechnical Projects NCHRP Project 20-05.halted CPT 7. Dense layer . 9. No 66% 43 Respondents Figure 7. 8. Approximately 64% of the respondent DOTs indicate they have made plans to increase their use of CPT in coming years for site exploration and geotechnical investigations (see Figure 8). pwps. Excessive inclination . Stopped by gravel and shale. 1. No consensus on N-bearing factor to obtain su value.Depth not achieved 2. it appears that the majority of DOTs are aware of the CPT technology and its availability. Cannot use BC equations reliably 2. Myriad of excuses by contractor. Electrical type penetrometers with both tip and friction readings were designed as research tools as early as 1949 and became commercially available in the 1960's (deRuiter. primarily due to rusting and bending. 1981). Battaglio. 1982). 1988). Electrical versions were developed circa 1948 by the Delft Soil Mechanics Laboratory (DSML) which offered continuous measurements of tip resistance with depth and direct strip chart plotting of the sounding record (Vlasblom. A representative schematic of a standard penetrometer cross-section is depicted in Figure 9. the advent of piezoprobes showed value in profiling penetration porewater pressures in soft clays and layered soils. In the 1970's. and porewater pressures (Tumay. particularly those that are highly stratified (Senneset. the penetrometer is linked via a wired cable through the hollow cone rods to a field computer at the surface for automated data acquisition. Inclinometer Geophone Friction Sleeve Strain Gauges Pressure Transducer Porous Filter u2 location Porous Filter u1 location Figure 9. Robertson. 1974. In the electrical systems. Initial cone systems were of the mechanical type designs with two sets of rods. a research piezocone was being designed for tip and porewater readings by DSML (Vlasblom. 1981). et al. sleeve friction. 1971.CHAPTER 3 . An outer set of steel rods was employed to minimize soil friction and protect an inner stack of rods that transferred tip forces uphole to a pressure gauge read-out at the ground surface. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 11 . et al. 1981. Basic Internal Schematic of an Electric Cone Penetrometer As early as 1962.1-foot) thick fill (Broms and Flodin. An inclinometer was incorporated to detect deviations from verticality and thus offer a warning to the user against excessive slope and/or buckling problems (Van De Graaf & Jekel. These solved noted problems associated with poor load readings acquired by mechanical cone systems because of frictional force buildups between the inner and outer sets of rods. 1985). Barentsen used a field cone to measure tip resistances with depth in a 4-m (13. but used for exploration in sands. 1985).CONE PENETROMETER EQUIPMENT History Cone penetrometers have been in use for soil exploration since 1932 when the Dutch engineer P. 1965). A step-wise pushing procedure applied at 20-cm (8-inch) increments permitted successive sets of tip and sleeve readings using the same load cell. NCHRP Project 20-05. Field readings were taken by hand. The merger of electric cone with the electric piezoprobe was an inevitable design as the hybrid piezocone penetrometer could be used to obtain three independent readings during the same sounding: tip stress. The electrical CPTs are also faster to perform than mechanical CPTs since they are conducted at a constant rate of push rather than stepped increments. 2001). Later a sleeve was added to provide a secondary measure of vertical loads over a cylindrical surface above the tip (Begemann. Baligh. et al. NCHRP Project 20-05. including: (a) temperature. Robertson. Figure 10 shows the basic styles of penetrometers in routine use and these are patterned after the original Fugro-type designs (De Ruiter. electronic systems have become available that contain the signal conditioning. 5 mm) at the upper portion.4-inch) diameter version having a corresponding crosssectional area Ac = 10 cm2 and sleeve area As = 150-cm2. sleeve friction (fs). and (2) 44-mm (1. 1995). and (5) data acquisition unit. Dimensions and Measurements Taken by Standard 10-cm2 and 15-cm2 Penetrometers. 2001). as discussed in the next section. (c) geophones.75-inch) diameter version (Ac = 15-cm2 and As = 200 to 300 cm2). More recently. and penetration porewater pressure (um).7-mm (1.Over the past three decades. (b) electrodes. (4) depth recorder. As rod sizes are normally 35. The front end consists of a 60º apex conical tip that has a small lip (approx. (f) vibrators. Figure 10. a number of other sensors or devices have been installed within the penetrometers. With digital cone penetrometers. (h) microphones for monitoring acoustical sounds. Briaud & Miran (1992). (g) radio-isotope detectors for density and water content determination. (3) cable or transmission device.7 mm in diameter. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 12 . and digital output directly within the penetrometer downhole. 1970). (e) fulldisplacement pressuremeters. Additional details on these topics may be found in Robertson & Campanella (1984). The penetrometers are normally available in two standard sizes: (1) a 35. While the 10-cm2 size is the original standard size. Other developments include a number of wireless CPT systems. & Powell (1997). many commercial firms have found the 15-cm2 version to be stronger for routine profiling and more easily outfitted with additional sensors in specific needs. and special designs for deployment in the offshore environment (Lunne. amplification. only 4 wires are needed to transmit the data uphole in series (in lieu of the parallel signals sent by cable). and (i) dielectric and permittivity measurements (Jamiolkowski. the 15-cm2 size cone also tends to open a larger hole and thus reduce side rod friction during pushing. (d) stress cells. (2) a hydraulic pushing system with rods. These items are briefly discussed in the following subsections. Equipment A CPT system includes the following components: (1) an electrical penetrometer. and Lunne. Penetrometers The standard cone penetrometer consists of a three-channel instrumented steel probe that measures: cone tip stress (qc). and 1-cm2 sizes). or else a reusable sintered metal or ceramic type. and 15-cm2 triple-element type. A second load cell is used to record the axial force either along the sleeve (Fs) within a "tension-type cone" design. although a few commercial units are available in stainless steel or brass. 5-. until proven otherwise (Campanella & Robertson. with a few deploying the 15-cm2 type. Periodically. Selection of Penetrometers from: (a) van den Berg series.g. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 13 . yet also in field applications (e. 1988). type 1. 15-. 10-. Tumay. or else subcontract to firms that use commercial equipment or maintain an arsenal of their own in-house penetrometers and data acquisition systems. miniature cone penetrometers are available. The reusable types can be cleaned in an ultrasonics bath. An internal load cell is used to register the axial force at the front of the penetrometer (Fc). the tip and sleeve elements are replaced due to wear or damage. dimensions. Appendix B lists various CPT manufacturers and service companies. NCHRP Project 20-05. These mini-cones have been used in laboratory testing programs. dual-piezo-element). else positioned behind the sleeve (Type 3). In the latter (termed "subtraction-type cone"). (a) (b) (c) Figure 11. Specifications on the machine tolerances. and (c) Georgia Tech collection (bottom to top): 5-cm2 friction.Depending upon the types of soils being tested. State and provincial DOTs which engage in cone penetration work either use commercially-manufactured CPT systems. and load cell requirements for electrical CPTs are outlined in ASTM D 5778 and in the international reference test procedure (IRTP. Based on the survey results. four 10-cm2 piezocones (type 2. Several are using advanced cones with seismic. the porous filter is usually located either at the apex or midface (termed Type 1) or at the shoulder (Type 2) just behind the cone tip. Also. Most penetrometers are constructed of tool-grade steel. data acquisition. 1998). et al. It is common to replace the porewater filter after each sounding with either a disposable plastic ring type. the Type 2 is required by national and international standards. both in calibration chambers and centrifuges. type 2 seismic. with reduced cross-sectional sizes of 5cm2 and 1-cm2 discussed in the open literature. guidelines per ASTM D 3441 still remain on the books. or else located in the back and records the total tip force plus sleeve (Fc + Fs). and postprocessing. In special instances. (b) Fugro series (left to right: 33-. while only 2 DOTs are using mini-cones and 1 DOT is operating a mechanical CPT. video.. or resistivity. including a 33-cm2 version and a 40-cm2 model which can be pushed into gravelly soils. For the older mechanical CPT systems. For the proper correction of measured cone tip resistance to total resistance. most DOTs are using 10-cm2 size penetrometers. and related websites for additional information on equipment. the combined force minus the separately-measured front force provides the sleeve force. 1999). large diameter penetrometers have been developed for special projects. Figure 11 shows a selection of various penetrometers. Perhaps not appreciated is that. and (2) field porewater pressures to be measured at the shoulder position (ub = u2). This too requires a correction. 1986. as illustrated in Figure 12. without the total resistance. Lunne et al. qc = Fc/Ac. This is an important finding in that.. beyond standard practice and not required by the ASTM nor international standards. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 14 . However. taken at both top and bottom ends of the sleeve. The total cone tip resistance is determined as: qt = qc + (1-an)u2 (1) In clean sands and dense granular soils. and requires two prerequisites: (1) calibration of the particular penetrometer in a triaxial chamber to determine the net area ratio (an). thus the correction is not paramount. The corrected tip stress or total cone tip resistance is designated as qt. et al. 1988). thus indicating excellent response (best fit line from regression shown). NCHRP Project 20-05. et al. It can be seen that the porewater transducer provides a one-toone correspondence with the applied chamber pressures. however. This stress must be corrected for porewater pressures acting on unequal tip areas of the cone. appreciable porewater pressures are generated and the correction can be very significant. fs = Fs/As. Also shown is the response of the sleeve reading with applied pressure (conceptually.g. 1997). and therefore. the correction is still needed. The uncorrected cone tip resistance (qc) shows significantly less response.. from 20% to 70% in some instances (Lunne. 1988. The measured axial force over the sleeve (Fs) is divided by the sleeve area to obtain the sleeve friction. et al. this should show no readings). especially important in soft to firm to stiff intact clays and silts (Jamiolkowski. Campanella and Robertson. Campanella & Robertson.. in soft to stiff clayey soils. the value qt ≈ qc. the interpretations of soil parameters and application of direct CPT methodologies may not be as reliable as they could be. Figure 12. 1997). 1985. 1985). with a corresponding net area ratio an = 0. A friction correction factor (bn = 0. et al.58 for this particular penetrometer.014) can be applied using the guidelines given in the Swedish CPTu standard (e. at this time.Penetration Tip and Sleeve Readings The measured axial force (Fc) divided by the area gives the measured tip resistance. two porewater pressure readings are needed. even with standard friction-type cones that do not measure porewater pressures. Determination of Total Cone Tip Resistance and Total Sleeve Friction (after Jamiolkowski. Lunne.. An example calibration of a (brand new) cone penetrometer within a pressurized triaxial chamber for all three readings is presented in Figure 13. Results from the survey indicate that only 48% of DOTs are correcting the measured tip resistances to total tip stress.. clearly indicating that the latter is around 35% greater than the uncorrected value. the importance of using a corrected cone resistance in profiling of soil parameters can be fully appreciated. Example CPTu Sounding Showing Uncorrected and Corrected Cone Tip Resistances. Using the same penetrometer. Thus. Cone Calibration Results in Pressurized Triaxial Chamber for Net Area Ratio Determination. Figure 14. NCHRP Project 20-05. The soil profile consists of a desiccated crust overlying soft clay and a layer of loose to firm sands to silty sands. Here. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 15 . the raw measured qc can be compared directly with the corrected total qt. with soft silty clays encountered at depths greater than 12 m within the termination depths of 22 m.Figure 13. an illustrative sounding in soft sediments near the City of New Orleans is presented in Figure 14. porewater pressures are monitored using a saturated filter element connected through a saturated portal cavity that connects to a pressure transducer housed within the penetrometer. usually located mid-face. A series of five piezocone penetration tests (CPTu) is presented in Figure 15 showing total cone resistance. However. Additional remarks are given on this issue in Chapter 4.1 (kPa) 1000 1500 2000 Depth (m) 10 12 14 16 18 20 22 fcpt01 fcpt02 fcpt03 fcpt04 fcpt05 fs qt u0 u2 u1 Figure 15.. and a fair number employ both u1 and u2 readings (11%).. and less common position located behind the sleeve (u3). the resulting measurements will appear either incorrect or sluggish. and even quad-element piezocones are also available (e. The tests were made very near the national geotechnical experimentation site at Northwestern University in Evanston. triple-. in stiff fissured clays and other geologic formations (e. Series of Piezocone Penetration Tests at Northwestern University. Tip Resistance Sleeve Friction Porewater Pressure qT (MPa) 0 0 2 4 6 8 2 4 6 8 10 12 14 0 fs (kPa) 100 200 300 0 500 u2. Without due care. Therefore. not realizing their full magnitude. Proper saturation of the filter elements and portal cavities in the penetrometer during assembly is paramount to obtaining good quality penetration porewater pressures. superior profiling capability is attained using a face porous element. Usually. although a good number use a face element (22%). The tests show very good repeatability in the recorded data. although dual-. in these cases. Chen & Mayne 1994). From the survey. because of trapped air pockets or gas within the system.g. sleeve friction. Illinois where a sandy fill layer approximately 4 meters thick is underlain by deep deposits of silty clays. zero to negative porewater pressures can be recorded. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 16 . CPTs used on DOT projects generally utilize a filter element position at the shoulder position (49%). Common filter locations include the tip or face (designated u1) or the shoulder (u2). NCHRP Project 20-05. designated ub = u2) because of the required correction to total tip stress discussed previously. Most penetrometers measure a single u-value. and penetration porewater pressure measurements at the shoulder (4 soundings) and midface (one sounding).g. The standard location is the shoulder element (just behind the tip.Penetration Porewater Pressures The measured porewater pressures (um) can be taken at a number of different positions on the penetrometer. although some apex versions have been used as well. residual soils). (a) (b) (c) (d) Figure 16. There are also many small lightweight CPT systems in the 18 to 50 kN range (2 to 6 tons) that utilize earth anchoring capabilities to gain capacity. Downhole CPTs can also be conducted step-wise in deep boreholes by alternating off and on with rotary drilling bits. trailer. and (d) Hogentogler & Company. A few specialized vehicles have been built with addon weights to provide up to 350 kN (40 tons) reaction. onshore CPTs have reached 100 m using direct-push technology from the ground surface. For difficult access sites. (c) Vertek Type Operated by MN DOT. or portable unit. yet are more mobile and portable than the deadweight vehicles (Figure 18). with depths up to 300 m or more achievable (e.. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 17 . all-terrain vehicle. A selection of truckmounted type CPT rigs is shown in Figure 16.g. In some special cases. After positioning the rig at the desired test location. 1990). Typical depths of penetration by CPT rigs depends upon the site-specific geologic conditions but most commercial systems are setup for up to achieve 30 m (100 feet). track. The track-mounted systems generally require a second vehicle (tractor trailer) in order to mobilize the CPT rig. A full capacity hydraulic system for CPT work is considered to be on the order of 200 kN (22 tons). These anchored rigs can obtain significant depths and penetrate rather dense and hard materials. the rig is usually leveled with hydraulic jacks or "outriggers". (b) ConeTec Investigations. Figure 17 shows selected tracktype and all-terrain rubber-tired vehicles for CPT. Robertson. The dedicated CPT systems push near their centroid of mass and usually rely on dead-weight reaction of between 100 to 200 kN (11 to 22 tons) for capacity. Hydraulic Pushing System The hydraulic pushing system can consist of a standard drill rig or a dedicated CPT hydraulic system mounted on a truck. Selection of Truck-Mounted Cone Penetrometer Rigs: (a) Fugro Geosciences. NCHRP Project 20-05. exception being the special track-truck design shown in Figure 17d. skid arrangement. CPT is two to five times more efficient than conventional rotary drilling. production rates of between 30 m/day to over 150 m/day (100 to 500 feet/day) were reported by the DOTs. (c) Vertek All-Terrain Rubber Tired Vehicle.79 in/s) per ASTM D 5778 and IRTP (1999). and (d) Fugro Track-Truck. In the survey questionnaire. with the majority indicating a typical rate of 60 m/day (200 feet/day). Typical rates of drilling of soil borings by state agencies is between 15 to 30 m/day (50 to 100 feet/day). Some limited ability exists for occasional soil sampling. Thus. in terms of lineal productivity. NCHRP Project 20-05. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 18 . but this is not routine. (b) Pagani. A disadvantage of the CPT rigs is that their basic abilities include only pushing and pulling the probes. (b) Remotely-Operated van den Berg Track Rig. CPT Vehicles for Difficult Access: (a) ConeTec Track Rig. The standard rate of testing is at a constant push of 20 mm/s (0. Anchored-Type CPT Rigs: (a) GeoProbe Systems. if necessary. and (c) Hogentogler. (a) (b) (c) Figure 18. The dedicated CPT systems are geared for production testing.(a) (b) (c) (d) Figure 17. and optical reader. thus an escape slot or special sub-connector piece must be provided for the electrical cable.7-mm outer diameter hollow steel rods in one-meter lengths with tapered threads. as well as obtain soil samples on-site.. rods must be pulled from the top. and (4) care in manual control of hydraulic pressure must be made to achieve constant 20 mm/s push rate. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 19 . gear box. Figure 19. Since NCHRP Project 20-05. Some common systems include: depth wheel. ultrasonics sensor. displacement transducer (either LVDT or DCDT). the DOT damaged the slide base of a CME 850 rig during pushing operation. In other cases. a cumulative tracking of each one-meter rod increment is made to determine depth. Cone Rods With Threaded Electronic Cabling and Grips with Hydraulic Rams for Pushing Cone Rods Cone rods consist of 35. The friction reducer is merely an enlarged section of rods (e. (2) during advancement. In one instance. and then continue the soundings to the desired depths. a set of standard "A" or "AW" drill rods (or standard cone rods) may be used with a sub-connector to convert the metric threads of the penetrometer to the drill rods. For hard ground. the actual total cable length is monitored. potentiometer (spooled wire). Depth Logger There are several methods to record depth during the advancement of the CPT. This reduces costs associated with mobilizing a dedicated CPT truck. A stack of 30 to 40 one-meter long rods is common (see Figure 19). thus a sub-connector piece must be added and removed for each rod break. a ring welded to the outside rod) on the sub-connector above the penetrometer that opens the pushed hole to a larger diameter. thereby reducing soil contact on all the upper rods. If a drill rig is used.or hydraulic-type) to grasp the sides of the rods during pushing and pulling. (3) during withdrawal. Major difficulties with CPTs performed using standard drill rigs include the following: (1) the dead weight reaction is only around 50 kN (5.5 tons). a larger diameter set of cone rods is also available (d = 44 mm). In most cases.g.Advantages of using standard drill rigs for CPT work include the added capabilities to drill and bore through hard cemented or very dense zones or caprock. A friction reducer is often provided to facilitate pushing operations. The hydraulic systems of dedicated CPT rigs are usually outfitted with grips (either mechanical. rods are pushed from the top. All are available from commercial suppliers and some designs are patented for a particular system. as necessary. using the same rig. if encountered. wireless (or cableless) digital CPT systems have been developed. GIS and Field GPS Coordinates Today. They are particularly favored when CPT is conducted using standard drill rigs and crews (since the cable might easily be damaged) and in offshore site investigation where wireline can deploy the units to great depths. and (3) data stored in battery-powered micro-chip until penetrometer retrieved back at surface. 2006) and CALTRANS (Turner. electrical current is supplied since the losses over long cable lengths is mitigated. since the same outer diameter of the cable had to be maintained in order to insert it through the hollow center of the cone rods and thus smaller wires internal to the cable were more fragile. it is quite inexpensive to provide a small hand held unit that contains a global positioning system (GPS) to give latitude and longitude coordinates. The cable is used to provide voltage (or current) to the penetrometer and to transmit data back up to the computer for storage. (2) audio-transmitted signals.each of the channel sensors is technically positioned at slightly different elevations. The disadvantage of the newer digital systems is that proprietary designs restrict the data coding and channel sequences from the output. Data Acquisition System A wide variety of data acquisition systems have been developed for electric CPTs. 16-. 24-. Data Transmission and Cabling All analog CPT systems and many digital CPT systems use a cable threaded through the rods for transmission of data uphole. however. based on the following technologies for data transmission or storage: (1) infrared signals conveyed uphole in glass-lined rods. it is standard practice to correct the readings to a common depth. These coordinates can be entered into a geographic information service (GIS) database for future referencing and archiving of geotechnical information. An advantage of the older analog systems is that they could be adapted to accommodate any type of commercial cone. Therefore. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 20 . cable. only a matched set of penetrometer. Alternate systems employed 12-. a special receiver is required uphole at the top end of the rods to capture the signals and decode them for digital output. The standard cables were 10-pin type. thus a maximum of 5 channels (2 wires per channel) could be read. and analog-digital converter at the surface. signal amplifier. Currently. initially starting with simple pen plotters and analog-digital converters to matrix dot printers. depending upon the manufacturer design. et al. usually taken at the tip of the penetrometer. The field record for each CPT sounding should be documented with its GPS location. NCHRP Project 20-05. A variety of wireless systems are available. With the above infrared and acoustic transmissions. in lieu of voltage. and 32-wires. 2006) both have established GIS programs to coordinate and organize statewide sets of soil borings and CPT records. In some of the newest designs. MN DOT (Dasenbrock. In the van den Berg system. and evolving to fully digital systems with ruggedized notebooks and micro-chip technologies with memory within the cone penetrometer itself. The initial electric CPT systems were analog types that required an external power supply. and data acquisition system can be used. A power supply is normally used to provide a voltage of between 5 to 20 volts. at the sacrifice of longevity. filter element preparation.CHAPTER 4 . 18% water. however. The protocol for environmental soundings recommends that sintered stainless steel filters be used. an annual production of 12. IRTP (1999). The rate will depend upon soils tested. This is best accomplished using a penetrometer having a male plug in the tip section to promote positive fluid displacement when the tip is screwed onto the chassis. as well as special testing practices. 1990. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 21 . and preparing the penetrometer and field advancement of the cone penetration test are well established. if a female plug is provided on the tip unit. the closure criteria depend upon the regional groundwater regime and aquifer characteristics.. The tip and sleeve should be replaced if damaged or if excessively worn. 1994. (1997). Procedures for calibrating. The plastic versions are common since they are disposable and can be replaced after each sounding to avoid any possible clogging problems particularly associated with plastic clays.g. as well as check on the net area ratio (an). Mulabdić. These ports and cavities must also be fluid-filled at all times. Filter Elements The filter elements used for piezocone testing are usually constructed of porous plastic or ceramic or sintered metal. field testing procedures for cone penetration testing are reviewed. it appears that the penetrometers and/or field computers are returned to their respective manufacturers to confirm the equipment is within calibration and tolerances. including: calibration. assembly. In the retraction of the cone penetrometer and completion of the sounding. the filter elements must be installed so that a continuity of fluid is maintained from the filter face through the ports in the penetrometer and cavity housing the pressure transducer. et al. calibrations can be conducted in-house to check for load cell compliance using a compression machine. a ceramic filter is preferred as it offers better rigidity and is less prone to abrasion over plastic filters. An alternative would be use of silicone oil as the saturation fluid. Lunne et al. and 7% a half-half mix of glycerine and water. 18% silcone oil. For most CPT operators.TESTING PROCEDURES AND SOUNDING CLOSURE In this section. It is normal practice to presaturate 10 to 15 elements overnight for use on next day's project. as sands are considerably more abrasive than clays. baseline readings. maintaining. The frequency of such depends upon the amount of usage and care taken during storage between soundings. 1997). For face elements. Lunne et al. Calibration and Maintenance of the Penetrometer The penetrometer requires calibration and maintenance on a regular basis. those fluids require much more care during cone assemblage. Yet. Chen & Mayne. procedures are quite different and vary across the US and Canada. and withdrawal. In many cases.000 m/year would likely require tips and sleeves being replaced once to twice per annum. Saturation of the filter elements should be accomplished using a glycerine bath under vacuum for a period of 24 hours. A sealed and pressurized triaxial apparatus can be used to check for pressure transducer calibrations. depending upon hole closure requirements established by the state or province. pushing. The fluid should be 100% glycerine (or silicone oil) that is easily applied using a plastic syringe. In the field. and other guidelines. per ASTM D 5778. The DOT survey indicated 39% used glycerine. however. since polypropylene types are from petroleum based manufacturer and may cross-contaminate readings. Otherwise. Sintered elements are not to be used for face filters however because of smearing problems. For a typical CPT rate of 60 m/day used 4 days/week. the penetrometer must be carefully assembled NCHRP Project 20-05. It is also possible to use water or a 50-50 mix of glycerine and water. Full details concerning the calibration of cone and piezocone penetrometers are given elsewhere (e. The sintered metal and ceramic filters are reusable and can be cleaned using an ultrasonics bath after each sounding. Thus in those case. Given sufficient time in all soils. Using a rotary drill rig. Thus. and desaturation of elements in the unsaturated vadose zone. usually applied in 1-m increments (standard cone rod length). there is an opportunity to conduct intermittent testing before the next succession of pushing as the next rod is added. This avoids problems associated with vacuum presaturation of elements. it is possible to use a very thin (0. this is considerable more effort than the aforementioned approach with a positive displacement plug on the tip. The installation of a full-displacement device such as a cone penetrometer results in the generation of excess porewater pressures (Δu) locally around the axis of perturbation. Two common procedures include: (1) dissipation testing. at the expense of a more sluggish transducer response and less detailing in the um profiling. the measured Δu will require a considerable time to equilibriate. it would be desirable to measure time as well as depth so that the actual rate can be ascertained. In clean sands. 1995). These baselines should be recorded in a field log booklet and checked periodically to forewarn of any mechanical or electronic shifts in their values.while submerged in the saturating fluid. electronic baselines or "zero readings" of the various channels of the penetrometer are recorded. Tests at Intermittent Depths At each one-meter rod break. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 22 . in lieu of a filter element and saturation procedure. the penetrometer porewater channel will eventually record the ambient hydrostatic condition corresponding to u0. assembly difficulties in the field. the hydraulic system is automatically established to adjust the pressures accordingly in order to maintain this constant rate. During the initial push into the ground. Baseline Readings Prior to each sounding. Larsson. it is common practice to tightly place a prophylactic containing saturation fluid over the front end of the penetrometer. as possible damage or calibration errors may occur. the Δu will dissipate almost immediately because of the high permeability of sands. 1995. Once assembled. Undoubtably. usually accomplished with a special cylindrical chamber designed for such purposes. Porewater Dissipation Tests Dissipation testing involves the monitoring of porewater pressures as they decay with time. this light rubber membrane will rupture automatically. the driller must be attentive in manually adjusting pressures to seek a rate around 20 mm/s. however. such that: um = Δu + u0 (2) NCHRP Project 20-05. and (2) downhole shear wave velocity measurements. It is also recommended to secure a set of baseline readings after the sounding has been completed and the penetrometer withdrawn to the surface. Advancing the Penetrometer The standard rate of push for CPT soundings is 20 mm/s. Several rubber bands are used to secure the rubber covering and help maintain the saturated condition. With dedicated CPT rigs. the measured porewater pressures (um) are a combination of transient and hydrostatic pressures. however. whereas in clays and silts of low permeability.3-mm) grease-filled slot to record porewater pressures (Elmgren. In new developments. Provision of a biaxial arrangement of two geophones at each elevation allows correction for possible cone rod rotation. designated t50. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 23 .. A more reliable Vs is achieved by true-interval downhole testing. it becomes impractical to wait for full equilibrium that corresponds to Δu = 0 and um = u0. Shear Wave Testing A convenient means to measure the profile of shear wave velocity (Vs) with depth is via the seismic cone test (SCPT). Dissipation readings are normally plotted on log scales.5. The shear wave arrival time can be recorded at the test elevation by incorporating one or more geophones within the penetrometer. Setup and Procedure for Pseudo-Interval Seismic Cone Testing (SCPT). At the one-meter rod breaks. et al. that provides a pseudo-interval downhole Vs (Campanella. the rate at which Δu decays with time can be monitored and used to interpret the coefficient of consolidation and hydraulic conductivity of the soil media. as long as the geophone axis is kept parallel to the source alignment (no rotation of rods or cone) and a repeatable shear wave source is generated at each successive one-meter interval.g.or 1. et al.0-m vertically apart). A standard of practice is to record the time to achieve 50% dissipation. This approach is sufficient in accuracy. 1986). since shear waves only have movement in their direction of motion and direction of polarization (only 2 of 3 Cartesian coordinate directions). Figure 20. a surface shear wave is generated using a horizontal plank or autoseis unit. as depicted in Figure 20. The vertical component could be used in a crosshole test arrangement (e.During the temporary stop for a rod addition at one-meter breaks. 1988). incorporation of a triaxial geophone with vertical component really offers no benefit. as the resultant wave can be used (Rv2 = x2 + y2). For downhole testing. The simplest and most common is use of a single geophone. NCHRP Project 20-05. but this requires two or more geophones at two elevations in the penetrometer (usually 0. Baldi. therefore in clays with low permeability. and 18% grout using a secondary deployment system (e. The grout or slurry sealants can be placed using surface pour methods. 1995b). Hole is backfilled using native soils or pea gravel or sand. 1995). 18% grout during retraction. gypsum. Results of the questionnaire on the subject of CPT hole closure indicated that 43% allow the hole to remain open. contamination. Hole sealing can be accomplished using either a bentonite slurry or a lean grout made from Portland cement. or special CPT systems that provide grouting during advancement or during withdrawal. Hole Closure Methods: (a) Re-Entry Techniques. or water transmission. hole is re-entered using a separate grouting system. After withdrawal. (b) CPT Retraction with Expendable Tip (Lutenegger & DeGroot. for CPTs advanced through asphalt pavements. Cavity is grouted during withdrawal using a special "loss tip" or retractable portal. 20% backfill with soil. such as a GeoProbe). d.g. Pozzolan-based grouts can also be adequate. The requirement of borehole closure can significantly reduce CPT production rates. 1998). The need for grouting or sealing of holes is usually established by the state or province. but they tend to setup more slowly (Lee. or by local and specific conditions related to the particular project. as depicted in Figures 21 and 22. or a bentonite-cement mix combination. sealing of the hole would be warranted to prevent water infiltration and/or long-term damage. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 24 . b..Hole Closure After the sounding is completed. a number of possible paths may be followed during or after extraction: a. CPT hole is left open. flexible or rigid tremie pipes. c. the state or province will deem the need or requirement for hole closure by grouting or sealing in specific geologic settings where the groundwater aquifer(s) need to be protected against vertical cross-talk. Most often. NCHRP Project 20-05. A full discussion of these systems and their advantages and disadvantages is given by Lutenegger & DeGroot (1995a. (a) (b) Figure 21. For instance. et al. perhaps these are the only graphical plots that should be presented. Geostratigraphic Profiling By recording three continuous measurements vertically with depth. the presentation of CPT data for use in detailing subsurface stratigraphic features. as illustrated by Figure 23. The data presentation from a CPT sounding should include the tip. it is common to report the um reading in terms of equivalent height of water. If the depth to the water table is known (zw). (b) Grouting Through Ports in Friction Reducer (Lutenegger & DeGroot. a simple conversion is: 1 tsf ≈ 1 bar = 100 kPa = 0. delineating the interfaces between soil layers. as they represent the raw un-interpreted results. and sleeve resistance (fs) and porewater pressures (um) in kPa.CPT DATA PRESENTATION AND GEOSTRATIGRAPHY In this chapter. where γw = 9. For conversion to English units. and stringers within the ground. inclusions.4 pcf for freshwater. calculated as the ratio of the measured porewater pressure divided by the unit weight of water. and detecting small lenses. sleeve. the CPT is an excellent tool for profiling strata changes. In that case. For SI units. The total cone tip resistance (qt) is always preferred over the raw measured value (qc).zw) γw. For DOT projects wishing to share CPT information to contractors in bidding documents.(a) (b) Figure 22. the hydrostatic pressure can be calculated from: u0 = (z .8 kN/m3 = 62. and identification of geomaterials will be presented. cone tip stress (qt) in either kiloPascals (1 kPa = 1 kN/m2) or MegaPascals (1 MPa = 1000 kN/m2). γw* = 10. and porewater readings plotted with depth in side-by-side graphs. the depth (z) is presented in meters (m). determination of soil behavioral type.0 kN/m3 = 64. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 25 . NCHRP Project 20-05. soil layering. it is convenient to show the hydrostatic porewater pressure (u0).1 MPa. In some CPT presentations. or hw = um/γw.0 pcf for saltwater. Hole Closure Methods: (a) Temporary Casing. if the groundwater regime is understood to be an unconfined aquifer (no drawdown and no artesian conditions). 1995) CHAPTER 5 . the ratio u2/u0 increases with clay hardness. showing (a) Total Cone Tip Resistance. Soil Type by Visual Interpretation of CPT Data Since soil samples are not normally taken during CPT. it is observed that friction ratios are low: Rf < 1%. the ratio may be around u2/u0 ≈ 3 ± which increases to about u2/u0 ≈ 10 ± for stiff clays. approaching zero in many instances. Below the water table.g. clean saturated sands show penetration porewater pressures often near hydrostatic (u2 ≈ u0). (b) Sleeve Friction. The friction ratio is defined as the ratio of the sleeve friction to cone tip resistance. degree of saturation. and (d) Friction ratio (FR = Rf = fs/qt) with Depth in Steele. In fact. reflecting the prevailing drained strength conditions. Presentation of CPTu Results. for the standard shoulder element. yet as high as 30 or more for very hard clays. the drilling of an adjacent soil boring with sampling can be warranted to confirm or verify any particular soil classification. For soft intact clays. moisture. (c) Shoulder Porewater Pressures. the friction ratio can be quite low. and other factors and should therefore be considered tentative. NCHRP Project 20-05. The friction ratio has been used as a simple index to identify soil type. At test depths above the groundwater table. However. As a general rule of thumb. designated FR = Rf = fs/qt. if the clays are fissured. Rf > 4%. while in clays and clayey silts of low sensitivity. However. Missouri. the magnitudes of CPT measurements fall into the following order: qt > f s and qt > u1 > u2 > u3. soil types must be deduced or inferred from the measured readings.. Mayne. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 26 . in soft sensitive to quick clays.Tip Resistance Sleeve Friction Porewater Pressure Friction Ratio qT (MPa) 0 0 10 20 30 40 0 fs (kPa) 200 400 600 -500 u2 (kPa) 0 500 1000 0 2 FR (%) 4 6 8 10 5 Depth (m) 10 15 20 25 Figure 23. 1990). and reported as a percentage. In clean quartz sands to siliceous sands (comparable parts of quartz and feldspar). measured porewater pressures depend upon the position of the filter element and groundwater level. porewater pressure readings vary with capillarity. while intact clays exhibit values considerably higher than hydrostatic (u2 > u0). The measured cone tip stresses in sands are rather high (qt > 5 MPa or 50 tsf). then zero to negative porewater pressures are observed (e. Correspondingly. et al. while measured values in clays are low (qt < 5 MPa or 50 tsf) and indicative of undrained soil response due to low permeability. In critical cases or uncertain instances. and the charts for all three piezocone readings presented by Robertson. for clean sands. u0 = γw·(z .10. where u0 = hydrostatic porewater pressure. a visual examination of the CPTu readings in Steele. qt (bar) In the simplified CPT chart method (Robertson & Campanella. Bq ≈ 0 while in soft to firm intact clays.u0. Robertson et al. Bq Figure 25.2 1. Bq ≈ 0. and γw = unit weight of water. CPTu Soil Behavioral Type (SBT) for Layer Classification (after Robertson. FR with paired sets of log qt vs. (1986) and Robertson (1990). FR = fs/qt (%) Cone Bearing. Generally. The method is depicted in Figure 24.5 m sand over desiccated fissured clay silt to 4. qt (bar) 3=clay 11. The soil behavioral type (SBT) represents an apparent response of the soil to cone penetration. soft clay to 24. The total overburden at each layer i is obtained from σvo = Σ (γti Δzi ) and effective overburden stress calculated from: σvo' = σvo .2 0. including well-known methods by Begemann (1965). ending in a sandy layer. clayey silts. Soil Behavioral Classification At least 20 different CPT soil classification methods have been developed.6 ± 0. and Robertson (1990). and clays. 1983). where σvo = total vertical overburden stress at the corresponding depth z as the readings. The method was expanded to include use of a normalized porewater pressure parameter defined by: 1000 Sands 100 Silty Sands Sandy Silts & Silts 10 Clayey Silts Clays Bq = u2 − u0 qt − σ vo (3) 1 0 1 2 3 4 5 6 Friction Ratio. (1986) 1000 Zone 10 Zone 12 . et al.4 0. as well as for conditions of full capillary rise above the water table.Clay Zone 1 1 -0. Simplified CPT Soil Type Classification Chart (after Robertson & Campanella. (1986) 1000 Zones 9.Returning to Figure 23.5 m. underlain by clean sand to 14 m. NCHRP Project 20-05.Clay 8=sand to silty sand Zone 2 . silty sands. FR = fs/qt (%) Figure 24. MO shows an interpreted soil profile consisting of five basic strata: 0. the most popular methods in use by North American DOTs include the simplified method by Robertson & Campanella (1983) for the electric friction cone.4 Zone 11 Very stiff fine grained soil 2=organic soil 9 100 8 7 6 5 4 10 Zone 1 Sensitive Clay 1 0 1 2 3 4 5 4=silty clay 5=clayey silt 6=sandy silt 7=silty sand Zone 3 . 1983). sandy silts. For dry soil above the water table. obtained by plotting log qt vs. Bq.sand to clayey sand 1=sensitive clay Robertson et al. et al.2 2 0 0. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 27 . u0 = 0.6 0. Robertson & Campanella (1983) Cone Bearing. 10 9 8 100 7 6 5 4 10 Zone 3 . Below the groundwater table. Schmertmann (1978).5 m.8 1 1.2. q t (bar) Cone Bearing. the friction ratio (FR) to delineate five major soil types: sands. The chart in Figure 25 indicates twelve possible SBT zones or soil categories.Organic 6 7 8 9=sand 10=gravelly sand Friction Ratio. the logarithm of cone tip resistance (qt) is plotted vs.zw) where z = depth.12 Bq = u 2 − u0 q t − σ vo Porewater Pressure Parameter. zw = depth to groundwater table. 1986). Based on the results of the survey. For general use.3 ⋅ (log F )]2 (6) The advantage of the calculated *Ic parameter is that it can be used to classify soil types per the general ranges given in Table 2 and easily implemented into a spreadsheet for post-processing results.90 < Ic < 2.u0 = effective vertical overburden stress at the corresponding depth.54 Sands 6 1. Robertson (1990) presented a nine-zone SBT chart that may also be found in Lunne et al. MO is re-evaluated in terms of the index Ic to delineate the layering and soil types. whilst a different SBT suggested by the Q-Bq chart. Soil Behavior Type (SBT) or Zone Number from CPT Classification Index. Jefferies & Davies (1993) showed that a cone soil classification index (*Ic) could be determined from the three normalized CPT parameters by: * Ic = {3 − log[Q ⋅ (1 − Bq )]}2 + [1.The overburden stress and depth influence the measured penetration resistances (Wroth. Therefore.82 < Ic < 3. since a SBT may be identified by the Q-F diagram. the CPTu data from Steele.5 + 1.25 *Notes: a. In this case. Using all three normalized parameters (Q. it is convenient to define normalized parameters for tip resistance (Q) and friction (F) by: Q = qt − σ vo σ vo ' fs ⋅ 100 qt − σ vo (4) F = (5) where σvo' = σvo . as well as low friction F < 1% Using the SBT approach from Table 1.82 Sand Mixtures 5 1. Zone Number per Robertson SBT (1990) b. Zone 1 is for soft sensitive soils having similar Ic values to Zones 2 or 3. Bq).54 < Ic < 2. 1988). F.22 Clays 3 2. (1997). Table 2. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 28 . Occasional conflicts arise when using the aforementioned 3-part plots. it is more rigorous in the post-processing of CPT data to consider stress-normalization schemes for all three of the piezocone readings. *Ic (after Jefferies and Davies.22 Silt Mixtures 4 2. in addition to the aforementioned Bq parameter. 1993) Soil Classification Zone Number* Range of CPT Index *Ic Values Organic clay soils 2 Ic > 3. NCHRP Project 20-05. as presented in Figure 26.25 < Ic < 1. The results are in general agreement with the previously described visual method.90 Gravelly Sands 7 Ic < 1. Boulanger & Idriss.7 psi. It is fully automated by computer software and available as a free download from the LTRC website (see App.5 = qt/(σvo'⋅σatm)0.Tip Resistance Sleeve Friction Porewater Pressure qT (MPa) 0 0 10 20 30 40 0 fs (kPa) 200 400 600 -500 0 u2 (kPa) 500 1000 Soil Classification by Visual Method SAND OC fissured Clayey SILT 6 5 3 4 4 3 4 6 5 6 6 6 6 6 6 6 7 6 6 3 2 2 2 2 2 2 2 2 2 2 3 2 2 4 SBT (1990) 1 2 3 4 5 6 7 8 9 SAND SILT MIX 5 Clean SAND SAND Depth (m) 10 15 Soft Silty CLAY 20 Organic Clay or Silt 25 SAND SAND Figure 26. 2006).. and clay fractions with depth. or else highly-interbedded lenses and layers of clays and sands. 1995.. MO Evaluated by Index Ic for Soil Behavioral Type. silt. For example. Alternate CPT stress-normalization procedures have been proposed for the cone readings (e. but are beyond full discussion herein. Moss. U' > 3 (clay). Other alternative and more elaborate stress-normalization procedures for the CPT have been proposed as well (e. Additionally. in clean sands. Jamiolkowski et al. The latter offers the simplicity that soil types can be simply evaluated by: U' < 1 (sand). et al. and sand has been developed by Zhang & Tumay (1999). the stress-normalized tip resistance is often presented in the following format: qt1 = (qt/σatm )/(σvo'/σatm)0. 2004. the normalized side friction can be expressed as: F' = fs/σvo'.5 (7) where σatm = 1 atm = 1 bar = 100 kPa ≈ 1 tsf ≈ 14. Values in between these limits are indicative of either mixed sand-clay soils or silty materials.. Using the same CPT sounding presented in Figures 23 and 26. Olsen & Mitchell. and normalized penetration porewater pressure given by U' = Δu/σvo'. 2001). Bq > 0.g.g.3 (clay). Kulhawy & Mayne. silt. In a recent and novel approach to indirect soil classification by CPT. a probabilistic method of assessing percentages of clay. The method uses the cone tip resistance and sleeve friction to evaluate probability of soil type. B). A similar relationship based on Bq readings can be adopted: Bq < 0. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 29 . NCHRP Project 20-05. CPTu Results from Steele. 1990.1 (sand). this approach has been applied to determine the probability of sand. as shown in Figure 27 with good results. ) which have undergone long periods of environmental.SOIL PARAMETER EVALUATIONS Soils are very complex materials because they can be comprised of a wide and diverse assemblage of different particle sizes. natural prestressing. analytical. etc. and thermal processes. relative density (DR). Gmax. drainage and flow characteristics. numerical. strain rate parameters (α). NCHRP Project 20-05. a rather large number of different geotechnical parameters have been identified to quantify soil behavior in engineering terms. and overconsolidation ratio (OCR). consolidation coefficient (cvh). Cs). alluvial. and more. Application of the Probability Method for Soil Type to Missouri CPT Sounding. hydrological. aeolian. Eu. glacial. deltaic. lacustrine. Cr. residual. These facets have imparted complexities of soil behavior which relate to their initial geostatic stress state. they can be created from various geologic origins (marine. subgrade reaction coefficient (ks). seasonal. φ'. including various post-processing approaches based on theoretical. creep (Cae). In this section. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 30 . D'. nonlinear stress-strain-strength response. In the survey results. These include state parameters. strength parameters (c'. as well as rheological and time-rate effects. cu = su). mineralogies. KP). the evaluation of select geotechnical parameters from CPT data is addressed. permeability (k). As such. estaurine. CHAPTER 6 . dilatancy angle (ψ). Poisson's ratio (ν'. spring constants (kz). and fabric. lateral stress parameters (KA.Tip Resistance Sleeve Friction qT (MPa) 0 0 10 20 30 40 0 fs (kPa) 200 400 600 PROBABILITY OF SOIL TYPES PL (%) 80 100 0 20 40 60 5 Depth (m) 10 15 20 25 Figure 27. Cc. and empirical methods. biochemical. Moreover. stiffness (E'. νu). DOT geotechnical engineers have indicated that CPT results are being used presently to assess several soil parameters that relate to highway design and construction. fluvial. compressibility (σp'. K0. K'). such as void ratio (e0). G'. packing arrangements. porosity (n). unit weight (γ). Shear Wave Velocity Shear wave velocity (Vs) is a fundamental measurement in all civil engineering solids (steel. It may be noted that no uniform and consistent methodology currently exists to interpret all necessary soil engineering parameters within a common framework..27 (8) NCHRP Project 20-05.Selected relationships utilized in the data reduction of the cone. (1996). cemented soils. discrete elements. It is best practice to measure the shear wave velocity by direct methods such as the DHT and SCPT. concrete.. in particular referring to "vanilla" clays and "hourglass" sands. finite differences. Baldi et al. residual and tropical soils. downhole. alluvial). silts. and degree of reliability needed in the assessed Vs profile. wood. rocks). suspension logging. However. Coutinho. collapsible soils. including: crosshole. For uncemented unaged quartzitic sands. ultrasonics. sensitive structured clays. and dislocation based theories. The Vs can be obtained for all types of geomaterials. (2004). various parameters have been derived from analyses based in limit equilibrium. refraction. 1994). including: resonant column. kaolin.g. fractured and intact rocks. Also. depending upon the equipment available. As noted earlier. and reflection (Campanella. loess. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 31 . 1997. For specific concerns in the interpretation of CPT data. plasticity. and special triaxial apparatuses (Woods. the subsequently noted procedures are based largely in part on mixed theories tempered with experience and available calibrations with laboratory test results and/or backcalculated values from full-scale load tests and performance monitoring. It can be noted that alternative evaluations of soil properties and parameters are available and that a spreadsheet format best allows for "tuning" and sitespecific correlations for particular geologic settings and soil materials. Schnaid 2005). elasticity. soils are grouped into either clays or sands. and validation be performed by a research institution working with the state DOT.. dispersive clays.) that will undoubtably not fall within the domain and applicability of these relationships. gravels. carbonate sands. etc. torsional shear. testing. strain path. A number of nontextbook geomaterials can be found throughout North America (e. (1989) suggested the shear wave velocity may be evaluated from the following relationship: Sands: Vs = 277 (qt) 0. in some instances. the correlations can be expected to apply to "well-behaved" soils of common mineralogies (i. stress path. For those materials. the correlative relationships may be employed to check on the reasonableness of obtained Vs readings obtained by SCPT and/or identify unusual geomaterials that may fall into the category of unusual or nontextbook type soils (Lunne.e.g. The procedures chosen herein represent a selection of methods based on the author's understanding and experiences in US and Canadian practices. the incorporation of one or more geophones within the penetrometer facilitates the conduct of seismic cone testing (SCPT). et al. care taken in execution of the test. cavity expansion. glacial till. including clays. At this time. (2004). it may be necessary to estimate the Vs profile via an empirical correlation if a seismic penetrometer is not available. et al. sands. spectral analysis of surface waves. quartz. 1978) and by a variety of different field geophysical tests. soils.13 (σvo') 0. bender elements. feldspar) and typical geologic origins (e. This is a version of the downhole geophysics test (DHT) and may be conducted either by pseudo-interval or true-interval method. That is. finite elements. Schnaid et al. and Schnaid (2005). marine. piezocone. Some guidelines and methods in assessing nontextbook geomaterials are given by Lunne et al. as well as mine tailings and fills. and seismic cone tests are presented in the subsequent subsections. fiberglass. The values of Vs can be readily determined by laboratory tests. As per conventional practice. it is suggested that site-specific calibration. as shown in Figure 28a. (11) (a) (b) Figure 28. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 32 .627 (9) The relationship was significantly improved for intact clays if both the tip stress (qt in kPa) and void ratio (e0) were known in advance. A similar database from well-documented experimental sites in saturated clays. NCHRP Project 20-05. silts. For clay soils. 1995). Of particular interest are interpretative methods that accommodate all types of soils. 1989.75 (qt) 0. and qt = corrected cone tip resistance (MPa).67 [fs/qt ·100]0. et al.5 where fs is in kPa.1· log qt . The relationship was derived from a database that included sands.where Vs = shear wave velocity (m/s). In one approach. Shear Wave Velocity Estimate from CPT data in (a) clean quartz sands (after Baldi. 1995): All Soils: Vs (m/s) = [10. 1995) that determined: Clays: Vs = 1.11.8 log (fs) + 18. and sands showed that (Mayne. thus is interesting in that it attempts to be global and not a soil-dependent relationship. 2006c): Vs = 118. and σvo' = effective overburden stress (MPa). silts. as well as mixed soil types. clays. Figure 28b shows a generalized interrelationship between shear wave and cone tip resistance for soft to firm to stiff intact clays to fissured clay materials (Mayne & Rix. and (b) clay soils (after Mayne & Rix. an estimate of the in-situ shear wave velocity can be made from (Hegazy & Mayne.4]1.3 (10) where qt = tip resistance and fs = sleeve resistance are input in units of kPa. Also shown are calibration chamber test data for four different carbonate sands (calcareous type).89 log(qt1) + 11. r2 = 0. An alternative approach uses results from large scale calibration chamber tests to evaluate the dry unit weight (γd) of sands from normalized cone tip resistance (qt1) given by eqn (7).Unit Weight The saturated unit weight of each of the soil layers is needed in the calculation of overburden stress and in the other calculations. loose to dense sands and gravels. Also shown for comparative purposes (but not included in the regression) are data from intact rocks whereby a maximum unit weight (γrock = 26 kN/m3) and maximum shear wave velocity (Vs = 3300 m/s) can be taken as limiting values. qT1 = (qt/σatm)/(σvo'/σatm)0. during CPT. as well as mixed geomaterials (n = 727. Based on the survey. Moreover. the saturated total unit weight depends on both Vs (m/s) and depth z (meters).4881 Normalized Tip Stress. The trend is presented in Figure 29. However. 2006). samples are not routinely obtained. clearly showing that the relationship should be used with caution in sands and that mineralogy and cementation can be important facets of geomaterials. (1997). nearly 40% of DOTs assume the unit weight (Appendix A. particularly clean sands. the correlation in Figure 30 is based on a large data set of soils. (Silica Quartz Sands) Regression on Quartz Sands: Dry Unit Weight. in many soils. as discussed by Lunne.488). et al. The unit weight is best achieved by obtaining undisturbed thin-walled tube samples from borings. Another 15% use an estimate based on the 12-part SBT classification. A regression line is given for uncemented unaged quartz to siliceous sands that has only a rather modest coefficient of determination (r2 = 0.5 Figure 29.8 100 1000 R2 = 0. Question 35). cohesionless silts. undisturbed samples are difficult to obtain. A set of alternate expressions for the dry and saturated unit weights is available in terms of Vs and σvo' (Mayne. For saturated soils. and gravels.808). NCHRP Project 20-05. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 33 . thus indirect methods for assessing unit weight are desirable. γdry (kN/m ) 3 γDRY = 1. 20 19 18 17 16 15 14 13 12 11 10 9 8 10 CPT Chamber Data on Sands Type of Sand Silica Quartz Sands Carbonate Quiou Sand Carbonate Dogs Bay Sand Carbonate Ewa Sand Carbonate Kingfish Sand Log. For these. Dry Unit Weight Relationship with Shear Wave Velocity and Depth. including soft to stiff clays and silts. 32 log Vs . Saturated Soil Unit Weight Evaluation from Shear Wave Velocity and Depth.28 Saturated Unit Weight. Saturated Unit Weight Evaluation from CPT Sleeve Friction Reading and Specific Gravity of Soil Solids. Vs (m/s) Figure 30.61 Log z with Vs (m/s) and depth z (m) n = 727 r2 = 0.E.05 Depth z (m) 1 10 100 Additional n = 163 Rock Materials Intact Clays Fissured Clays Silts Peat Sands Gravels Weathered Rx Intact Rocks z=1m z = 10 m z = 100 m 100 1000 10000 Shear Wave Velocity. Figure 31. NCHRP Project 20-05.1.808 S. γT (kN/m3) 26 24 22 20 18 16 14 12 10 10 Saturated Soil Materials: γT (kN/m3) = 8. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 34 . = 1. the stiffness can be expressed in terms of an equivalent Young's modulus of soil through elastic theory: Emax = 2Gmax (1+ν) where ν' = 0. as discussed by Jardine et al.1 to 0. is a fundamental stiffness that relates to the initial state of the soil. the value of drained ν' ranges from 0. G/Gmax vs. fissured clays. (1986. Burland. as presented in Figure 31. as discussed by Tatsuoka & Shibuya (1992). 1989. the evaluation appears good for soft to stiff clays of marine origin. increasing to larger values as failure states are approached. 1989. For cyclic loading and dynamic problems in geotechnical engineering. including static. Based on recent local strain measurements on samples with special internal high-resolution instrumentation (e. also known as the initial tangent dynamic shear modulus (Gdyn). silts. and dynamic types of loading. Leroueil & Hight. Small-Strain Shear Modulus The slope of a shear stress (τ) vs. The mobilized shear stress is analogous to the reciprocal of the factor of safety (τ/τmax = 1/FS). The value for undrained loading is νu = 0. 2000). or alternatively in terms of mobilized shear stress (τ/τmax). and a variety of clean quartz sands. Vucetic & Dobry (1991) present G/Gmax curves in terms of soil plasticity and shear strain (γs). mobilized stress level (τ/τmax) plots are visually biased towards the intermediate. Mayne.5. G/Gmax vs. In contrast. log γs curves tend to NCHRP Project 20-05. Soil Stiffness The value of small-strain shear modulus.g. In general. For loading levels at strains higher than these. 2001.to large-strain regions of the soil response.8 m/s2 = gravitational acceleration constant: Gmax = ρT Vs2 (12) In lieu of shear modulus. Gmax (and corresponding Emax) applies strictly to the nondestructive range of strains. 2005) and Atkinson (2000). G. The small-strain shear modulus (termed G0. The appropriate value of shear modulus is then obtained from: G = Gmax ⋅ (G/Gmax) (14) (13) The G/Gmax curves can be presented in terms of logarithm of shear strain (γs). modulus reduction curves (G/Gmax = G/G0) must be implemented.5 for undrained conditions. or Gmax). the unit weight of the freshwater glacial lake clays at the Northwestern NGES are underpredicted by this approach. This stiffness applies to the initial loading for all stress-strain-strength curves. The smallstrain shear modulus is calculated from the total soil mass density (ρT = γT/g) and shear wave velocity (Vs). where γs < 10-4 as a decimal (or γs < 10-6%). where g = 9. 2003).2 applies for drained and νu = 0. as well as undrained and drained conditions (Burland. Fahey & Carter (1993). (1998).2 for all types of geomaterials at working load levels. cyclic. shear strain (γs) curve is the shear modulus. however. Lehane & Cosgrove.Using the results from Figure 30 and an evaluation of Vs from the sleeve friction.. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 35 . and LoPresti et al. Poisson's Ratio The value of Poisson's ratio (ν) is normally taken for an isotropic elastic material. In terms of fitting stress-strain data. the total unit weight of saturated soils can be directly estimated from CPT fs (kPa) and depth z (m). 2005). A selection of modulus reduction curves.2 < g < 0. An equivalent stiffness of soils is also afforded via the constrained modulus (D') obtained from onedimensional consolidation tests.accentuate the small.4 g = 0.2 0. q/qmax.7 0.4) tends to encompass many of the laboratory TS and TX data.7 0.6 0.(τ/τmax)g with g ≈ 0. τ/τmax (a) (b) Figure 32. The results are presented in Figure 32a.9 1 0 0. et al. and the sands were tested under drained shearing conditions (except Kentucky clayey sand).3 ± 0.8 0.6 0. and (b) Using Modified Hyperbolic Expression Proposed by Fahey & Carter (1993).5 0. 1993.6 g = 0. τ/τmax or 0.4 0.83 Ham River Sand Ticino Sand Kentucky Clayey Sand Kaolin Kiyohoro Silty Clay Pisa Clay Fujinomori Clay Pietrafitta Clay Thanet Clay London Clay Vallericca Clay 0. In general. 2006).B. Monotonic Modulus Reduction Curves from (a) Static Torsional and Triaxial Shear Data on Clays and Sands.2 0.1 0.6 0.g. Similar trends for the various curves are noted for both undrained and drained tests on both clays and sands.B. mobilized stress (τ/τmax = 1/FS) shown in Figure 32b.9 1 q/qmax Mobilized Strength. represented by the ratio (G/Gmax). The ratio (G/Gmax) is a reduction factor to apply to the maximum shear modulus. the clays were tested under undrained loading (except Pisa).1 0 ⎡ τ g⎤ G = G max ⋅ ⎢1 − ( ) τ max ⎥ ⎣ ⎦ g=1 g = 0..8 0. Sand Hamaoka Sand Hamaoka Sand Toyoura Sand e = 0. One rather simple algorithm involves a modified hyperbola (Fahey & Carter.2 0.9 0.2 0. LoPresti et al.1 0 0 Modulus Reduction. Jardine.8 0.6 0.. Fahey 1998) with presented results for modulus reduction (G/Gmax) vs. It can be seen that a limited range of the exponent (0. depending on current loading conditions.1 0.3 0. the data may be plotted in terms of vertical stress vs.9 Modulus Reduction. where G = τ/γs = secant shear modulus.5 0. Undrained tests are shown by solid dots and drained tests are indicated by open symbols. has been collected from monotonic laboratory shear tests performed on an assorted mix of clayey and sandy materials (Mayne. The nonlinear representation of the stiffness has been a major focus of the recent series of conferences on the common theme: Deformation Characteristics of Geomaterials (e.to intermediate-strain range.8 0.3 0.4 0. not highly-structured). G/Gmax NOTES: Solid Dots = Undrained Open Dots = Drained Tests NC S. G/Gmax or E/Emax 0. The modulus reduction can be given by: G/Gmax = 1 .g.5 0. vertical strain and the tangent slope is defined as the (15) NCHRP Project 20-05. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 36 . where q = (σ1-σ3) = deviator stress.7 0.4 0.3 0.4 0.5 0. A number of different mathematical expressions can be adopted to produce closed-form stress-strain-strength curves (e.2 Note: f = 1 TS and TX Data Mobilized Strength. In lieu of e-logσv' graphs developed from consolidation tests. 1998).3 0.L.3 g = 0.7 0. insensitive. These lab tests include static torsional shear and special triaxial tests with internal local strain measurements.67 Toyoura Sand e = 0. Sand OC S. An assumed constant value of ν has been applied with the conversion: E = 2G(1+ν) to permit plotting of E/Emax vs. 1 1 0.L.1 for "well-behaved" soils (uncemented. However. 2002): D' ≈ αG' ⋅ Gmax (18) with assigned values of αG' ranging from 0. additional studies with multiple regression.. From elastic theory. as presented in Figure 33b. For cemented (Fucino) clay. and (b) Small-Strain Shear Modulus of Various and Diverse Soils. the constrained modulus relates to the equivalent elastic Young's modulus (E') and shear modulus (G') for drained loading conditions: D' = E '⋅ (1 − ν ' ) (1 + ν ' )(1 − 2ν ' ) = 2G (1 − ν ' ) (1 − 2ν ' ) (16) For foundation settlement analyses. In the future. Trends Between Constrained Modulus and (a) Net Cone Resistance. G0 (MPa) (a) (b) Figure 33. From a collection of diverse geomaterials ranging from sands. D' (MPa) Constrained Modulus.02 for the organic plastic clays up to 2 for overconsolidated quartz sands. Figure 33a shows that a relationship for "well-behaved" soils might take the form: D' ≈ αC' ⋅ (qt-σvo) (17) with an overall representative value of αC' ≈ 5 for soft to firm "vanilla clays" and NC "hourglass sands". silts. Schmertmann 1978.constrained modulus: D' = Δσv'/Δεv. intact organic and inorganic clays. an alternate correlation can be sought between D' and small-strain shear modulus (Gmax). and numerical modeling may help guide the development of more universally-applied global relationships. where Δεv = Δe/(1+e0). and fissured soils (Mayne.σvo (MPa) Small-Strain Shear Modulus. 1985).2 0. With SCPT data. Jamiolkowski. a representative constrained modulus of the supporting soil medium is usually sought. a similar adopted format could be (Burns & Mayne.05 Intact CLAYS Organic Plastic CLAYS 100 1000 0.02 10 Net Cone Tip Stress.1 0. 1000 1000 D' = 5 (qt-σvo) Constrained Modulus. artificial neural networks. 2006).1 1 10 100 0. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 37 . 1975. a considerably lower αC' ≈ 1 to 2 may be appropriate. NCHRP Project 20-05. Mitchell and Gardner. et al.g. qt . it has been usual to correlate the modulus D' to a penetration resistance (e. In this case. D' (MPa) 100 cemented SANDS 100 Ratio D'/G0 = 2 0.1 0. a value αC' ≈ 10 to 20 may be more appropriate. for organic plastic clays of Sweden. In practice.1 1 0.7 SANDS cemented fissured clays 10 SILTS 10 SILTS Intact CLAYS 1 Organic Plastic CLAYS 1 0. CA Lower 232rd St BC Port Huron MI St.. An example of the profiling of preconsolidation stress with depth by cone penetrometer data is illustrated in Figure 35 using results from the national experimental test site at Bothkennar in the UK (Nash. MA Colebrook Road BC Empire LA Evanston IL SF Bay Mud. LA Strong Pit.33(qt . Italy Brent Cross UK Madingley UK Surrey UK Canons Park UK Cretaceous DC Bothkennar Trend Preconsolidation Stress. BC Ottawa STP. QE Surry. any fissures or cracks within the small laboratory oedometric specimens are closed up during constrained compression in one-dimensional loading. 1992). Ontario Varennes.5 to 1. and in-situ field tests have been conducted in the soft clays having thicknesses up to 30 m and a shallow groundwater table around 0. QE Taranto. σp' (kPa) Fissured Intact Clays: 1000 σp' = 0. Quebec NRCC. et al. NCHRP Project 20-05. as shown in Figure 34. 1995. a reference profile of σp' has been established from consolidation tests using three laboratory devices at different universities including: (a) incremental loading (IL) oedometers. a first-order estimate of the preconsolidation stress can be obtained from the net cone tip resistance (Mayne. formerly designated σvmax' or Pc'. OCR = (σp'/σvo'). First-Order Relationship for Preconsolidation Stress from Net Cone Resistance in Clays.e. the degree of preconsolidation is termed the overconsolidation ratio. 2002). Extensive geological.33 (qt-σvo) (19) It can be seen that this expression underestimates values for fissured clays. Alban. 10000 Amherst. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 38 . σp' = 0. MA Washington DC Atchafalyala LA Boston Blue Clay. Demers & Leroueil. The yield point in one-dimensional loading (i.. VA Baton Rouge. q t . (b) constant rate of strain (CRS) consolidometers.σvo (kPa) Figure 34. consolidation test) denotes the preconsolidation stress (σp').Stress History Clays The stress history of clay soils is classically determined from one-dimensional oedometer tests on highquality undisturbed samples. Ontario Yorktown VA St. The firstorder evaluation from net cone resistance is shown to be rather good agreement with the lab results. and (c) restricted flow (RF) tests. This is because the macrofabric of cracks and fractures affect the field measurements of the CPT as the blocks of clay are forced away from the axis of penetration.Jean Vianney.0 m below grade. Using a variety of different sampling techniques. In normalized form. laboratory. For intact clays. In contrast.σvo) 100 σvo } qt 10 100 1000 10000 Net Cone Resistance. et al.Stress. First-Order Trends of Preconsolidation Stress in Clays with Excess Porewater Pressures Measured by (a) Type 1 Piezocones and (b) Type 2 Piezocones. With piezocone testing.40 (u1-uo) Shoulder Filter Element: σp' = 0. qt (MPa) 0.0 0 2 4 6 0.2 0 2 4 6 Preconsolidation Stress.2 0. The first order relationships for intact clays can be expressed (Chen & Mayne. Comparison of CPT-based Evaluation of Preconsolidation Stress with Laboratory Consolidation Tests in Bothkennar Soft Clay (data from Nash. as shown in Figure 36. 1996): Midface Filter Element: σp' = 0. 1992). data for fissured clays lie above the trends.53 (u2-uo) (20) (21) (a) (b) Figure 36. NCHRP Project 20-05.4 0. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 39 . σp' (kPa) 0 50 100 150 200 250 300 CPTU 1992 CRS IL Oed Depth (meters) 8 10 12 14 16 18 20 Depth (meters) 8 10 12 14 16 RF svo' σvo qt 18 20 Figure 35. Notably.0 1.8 1..6 0. a separate and independent assessment of σp' in intact clays can be made from the porewater pressure measurements. 4 0. ranging from soft to hard intact clays to fissured deposits.0 0 2 4 6 0. CRS consolidometer) may be warranted. the penetration porewater pressures are positive for all clay consistencies. a slight additional trend with plasticity index (PI. as with the earlier profiles in Figure 35. For the Bothkennar site. yet this is interesting as it lends support to the values obtained should they agree. however.8 Overconsolidation Ratio.. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 40 .u2) (22) (23) The above relationships give redundancy to the interpretation of yield stress in clays via CPTu data.6 0. OCR 0 0 2 4 6 1 2 3 4 5 6 7 8 Depth (meters) Depth (meters) 8 10 12 14 16 18 20 8 10 12 14 16 18 Excess u2 Effective qt-u2 CRS IL Oed RF uo u2 20 Figure 37. thus providing a non-unique relationship. the trend is similar for soft to firm to stiff intact clays. For type 1 piezocones. Porewater Pressure.e.75 (qt . Comparison of CPTu-based Evaluations of Preconsolidation Stress with Laboratory Consolidation Tests in Bothkennar Soft Clay Using Excess Porewater Pressures and Effective Cone Resistance (data from Nash. multiple methods give an opportunity to confirm and corroborate the interpreted soil parameters. the additional evaluations of stress history using excess porewater pressures (Δu2) and effective cone resistance (qt-u2) are presented in terms of overconsolidation ratio in Figure 37. for overconsolidated fissured clays the porewater pressures can be negative.. as well as a caution that additional testing (i. u (MPa) 0. Again.As indicated by Figure 36. oedometer. good agreement between the laboratory consolidation data and field methods is evident. That is. 1992). From a theoretical perspective.60 (qt . particularly in unusual soil formations. For type 2 piezocones. the value of preconsolidation stress can also be ascertained from the effective cone tip resistance (Mayne.u1) Shoulder Filter Element: σp' = 0. Else. NCHRP Project 20-05.2 0. or IP) was determined from the database using multiple regression analyses. 2005): Midface Filter Element: σp' = 0. a noted discrepancy offers reason to investigate why the conflict exists. et al. unaged quartz sands is a more challenging assignment due to two primary reasons: (1) oedometric e-logσv' curves for sands are very flat. 1990.62 Figure 38. Multiple regression analyses of the chamber test data (n = 636) from anisotropically-consolidated sands indicate the induced OCR is a function of the applied effective vertical stress (σvo'). Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 41 .1 26 Different Sands n = 636 1 < OCR < 4 4 < OCR < 6 0.σvo)/σvo'. moist. 2005): ⎡ 0. effective confining stress levels. 1997. and measured cone tip resistance (qt). and (2) undisturbed sampling of clean quartz to siliceous sands is quite difficult. Here. the sands are primarily siliceous (quartz and feldspar) with applied stress histories ranging from normally-consolidated (NC) to overconsolidated states (1 ≤ OCR ≤ 15).001 10 100 1000 10000 +(qt)0.31 ⎥ ⎣ (1 − sin φ ' ) ⋅ (σ vo ' / σ atm ) ⎦ 0.192 ⋅ (qt / σ atm ) ⎤ OCR = ⎢ 0. σvo' = effective overburden stress. and σatm = a reference stress equal to one atmosphere = 1 bar = 100 kPa ≈ 1 tsf. effective horizontal stress (σho' = (K0 · σvo'). 2001). The results can be presented by the following closed-form expression (Mayne.828(σvo')0. a relationship for obtaining OCR in clean quartz sands has been empirically derived from statistical evaluations on 26 different series of CPT calibration chamber tests (Kulhawy & Mayne. saturated) at the desired relative density.5 m. Cone penetration is conducted after preparation of the sand sample (dry. uncemented. NCHRP Project 20-05. Therefore. Mayne. as indicated by Figure 38. et al.27 ⎟ ⎠ ⎝ (24) where φ' = effective stress friction angle of the sand. thus making detection of a yield stress problematic. the OCR is shown normalized by Q = (qt . et al. Chamber Test Data Showing Trend of OCR/Q for Clean Quartz and Siliceous Sands. 2001).22 ⎞ ⎛ 1 ⎟ ⎜ ⎜ sin φ ' −0. and though now attainable by new freezing methods. remains very expensive. 1 Note: Stresses in atmospheres (1 atm = 1 bar = 100 kPa ≈ 1 tsf) NC Sands Ratio OCR/Q 0. Lunne. For purposes herein.Sands The evaluation of stress history for clean.9 to 1. and stress history (Jamiolkowski.01 6 < OCR < 10 10 < OCR < 15 0. Chamber tests are very large diameter triaxial specimens having diameters and heights on the order of 0.853(σho')-1. From the overconsolidation ratio, the apparent preconsolidation stress of the sand can be calculated from: σp' = OCR ⋅ σvo' (25) An example of the procedure for evaluating stress history from cone tip stress measurements in clean sands is afforded from a quarry site near Stockholm investigated by Dahlberg (1974). The site was comprised of a Holocene deposit of clean glacial medium-coarse sand, having an initial 24 m thickness overlying bedrock. After the upper 16 m was removed by quarrying operations, a series of in-situ testing (SPT, CPT, PMT, SPLT) were performed in the remaining 8 m of sand, in addition to special balloon density tests in trenches. The groundwater table was located at the base of the sand just above bedrock. Index parameters of the sand included: mean grain size (0.7 < D50 < 1.1 mm); uniformity coefficient (2.2 < UC < 3), mean density ρT = 1.67 g/cc, and average DR ≈ 60%. Using results from 4 Borros-type electric CPTs at the site, Figure 39 shows the measured qt readings and interpreted profiles of OCR and σp' in the Stockholm sand. Results of screw plate load tests (SPLT) were used by Dahlberg (1974) to interpret the preconsolidation stresses in the sand, which are observed to be comparable to the known values from mechanical overburden removal, where the stress history can be determined by calculating the OCR = (Δσv + σvo')/σvo', using the prestress Δσv = (16m)(16.4 kN/m3) = 262 kPa (2.72 tsf). Cone Tip Stress, qt (bars) 0 0 CPT 12 CPT 03 2 CPT 11 CPT 06 3 3 2 50 100 150 200 250 0 0 Overconsolidation Ratio, OCR 5 10 15 20 25 0 0 Preconsolidation σp' (kg/cm2) 1 2 3 4 5 6 7 8 1 1 1 Calculated Δσv from Quarry Excavation 2 Depth (meters) 4 4 Calculated OCR from Quarry Removing 16 m of Sand CPT Evaluation 3 4 σp' from Screw Plate Load Tests 5 5 5 6 6 6 CPT Evaluation 7 7 7 8 8 8 Figure 39. Results from Stockholm Quarry Sand Site Showing (a) Cone Tip Resistances; (b) OCR Profiles from Excavation; and (c) Preconsolidation Stresses (data from Dahlberg, 1974). NCHRP Project 20-05; Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 42 Mixed Soil Types If seismic cone data are obtained, then the small-strain stiffness may be used together with overburden stress level to evaluate the effective preconsolidation stress in all soil types (clays, silts, and sands). An original database compiled by Mayne, et al. (1998) on a variety of 26 intact clays worldwide has been supplemented with recent data on two cemented clays (Fucino, Italy and Cooper Marl from Charleston, SC) in Figure 40. In addition, data from Po River sand (Ghionna, et al. 1995) and Holmen Sand (Lunne, et al. 2003) where the stress histories of the granular deposits are well-documented are also included. Finally, results from Piedmont residual fine sandy silts at the National Geotechnical Test Site (NGES) at Opelika, AL are also considered (Mayne & Brown, 2003). The overall relationship for intact geomaterials is shown in Figure 40 and expressed by: σp' = 0.101 σatm 0.102 G0 0.478 σvo' 0.420 (26) with a statistical coefficient of determination r2 = 0.919 for intact soils. The approach is evidently not valid for fissured geomaterials. The advantage of this particular approach is that all soil types may be considered in a consistent manner, whereas the separation of soil layers into "clay-like" and "sand-like" often result in mismatched profiles of preconsolidation stress with depth. Preconsolidation Stress, σp' (kPa) 10000 1000 Multiple Regression: n = 428 2 r = 0.919 S.E. = 0.117 100 10 10 Units: kPa 100 Intact Clays Cooper Marl Fucino Clay Po River Sand Holmen Sand Piedmont Silt Fissured Clays 1000 0.478 10000 0.42 σp' = 0.161(Go) (σvo') kPa Figure 40. Preconsolidation Stress Evaluation from Small-Strain Shear Modulus in Soils. NCHRP Project 20-05; Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 43 Effective Stress Strength Sands The strength of soils is controlled by the effective stress frictional envelope, often represented in terms of the Mohr-Coulomb parameters: φ' = effective friction angle and c' = effective cohesion intercept. For clean sands, a commonly-used CPT interpretation is based on considerations of an inverted bearing capacity theory supplemented with CPT calibration chamber data from 5 sands (Robertson & Campanella, 1983). However, the flexible-walled chamber test results were not corrected for boundary size effects. In that approach, the expression for peak friction angle of clean quartz sands is given by the approximation (c' = 0): φ' = arctan [0.1 + 0.38 log (qt/σvo')] (27) An alternate expression derived from a much larger compilation of calibration chamber database from 24 sands where the cone tip stresses were adjusted accordingly for relative size of chamber and cone diameter (D/d ratio) was proposed by Kulhawy & Mayne (1990): φ' = 17.6° + 11.0º · log (qt1) (28) where qt1 = (qt/σatm)/(σvo'/σatm)0.5 is a more appropriate form for stress-normalization of CPT results in sands (e.g., Jamiolkowski, et al. 2001). The relationship for φ' with qt1 is shown in Figure 41. 50 Japan Canada Norway China Italy K&M'90 Total %: kaolinite, smectite, calcite, illite, and chlorite 0 0 0 0 0 0 0 Triaxial φ' (deg.) 45 40 18 26 10 10 5 35 5 30 0 50 7 φ '(deg) =17.6 +11⋅ log⎜ ⎛ ⎞ qt ⎟ ⎜ σ '⋅σ ⎟ vo atm ⎠ ⎝ 200 250 0.5 qt/(σvo') 300 100 150 Normalized Tip Stress, qt1 = Figure 41. Peak Triaxial Friction Angle from Undisturbed Sands with Normalized Cone Tip Resistance. Recently, a database was developed on the basis of undisturbed (primarily frozen) samples of 13 sands. These sands were located in Canada (Wride et al., 1999, 2000), Japan (Mimura, 2003), Norway (Lunne, et al. 2003), China (Lee, et al. 2000), and Italy (Ghionna & Porcino, 2006). In general, the sands can be considered as clean to slightly dirty sands of quartz, feldspar, and/or other rock mineralogy, excepting two of the Canadian sands derived from mining operations that had more unusual constituents of clay and NCHRP Project 20-05; Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 44 particle size (D50 = 0. Effective Stress Friction Angle for Sands.03).19). Each undisturbed sand was tested using a series of either isotropically.36 ± 4.23 mm). fines content (FC = 4. these granular geomaterials include 10 fine sands. Figure 42.and/or anisotropically-consolidated triaxial shear tests. NCHRP Project 20-05. In the fully-developed version. The two outliers from LL and Highmont Dams are mine tailings sands that contained high percentages of clay minerals (as noted) and are both underpredicted by the CPT expression. In terms of grain size distributions. 4 medium sands.other mineralogies. then the effective friction angle can be determined from normalized CPT readings Q = (qt-σvo)/σvo' and Bq = (u2-u0)/(qt-σvo) using the chart shown in Figure 42. Here.35 ± 0. Additional details are discussed by Mayne (2006). 1988. Mixed Soil Types An interesting approach by the Norwegian University of Science & Technology (NTNU) is an effective stress limit plasticity solution to obtain the effective stress friction angle for all soil types (Senneset. & Clays from NTNU Method. et al. The sand database was used to check the validity of the friction angle determinations from in-situ CPT tests. For the simple case of Terzaghi-type deep bearing capacity (angle of plastification βP = 0) and adopting an effective cohesion intercept c' = 0. At all sites. and one coarse sand (Italy). The sands from Canada were slightly dirty having fines contents (FC) between: 5 < FC < 15%. results from electric SCPTu were available.66 ± 0. Silts. Mean values of index parameters (with plus and minus one standard deviation) of these sands indicated: specific gravity (Gs = 2. the CPT proves to be an excellent predictor in evaluating the drained strength of the sands. the NTNU theory allows for the determination of both the effective friction angle (φ') and effective cohesion intercept (c') from CPTU data in soils. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 45 .80 ± 1. The relationship between the triaxial-measured φ' of undisturbed (frozen) sands and normalized cone tip resistance is presented in Figure 41. except the China site where only CPTu was reported.49). 1989). whereas the other sands were all relatively clean with FC < 4%. and uniformity coefficient (UC = D60/D10 = 2. 7 ≤ Λ ≤ 0.256 + 0.121 [0. 1986. 1990). Cs = swelling index. CK0UC.0 can be observed. UU. Trak. NCHRP Project 20-05. PSE. 1980. In fact. stress level. and other factors (Ladd. CK0UE. a representative mode for general problems of embankment stability. The classical approach to evaluating su from CPT readings is via the net cone resistance: su = (qt-σvo)/Nkt (30) where Nkt is a bearing factor. 2005): φ' (degrees) = 29.5° Bq 0. different test modes will be chosen to benchmark the su for the CPT. while for sensitive and structured clays. the OCR profile already evaluated by the CPT results can be used to generate the variation of undrained shear strength with depth in a consistent and rational manner. this simple shear mode can be expressed in normalized form (Wroth.g. More papers and research programs have focused on the assessment of relevant value of NkT for an interpretation of su than for any other single parameter (e. (2) empirical normalized strength ratio approach. 1987). For the basic laboratory shear modes. Undrained Shear Strength of Clays For geotechnical applications involving short term loading of clays and clayey silts. In lieu of the classical approach. and (3) empirical method at low OCRs. and slopes and excavations in clays and clayey silts can be taken as that for direct simple shear (DSS). in part. foundation bearing capacity. 1984): su/σvo'DSS = ½ sinφ' OCRΛ (31) where Λ = 1. Keaveny & Mitchell.336 Bq + log Q] (29) that is applicable for 0.. Konrad & Law. Leroueil and Hight. strain rate. 0. et al. sample disturbance effects.8.Cs/Cc = plastic volumetric strain potential. as well as hollow cylinder. Depending upon the particular agency. a higher range between 0. From considerations of critical-state soil mechanics (CSSM). true triaxial.9 ≤ Λ ≤ 1. For many clays of low to medium sensitivity. DS. or institution given responsibility for assessing the appropriate Nkt. 1985. and Cc= virgin compression index of the material. the previous expression for clean sands would apply. a suite of different undrained shear strengths are available for a given clay soil.1 corresponding to granular soils. DSS. et al.An approximate form for a deterministic line-by-line evaluation of f' for the NTNU method is given by (Mayne & Campanella.0 and range: 20° < φ' < 45°. an alternate and rational approach can be presented which focuses on the assessment of σp' from the CPT. there are many available apparatuses. firm. PSC. including: CIUC. the value of su is not unique but depends upon the direction of loading. For all cases. Kulhawy & Mayne. 1991). 2003). Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 46 . UC. This is because. the undrained shear strength (su = cu) of the soil (formerly termed: c = cohesion) is commonly sought for stability and bearing capacity analyses. For Bq < 0.. Therefore.g. The influence of OCR in governing the undrained shear strength of clays is very well-established (e. and torsional shear (Jamiolkowski. The magnitude of preconsolidation stress (σp') is uniquely defined as the yield point from the e-logσv' plot obtained from a consolidation test. as discussed below. boundary conditions. without any consensus reached. A three-tiered approach can be recommended based on: (1) critical-state soil mechanics.1 < Bq < 1. 8 Figure 43. For type 1 piezocones. It is important here to note the exception for fissured clay materials (specifically. 0. it has been shown that the mobilized undrained shear strength may be taken simply as (Trak. Fissured soils can exhibit strengths on the order of one-half of the values associated with intact clays. as shown by Figure 43.3 0. the normalized undrained shear strength to effective overburden stress ratio (su/σvo')NC increases with effective friction angle. Normalized DSS Undrained Shear Strength vs. In Figure 44.0 0. 1996): su ≈ 0. London clay from Brent Cross) that have a macrofabric of cracking and pre-existing slip surfaces.80 which is clearly a subset of the CSSM equation for the case where φ' = 26º and Λ = 0.7 (Mayne. Notably. Available experimental data support the CSSM approach. the backanalyses of failure case records involving corrected vane strengths for embankments. For these cases. et al. et al.4 0. 1980. et al.6 0.4 in comparison with intact clays that exhibit characteristic ratios on the order of u1/qt > 0. et al. zones of fissured clays can be demarcated by a low ratio u1/qt < 0. at low OCRs < 2. For normallyconsolidated clays. fissured clays occurring below the groundwater table can be identified by zero to negative porewater pressures taken at the shoulder position (type 2). the CSSM adequately expressed the increase with OCR in terms of a power function..If the compression indices and φ' are not known with confidence. and excavations. et al. 2003): su/σvo'DSS = 0. thus: u2 ≤ 0 (Lunne. Ladd & DeGroot.22 σp' which is subset of both the CSSM and MIT approaches.80.22 OCR0.5 sinφ' 0. 1991. Effective Friction Angle in NC Clays. Ladd.3 0. Finally.2 Bothkennar Amherst Hackensack James Bay Porto Tolle Boston Blue Silty Holocene Wroth (1984) Mexico City AGS Plastic Onsoy Rissa Portsmouth San Francisco Cowden 0. a recommended default form based on three decades of experimental laboratory work at MIT has been proposed (Jamiolkowski. footings. NCHRP Project 20-05. Terzaghi.7 0. 1997). 1985. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 47 . the larger influence of stress history is shown to dominate the ratio (su/σvo')OC for overconsolidated soils.1 0.4 (32) (33) su/σvo'NC (DSS) = ½ sinφ' su/σvo' NC (DSS) 0. 1990). NCHRP Project 20-05. These were used in turn with an effective φ' = 21º in eqn (31) to obtain the profile of undrained shear strengths.. For instance. et al. The net cone resistances were processed to evaluate σp' values per eqn (19) and produce the overconsolidation ratios shown in Fig. the value of St is sought for soft clays. it is warranted to perform additional strength testing to confirm and support the CPT interpretations. su/σvo' Page 48 . 1988). as seen in Fig. or by laboratory strength testing on high-quality samples (e. Mostly. 2003). The results are in good agreement with the lab reference oedometer tests and corresponding direct simple shear strength tests at the site. The reference test for determining St is the field vane shear (Chandler.g. and Degree of Fissuring in Clays. Ladd & DeGroot. the friction sleeve reading can be considered indicative of a remolded undrained shear strength: fs ≈ sur (e. an indicator as to the sensitivity (St) of the deposit may be obtained by taking the ratio of peak shear strength to remolded value. Task 37-14: Synthesis on Cone Penetration Test (February 2007) DSS Undrained Strength. OCR. Leroueil & Jamiolkowski.8 0. Relationship for DSS Undrained Strength with φ'.. OCR 100 Drammen Portsmouth Portland Maine Boston Blue Silty Holocene Haga Upper Chek Lok Lower Chek Lok Bangkok Atchafalaya Conn Valley Paria Hackensack McManus Cowden Brent Cross 20 30 40 Fissured 20 Figure 44.. An illustrative example of post-processing CPTs in clays to determine the undrained shear strength variation with depth is shown in Figure 45.g. rather than rely solely on one test method. and fall cone. therefore using the aforementioned relationship for peak strength at low OCRs (i. The groundwater lies about 0. 1991). although lab testing methods can include the unconfined compression test. The site is the national geotechnical experimentation site in soft varved clay at the University of Massachusetts-Amherst (DeGroot & Lutenegger. Thus.5 to 1 m below grade. reference benchmarking of su values can be established using field vane tests with appropriate corrections (e..1 1 10 Λ Overconsolidation Ratio. With the CPT.10 φ' = 40o Intact 30o 20o Fissured 1 su/σvo' = ½ sinφ' OCR Note: Λ = 0. A series of five CPTs produced the total (corrected) cone tip resistances presented in Figure 45a showing a subsurface profile with 1 m clay fill over a desiccated clay crust to about 4 m depth overlying soft silty clay. Gorman.e. miniature vane. Sensitivity In soft clays and silts. 2003). 45c. the sensitivity (St) is considered as an index to problematic construction and field performance difficulties. On particularly critical projects. 45b.g. 1975). Jamiolkowski. 1985). it is common practice to assess the relative density (DR) by in-situ tests. NCHRP Project 20-05. 1978.g.su ≈ 0. follow-up testing with the vane shear is prudent.675⎥ ⎟ ⎥ ⎠ ⎦ (35) and the effects of relative sand compressibility can be considered by reference to Figure 46. et al. Robertson & Campanella. (Data from DeGroot & Lutenegger. 1983. qt (MPa) 0 0 1 2 3 4 0 0 2 OCR 4 6 8 10 0 0 20 suDSS (kPa) 40 60 80 100 120 2 2 2 4 4 4 Depth (m) 6 6 CPT Depth (m) Lab DSS Strength Tests 6 8 8 Lab Oedometer 8 10 10 10 CPT 12 12 12 Figure 45. 2003)..22σp') combined with the evaluation of preconsolidation stress from net cone resistance [i. a number of different expressions have been developed from large scale chamber tests (e. Schmertmann.268 ⋅ ln⎜ t atm ⎜ σ '/σ ⎢ vo atm ⎝ ⎣ ⎤ ⎞ ⎟ − 0. however. (2001) which incorporates a correction factor has found that a mean relationship in terms of normalized cone tip stress can be expressed by: DR ⎡ ⎛ q /σ = 100 ⋅ ⎢0.. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 49 . 1998).073(qt-σvo)/fs (34) If a direct and accurate measure of in-place sensitivity is necessary.e. σp' = 0. and (c) Undrained Shear Strengths. et al. A recent re-examination of a large CCT dataset by Jamiolkowski. et al. those correlations did not consider the boundary effects which causes reduced values of qt measured in flexible walled chambers (e. Salgado. Relative Density of Clean Sands In clean sands with less than 15 percent fines content. For the CPT. Results from Amherst Soft Clay Site Showing: (a) Corrected Cone Tip Resistances.g.33(qt-σvo)] suggests that (OCRs < 2): St ≈ 0. (b) Overconsolidation Ratios. 100 90 Relative Density. Normalized Cone Tip Resistance. 1991).7 psi DR ⎡ ⎛ q t / σ atm = 100⋅ ⎢0. In sands of carbonate and calcareous composition. D R (%) 80 70 60 50 40 30 20 10 0 10 Corrected Calibration Chamber Test Data (n = 456) NOTE: σatm = 1 atm = 1 bar = 100 kPa ≈ 1 tsf ≈ 14. Relative Density Relationship with Normalized Tip Stress and Sand Compressibility from Corrected Chamber Test Results (after Jamiolkowski. 2000). 100 Relative Density. 1991). the corresponding CPT data on these 15 sands fall generally within the bounds established from the CCT results.675⎥ ⎟ ⎥ ⎠ ⎦ 1000 Normalized Tip Stress (bars): qt1 = qt/(σvo')0.268 ⋅ ln⎜ ⎜ σ ' /σ ⎢ vo atm ⎝ ⎣ 100 ⎤ ⎞ ⎟ − 0. For this. NCHRP Project 20-05. 1998). Ewa Sand (Moriota & Nicholson. D R (%) 90 80 70 60 50 40 30 20 10 0 10 Japan Canada Norway China Italy Mean High Low Undisturbed Sands DR ⎡ ⎛ q t / σ atm = 100 ⋅ ⎢ 0 . with the Canadian sands indicative of high compressibility materials and the Japanese sands trending on the low compressibility side. 2001). the expected trend would follow that for high compressibility bounds because of particle crushing (Coop & Airey. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 50 . Relative Density of Undisturbed (Frozen) Quartz Sands vs. These data confirm that carbonate sands would fall at the higher side of trends reported for quartzitic sands because of their higher compressibility. a newly created dataset from CPT chamber testing on carbonate sands has been compiled including: Quiou Sand (Fioravante. et al. and Kingfish Platform (Parkin. as shown by Figure 48. 2003). et al. Dogs Bay Sand (Nutt & Houlsby.268 ⋅ ln ⎜ ⎜ σ '/σ ⎢ vo atm ⎝ ⎣ 100 ⎤ ⎞ ⎟ − 0 .675 ⎥ ⎟ ⎥ ⎠ ⎦ 1000 Normalized Tip Stress (bars): qt1 = qt/(σvo') 0. As seen in Figure 47.5 Figure 47.5 Figure 46. The aforementioned database on undisturbed (frozen) sands also lends an opportunity to assess this revised expression. Lateral Stress Coefficient K0 from TSC Field Measurements vs. 5 Strong Pit Lower 232nd St Backbol Ugglum Grangemouth Saint Alban Cowden 20 30 40 Lateral Stress Coefficient. Figure 49 shows field K0 data from total stress cell (TSC) measurements (or spade cells) in clays that generally agree with the above relationship.268 ⋅ ln⎜ t atm ⎜ σ ' /σ ⎢ ⎝ vo atm ⎣ ⎤ ⎞ ⎟ − 0. D R (%) 90 80 70 60 50 40 30 20 10 0 10 100 Carbonate Sands Quiou Sand Dogs Bay Sand Ewa Sand Kingfish Platform High Medium Low DR ⎡ ⎛ q /σ = 100 ⋅ ⎢0.5 1000 Normalized Tip Stress (bars): qt1 = qt/(σvo') Figure 48. Geostatic Lateral Stress State The geostatic horizontal stress is represented by the K0 coefficient. somewhat related to the clay sensitivity (Hamouche et al.100 Relative Density. 1990). where K0 = σho'/σvo'. OCR in Clays. Relative Density of Carbonate Sands in Terms of Normalized Cone Tip Resistances. K 0 Norrkoping 4 Ames British Lib Canons Park Saugus BBC 3 Drammen Essex Tokyo = φ' Kp = tan2(45º+½φ') 2 Total Stress Cells 1 K 0 = (1 − sin φ ' ) ⋅ OCR 0 1 10 sin φ ' 100 Overconsolidation Ratio. In general. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 51 . Results from selfboring pressuremeter tests (SBPMT) also give a similar trend between K0 and OCR for a variety of clay soils (Kulhawy & Mayne.675⎥ ⎟ ⎥ ⎠ ⎦ 0. OCR Figure 49.sinφ') OCR sinφ ' (36) For structured and cemented soils. 1995). NCHRP Project 20-05. higher values of K0 can be realized. laboratory data on small triaxial specimens and instrumented oedometer tests indicate the following relationship can be adopted in uncemented sands and well-behaved clays of low to medium sensitivity: K0 = (1 . OCR from Laboratory Tests on Sands. K 0 3 2 Triaxial: n = 121 Oedometer: n = 307 Chambers: n=597 Po River: n = 28 Stockholm: n = 33 Holmen: n = 2 Thanet Sands: n =20 30 40 = φ' (deg) 50 0 Kp 1 0 1 K 0 = (1 − sin φ ' ) ⋅ OCR sin φ ' 10 100 Overconsolidation Ratio.30 qc 0.27 (37) 1000 Anisotropically-Consolidated Sands NC Sands 1 < OCR < 4 4 < OCR < 6 6 < OCR < 10 10 < OCR < 15 Applied Lateral Stress. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 52 .69 σho' = 0.192 ⋅ ⎛ t ⎜ σ ⎟ atm ⎠ ⎝ 0. Lateral Stress Coefficient K0 vs. For clean sands. σhc' 100 (kPa) 26 Series n = 636 Note: stresses in kPa 10 10 100 1000 0.OCR trends in Figure 50.22 σ ⎞ ⋅ ⎛ atm ⎜ σ vo ' ⎟ ⎝ ⎠ 0. OCR Figure 50.4 Lateral Stress Coefficient.22 σvo' OCR 0. Related to the prior OCR eqn (24). data from large calibration chamber tests and small lab triaxial and oedometer test series show the K0 . Lateral Stress Evaluation of Quartz Sands from CPT Results in Chamber Tests. NCHRP Project 20-05.31 ⋅ (OCR )0.27 Figure 51. the derived formulation for the lateral stress coefficient from chamber tests is shown in Figure 51 and expressed by: K0 q ⎞ = 0. The intercept is actually a projection caused by the forced fitting of a straight line (form: y = mx+b) to a strength envelope that is actually curved (Singh.. i. K0 1 2 3 4 50 100 150 200 250 1 CPT 03 CPT 11 1 Triaxial Data 1 2 CPT 06 2 Depth (meters) CPT Data 6 2 3 3 3 4 4 4 5 5 5 PMT 6 6 6 CPT 03 CPT 06 7 7 7 CPT 11 CPT 12 8 8 8 Figure 52. an a priori relationship between K0 and OCR must be made. respectively. In order to utilize eqn (37) for evaluation of K0. equation (36). Effective Cohesion Intercept For long-term stability analyses. Cone Tip Stress. The CPT data together with φ' are utilized in eqn (24) to obtain the OCR (Figure 39) in either equations (36) or (37) to produce the profiles of K0. Samples of the sand were reconstituted in the laboratory at the measured in-place densities and subjected to consolidated drained triaxial shear testing to determine φ' = 40º (Mitchell & Lunne. these are in agreement with the reported field K0 values determined from liftoff pressures in pressuremeter tests performed at the site (Dahlberg.A maximum value for K0 can be set by the passive stress coefficient (KP) which for a simple Rankine case is given by: KP = tan 2 (45° + φ ' / 2 ) = 1 + sin φ ' 1 − sin φ ' (38) The Kp limit is shown in Figure 49 and 50 for the K0-OCR relationships for clays and sands. 1974). CPT Post-Processing for Peak φ' and Coefficient K0 in Stockholm Sand. and age of the deposit (t). Several difficulties are associated with assessing a reputable value of c' to a particular soil. the effective cohesion intercept (c') is conservatively taken to be zero. et al. 1978).e. Using eqn (28) provides a comparable evaluation of φ' from the four CPTs. 1974). qt (bars) 0 0 CPT 12 Friction Angle. Illustration of the approach for K0 profiling in sands by CPT is afforded from the previous case study of quarried glacial sand near Stockholm (Dahlberg. as seen by Figure 52. φ' (deg) 30 0 35 40 45 50 55 0 0 Lateral Stress Ratio. As seen by Figure 52. The projected c' is actually a manifestation of the three-dimensional yield surface which extends above the frictional NCHRP Project 20-05. strain rate of loading (dε/dt). Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 53 . 1973). including its dependency on the magnitude of preconsolidation stress (σp'). et al. Figure 53. 1990).envelope. For most natural soft marine clays. as discussed by Hight & Leroueil (2003). 1994. 1993): c' ≈ 0. D' = constrained modulus. the horizontal permeability is only around 10% to 20% higher than the vertical value (Mesri. NCHRP Project 20-05. Leroueil & Hight. 2003). the excess pressures will decay with time and the transducer reading will eventually reach equilibrium corresponding to the hydrostatic value (u0). and γw = unit weight of water. Mesri & Abdel-Ghaffar. an apparent value of c' may be assessed from the stress history (Mayne & Stewart. Once the penetration process is halted. 1988.02 σp' Coefficient of Consolidation Porewater pressures generated during cone penetration in fine-grained soils are transient. Comparison of Horizontal and Vertical Permeabilities on Natural Clays (Leroueil. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 54 . The rate of dissipation is governed by the coefficient of consolidation (cvh): (39) cvh = k ⋅ D' γw (40) where k = coefficient of permeability. For varved clays and highly stratified deposits. and very rarely approaches 10. For short-term loading conditions. the ratio of horizontal to vertical permeabilities may range from 3 to 5. A summary of laboratory series of permeability tests on different natural soft clays is given in Figure 53 whereby both standard vertical measurements of hydraulic conductivity (kv) are compared with horizontal values (kh) using radial permeameter devices. Guidelines to permeability anisotropy are given in Table 3. T* Figure 54.2 0.001 U 1 * ≈ (1 + 27T *) −0. (1985).5 0.5 to 15 permeable layers Note: kh = horizontal hydraulic conductivity. kv = vertical hydraulic conductivity The most popular CPTù method to evaluate cvh in soils at present is the solution from the strain path method (SPM) reported by Houlsby & Teh (1988).01 0. Jamiolkowski (1995). and others (Abu-Farsakh & Nazzal. NCHRP Project 20-05.7 0. The degree of excess porewater pressure dissipation can be defined by: U* = Δu/Δui. Sedimentary clays with discontinuous lenses 2 to 4 and layers. 1989). (1997). (1988. The SPM solutions relating U* and T* for midface u1 and shoulder u2 piezo-elements are shown in Figures 54 and 55. Homogeneous clays 1 to 1.1 0. Teh & Houlsby (1991) provide time factors for a range of porewater pressure dissipations. thus offering a means to implement matching data on a spreadsheet. For the SPM solution. The modified time factor T* for any particular degree of consolidation is defined by: T *= cvh ⋅ t a ⋅ IR 2 (41) where t = corresponding measured time during dissipation and a = probe radius. Senneset et al. These can be conveniently represented using approximate algorithms as shown. Gupta & Davidson (1986). well-developed macrofabric 3. 4 − 0.3 0.8 0. 1991) Nature of the Clay Ratio kh/kv 1. Strain Path Solution for CPTu1 Dissipation Tests (after Teh & Houlsby 1991). Burns & Mayne (1998. Permeability Anisotropy in Natural Clays (adapted after Leroueil & Jamiolkowski.0 0. 2002). Danzinger et al.5 2. 1.06 Strain Path Approximation T*= c vh ⋅ t a ⋅ IR 2 u1 0.4 0.0 U1* = Normalized Δu1/Δu1i 0. although other available procedures are discussed by Jamiolkowski et al. Varved clays and silts with continuous 1. where Δui = initial value during penetration. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 55 . 2005).9 0.Table 3.6 0.1 1 10 100 Modified Time Factor. respectively. 2 0.1 1 10 100 Modified Time Factor. Measured Dissipation at Amherst NGES and Definition of t50 at 50% Consolidation. The study compared laboratory-determined results with the SPM solution (Teh & Houlsby 1991) using data from type 1 piezocones (22 sites) and type 2 piezocones (23 sites).2 m Strain Path Solution Hydrostatic uo (zw = 1m) Shoulder Porewater Pressures.3 0.2*9.6 0. (1992). With the SPM approach in practice.1 1 10 t50 = 9.1 0.08 Strain Path Approximation T*= c vh ⋅ t a ⋅ IR 2 u2 0.Piezocone Dissipations (NGES) 600 Penetration u2 = 450 kPa 500 400 300 200 100 Hydrostatic u0 = 11.7 0.1.01 0. a fairly comprehensive study between lab cv values and piezocone ch values in clays and silts was reported by Robertson.5 minutes 100 1000 Porewater Pressure at U = 50% = ½ (450+110) = 280 kPa Measured PWP at 12. the measured t50 = 9. Strain Path Solution for CPTu2 Dissipation Tests (after Teh & Houlsby 1991).5 0. u2 (kPa) Time (minutes) Figure 56.001 U 2 * ≈ (0. At a dissipation test depth of 12.0 0.8 0. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 56 . designated t50.01 0. it is common to use only the measured time to reach 50% consolidation. et al.4 0. An illustrative example of determining t50 for a 15-cm2 type 2 piezocone dissipation in soft varved clay at the Amherst NGES is shown in Figure 56.9 0. In terms of calibrating the approach.85 + 10T *) −0.45 − 0.0 U2* = Normalized Δu2/Δu2i 0.8 = 110 kPa 0 0. 55. Amherst .5 minutes. Assumptions were made between the ratio of horizontal to vertical permeability to address possible issues of anisotropy during interpretation. T* Fig.2 m and groundwater located at zw = 1 m. NCHRP Project 20-05. as well as 8 sites where backcalculated field values of cvh were obtained from full-scale loadings. 2 0. 2001): IR ⎡⎛ 1. ub. t50 (s) 100 1000 10000 0 Shear Wave. the calculated coefficient of consolidation is determined from: cvh T50 * ⋅ac ⋅ I R = t50 2 (42) where ac = probe radius and IR = G/su = rigidity index of the soil.VS (m/s) 100 200 300 400 0 2 4 6 Depth (m) 8 10 12 14 16 Figure 57. Rigidity Index The rigidity index (IR) of soil is defined as the ratio of shear modulus (G) to shear strength (τmax). thus from considerations of cavity expansion and critical-state soil mechanics. fs. the undrained value of rigidity index (IR = G/su) can be evaluated directly from the CPTu data (Mayne.245 for Type 2 shoulder elements.79 cm2/min. The SPM may also be used to fit the entire porewater decay curve.Using a standard adopted reference at 50% dissipation. a fairly optimized data collection is achieved by the SCPTù since five measurements of soil behavior are captured in that single sounding: qt.0 0. For a 15-cm2 penetrometer. qt (MPa) 0 1 2 3 4 5 Porewater ub (MPa) 80 -0. Seismic piezocone test with dissipations (termed SCPTù) at the Amherst soft clay test site. ac = 2.5 ⎞⎛ q − σ vo = exp ⎢⎜ + 2. Using a value IR = 40 and the measured t50 = 9.925⎥ ⎟ ⎥ ⎠ ⎦ (43) NCHRP Project 20-05.2 cm.925 ⎟⎜ t ⎠⎜ q t − u 2 ⎢⎝ M ⎝ ⎣ ⎤ ⎞ ⎟ − 2.5 min gives cvh = 0. the modified time factors are T50* = 0. Sleeve fs (kPa) 0 20 40 60 Tip Stress. and Vs. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 57 . t50. as shown on Figure 56.118 for Type 1 midface elements and T50* = 0.4 0. If dissipation tests are carried out at select depth intervals during field testing. Here the results of a seismic cone sounding are augmented with data from a separate series of dissipations conducted by DeGroot & Lutenegger (1994). Then. The results of a (composite) SCPTù in the soft Amherst clays are depicted in Figure 57.2 0.6 10 Time Rate. 1986). the rigidity index can be measured in laboratory direct simple shear (DSS) or triaxial compression (TX) tests on undisturbed samples. Evaluation of Rigidity Index from Plasticity Index and OCR (after Keaveny & Mitchell. and an estimate of D' is obtained from either of the relationships with net cone resistance or small-strain shear modulus (or both). Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 58 . the derived values are particularly sensitive to accurate CPT measurements and therefore require proper saturations for the filter and cone assembly to obtain u2 readings and correction of measured qc to total qt. An approximate expression for the overall mean trend (dashed line) is also shown. as presented in Figure 59. results from piezo-dissipation testing are used together with an appropriate rigidity index to evaluate cvh. Permeability The permeability may be evaluated via the interrelationship with the coefficient of consolidation and constrained modulus (D'). a direct empirical method has provided by Parez & Fauriel (1988) based on the measured t50 value from the dissipation curves.385(OCR − 1) }] 3.2 exp[ 0. as discussed previously. NCHRP Project 20-05. as presented in Figure 58.0435(137 − PI )] 0. such that: k = cvh ⋅ γ w D' (44) For this approach.where M = 6sinφ'/(3-sinφ'). If undisturbed samples of the material are available. Alternatively. An empirical correlation for IR developed from triaxial test data has been related to clay plasticity index and OCR (Keaveny & Mitchell. or alternatively estimated from expressions based in critical state soil mechanics (Kulhawy & Mayne. 300 Keaveny & Mitchell (1986): I CK = UC Triaxial Data R PI 0 10 ≈ Rigidity Index. (IR)50 = G/su 250 [1 + ln{1 + 0.8 200 20 150 30 40 50 50 PI >50 100 0 1 2 10 Overconsolidation Ratio. As this is an exponential function. 1986). 1990). OCR Figure 58. 25 Clay 100 1000 10000 Measured Dissipation Time. NCHRP Project 20-05. then the rod pressure should likely be maintained during the time decay readings since the release of the rod pressure may cause a stress drop in the initial readings. During the hydraulic push. If a dissipation test is to be performed. and (b) dilatory responses. yet beyond the scope covered herein.E-01 Hydraulic Conductivity. k (cm/s) 1. porewater decay with time is always monotonic (decrease with time). climb to a peak value.1 Sand and Gravel Sand u2 Silty Sand to Sandy Silt Silt ⎛ ⎞ 1 k h (cm / s ) ≈ ⎜ ⎜ 251 ⋅ t (sec) ⎟ ⎟ 50 ⎝ ⎠ 1 10 1. a dilatory response can occur. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 59 . Interpretations of piezocone data for dilatory response are discussed by Sully & Campanella (1994) using an empirical approach and by Burns & Mayne (1998. This is especially evident in Type 1 piezocone dissipation (Campanella & Robertson.E-06 1. Other Soil Parameters A number of additional soil parameters may be determined from cone penetration results. 2002) within a model based in cavity expansion and critical-state soil mechanics framework. then decrease with time. can also occur in Type 2 readings conducted in stiff clays and silts. during type 2 dissipation tests in stiff clays and silts.E-02 1. pressure is placed on the cone rods in the advancing penetration.E-08 0. t50 (sec) Figure 59.E-07 1. a similar monotonic decay is observed. For the latter. a simplified method to address this is presented in Mayne (2001).1. 1991) Some additional considerations in the evaluation of piezo-dissipation tests include (a) stress release of rods.E-05 1. For type 1 piezocones. however. Direct Evaluation of Soil Permeability from t50 Measured in Piezo-Dissipation Tests (after Parez & Fauriel. Some guidance towards references sources which address selected parameter topics is given in Table 4. However. whereby the measured porewater pressures initally increase after the halt of penetration. Leroueil & Jamiolkowski. 1988.E-04 1. For type 2 piezocone filters in soft to firm soils. 1988).E-03 1. 77]2 } (45) Of course. fs. Additional Geotechnical Parameters Determined by Cone Penetration Testing Soil Parameter Attraction. (1986. 1988) Randolph (2004) ADDITIONAL CONSIDERATIONS Layered Soil Profiles When pushing a cone penetrometer in layered soils. A recommended conservative correction is given by the lower bound that can be expressed: qtA* = qtA {1 + 0. CBR Effective Cohesion Intercept. MR State Parameter of Sands. Thus the interface layering can be best ascertained by cross-referencing the qt. (1989) Remarks Defined as intercept from plot of net resistance (qt-σvo) vs. ks Resilient Modulus. NCHRP Project 20-05. the thickness of the sand (Hs). Efforts to investigate these relative effects have been made using numerical simulations by finite element analyses (e. it should also be realized that modern electronic piezocone testing involves three or more continuous recordings with depth. et al.. Related to c' (below) Relates to pavement design Mohr-Coulomb strength parameter. 1994) and experimentally using miniature CPTs in chamber tests with alternating deposited layers of sand and clay. c' Modulus of Subgrade Reaction.25 [0. et al. the tip resistance in a uniform clay underlain by sand will register an increase in qt before the sand layer is actually penetrated. a' = c' cotφ' California Bearing Ratio. Vreugdenhil. as per Robertson & Wride (1998).059(Hs/dc) .Table 4. a cone advancing through a sand layer underlain by softer clay will start to "feel" the presence of the clay before actually leaving the sand. The result is that there will be an apparent false sensing of soil interfaces when CPTs are conducted in layered stratigraphies having large contrasts between different soil types. Relates to attraction term above.g. For instance. thus the qt will reduce as the lower clay is approached. Ψ Strain Rate and Partial Saturation Reference Senneset et al. In the case of a sand layer that is sandwiched between upper and lower clay layers. et al. The apparent measured value of cone tip resistance in the middle sandy layer (qtA) is influenced by the value of apparent cone resistance in the clay layers (qtB).1. (2002) Been. Ahmadi & Robertson (2005) discuss means to correct the apparent measured qtA in the sand to an equivalent qtA* for full thickness layer. the advancing probe will sense portions of a deeper layer before that stratum is physically reached. (1988) Newcomb & Birgisson (1999) Mohammad. 1987. Similarly. The problem is depicted in Figure 60. NCHRP Synthesis Used in the design of highway pavement sections Critical State Approach for Sands Conduct CPTu "twitch testing" at variable rates of penetration Pamukcu & Fang (1989) Amini (2003) Senneset et al. and u2 readings of the CPTu next to a carefully controlled log from an adjacent soil test boring and evaluated within the context of the available engineering geology understanding of the region. and diameter of the penetrometer (dc). effective overburden (σvo'). Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 60 . Results from numerical simluations and limited field data are presented in Figure 61. 1994) and Field CPT Data (Ahmadi & Robertson. NCHRP Project 20-05. Situation of Thin Layer Effect on Measured Cone Tip Resistance (Robertson & Wride.Figure 60. Thin Layer Correction Factor Based on Numerical (Vreugdenhil. et al. 1998) Figure 61. 2005). Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 61 . In both cases. and silts. Idaho. Figure 63 shows a representative cross-section derived from four CPT soundings at a test embankment site.CPTs FOR SHALLOW FOUNDATIONS AND EMBANKMENTS Cone penetration testing is directly suited to evaluating ground response for support of shallow foundations and embankments. soil layering. These are needed to evaluate the thickness and extent of compressible soil layers in calculating the magnitudes of settlements and time duration for completion for embankments and shallow foundation systems. In fact. the CPT is first employed to delineate the subsurface stratigraphy. thin layers. The exceptional detailing of the silty clay with interbedded sand layers and small stringers is quite evident. and groundwater regime. according to the results of the TRB survey (Figure 5). As noted previously. the top two major uses of CPT by the DOTs include embankment stability and investigations for bridge foundations. Figure 62 provides an illustrative example of a piezocone record for an Idaho DOT bridge and embankment construction. and sand stringers. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 62 . the digital data are post-processed to provide numerical values. Figure 62. and time rate of consolidation.CHAPTER 7 . CPTu offers an excellent means for profiling the subsurface geostratigraphy to delineate soil strata and detect lenses. clearly indicating the various strata designated A through F with alternating layers of clays. sands. Representative Piezocone Sounding for Soil Layering Detection at Sandpoint. This chapter addresses the application of CPT penetration data for: (1) calculating the magnitudes of bearing capacity and settlements of shallow spread footing foundations. magnitude of consolidation settlements. Results from multiple soundings can be combined to form cross-sectional subsurface profiles over the proposed construction area. and (2) embankment stability. This sounding was conducted to a rather extraordinary final penetration depth of 80 m (262 feet) below grade. Afterwards. NCHRP Project 20-05. In the rational approach. The method may be based either on one of the aforementioned theories or else empirically derived from statistical evaluations of field foundation performance. FLAC) towards this purpose. In practice. CRISP. Subsurface Cross-Section Developed from Piezocone Soundings at Treporti Embankment (Gottardi & Tonni. or (b) direct CPT methods. also in combination with (Bousinessq) elastic theory in order to provide calculated stress distributions beneath the surface loaded footing. The CPT data could be post-processed to provide relevant input parameters for these simulations. 2004). the allowable bearing stress of the footing (qallow) is obtained by dividing the ultimate bearing capacity (qult) by an adequate factor of safety (FS): qallow = qult/FS.. Most recently. the CPT results can be utilized in one of two ways to evaluate bearing capacity: (a) rational (or indirect) CPT methods. SIGMA/W) or finite differences (e. For the calculation of foundation settlements. In simplistic terms. PLAXIS. Shallow Foundations For shallow spread footings.. or alternate approach using compressibility parameters in an e-logσv' framework. and cavity expansion. In a direct CPT approach.g. su) which are subsequently input into traditional theoretical bearing capacity (BC) equations. the CPT readings are employed within a methodology that outputs the ultimate bearing capacity directly.g. φ'. the measured CPT resistances are used to assess soil engineering parameters (c'. these BC solutions are based in limit equilibrium analyses. For both approaches. the resistance factor NCHRP Project 20-05. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 63 . it has become feasible to use numerical modeling simulations by finite elements (e. It is normal geotechnical practice to adopt FS ≥ 3 for shallow foundations. the CPT results are post-processed to provide an equivalent soil modulus for use in elastic continuum theory. theorems of plasticity. An alternative to the application of the FS approach is the use of load resistance factored design (LRFD).Figure 63. In the case of rectangular footings. the drained and undrained bearing capacity calculations proceed in the same manner. where in essence it is the reciprocal of the safety factor. yet γtotal = γsat in clays with capillarity 3. for overconsolidated materials. then the operational unit weight may be determined as follows: 1. there are two major improvements offered by LRFD: (1) the RF takes on differing values depending upon the quality and source of the data being used in the evaluation. the undrained condition will be the critical case for footings situated over soft clays and silts. However. the appropriate equation is: qult = ½ B *γ *Nγ (47) where the bearing factor *Nγ is a function of effective stress friction angle (φ') and footing shape (see Figure 64). Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 64 . In all cases. partly undrained).(RF) is used as a reduction term: qallow = RF·qult. silts. RF = 1/FS. either drained or undrained conditions may prove to the critical case. The assigned RF values are based on risk and reliability indices. thus volumetric strains are zero (ΔV/V0 = 0). The appropriate value of soil unit weight (*γ) depends upon the depth of the groundwater (zw) relative to the bearing elevation of the footing. submerged or buoyant unit weight) 2. For undrained loading conditions. The value of undrained shear strength (su) is taken as an average from the bearing elevation to a depth equal to one footing width (B = smaller dimension) below the base of the foundation. however. Details on the LRFD approach are given by Goble (2000).15 for a strip foundation and *Nc = 6. ze ≤ zw. However. ze < zw < (ze+B): *γ = γtotal − γw ⋅[1-(zw-ze)/B] NCHRP Project 20-05. thus both should be checked during analysis. and gravels which are very old. Technically. For bearing capacity problems. the relatively high permeability allows for drained response (Δu = 0). For static loading conditions involving sands. the allowable stress might be ascertained as qallow = RFx · qx + RFz · qz.e. Rational or Indirect CPT Approach for Shallow Foundations For the rational CPT approach. the limit plasticity BC solution of Vesić (1975) and elastic continuum solutions (Harr. all soils are geological materials and therefore drained loading will eventually apply to clays. the ultimate bearing stress for shallow foundations can be calculated: qult = *Nc su (46) where the bearing factor *Nc = 5. 1966. The simple shear mode (suDSS) is appropriate and should be calculated per the three-tiered hierarchy. especially if liquefaction occurs. If the foundation has a width B and bears at a depth ze below grade. and (2) multiple RF values are utilized on different components of the calculated capacity. sands. For instance. however. In the case of seismic loading of sands. then: *γ = γtotal where γtotal = γdry for sands. then: *γ = γsat − γw = effective unit weight (also. because of relatively fast rates of loading relative to the low permeability of these soils. while drained loading conditions are adopted for sands and gravels. assume that the ultimate stress depends on two calculated components: qult = qx + qz.14 for square and circular foundations. Drained and undrained cases are considered to be extreme boundary conditions. In fact. it is possible for undrained bearing capacity to happen during large earthquake events. yet it is plausible that intermediate drainage conditions can arise (i. 1974) for foundation displacements will be adopted herein. For drained bearing capacity of shallow foundations where c' = 0. it is common practice to address short-term loading of clays and silts under the assumption of undrained conditions. the plan dimensions are length (denoted "c" or "A") and width (denoted "d" or "B"). zw ≥ (ze + B). Poulos & Davis.. as discussed previously. Then. semi-drained. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 65 .92 5. For the simple case of a flexible rectangular foundation resting on the surface of a homogenous layer (modulus E constant with depth) which has finite thickness.28 4.03 11.0 30.70 46.81 33.52 7. (3) embedment.79 55. the elastic continuum solution for the centerpoint displacement (sc) is: sc = q ⋅ d ⋅ I H ⋅ (1 − ν 2 ) Es (48) where the equivalent elastic modulus and Poisson's ratio are appropriately taken for either undrained conditions (immediate distortion) or drained settlements (due to primary consolidation).16 77.71 19.0 29.e.20 109.0 33. Also.0 23.47 16.33 22.09 (deg) Footing or Square Bearing Factor. with excellent agreement.72 4.68 10.0 22.59 18. (2) foundation rigidity.36 130.13 8.52 8.17 41.44 10.0 37.87 12. Displacement influence factors for various distortions of rectangles of length "c" and width "d" are given by Harr (1966) and shown in Figure 65 for a compressible layer of thickness "h". 1996. include: (1) soil modulus increase with depth (i.0 25.61 78.20 9.0 28.0 34.0 36.29 66. the applied stress q is determined and used to evaluate the displacement of the foundation at working loads.0 21.20 35.0 39.23 3.Bearing Factor.63 28.60 13. plus the standard utilization of elastic theory for calculating stress distributions (Fellenius. In a simplified approach.0 41.19 7. Additional variables can be considered in the evaluation of displacements beneath shallow footings and mats.12 21.99 30. φ' (deg) 25 30 35 40 45 38. 2002).0 40.77 39. the use is synonymous with the e-logσv' approach within the context of recompression settlements due to the close interrelationship of D' and E'.0 24. With the appropriate FS. an approximate solution using a spreadsheet integration of the Boussinesq equation is also given by the method described by Mayne & Poulos (1999).32 65.66 6.99 92. Bearing Factor for Shallow Foundations under Drained Loading (Vesić Solution).15 Circle 3.54 14. and (4) approximate nonlinear soil stiffness with load level. *Nγ 1000 Drained Loading .0 32. *Nγ 100 10 31.0 1 20 Effective Friction Angle. Mayne NCHRP Project 20-05. "Gibson Soil").0 35.0 26.39 6.Shallow Foundations Strip (d/c = 0) Rectangle (c/d = 5) Rectangle (c/d = 2) Square/Circle (c/d = 1) φ' 20. That is.0 Figure 64.01 56.44 15.0 27..40 25.05 48.10 24.0 Strip 5. Mayne 2003). Mayne 1994. Fahey.3 ± 0.0 sc = q ⋅ d ⋅ I H (1 − ν s 2 ) c/d 10 5 3 2 1.1 for uncemented sands and finegrained silts and clays of low to medium sensitivity. Here. IH 1. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 66 . Relevant terms are defined in Table 5 with the elastic displacement influence factor for homogeneous to Gibsontype soil shown in Figure 66. and Eso = soil modulus at the bearing elevation of the foundation base.5 Harr (1966) Approximation 0. c width d 1.0 0. IE = modifier for foundation embedment. or an approximate nonlinear approach can be taken by adopting the modified hyperbolic algorithm for modulus reduction with level of loading.5de)2]. h/d Figure 65.g. This approach has been used successfully in the prediction of footings on sands (e. h Incompressible Layer 3 4 5 6 Finite Layer Thickness. Displacement Influence Factors for Flexible Rectangular Surface Loading over Finite Layer & Poulos (1999) showed that the first three of these factors could be expressed by: sc = q ⋅ d e ⋅ I GH ⋅ I F ⋅ I E ⋅ (1 − ν 2 ) E so (49) where de = diameter of an equivalent circular foundation in plan area [AF = c⋅d = π(0. the magnitude of mobilized shear stress (τ/τmax) can be evaluated as the level of applied loading to ultimate stress from the bearing capacity calculations..5 sc length.0 0 1 2 Elastic Soil Es'. 1994) and clays (e. et al.. IF = modifier for relative foundation flexibility. which is equal to the reciprocal to the calculated factor of safety: q/qult = 1/FS.g. as described previously (Figure 21). NCHRP Project 20-05. Combining this aspect into the generalized equation gives: sc = q ⋅ d e ⋅ I GH ⋅ I F ⋅ I E ⋅ (1 − ν 2 ) E max ⋅ [1 − (q / qult ) g ] (50) where the exponent g may be assumed to be on the order of 0. The analysis can proceed as an equivalent elastic analysis using an appropriate modulus (e.2. v' width d finite depth.5 1 q E Influence Factor..g. the factor IGH = displacement influence factor. D' = E' from Figure 32). 6 + d / ze ) EFDN = foundation modulus t = thickness of foundation a = ½ d = footing radius ze = depth of embedment Finite Gibson Soil Case 1.4)(1.3 0. Terms for Circular Shallow Foundation Displacement Calculations Term or Factor Soil Modulus. Es Equation Es = Eso + kE⋅d Remarks/Notes Eso = modulus at footing bearing elevation.2 0.6 0.6 + 10 K F Foundation Embedment Modifier IE ≈ 1− 1 3.1 1 10 100 Gibson Parameter.235 ⎞ ⎜ ⎟ +⎜ + 1⎟ (h / d ) ⎝ ⎠ Es 2 0.8 0. q d Eso 0 .7 0.8 ] + [ (0h. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 67 . Displacement Influence Factor for Finite Homogeneous to Gibson-Type Soil for Shallow Circular Footings and Mat Foundations. βG = Es0/(kEd) Figure 66.9 0. IGH 0.5 z h kE Numerical Integration of Boussinesq Solution 0.Table 5.1 0.5 exp(1.8 ⎛ 0 . NCHRP Project 20-05.0 Homogeneous Case β=∞ 30 10 5 2 1 Displacement Influence Factor.56 G IF ≈ π 4 + 1 4.01 I GH ≈ 1 0 .56 βG stress. kE = ΔEs/Δz = rate parameter d = equivalent diameter Homogeneous case: kE = 0 Homogeneous Case: βG → ∞ For finite homogeneous to Gibson-type soils where h = thickness of compressible layer KF = Footing rigidity factor t K F = ( E sFDN) ) ⋅ ( a )3 ( av E Normalized Gibson Rate Elastic Displacement Factor Foundation Rigidity Modifier βG = Eso/ kE⋅d I GH ≈ 1 [ β 0.5 0.0 0.2 h/d ratios 0.22ν − 0.235) + 1]2 /d 0.4 0. 785 (51) (52) where σatm = reference stress equal to one atmosphere (1 atm = 100 kPa ≈ 1 tsf). a direct relationship between the measured CPT qt and foundation bearing capacity (qult) has been sought (e. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 68 . Here. two methods will be presented: one each for sands and clays. Frank & Magnan. via empirical methodologies and or experimental studies. q ult (tsf) Square Footing Strip Footing Note: 1 atm = 100 kPa ≈ 1 tsf 0 0 20 40 60 80 100 120 140 160 Cone Tip Resistance. 1995. Thus.36 σatm (qt/σatm)0. Direct Relationship for Ultimate Bearing Stress and CPT Measured Tip Stress in Sands (after Schmertmann. Lunne & Keaveny. For the range of measured cone tip resistances 20 ≤ qt ≤ 160 tsf. Schmertmann (1978) presents a direct relationship between qult and qt shown in Figure 67 as long as the following conditions are met relative to foundation embedment depth (ze) and size (B): When B > 0. embedment ze ≥ 1.g. (1986) defined a parameter Rk as follows: Rk = qult − σ vo qt − σ vo (53) NCHRP Project 20-05. 1978). then embedment ze ≥ 0.. the ultimate bearing capacity stresses can be approximated by: Square Footings: Strip Footings: qult = 0.9 m (3 feet). Eslami.Direct CPT Approaches for Shallow Foundations The CPT point resistance is a measure of the ultimate strength of the soil medium. 1995.45 m + ½ B 25 20 15 10 5 [or: ze ≥ 1. Tand et al. 1972. qt (tsf) Figure 67. For shallow footings on sands.9 m (3 feet).55 σatm (qt/σatm)0. Sanglerat.2 m (4 feet) When B ≤ 0.5' + ½ B (feet)] Ultimate BC Stress. For shallow footings on clays. 2006).785 qult = 0. a form of the elastic theory solution described earlier where the CPT resistance is used to provide a direct evaluation of modulus via: D' ≈ E' = α qt (55) or alternate form: D' ≈ αc (qt . The term Rk depends upon the embedment ratio (He/B). in fact. 1995). since qt is actually a measure of strength. 1965. the use of the same measurement for estimating stiffness has noted a wide range in α values from as low as 0. Results are taken from the load test program involving large square footings on sand NCHRP Project 20-05.g. Lunne & Keaveny.which is obtained from Figure 68. The use of Gmax to obtain a relevant stiffness may therefore be more justifiable (e. to α = 40+ for OC sands at low relative densities (Kulhawy & Mayne. Notably.. Rearranging. Fahey. et al. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 69 . 1986) For the direct assessment of footing settlements at working loads by CPT. 1 < α < 10 for clays and sands (Mitchell & Gardner. Meyerhof. as discussed previously. Schmertmann.σvo) (54) Figure 68.4 for organic clays (Frank & Magnan. 1995). 1994). a number of methods have been proposed (e. et al. where He = depth of embedment and B = foundation width. Direct CPT Method for Determination of Ultimate Bearing Stresses on Clay (after Tand. 1975). as well as whether the clay is intact (upper curve) or fissured (lower curve). Many of these approaches are. 1990).g. Footing Case Study A case study can be presented to show the approximate nonlinear load-displacement-capacity response from eqn (50). the BC for shallow foundations on clay becomes: qult = σvo + Rk ⋅ (qt . 1970.σvo). 00 0.4 2.5 8. IGHFE = 2 2 q/qult 3 72 250 0.00 0.50 0.9 9.55 0.031 q (kPa) 0 152 303 455 531 606 682 758 834 909 985 1061 1137 1213 1288 1364 Q (kN) 0.90 E/Emax 1.2 MPa = 72 atm 1 2 3 4 5 6 7 8 9 10 Depth (m) 4 5 6 7 8 9 10 VS (ave) = 250 m/s SCPT (1997) 8 CHT (1993) 9 10 Figure 69.5 m deep.7 tsf) from Vesic bearing capacity solution per eqn (47). The groundwater table lies about 5.8 5.121 0.55 (qc)0.7 MPa (17.240 0.3 30.65 0. The initial stiffness Emax is obtained from the shear wave velocity measurements and can be used in eqn (50) to generate the curve in Figure 70.0 49.9 132. ν' = Dry Unit Weight γd = Mass Density.Sand Site .9 24. s (mm) Figure 70.1 10 8 6 4 2 0 0 50 100 150 200 Measured B=3m s= q ⋅ d e ⋅ I GHFE ⋅ (1 − υ 2 ) E max ⋅ [1 − ( q / q ult ) 0.1 1. B = qc (average) = Vs (average) = Poisson's ratio.7 180.6 MPa (16.79 0.5 6.79 14211 9 3. Good agreement is shown in comparison to the measured load-displacement response of the footing. IE = Influence. Calculated and Measured Response of Large 3-m Square Footing at TAMU Sand Site.7 261.0 2.reported by Briaud & Gibbens (1994).1 6.000 0.39 1.9 39.6 12.74 109 262 15.7 4.8 7.9 7. IF = Embed Factor.0 61. Using the direct Schmertmann CPT approach per eqn (51).303 0.30 0.270 0.6 tsf). Af = Equivalent de = Influence IGH = Flex Factor.8 78.2 MPa (72 tsf) and mean shear wave velocity Vs (ave) around 250 m/s.6 14.5 10. Q (MN) 0.142 0. The site is located at Texas A&M University and underlain by clean sands to about 5 to 6 meters whereby the sands become slightly silty and clayey with depth.9 11.78 m atm m/s kN/m3 g/cc MPa MPa atm kN m2 m 0.45 0. ρT = Gmax = ρt Vs = Emax = 2Gmax(1+ν') = qult = 0. the CPT data can be post-processed to determine an effective stress friction angle φ' = 40. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 70 .80 0.5 18.60 0.1 4.383 0.85 0.20 0. The large north footing (B = 3 m) can be used with data from SCPT conducted by Lousiana Transportation Research Center (LTRC). NCHRP Project 20-05. Results from Seismic Cone Tests at Texas A&M Experimental Test Site Texas A&M NGES .3 423.065 0.4 100. the ultimate bearing stress is calculated as qult = 1.213 0.99 0.70 0.2 8.75 0. Results from the seismic cone testing indicate a representative mean value of cone tip resistance qc (ave) = 7.164 0.083 0.101 0. The calculation procedure is detailed in Figure 70.048 0.40 0.785 = Qult = qult B = Footing Area.North Footing Load Test INPUT PARAMETERS Footing Width. as reported by Tumay (1998) and presented in Figure 69.499 0.1º which determines qult = 1.35 0.188 0. qT (MPa) 0 0 1 2 3 10 20 30 0 0 100 fS (kPa) 200 300 400 0 1 2 3 4 5 6 7 0 100 Vs (m/s) 200 300 400 qt (ave) = 7.0 1.2 10.2 17.3 s (mm) 12 Applied Axial Load.3 ] Deflection.10 0. Alternatively. Settlements due to primary consolidation of the underlying soft ground are calculated using one dimensional consolidation theory to evaluate both their magnitudes and time rate behavior. else the conventional calculations for one-dimensional consolidation due to primary settlements: drained settlements: sc = Cr ⋅ Δz ⋅ log(OCR ) + (1 + eo ) Cc ⋅ Δz ⋅ log(σ vf ' / σ p ' ) (1 + e p ) (58) NCHRP Project 20-05. These displacements are determined in the same manner as described previously for shallow footings.5. Displacements Beneath Embankments For embankments on soft ground. Cαe/Cr = 0. and up to Cαe/Cc = 0. i. it is common practice to use elastic theory to calculate the magnitudes of undrained distortion (immediate displacements). Leroueil & Hight. STABL. The key advantages of using CPTu for embankment settlement calculations include: (1) ability to obtain a continuous profile of OCR in soft ground. provided that the applied embankment stresses do not exceed the natural preconsolidation stresses: σvo' + Δσv' < σp'. and (2) in-situ assessment of cvh from dissipation testing. but applying displacement influence factors that account for the side slopes and height of the embankment. are then given by: [undrained distortion] [drained settlements] 2 [secondary compression] sc = q ⋅ d ⋅ I H '⋅(1 −ν u ) Eu + q ⋅ d ⋅ I H '⋅(1 −ν '2 ) E' + screep (56) The calculation of long-term displacements caused due to creep can be assessed from: screep = Cαe ⋅ Δz ⋅ log(t ) (1 + eo ) (57) where Δz = thickness of layer undergoing creep.Embankment Stability and Settlements In geotechnical practice. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 71 . plus additional displacements due to long-term creep. the total vertical displacements for undrained distortion and drained primary consolidation settlements. Cαe/Cc = 0.01 for organic materials. as detailed by Foott & Ladd (1981). 1974) and a drained stiffness (E') and drained Poisson's ratio (ν'). The calculation of consolidation settlements can proceed in a similar manner. The same constant also applies to that soil in overconsolidated states. GeoSlope. and Cαe = coefficient of secondary consolidation. 1994.e.04 ± 0.01 for inorganic clays and silts. such as UTEXAS4. and others.025 ± to 0.04 for inorganic clays. 2003).. either the special method described by Schmertmann (1986) can be used.01 for sands. Extensive lab testing on various soils has shown the ratio of Cαe/Cc is constant for a given normally-consolidated soil (Mesri. In the case of embankment loadings where the imposed earth loadings exceed the preconsolidation stresses. including Cαe/Cc = 0. stability analyses of embankments are handled by limit equilibrium analyses.06 ± 0. but using the recompression index in the ratio. The stiffness is assessed in terms of an undrained soil modulus (Eu) and corresponding νu = 0. usually by trial and error search routines within computer software codes. t = time. using elastic theory with the appropriate displacement influence factors (Poulos & Davis. At the centerpoint of the embankment. presence of lower sand drainage layers. (59) Time Rate Behavior Large areal fills and embankments constructed over soft ground may require long times for completion of primary consolidation. Dissipation testing by CPTù helps assess cvh needed in one-dimensional rate of consolidation analysis. and laboratory testing budgets. ranging from months to tens of years. as only discrete points are obtained in limited numbers because of high costs in sampling. In addition. The time for completion of one-dimensional consolidation for a doubly-drained soil layer (top and bottom) can be estimated from: t = Tv hp2/cv (60) where Tv ≈ 1. coefficient of consolidation. thus aiding in a more definitive calculation of settlements. 1994). et al.933 log(100-U%) (61a) (61b) NCHRP Project 20-05. For the underlying natural soft ground. Results from CPTu soundings can provide information on layer thickness.785(U%/100)2 Tv ≈ 1.781 . 1986). This is perhaps advantageous when large fills are made using the hydraulic fill process (e. and available drainage paths. the CPTu can provide the profile of preconsolidation stress which controls the undrained shear strength for the stability analysis: suDSS ≈ 0. time.The CPTu is particularly suited to the in-situ and continous profiling of the effective preconsolidation stress (σp') and corresponding OCRs with depth. as described previously.g. as well as the calculated spacing of vertical wick drains. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 72 . with adequate side slopes and use of suitable soil fill materials. sample disturbance effects tend to lower and flatten the elogσv' curves and imply yield values which are lower than true in-situ σp' profiles (Davie. In contrast.0. For control of constructed fills. depending upon the thickness of the consolidating layer. the CPTu can be used as a measure of quality control and quality assurance. Time factors for other percentage degrees of consolidation are given by Holtz & Kovacs (1981) with approximations cited as: U < 60%: U ≥ 60%: Tv ≈ 0... determining OCRs from oedometer and/or consolidometer testing are rather restricted. sand drains.22 σp' which applies for OCRs < 2.2 = time factor (assuming 96% consolidation is essentially "complete") from onedimensional vertical consolidation and hp = drainage path length (= ½ layer thickness for double drainage). Embankment Stability Stability analyses of embankments include: (1) the evaluation of the soft ground conditions beneath large fills and (2) the constructed embankment itself. and the detection of sand lenses or stringers that may promote consolidation. or stone columns that may be required by the geotechnical engineer to expedite the consolidation process. Yilmaz & Horsnell. 1993) due primarily to a Poisson effect. The DOTs employ various direct CPT methods (28%). qt. For undrained loading. Emphasis is on the axial pile response (compression and tension modes).Wp Qside = Σ (fp dAs) Qbase = qb Ab fp fp = unit side friction Rational or “Indirect” CPT Method “Direct” CPT Method (Scaled Pile) fp = fctn (soil type. α factor. pile type. and both methods (29%). or by direct CPT methods whereby the measured readings are scaled up for evaluation of full-size pilings. the analysis of pile foundations can be accomplished using classical soil mechanics principles (i. and degree of movement. or fs and Δu) fp = cmck Ko σvo’ tanφ’ unit end bearing = qb qb = unit end bearing Undrained: qb = Nc su Drained: qb = Nq σvo’ qb = fctn (soil type. the cone penetrometer can be considered as a mini-pile foundation. whereby the measured point stress and measured sleeve resistance correspond to the pile end bearing and component of side friction. Direct CPT versus Rational (or Indirect) Method for Evaluating Axial Pile Capacity From those DOTs responding to the questionnaire summary. The two concepts are depicted in Figure 71. s/B ) Figure 71. A review of various methods is summarized here. numerical and analytical studies supplemented by experimental results have shown the magnitude of unit side resistance in tension is around 70% to 90% of that in compression loading (DeNicola & Randolph. Thus.APPLICATION TO PILINGS AND DEEP FOUNDATIONS In one viewpoint. NCHRP Project 20-05. particularly noting newly available approaches that have recently been developed. For axial uplift or tension loading. tanφ'.CHAPTER 8 . AXIAL PILE CAPACITY unit side friction = QTotal = Qs + Qb . yet mention to lateral and moment loading will be made..e. For drained loading. indirect methods (11%). via indirect CPT assessments of su. the analysis may consider only the side resistance component. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 73 . The axial compression capacity of deep foundations is derived from a combination of side resistance and end-bearing (Poulos & Davis. qt-ub. Calculation procedures given subsequently refer to compression-type loading. 1980). the side resistance in compression and uplift will generally be of the same magnitude. approximately 68% utilize the results for axial pile capacity determinations (see Figure 72). β factor). K0. Additional details on the methodologies used are given in Appendix A. Axial Pile Capacity from CPT Not Applicable 11% None 21% Both Direct & Indirect Methods 29% Direct CPT Method 28% 28 Respondents Indirect CPT Method 11% Figure 72. or both. sands. 1997). The unit side resistance (fp) of driven piles and drilled/bored shafts can be calculated using the in-situ CPT results by either direct methods or rational (indirect) approaches. 2005). O'Neill & Reese. Survey Results on DOT Use of CPT for Axial Pile Foundation Design The summation of the unit side resistances acting along the perimetric area of the sides of the pile shaft provides the total shaft capacity (Qs). followed by an engineering analyses of unit side resistance (fp) and end-bearing resistances (qb) within a theoretical framework. 1983): fp = CM CK K0 σvo' tanφ' (62) NCHRP Project 20-05. improved interpretation procedures for soil engineering parameters from CPTu have been introduced (Schnaid. and/or u2) to obtain axial capacities.. the in-situ test data are used first to calculate soil parameters and properties. 1989). In a generalized betamethod for different pile types and methods of pile installation. silts. many of the early studies were based on data obtained with mechanical or electrical friction-type penetrometers. et al. total stress analyses (alpha-method) are used in clays and effective stress approaches (beta-methods) applied to sands (Poulos. One reason for these improvements is that the measured cone tip resistance is corrected for porewater pressures acting behind the tip. the beta-method has shown usefulness and reliability for all soil types (clays. the unit pile side resistance is calculated (Kulhawy. the unit end bearing resistance (qb) of driven pilings or drilled piers can be evaluated by direct or indirect methods to obtain the capacity at the toe or base (Qb). Finally. The total axial capacity is obtained as shown in Figure 71. with two decades use of the SCPTu. Yet. many of which were strictly based solely on qc. as opposed to the older methods. offers improved correlations for both rational and direct CPT analyses. 2005. Mayne.. Thirdly. et al. 1980. fs. Poulos & Davis. The more recent utilization of piezocones. Although the basic concepts remain. et al. with three separate readings with depth. Rational or Indirect CPT Method for Axial Pile Capacity With the rational CPT method. (1988) and Poulos (1989. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 74 . Vesić. A second benefit relates to methodologies based on two or three readings (qt. and gravels).. 1977. 1999). it has now become possible to provide an evaluation of the entire load-displacement-capacity curve for axial pile foundations. Commonly. Prior reviews on selected available direct and indirect approaches for evaluating axial pile capacity from CPT results are given by Robertson.g. Likewise. including the well-known alpha methods for clays and beta methods for sands (e. as detailed previously (Lunne. 1999. et al.1 Nq ⋅σvo' The total axial compression capacity (Qtotal = Qult) of the deep foundation is calculated from: Qult = Qs + Qb . For very large diameter piles in sands.g.g.and pipe pilings) Soil/Smooth Steel (cone penetrometer) Soil/Stainless Steel (flat dilatometer) CK = 0. closed-ended pipe. a practical value for end bearing resistance at working loads is: Operational Drained: qb = 0. and Ab = base/toe area of the pile tip.1 to 1. Modifier Terms for Pile Material Type (CM) and Installation Effects (CK). 1977). et al.6 CM = 0. Lee & Salgado.33 for circular or square foundations and su is the representative undrained shear strength beneath the foundation base from depth z = L to depth z = L+d. (adapted after Kulhawy.0 CK = 1. the theoretical end-bearing resistance will not be realized within tolerable displacements.7 CM = 0. The relevant values for evaluating the lateral stress coefficient (K0) and frictional characteristics (φ') for the soil layers from the CPT have been discussed in prior sections. Thus. as per Table 6. The limit plasticity solution for undrained loading is given as: Undrained: qb = *Nc ⋅su (63) where *Nc = 9.W = Σ (fpi π d Δzi) + qb Ab . 1993. NCHRP Project 20-05. W = weight of the pile.0 to 1. Ghionna.9 CM = 0.where CM = modifier term for soil-structure interaction (pile material type) and CK = modifier term for installation. As such. This is because the full mobilization of base resistance requires the toe/tip to undergo considerable movements on the order of s ≈ B for complete development of these resistances. Vesić. reduction factors have been recommended for large-diameter drilled shafts and piles that result in only 5 to 15% of the calculated capacities can be utilized under normal acceptable movements (e. 1995.8 CM = 0..2 CM = 1.1 CK = 1.9 to 1. precast) Soil/Rough Concrete (drilled shafts) Pile Material Effects Modifier CM Soil/Smooth Concrete (precast) Soil/Timber (wood pilings) Soil/Rough Steel (normal H. open-ended pipe) High-Displacement Driven Piles (e. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 75 .W (65) (64) where Qs = side capacity.0 CM = 0. Fioravante.5 The unit end bearing (qb = qult) can be calculated from theoretical solutions for *Nq based in limit plasticity or cavity expansion (e...g. especially drilled shaft foundations. For drained loading.g. H-piles. 1983) Pile Installation Effects Jetted Pile Modifier CK Drilled and Bored Piles Low-Displacement Driven Piles: (e. et al. Table 6. fpi = unit side resistance at each soil layer. Qb = base capacity.6 CK = 0... qb is a function of φ' and presented in Figure 73.5 to 0. 2003). jacked. NCHRP Project 20-05.0 25.17 26. *Nq Drained Loading . (1997).51 19.89 14.99 69.99 14.49 53.81 66.0 21.65 12. Additional details on the appropriate design values and the specific averaging procedures to obtain a characteristic qc beneath the pile tip.43 52. including: driven.36 18.09 36.0 24. *Nq Strip (d/c = 0) 100 28.0 32.35 ± 0.0 31.0 39.31 33.0 Effective Friction Angle.0 qult = *Nq σvo' 10 20 25 30 35 40 45 50 33. respectively.80 17.0 26. for clays and sands. The approach offers versatility in the variety and types of different deep foundation systems and geomaterials that can be accommodated.67 33.32 136.0 35.11 103.12 118.0 37. Bustamente & Frank. and Takesue methods.0 29.32 78.43 75.77 60.41 21. LCPC Method for Driven & Drilled Piles For the CPT direct methods. high-pressure grouted.0 36.54 11. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 76 .24 85. including the LCPC.00 37. 1995. Full details for obtaining fp and qb are given in Bustamente and Gianeselli (1982). Table 8 provides the kc factors using a simplified LCPC approach (Frank & Magnan. End Bearing Resistance from Limit Plasticity Solution (after Vesić. NGI-BRE. Unicone. particularly for layered soil profiles.0 34. Table 7 lists the various pile categories. Summary graphical approaches for unit side friction (fp) by the LCPC method are provided by Poulos (1989) and these are presented in Figures 74 and 75.94 26.05 29. the well-known LCPC (Laboratoire Central des Ponts et Chaussées) was based on results from 197 pile load tests (Bustamante & Gianeselli. bored.Deep Foundations 1000 φ' Strip 9.48 Circle or Square 12.21 42.63 29.20 (deg) Footing 20.17 157.0 Square/Circle (c/d = 1) 22.98 41.01 47. The LCPC method has a primary reliance on qc for evaluating fp along the pile sides and for qb beneath the pile toe.62 23.0 Bearing Factor.0 23.55 23. and screwed piles. Direct CPT Methods for Axial Pile Capacity Several direct CPT procedures will be reviewed in this section.Let d = minimum plan dimension.53 10.19 58. c = breadth (in plan) Bearing Factor. 1997).94 90. For piles in soils. 1982).40 46.0 30. Polytecnico di Torino.36 20.58 16.0 38. 1977). augered. φ' (deg) Figure 73.0 40.0 27. the end bearing is determined from: LCPC Unit End Bearing: qb = kc · qc (66) The reduction factor kc is obtained from the pile type and ground conditions and averages 0. are discussed by Lunne et al.2.27 15. Specifically. Barrettes Cased bored piles.50 LCPC Direct CPT Method for Clays 180 Note: Lower limit applies for unreliable construction. Upper Limit for very careful construction l PILE TYPE = IIIB Side Resistance. 1982.Table 7. (based on Bustamente & Gianeselli. Case screwed piles. Piers. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 77 . NCHRP Project 20-05. Bustamante & Frank (1997). LCPC Method for Pile Side Resistance Evaluation from CPT in Clays.40 0. Jacked steel piles Driven grouted piles. 1989). 1995.25m). Soil Type Clay and/or Silt Sand and/or Gravel Non-Displacement Pile 0. Prestressed tubular piles. adapted from Poulos. Driven rammed piles High pressure grouted piles (d > 0. Driven cast piles Driven precast piles. Hollow auger bored piles. Type II micropiles Table 8. Type I micropiles. Mud bored piles. qc (MPa) Figure 74. Base Bearing Capacity Factors kc for LCPC Direct CPT Method (Simplified approach by Frank & Magnan. IIA lower upper IB lower IIB 5 10 15 20 Cone Tip Resistance. Jacked concrete piles Driven steel piles. fp (kPa) 160 140 120 100 80 60 40 20 0 0 IIIA upper IA.55 0. Various Pile Categories for LCPC Direct CPT Method Pile Category IA IB IIA IIB IIIA IIIB Type of Pile Plain bored piles.15 Displacement Type Pile 0. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 78 . (1996) relate the driven pile side resistance in clays to total cone tip stress. 1988).5 to 3. the side friction is obtained from: clays: f p = q t − σ vo 10. et al. Almeida et al. NkT may be as high as 25 to 35 (Powell & Quarterman. 1989). 1996). In an updated form given by Powell et al. IIA upper lower upper lower Side Resistance.3 log Q (67) where Q = normalized cone tip resistance. The unit end bearing (qb) is determined from: clays: q b = q t − σ vo k2 (68) where k2 = Nkt/9 and a value of NkT = 15 is often appropriate for piles in soft to firm intact clays where DSS mode controls the bearing mechanism.4 for just three piles in intact clays (Almeida. (2001). LCPC Method for Pile Side Resistance Evaluation from CPT in Sands. London. Direct backfigured values of k2 were reported to range from 1. Yet in fissured to hard clays. adapted from Poulos. 1982.5 + 13. NGI-BRE Method for Driven Piles in Clay An empirical approach for driven piles in clay has been developed jointly by the Norwegian Geotechnical Institute (NGI). (based on Bustamente & Gianeselli. Oslo and Building Research Establishment (BRE).LCPC Direct CPT Method for Sands 300 Note: Lower limit applies for unreliable construction. Upper Limit for very careful construction PILE TYPE = IIIB IIIA 150 100 50 0 0 10 20 30 40 IA. fp (kPa) 250 200 IIB IB Cone Tip Resistance. qc (MPa) Figure 75. Using the total cone resistance. NCHRP Project 20-05. Politecnico di Torino Method for Drilled Shafts in Sand A direct CPT method for drilled shafts in sands has been developed by the Politecnio di Torino, Italy. For clean quartzitic uncemented NC sands, the side resistance of drilled shafts may be estimated from the CPT resistance (Fioravante, et al. 1995), where an average trend can be represented by: sands: ⎛ q ( MPa) ⎞ f p ( MPa) ≈ ⎜ t ⎟ ⎝ 274 ⎠ 0.75 (69) The unit end-bearing depends upon the actual movement of the base, yet can be taken as approximately 10% of the cone resistance: qb ≈ 0.10qt (e.g., Ghionna, et al., 1993). In this regard, an improved relationship has been developed by Lee & Salgado (1999) based on numerical analyses. Neglecting the minor effects of sand relative density, an average relationship can be expressed by: sands: qb ≈ 0.62 1.90 + (s / d ) qt (70) where s = pile base deflection and d = pile base diameter. A nominal value of s/d = 0.10 is often taken for a relative capacity, thereby giving qb ≈ 0.12qt and in general agreement with the aforementioned. An updating of this approach is presented by Jamiokowski (2003). Unicone Method for Driven and Bored Piles A generalized direct CPTu method for sands, silts, and clays has been proposed by Eslami & Fellenius (1997) based on 106 load tests from both driven and bored piling foundations. The method uses all three piezocone readings (qt, fs, and u2). In this Unicone approach, the soils are classified into one of five SBT zones per their effective cone resistance (qE = qt - u2) and sleeve friction (fs) according to Figure 76. 100 1- Very soft clays, sensitive soils 2- Soft clays 3- Silty clays - Stiff clays 4- Silty sands - Sandy silts 5- Sands, Gravelly Sands qE = qt-u2 (MPa) 5 4 3 10 1 1 0.1 1 10 100 2 1000 Sleeve Friction, fs (kPa) Figure 76. Unicone Chart to Determine Zone Number and Soil Type (after Eslami & Fellenius, 1997) NCHRP Project 20-05; Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 79 Table 9. Unicone Method for Soil Type, Zone, and Assigned Side Friction Coefficient* Zone Number 1 2 3 4 Soil Type Soft Sensitive Soils Soft Clay and Silt Stiff Clay and Silt Silty Sandy Mixtures Sands Side Factor, Cse 0.08 0.05 0.025 0.01 0.004 5 *Note: after Eslami and Fellenius (1997) For each layer, the unit side resistance is obtained from: fp = Cse ·qE (71) where Cse = the side correlation coefficient obtained from Table 9. The unit end bearing resistance can be obtained from the effective cone tip resistance beneath the pile toe: qb = Cte · (qt – ub) (72) where Ctee = toe correlation coefficient, generally taken equal to 1. If the CPT record indicates high spikes and variability in the effective cone resistance profile, Eslami & Fellenius (1997) recommend use of a geometric mean for averaging readings, rather than the more common arithmetic mean. The geometric mean is given by: qE(ave) = [qE1⋅ qE2⋅ qE3⋅⋅⋅⋅⋅⋅qE1](1/n) where n = number of data. (73) Takesue Method for Driven & Drilled Piles In the method of Takesue, et al. (1998), the unit pile side resistance (fp) is estimated from the measured CPT fs which is scaled up or down depending upon the magnitude of the measured CPT excess porewater pressures (Δu2), as presented in Figure 77. The data used to derive the correlation were obtained from both bored and driven pile foundations in clays, sands, and mixed ground conditions. As such, the method has been shown to work well in the nontextbook sandy silts to silty fine sands soils of the Atlantic Piedmont residuum (e.g., Mayne & Elhakim, 2003) that typically show negative CPTu porewater pressures during penetration. From Figure 77, the scaling factors are divided into two porewater pressures regimes at Δu2 = 300 kPa, with a maximum Δu2 < 1200 kPa. The Takesue method does not specifically indicate a means for evaluating unit end bearing of piles, therefore either the aforementioned NGI-BRE, LCPC, Torino, or Unicone methods could be used for that purpose. NCHRP Project 20-05; Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 80 6 Clay Pile-CPT Friction Ratio, fp/fs 5 Mix Sand Clay Bored cast-in-situ piles 4 Mix Sand Driven steel piles 3 fs ub fp 2 fs ≈ Δu b + 0 .76 1250 fp 1 fs 0 300 600 ≈ Δub − 0.5 200 900 1200 (Takesue, et al., 1998) 0 -300 Excess Pore Pressure, Δub (kPa) Figure 77. Direct CPTu Method for Evaluating Side Friction of Bored and Driven Piles in Different Soils (after Takesue, et al. 1998). Other Direct CPT Methods for Axial Capacity A number of new direct CPT methods for driven piles in sands have emerged recently from work funded by the offshore oil industry, including: (1) Imperial College Pile (ICP) Method (Jardine, et al. 2005); (2) UWA Procedure (Lehane, et al. 2005); (3) NGI Method (Clausen, et al. 2005); and (4) Fugro Method (Kolk, et al. 2005). For evaluating driven piles in clay from CPT data, new interpretative approaches include: (1) ICP method (Jardine, et al. 2005); and (2) the NGI Method (Karlsrud, et al. 2005). Caution should be exercised when using any calculated pile capacity method without proper checking and validation. On critical projects, verification by full-scale load tests and/or calibration at well-established experimental test sites may be warranted by the particular DOT. Foundation Displacements The load-displacement behavior of deep foundations subjected to loading may be analyzed using empirical, analytical, and/or numerical methods. In all cases, data input on the soil parameters and geostratigraphy must be supplied. Elastic continuum theory is one popular method that is also consistent with the analysis of shallow foundation systems, as discussed in the next section. Elastic Continuum Solutions Elastic continuum theory provides a convenient means for representing the load-displacement response of pile foundations under axial loading, as well as lateral loading and moments (Poulos & Davis, 1980). An approximate closed-form solution has been developed that can account for axial piles either floating or end-bearing, situated in homogeneous or Gibson-type soils, as well as accommodate pile compressibility NCHRP Project 20-05; Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 81 effects and belled pier situations (Randolph & Wroth, 1978, 1979; Fleming, et al. 1992). For a pile of diameter d and length L residing within an elastic medium, the top displacement (wt) is given by: wt = Qt ⋅ I ρ d ⋅ Es (74) where Qt = applied axial load and Iρ = displacement influence factor. For the simple case of a rigid pile embedded in a homogeneous soil: Iρ = 1 1−υ 2 + (L / d ) (1 + υ ) ln[ 5 ( L / d )(1 − υ )] ⋅ π 1 (75) where Figure 50 shows that the classic boundary element solution (Poulos & Davis, 1980) agrees well with the approximate closed-form approach. Randolph & Wroth (1979); Poulos & Davis (1980) Rigid Pile in an Infinite Elastic Medium 1.00 Boundary Elements Influence Factor, Io Closed Form v = 0.5 Closed Form v = 0.2 Closed Form v = 0 0.10 0.01 0 10 20 30 40 50 60 70 80 90 100 Slenderness Ratio, L/d Figure 78. Displacement Influence Factors for Rigid Pile in Homogeneous Soil. By the closed-form solution, the percentage of total load at the top (Qt) which is transferred to the toe or base (Qb) is given by: Qb Qt = Iρ 1−υ 2 (76) For the more generalized case involving friction- to end-bearing type shafts and homogeneous to Gibson soil profiles with Es either constant or increasing with depth, and pile compressibility effect, the specific solutions for Iρ and percentage load transfer to the tip or toe (Qb/Qt) are given elsewhere (Randolph & Wroth, 1978, 1979; Fleming, et al. 1992; O’Neill & Reese, 1999; Mayne & Schneider, 2001). NCHRP Project 20-05; Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 82 Approximate Nonlinear Pile Load-Displacements The stiffness of soils is highly nonlinear at all levels of loading. The most fundamental stiffness is that measured at small strains (Burland, 1989), as it represents the beginning of all stress-strain curves at the initial state. The small-strain modulus (Emax) combined with the aforementioned modified hyperbola (exponent g = 0.3 ± 0.1) for modulus reduction allows for an approximate nonlinear load-displacementcapacity representation of the form: wt = Qt ⋅ I ρ d ⋅ E max ⋅ [1 − (Q t / Q tu ) ] g (77) With the utilization of seismic cone penetration tests, the advantage here is that the CPT resistances (qt, fs, and u2) are employed to determine the ultimate axial pile capacity (Qtu) and the downhole shear wave velocity (Vs) is used to determine the initial stiffness (Emax). The ratio of applied axial load to the calculated axial capacity represents the reciprocal of the current factor of safety: (Qt/Qtult) = 1/FS. The overall concept of this procedure is depicted in Figure 79 whereby all four readings of the SCPTu are utilized in evaluating the response of the deep foundation under axial compression loading. RIGID PILE RESPONSE CPT PILE Qtu = Qs + Qb Qsu = Σ (fp dAs) Qbu = qb Ab Top Displacement, wt unit side friction, fp wt = Iρ = Qt ⋅ I ρ d ⋅ E max [1 − (Qt / Qtu ) 0.3 ] 1 1 + (L / d ) (1 + υ ) ln[5( L / d )(1 − v)] ⋅ Vs → Emax = 2 ρt Vs2 (1+ν) fs ub qt π } V 1−υ 2 fp = fctn ( fs and Δu) Clays: qb = qt - ub Sands: qb ≈ 0.1 qt Load Transfer } Iρ Qb = Qt 1 − υ 2 qb = unit end bearing Figure 79. Concept of Using SCPTu for Evaluating an Approximate Nonlinear Axial Pile Response. NCHRP Project 20-05; Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 83 Illustrative Example An application of the methodology will be shown for a drilled shaft foundation constructed at the National Geotechnical Experimentation Site (NGES) near Opelika. It can be seen that the overall axial load-displacement response is in excellent agreement with the measured top-down shaft response. Using the Takesue.2 gives an initial small-strain stiffness of Emax = 190 MPa (1979 tsf). Results of the equivalent elastic continuum method with the modulus reduction scheme (g = 0. a representative shear wave velocity of Vs = 216 m/s (708 ft/s) with a corresponding total mass density ρT = 1.6 m2 (340 ft2). Alabama. Adopting a homogenous case for the modulus variation with depth. qt (MPa) 0 0 2 4 6 8 0 0 fs (kPa) 100 200 300 -100 0 u2 (kPa) 0 100 200 0 0 100 Vs (m/s) 200 300 400 2 2 2 2 Drilled Shaft (Cased Method) d = 0. the total side capacity is Qs = 3032 kN (340 tons). Alabama. the total end bearing capacity is Qb = 223 kN (25 tons). With the pile side area As = 31. The elastic continuum solution appropriately proportions the amounts of load transfer carried by the sides and base components. In addition.7 g/cc and drained ν' = 0. NCHRP Project 20-05. the modified hyperbola nicely fits the observed nonlinear response of the deep foundation. (1998) method for side resistance. the CPT-measured fs is reduced to an operational pile side friction of fp = 96 kPa (1 tsf) because of the CPTumeasured negative Δu throughout most of the profile.3) is presented in Figure 81. A dry cased-type installation was used to form the drilled shaft foundation with a diameter of 0. an equivalent sand method is considered applicable.914 m (3. Representative SCPTU in Piedmont Residuum at Opelika NGES.0 m 4 4 4 4 Depth (meters) 6 6 6 6 8 8 8 8 Piedmont Residuum (ML to SM) SCPTu CHT 10 10 10 10 12 12 12 uo 12 14 14 14 14 Figure 80.66 m2 (7. This gives a total axial compression capacity: Qt = Qs = Qb = 3255 kN (365 tons). A representative SCPTU from the site is presented in Figure 80 (Mayne & Brown.914 m L = 11. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 84 .0 feet) and embedment length L = 11 m (36 feet). 2003). et al. With a base area Ab = 0. comprised of fine sandy silts and silty fine sands derived from the inplace weathering of the underlying parent gneiss and schist bedrock. The shaft was load tested in axial compression and results are reported by Brown (2002). The facility is operated for the Alabama DOT. Therefore.1 ft2). Since these soils drain relatively rapidity. the end bearing resistance is taken as 10% of the measured cone tip stress beneath the foundation base (average qt = 3380 kPa = 35 tsf). The natural soils consist of residuum of the Atlantic Piedmont geologic province. Axial Load. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 85 . Base Qs d = 0.914 m L = 11. Shaft Meas. Q (MN) 0 0 Qtotal = Qs + Qb 1 2 3 4 Qt Top Deflection (mm) 5 10 15 20 25 30 Pred. NCHRP Project 20-05.0 m Qb Figure 81. Total Meas. Qb Meas. Qs Pred. Measured and SCPTu-Predicted Axial Response of Opelika Drilled Shaft. (2006) Schneider & Mayne (2000) Mitchell & Solymar (1984) Alperstein (2001) Howie. et al. In-situ testing by CPT allows quantification of the level & degree of quality control and effectiveness of the site improvement program. (2000) Lutenegger. et al. Quality control Lessons in sands and silts CPT before and after installation Measure degree of improvement Measure CPT increases with time Verify sand condition by CPT SCPTU for degree of change Evaluate cvh from dissipations An important aspect of quality control testing is to realize that time effects may occur after implementation of the site improvement program. with its strong theoretical basis and documented calibrations with laboratory test data. it may be necessary to conduct series of CPTu soundings at various times after the ground modification has been applied. Puppala. et al. 1986). et al. et al. Alternatively. (2004) Ghosh (1995) Huang. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 86 . Ground Modification Method Blast Densification Compaction of Trench Backfill Compact Natural Sands by Rollers Compaction Grouting Controlled Modulus Columns Deep Soil Mixing Dynamic Compaction GeoPiers (Rammed Aggregate Piers) Jet Grouting Stone Columns Surcharging Vibro-Compaction Vibro-Replacement Wick Drains Reference Source Mitchell (1986) Islam & Hashmi (1995) Alperstein (2001) Chun. (1988) Use of CPT Quantify time increases with qt Determine relative compaction CPTs to quantify improvement CPTUs for check on remediation of foundation settlements CPTs for initial investigation. consistency. et al. Table 10. columns too hard for CPTs after cement grouting Quality assurance and verification Quality control by CPT CPT for quality assurance Piers in clay at NGES-Amherst CPTs initially. Therefore. Selected Ground Modification Methods and Relevance of CPT for QA/QC. too hard for CPTs after cement grouting. NCHRP Project 20-05. et al. and/or properties from its initial state. et al. (1995) Chen & Bailey (2004) Shenthan. (1998) Lillis. (2004) Collotta. Selected applications of ground modification techniques which have benefited from CPT verification programs for these purposes are addressed in this chapter. The quality assurance can be checked by simple and direct comparisons between the original CPT measurements and results taken following site improvement. the soil is changed in its condition. in order to properly quantify the degree of effectiveness and to fully-appreciated the benefits of the improvement program (Mitchell. Table 10 lists a number of different site improvement techniques and representative reference sources for additional details on the results. (2004) Puppala & Porhaba (2004).CHAPTER 9 – CPT USE IN GROUND MODIFICATION During ground modification works. (2004) Durgunoğlu. et al. the CPT can be used to provide engineering parameters for re-evaluation of the completed works and modified ground conditions. et al. (2003) Plomteux. W = weight of falling mass. times number of drops per grid (n). fs (kPa) 0 50 100 150 -100 3 2 1 0 -1 -2 -3 Before DDC Porewater. dynamic compaction was carried out using a 15-tonne weight dropped repeatedly from 18 m height.An example of the use of CPTs to detail the depth and degree of improvement following dynamic compaction at a resort development near San Juan is shown in Figure 82. This energy intensity can be calculated as the sum of the energy per drop (W⋅H). For sands. P. and H = drop height. The average cone tip resistance following site improvement by dynamic compaction depends upon the applied energy intensity over the project site area. Figure 83. NCHRP Project 20-05. Figure 83 shows the observed trend between final average qt within the depth of improvement vs. CPT Results Before and After Dynamic Compaction near San Juan. ub (kPa) 0 100 200 Before DDC After DDC uo Friction Ratio.R. Cone Tip Stress. whereas the porewater measurements and friction ratio show little changes. The cone tip and sleeve resistances both show improvement occurring to depths of 7 m (22 feet) below grade. At this site. fs/qT (%) 300 3 2 1 0 -1 -2 -3 Before DDC 0 1 2 3 4 5 6 Elevation (m) 0 -1 -2 -3 After DDC -3 -4 -5 -4 -5 After DDC -4 -5 -4 -5 After DDC Figure 82. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 87 . Measured Cone Tip Stress in Sands following Ground Improvement by Dynamic Compaction. qT (MPa) 0 3 2 1 5 10 15 20 3 2 1 0 -1 -2 Before DDC Sleeve Friction. the applied UE for a number of DDC projects. divided by the area of the area treated: UE = Σ (n ⋅ W ⋅ H)/s2 (78) where s = spacing per grid. Use of seismic cone penetrometer for evaluating site-specific soil liquefaction concerns. NCHRP Project 20-05. A clear advantage of the SCPTU is its ability to identify possible loose silty to clean sand layers within the subsurface profile and then provide the required measured values at the site (Gmax. (d) deterministic and probabilistic means to assess soil liquefaction potential by stress-normalized shear wave velocity (Vs1). termed the cyclic stress ratio: CSR = τcyc/σvo′. as well as (e) post-cyclic undrained strength of sands for stability considerations. An attractive feature of the SCPTU is the ability to use the data directly in assessing the site-specific ground amplification of the soil column and evaluation of soil liquefaction potential in seismic regions. is described with reference to the following topics: (a) determination of soil stratigraphy and identification of potentially liquefiable soils. DEEPSOIL. all from the same sounding. The amount of soil resistance available to counter the effects of liquefaction is represented by either the stress-normalized cone tip resistance (qt1) or alternatively expressed by the stress-normalized shear wave velocity (Vs1). (b) collection of shear wave velocities for use in either International Building Code (IBC 2000) class or site-specific ground shaking analyses. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 88 . Figure 84. DESRA. These can be compared with the cyclic resistance ratio curves (CRR) for natural sands to silty sands to assess the tendency or risk of liquefaction. particularly the seismic piezocone. RASCALS. (c) deterministic and probabilistic means to assess soil liquefaction potential by stress-normalized cone tip resistance (qt1). or other computer codes available for site amplification. The small-strain stiffness (G0 = Gmax) is required input for determining the level of ground shaking via SHAKE. The CRR line demarcates two regions corresponding to liquefaction-prone versus liquefaction-resistant.CHAPTER 10 – SEISMIC GROUND HAZARDS For seismicity issues. and Vs1) for analysis. qt1. The procedure is illustrated by Figure 84. the utilization of cone penetrometer technology. Their output includes an evaluation of the applied ratio of cyclic shear stress normalized to effective overburden stress. et al. SHAKE2000. Deterministic approaches include procedures based on stress-normalized tip resistance (e. 1998. or 10% probability earthquake for a certain period of time. SHAKE. 2001) and/or stress-normalized shear wave velocity (e. 2001). Stark & Olson. the soil has a high likelihood of liquefaction.50 (80a) (80b) (80c) (80d) The value amax is taken from the appropriate design events for a given project (i. in terms of probabilistic curves of increasing risks of liquefaction. Using the conventional simplified procedures. Holocene) clean to silty sands below the groundwater table (Youd.. 5%. or alternatively evaluated more properly using site-specific Gmax data within commercial codes (e. if the CSR value is higher than the CRR. the likelihood of liquefaction is small. For the CPT-based method shown in Figure 85a. the level of ground shaking from seismic loading is expressed in terms of the cyclic stress ratio (CSR).5 ≤ rd ≤ 1. or NEHRP (National Earthquake Hazard Research Program).z)/125 rd = 0.g. Youd et al. Robertson & Wride.g.. the International Building Code (IBC). Andrus & Stokoe. EduSHAKE. Therefore.e. or DEEPSOIL). rd can be obtained with depth z (meters) as follows: For depth z ≤ 9. Youd et al.0).. RASCALS. σvo and σvo′ are the total and effective vertical stresses.65⎜ max ⎟⎜ vo ⎟rd ' ⎜ g ⎟⎜ σ ' ⎟ σ vo ⎠⎝ vo ⎠ ⎝ (79) where τave is the average equivalent uniform shear stress generated by the earthquake (assumed to be 65 percent of the maximum induced stress).15 m: For 9.e.g.. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 89 . The cyclic resistance ratio (CRR) can be expressed using conventional deterministic approaches which give a binary decision (liquefaction or no liquefaction). The cyclic resistance ratio (CRR) is the threshold for liquefaction and used to compare the available soil resistance with level of ground shaking represented by the cyclic stress ratio (CSR). 1971): CSR = ⎛ a ⎞⎛ σ ⎞ τ ave = 0. Per the recommendations of the NCEER Workshop on soil liquefaction (Youd et al. and rd is a stress reduction coefficient that accounts for the flexibility of the model soil column (0. the maximum credible event for a known fault located a certain distance from the site. the cyclic stress ratio (CSR) is expressed as (Seed & Idriss.. The CSR can be estimated using seismic ground hazard maps published by the USGS. 2001). respectively. g = the gravitational acceleration constant (g = 9.z)/131 rd = (44 . For clean quartz sands: NCHRP Project 20-05.z)/37 rd = (93 . state geological agencies. 1990). Determine Level of Ground Shaking In liquefaction analyses. The use of the aforementioned classification charts for soil behavioral type (SBT) for the CPTu can be used to identify and delineate the sand and silty sand layers in the soil profile (e. 2001). It the CSR falls beneath the CRR.8 m/s2 = 32 ft/s2). amax is the peak ground acceleration (or PGA). or alternatively. 1995..Identification of Liquefaction Prone Soils Soils which are prone or susceptible to liquefaction during large seismic events include loose young (i.g.15 m ≤ z ≤ 23 m: For 23 m ≤ z ≤ 30 m: For z > 30 m: rd = (131 . Robertson. the cone tip resistance is normalized as a function of the effective stress (actual normalization criteria depends upon the CPT soil classification) and is designated qc1N. the 2%. or a code-based response spectrum). 2000.. 1998): If 50 ≤ (qc1N )cs < 160 If (qc1N )cs < 50 CRR7.60 Clays 3 2.31 < Ic < 2. the index is re-defined by Robertson & Wride (1998) using only CPT Q and F data because porewater pressures are often near hydrostatic for loose to firm clean sands (thus both Δu and Bq ≈ 0).05 < Ic < 2.6 (see Robertson and Wride.88 (84b) This requires iteration as the value of Q is adjusted to qc1N for stress normalization if Ic < 2.60 Sands 6 1.08 ⎣ 1000 ⎦ 3 (85a) ⎡ (q ) ⎤ CRR7. The modified CPT soil type index is: Ic = [3.60 Silt Mixtures 4 2.60 < Ic < 2.403I c + 5. designated (qc1N)cs. which is its equivalent clean sand value. by the relationship: (qc1N)cs = Kc ⋅ qc1N (82) where Kc is the correction factor for the apparent fines content and is empirically calculated from a modified CPT soil classification index.05 ⎣ 1000 ⎦ (85b) NCHRP Project 20-05.5 = 0.75 I c − 17. 1998) Soil Classification Zone Number* Range of CPT Index Ic Values Organic clay soils 2 Ic > 3.. 1998).63 I c + 33. CPT Index Ic for Sand Liquefaction Evaluation (after Robertson & Wride.0 (84a) 4 3 2 For Ic > 1.5 (σ vo ' / σ atm ) σ vo '⋅σ atm (81) where atmospheric pressure is used to make the form dimensionless (Note: 1 atm = 1 bar = 100 kPa).64: Kc = 1.22 + log F ] 2 (83) Table 11.5 (Youd et al.64: K c = − 0.31 *Note: Zone Number per Robertson SBT (1990) Specifically. Here. Ic. For clean sand.95 < Ic < 3. the CRR is calculated by the following equation for an earthquake moment-magnitude of 7. The level of ground motion (CSR) and the adjusted tip resistance (qc1N)cs are compared with the cyclic resistance ratio (CRR) to determine whether liquefaction will or will not occur.47 − log Q] 2 + [1.581I c − 21. Robertson & Wride.833⎢ c1 N cs ⎥ + 0. Kc is evaluated from: For Ic ≤ 1.qc1N = (qt / σ atm ) qt = 0.95 Sand Mixtures 5 2.5 ⎡ (q ) ⎤ = 93⎢ c1N cs ⎥ + 0.05 Gravelly Sands 7 Ic < 1. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 90 . For silty sands. the stress-normalized cone tip resistance is modified to the adjusted tip resistance. 2001. In more recent evaluations. Based on a database of 225 CPT-based cases reported by Juang and Jiang (2000) for qc1N probability curves: PL = 1 1 + (Fs 1.2 0. Vs1c = 210 m/s for FC = 20%. a deterministic chart procedure is shown in Figure 85b using the stress-normalized shear wave velocity that is designated Vs1 and determined as: Vs1 = Vs (σ vo ' / σ atm ) 0.0.5 0.4 Liquefaction 0. qc1N Normalized Shear Wave Velocity.34 ] (88) NCHRP Project 20-05. V s1 (m/s) Figure 85.5 is found from Andrus & Stokoe (2000) and Youd et al.6 Cyclic Resistance Ratios (CRRs) for clean sand (Robertson & Wride.9 are fitting parameters and Vs1c is an asymptote related to fines contents (FC): Vs1c = 220 m/s for FC ≤ 5%.25 (86) where Vs is in m/s. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 91 . 2000.1 No Liquefaction 0 0 50 100 150 200 250 300 0 100 150 200 250 Normalized CPT Tip Resistance.5 0.2 0. and Vs1c = 200 m/s for FC ≥ 35%. 1997) Cyclic Stress Ratio (CSR) Cyclic Stress Ratio (CSR) 0.1 No Liquefaction 0.3 0.6 Cyclic Resistance Ratios (CRRs) for clean sand (Andrus & Stokoe. 2000). (2001): CRR7. Deterministic Approaches for Liquefaction Analysis of Clean Sand Based on: (a) Cone Tip Resistance (Robertson & Wride 1998) and (b) Shear Wave Velocity (Andrus & Stokoe.3 0. Juang & Jiang.03 and b = 0.0 ) [ 3. CRR curves of different probabilities of occurrence have been developed from mapping functions (Chen & Juang. For liquefaction evaluation based on shear wave velocity.4 Liquefaction 0.5 = a (Vs1 100 ) + b[1 (Vs1c − Vs1 ) − 1 Vs1c ] 2 (87) where a = 0. 1998) 0. A calculated factor of safety (Fs) can be defined as Fs = CRR/CSR for a particular earthquake magnitude and set of data. 2000) to relate the safety factor Fs to the liquefaction probability PL. The CRR for an earthquake moment-magnitude of 7. 3 0.7 0.5 Different Levels of Probability (Juang et al. Probabilistic Cyclic Resistance Ratios (CRRs) for Clean Sands based on (a) Cone Tip Resistance and (b) Shear Wave Velocity (after Juang & Jiang. 2000) 0.1 0. These include special stress normalization procedures for the CPT resistances and have been specifically developed to better address the reliability of seismic ground hazards in sands having various percentage fines contents. respectively.5 Different Levels of Probability (Juang & Jiang.3 P L=0.4 0.5 0. there is a similar mapping function (Juang et al.7 0.4 Liquefaction 0.2 0.2 0. 0.6 Cyclic Resistance Ratios (CRRs) at 7. CSR Liquefaction 0..1 No Liquefaction No Liquefaction 0 0 50 100 150 200 250 0 100 150 200 250 Norm alized CPT Tip Resistance.9 0. NCHRP Project 20-05. CSR 0.9 0.1 ] (89) Separate CRR curves corresponding to different probabilities of liquefaction ranging from 10% to 90% using qc1N and Vs1 are presented in Figures 86a and 86b. 2001) Cyclic Stress Ratio. V s1 (m /s) Figure 86. Alternate methods of post-processing CPT data to obtain probabilistic assessments of soil liquefaction potential have been recently proposed by Moss.2 0. q c1N Norm alized Shear Wave Velocity. 2001): PL = 1 1 + (Fs 0. 2006).5 P L=0.72 ) [ 3.For the normalized shear wave velocity (Vs1). et al. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 92 .1 0.5 0.1 0.3 Cyclic Stress Ratio.3 0. (2003.6 Cyclic Resistance Ratios (CRRs) at 7.5 0. 2000). (1995) Romani. (1988) Newcomb & Birgisson (1999) Foshee & Bixler (1994) Landslide Forensics and Slope Stability Pavements Investigations Sinkhole Detection in Limestone Terrain The CPT has enjoyed particular use on geo-environmental site investigations because the test produces no samples. Although downhole probes (e. the process often requires casing of the hole and is much more destructive than CPT invasion (i.5 m).e. (1995) Hight and Leroueil (2003) Badu-Tweneboah. (1998) Lambson & Jacobs (1995) Lightner & Purdy (1995) Mlynarek et al. et al. the conductivity cone is commercially available from manufacturers and CPT service firms as an expedient means to map contaminant plumes and detect the presence of underground pollutants (Campanella & Weemees. no cuttings. Special Applications of Cone Penetrometer Technology CPT Application Environmental Site Investigation and Detection of Soil Contamination Reference Source • • • • • • • • • • • • • • • • • Campanella & Weemees (1990) Auxt & Wright (1995) Bratton & Timian (1995) Campanella. Table 12. (1995) Pluimgraff et al. NCHRP Project 20-05. pavement investigations. The electrodes are provided as an array of either four axial rings at set vertical spacings.g. the continuous nature of CPT helps provide detailed logging with 3 or more channels. Electrical conductivity can be used to identify soil types.. and environmental investigations. An example resistivity piezocone sounding (RCPTu1) from downtown Memphis. cone penetrometer technology has been employed to assist in delineating and detecting anomalous conditions or unusual features in the ground. et al. so the device is also referred to as the resistivity cone (see Figure 87). Special Utilization of CPT In certain circumstances. 1990).MISCELLANEOUS USES OF CPT and SPECIALIZED CPT EQUIPMENT This section discusses a variety of other applications for the CPT. & Powell (1998) Shinn and Bratton (1995) Collotta. thereby minimizing the generation of above ground cleanup in sensitive areas and contaminated ground.CHAPTER 11 .4-inch or 36-mm pushed-place hole). Since traditional drilling and sampling is intermittent at say 5-foot depth increments (1. et al. sinkholes. It is also employed in coastal areas to distinguish the upper freshwater table from the lower salt water regime.. Resistivity (ohm-m) is the reciprocal of electrical conductivity. et al. (1995) Robertson. Lunne. Tennessee is presented in Figure 88. et al. The special membrane interface probe (MIP) offers a single button electrode for an index determination of in-situ resistivity penetration and gas sampling. geophysical tools or videocamera) can be lowered down a pre-drilled borehole. Of well-known acclaim. including: slope stability investigations and landslide forensics. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 93 . a 1. an augered 8-inch or 200-mm diameter hole vs. and no spoil. or with a button array (positioned diametrically). A listing of select special applications by CPT is presented in Table 12 with a cited reference source given should additional details be desired. (1989) Leroueil. and (c) Diametric (Button) Electrode Array for Resisitivity. et al. a similar approach can be provided using alternating current (AC) and thus established to obtain dielectric measurements (permittivity). 1998). For portions of the sounding that extend below the groundwater table. NCHRP Project 20-05. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 94 .Whereas resistivity induces a direct current (DC) electrical current into the ground. the gravimetric water content can be mapped. Figure 87b shows a special dielectric CPT penetrometer developed for this purpose (Shinn. qT (MPa) 0 10 20 30 40 0 fs (kPa) 100 200 300 400 0 U1 (kPa) 500 1000 1500 0 1 Ω (mS/m) 10 100 1000 0 5 10 Depth (m) 15 20 25 30 35 Figure 88. Tennessee. Example of Conductivity Piezocone Test at Mud Island. Electrical Conductivity Measurements: (a) Fugro Conductivity Cones. These dielectric readings can be interpreted to provide direct realtime profiles of volumetric water content. Memphis. (b) Vertek Dielectric and Hogentogler Resistivity Cones. Figure 87. Illustrative examples of these CPT technologies include the use of cableless systems to transmit or store data. fs. including: (a) memocone. In the audio-signal cone. (1998) Hryciw & Shin (2004) Notes/Remarks Indicator of soil type Delineate soil type and lenses Portable remote shear wave source Obtains soil sample when needed Maps volumetric water contents Towards tunnel investigations Measures total horizontal stress Total lateral stress during penetration Dual-element piezocone Tripe-element piezocone Quad-element piezocone Four friction sleeves of different roughness for pile interface studies Measures density and water content in real time Penetrometer with 100-cm2 head to increase load cell resolution in soft soils Evaluate site-specific soil liquefaction Vibration to locally cause liquefaction Real-time videocam of soil profile Detection of thin layers & lenses Figure 89. (2000) Hryciw. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 95 . In the case of the memocone. (2000) Bonita. and (3) wireline systems and deep soundings. and u2 are matched with the depth logger readings of time t and depth z. NCHRP Project 20-05. Specialized Sensors or Modifications to Cone Penetrometer Technology Specialized CPT System Acoustic Emission CPT AutoSeis Generator CPT Soil Sampler Dielectric CPT* Horizontal CPT Lateral Stress Cone Multi-Element Piezocones Multi-Friction Sleeve Penetrometer Radio-Isotope CPT T-bar penetrometer Vibro-Piezocone Vision Cone Penetrometer (VisCPT) *Note: also termed "soil moisture probe". (2006) Randolph (2004) McGillivray et al. et al. the data are stored downhole until the penetrometer is retrieved back at the ground surface and the readings of time t. (b) audio-signal cone. These systems are advantageous in the following situations: (1) when conducting CPTu with drill rigs where the crew are not sensitive to working with electronic cables. Cableless CPT Systems: (a) Memocone (ENVI) and (b) Audio-Signal Unit (Geotech AB). A similar approach is used for infrared signals. et al.New developments in sensors and testing procedures for CPT have been introduced to enhance the capabilities of direct-push technologies. Selected instruments and innovations are listed in Table 13. et al. the data are transmitted by sound waves up through the center of the rods in realtime and a special microphone used to capture the sounds that are digitally decoded for the data logger. et al. et al. as shown in Figure 89. qt. (1990) Juran & Tumay (1989) Skomedal & Bayne (1988) Danzinger. and (c) special glass-lined rods to allow transmission by infrared signals. et al. et al. (2) offshore deployment. (1998) Broere &Van Tol (2001) Takesue &Isano (2001) Campanella. (2004) Shrivastava & Mimura (1998) Dasari. Table 13. (1997) DeJong & Frost (2002) Hebeler. Reference Source Houlsby & Ruck (1998) Menge & Van Impe (1995) Casey & Mayne (2002) Shinn. (c) Fugro triple-element cone. Multi-Piezo-Element Penetrometers: (a) Dual-element type with midface and shoulder filters (v. then special CPT samplers have been developed that can obtain a disturbed pushed sample from specified depth (Figure 92). The T-bar is actually a penetrometer with a larger 100-cm2 hammerhead that replaces the standard 10-cm2 cone tip to increase resolution on the force gage.d. often because the electronic cable was of the 10pin type (10 wires). and inclination (i). the basic logging of was restricted to depth (z). (c) T-Bar NCHRP Project 20-05. cone tip stress (qt). Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 96 .With standard analog systems. (b) Vibro-Piezocone. porewater pressures (u). Figure 91. If soil samples are deemed absolutely critical. and (c) Quad element (Oxford University). Thus. Berg type). 91c) for defining shear strengths of very soft clays and silts (Long & Gudjonsson. the restriction on the numbers of simultaneous channels has been lifted. electronic digital cones can now process the readings downhole and the data can be transmitted uphole in series (rather than parallel with analog). wiring is shared during different portions of testing (such as the Fugro true-interval seismic cone). 2002). 16-. sleeve friction (fs). Figure 91a shows a multi-friction sleeve penetrometer that utilizes several sleeves of different roughness and textures to quantify soil-pile interface response (DeJong & Frost. 91b) for site-specific evaluation of soil liquefaction potential (without the use of empirical CRR curves) and T-bar testing (Fig. Most recently. and even 32-pin wires have been available. Other developments include vibrocone penetrometers (Fig. A few analog systems could circumvent the 10-wire limitations by either forgoing the inclinometer or friction readings. Figure 90. In other novel analog systems. 2004). While 12-. 24-. they are fragile with short lives because of the restrictive inner diameter of the cone rods that the cable must be threaded through. CPT Modifications: (a) Multi-Friction Sleeve Penetrometer. The multi-piezo-elements shown in Figure 90 are all analog penetrometers that allow simultaneous porewater pressure readings. 2004) In lieu of sampling. as well as show clear evidence of soil contamination. Vision Penetrometer System for Realtime VideoCam Soil Viewing (Hryciw & Shin. NCHRP Project 20-05.Figure 92. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 97 . The VisCPT has been used with digital image analysis processing to better define soil type and particle characterization. CPT Sampling Devices for Necessary Retrieval of Soil Samples Sapphire window Clean Sand Silty Sand Silty fine Sand Visual data acquisition recording system Silty Clay Figure 93. several vision or video cone systems have been built that allows a realtime digital camera viewing of the soils via a small window port (Figure 93). Figure 95.and right strikes to define the first crossover point that was used in the pseudo-interval downhole procedure (Campanella.For seismic cone testing. A selection of autoseis sources are shown in Figure 94. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 98 . Figure 94. Rig with Mounted Hydraulic Autoseis Unit for Deep Downhole Testing NCHRP Project 20-05. et al. pneumatic. With the advent of autoseis units. ShearPro). Heavy-duty hydraulic units for generating deep (60 m) waves are also available that are mounted to the truck belly (Figure 95). the downhole testing offers a quicker field testing time and only left (or right) series of strikes are needed since computers can easily post-process the consecutive waveforms and match them using cross-correlation (e.g. MatLab. When the SCPT was devised. and electro-mechanical units. the recording of analog wavelet signals required the paired matching of left. automatic seismic sources have been constructed to produce repeatable transient shear waves that can be detected by the geophone(s). 1986). including portable electric.. Autoseis Units for Surface Shear Wave Generation During Seismic Cone Testing. push CPTs. Towards this purpose.CHAPTER 12 . Figure 97. (b) Barge. In areas of high water table. Remote Access CPTs Other innovations include the construction of special deployment vehicles for cone penetrometer technology. Special CPT Deployment Systems: (a) Single Personnel Track Vehicle (Sweden). Several PROD components are shown in Figure 98. or that a dense impenetrable shallow layer prohibited advance of the CPT. with brief overviews given regarding novel approaches to solving these situations and special systems developed to cope with such difficulties. and (c) New Orleans Swamp Buggy Of particular interest is the completely automated PROD (portable remotely operated drill) that was developed for offshore use with capabilities to drill. A common response to the DOT survey question regarding the limited use of CPT in exploration in their state indicated that the ground conditions were often too hard for penetration. particularly in urban areas. and obtain rock coring to depths of up to 100 m (330 feet) below mudline (Randolph. a section is devoted herein to describing special systems that have been developed towards overcoming cone penetration in hard ground. 2005). and remote arctic weather. sample. vane shear testing.CPT MODIFICATIONS FOR DIFFICULT GROUND CONDITIONS Some obstacles to advancing CPTs in certain geologic formations and working in problematic soils are discussed. and (c) Portable Unit for Arctic Work (Canada). (b) CherryPicker for Urban Access (New Zealand). small limited access locations. and/or swamp buggy (Figure 97). barges. NCHRP Project 20-05. as shown by the selections presented in Figure 96. Figure 96. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 99 . et al. Special CPT Deployment by (a) Airboat. special deployment of CPTs can be accomplished by airboats. Components of Portable Remotely-Operated Drill (PROD): (a) 3000-m long umbilical cable. CPTs in Hard Ground From the TRB Questionnaire Survey (Appendix A. With the proper techniques. and transitional zones of residuum to saprolite. Questionnaire Response Concerning Majore Obstacles to Use of CPT Various creative and novel means of deploying CPTs have been designed to achieve depths of penetration in very dense sands. based in large part on the long experience of the Dutch and efforts advanced in the offshore site exploration industry. (b) remote control panel and data acquisition. weak rocks (chalks. available means to overcome these obstacles are discussed. as well as cemented layers and caprocks. Number 55). varies) Unreliable Results from SBT 0 5 38 Respondents QUESTION 55 Number of Replies 10 15 20 Figure 99. one of the biggest obstacles to use of CPTs by the DOTs is that the ground is too hard for static penetration. electric cone tip stresses over 100 MPa (1000 atms) have been recorded and mechanical CPT resistances up to 150 MPa (1500 atms).Cannot Penetrate Presence of Gravels or Stones Obstacles to CPT Use Soil Samples Not Normally Obtained Consultant Services Not Available Initial Equipment Expense Too High More Expensive Than Borings None Since CPT Use in Growing Requres Too Much Expertise Too Many Numbers & Data to Handle Equipment Too Easily Damaged Other (simple data. NCHRP Project 20-05. (c) CPT and rotary drill platform. An excellent overview on conducting CPTs in very hard soils and weak rocks is provided by Peuchen (1998). tuff). Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 100 . verify. one service. mudstones.Figure 98. as shown by Figure 99. Table 14 provides a general overview on methods developed to overcome CPT in hard ground. In this section. The second highest reported obstacle was the presence of gravels or stones. Ground Too Hard . having considerable more pushing reaction compared with drill rigs. et al. The anchors can be installed with variable size plates and depths of 1-. (1995) Zuidberg (1974) Bratton (2000) Stercks & Van Calster (1995) Sacchetto et al. Trucks with weights as high as 360 kN (40-tons) have been built to facilitate CPTs in very dense sands and gravels (Figure 100a) for routine application at the Hanford nuclear site in Washington state (Bratton 2000).Table 14. 44-mm cone. (1995) van de Graaf & Schenk (1988) Shinn (1995. 1998) Advancing Technique Heavy 20-ton Dead Weight CPT Trucks and Track Rigs Friction Reducer Cycling of Rods (up and down) Large diameter penetrometer (i. Farrington & Shinn (2006) For increased penetration in dense ground. depending upon local conditions (Figure 100b). but not so in very dense sands Local encounter in thin hard zones of soil Works like friction reducer Works well in situations involving soft soils with dense soils at depth Alternate between drilling and pushing Needs pump system for bentonitic slurry Increases capacity for reaction Switches from static mode to dynamic mode when needed Single push stroke usually limited to 2 or 3 m After large 20-ton rig arrives at site.) van de Graaf & Schenk (1988) Peuchen (1988) Comments/Remarks Increased weight reaction over standard drill rig Effective in frictional soils. or 3-m deep. NCHRP Project 20-05. personal comm.and 40-ton Rigs ROTAP . large dead weight vehicles on the order of 180 kN (20 tons) are available. (2004) Bratton (2000) Farrington (2000). Anchoring permits small lightweight CPT rigs (60 kN) to achieve depths of 30 to 40 m and successful penetration in fairly dense sands (N > 30 bpf). prevents buckling Drill Out (Downhole CPTs) Mud Injection Earth Anchors Static-Dynamic Penetrometer Downhole Thrust System Very Heavy 30. Special Techniques for Increased Success of Cone Penetration in Hard Geomaterials (adapted and modified after Peuchen. Special drilling capabilities through cemented zones Cone penetration test while drilling Use of a vibrator to facilitate penetration through gravels and hard zones Wireline systems for enhanced access penetrometer system NNI (1996) Van Staveren (1995) Pagani Geotechnical Equipment Geoprobe Systems Sanglerat et al. Another means to increase the reaction capacity is to employ earth anchors. standard 36-mm rods supported inside larger 44-mm rods. added mass for reaction. 2-. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 101 . Shinn & Haas (2004). 36-mm rods) Guide Casing: Double set of rods.e.outer coring bit CPTWD Sonic CPT EAPS Reference Mayne. These vehicles are too heavy to meet roadway load requirements at full capacity.. therefore they are mobilized to the site at acceptable weight limits (say 180 kN) and the additional 180 kN deadweight are added at the testing location. NCHRP Project 20-05.Figure 101.5 -0. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 102 . Note the characteristic negative porewater pressures in the Piedmont upon reaching the groundwater table. 1999).5 1 1.MARTA by Fugro Geosciences for Golder Associates qT (MPa) 0 0 1 2 3 4 5 10 20 30 40 50 0 fs (MPa) 0. CPT Vehicles for Hard Ground including: (a) 40-ton truck. Nevertheless.2 (bpf) SPT N60 23 34 71 34 56 67 Piedmont Profile Residuum: Silty Fine SAND to fine sandy SILT (SM/ML) Depth (m) 6 7 8 9 10 11 12 13 14 15 Saprolite (SM/ML) 50/6" 50/6" 50/2" 50/3" PartiallyWeathered Rock Figure 101. (b) anchored track rig An illustrative example of a CPTu conducted in hard saprolite and partially-weathered rock of the Piedmont in north Atlanta is shown in Figure 101 (Finke & Mayne. Piezocone Advanced into Very Hard Partially Weathered Gneiss.1 0.1 u2 (MPa) 0 0. CPTU . the piezocone sounding was successfully advanced into these hard residual soils. The very high resistances measured by the SPT-N values in an adjacent soil boring clearly shows the dense ground conditions. the results of part A of the sounding can be added to part B of the sounding to produce a complete depth profile. porewater pressures. NCHRP Project 20-05. The backfill helps to stabilize the cone rods and prevent buckling. caprock. Example results of static-dynamic penetration in dense sandstone are shown in Figure 103 with all 3 phases of testing shown.or 50-cm2 electrical penetrometer (qc and fs) pushed at 20 mm/s until hard static refusal is met at 30 MPa (300 tsf). If the geologic conditions of the region normally encounter an embedded hard cemented layer.e. The sounding is resumed using a 12-cm2 mechanical cone in static push mode until 120 MPa (1200 tsf) is reached. if conditions permit. rocks. perhaps the CPT user would wish to obtain a combine rig (as shown in Figure 102) that has capabilities of both static CPT push and rotary drilling operations. a rotary drill rig can be set up and used to bore through the cemented zone. the CPT sounding can be halted and the penetrometer can be withdrawn. A heavy duty van den Berg track truck is used for the hydraulic pushing. resume again with the static push phase. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 103 . sleeve friction. The ROTAP tool is specially designed to advance CPTs through hard cemented zones (Sterckx & Van Calster. The sounding has three distinct phases: (a) static electric CPT push. While this is somewhat unattractive to routine production type CPTs. the penetrometer is reinstalled to continue the sounding. the CPT is advanced in a normal procedure until the hard caprock or concretion is encountered. and (c) dynamic mechanical CPT. 1995). it does in fact help obtain the desired results which include electronic readings of tip stress. Once through the desired hard layer. and other obstacles. The test begins as a standard CPT with either a 44. Initially. et al. gravels. Then the penetrometer is removed and the ROTating AParatus (i. Then. and shear wave velocities. Combine Rig with Both CPT Hydraulic Rams and Drilling Capabilities. To penetrate very dense sands. Then. (b) static mechanical CPT push. The prebored hole can be filled with a backfilled sand or pea gravel and the CPT sounding can be resumed. When cemented layers. a special dynamic fast-action hydraulic hammer is used to advance the cone and. or hard concretions are encountered in the profile. as indictated. 1995). A special series of AMAP static-dynamic penetrometer systems have been developed for testing a range of soft soils to very hard and dense geomaterials with reported qc up to 140 MPa (1400 tsf) and depths up to 100 m (Sanglerat. upon completion.Figure 102. ROTAP) is installed and used to drill through the hard zone. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 104 . The vibrations are in the range of 25 to 125 Hz and used to facilitate CPT penetration through dense sands and gravels. Figure 104. AMAP Static-Dynamic Penetration System in Dense Sandstone (after Sanglerat. et al. NCHRP Project 20-05. and (b) sonic vibrator unit. 1999). Sonic CPT System with (a) dead-weight truck. The sonic vibrator is installed in the CPT truck and uses two twin 25-hp hydraulic motors with eccentric masses at the top of the rods.. A Sonic CPT system is detailed by Bratton (2000) whereby a vibrator can be intervened when the soil resistance becomes too great for normal static CPT pushing.Figure 103. Figure 104 shows the Sonic CPT unit within an ARA truck. An Envi-type memocone penetrometer is used to store the CPTu data downhole in a memory chip. and fluid pressure (FP). This is based on a wireline system (Farrington. et al. The CPTWD Wireline Based System (Sacchetto. NCHRP Project 20-05. When hard impenetrable layers are encountered (too hard for CPT). Some aspects are illustrated in Figure 106. (2004). including piezocone measurements (qt. Figure 105 shows the basic CPTWD scheme. equipment. The Enhanced Access Penetration System (EAPS) for Penetration of Hard Geomaterials (after Farrington. and a full set of results of 6 measurements from a site near Parma. and u2) as well as the MWD readings of penetration rate (PR). Figure 106. Italy.Figure 105. 2000).. 2000) and offers a means to interrupt the CPT steady-state rate of penetration and utilize downhole wireline coring intermittently and advance soundings through very dense or cemented zones or dense or hard geomaterials. A special modified wireline type core barrel has been developed to house the cone penetrometer. The system also employs MWD (Measurements While Drilling) during simultaneous operation of the CPT. thus two sets of penetration readings are obtained. et al. An enhanced access penetrometer system (EAPS) is presented by Shinn & Haas (2004) and Farrington & Shinn (2006). fs. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 105 . torque (T). 2004) A special wireline based system that combines CPT with drilling capabilities has been termed: CPTWD (Cone Penetration Test While Drilling) and is detailed by Sacchetto. The EAPS also has the ability to take soil samples as needed. then the sounding advances strictly on the basis of wireline coring techniques with MWD data still obtained. particularly bridges over rivers or streams or waterway canals. Figure 107 shows four sets of superimposed piezocone soundings at a test site with two CPTUs produced by the EAPS downhole wireline method (Nos. movable pontoons are used that are floated empty to their location. NCHRP Project 20-05. In nearshore environments. Jackup Rigs for Nearshore CPT Deployment: (a) SeaCore. The CPTs can be mobilized to conduct soundings from floating barges. 2000) Nearshore and Offshore Deployment On some highway projects. then a special CPT ship can be deployed to conduct offshore site investigations (Figure 109). with very good agreement seen for all cases. the highway alignment crosses over a body of water. 2A and 2B) and two CPTUs per normal push methods (Nos. In these cases. highways may follow the coastal shoreline or connect small islands and land masses. Figure 108. then filled with water to prepare a working platform for the CPT truck or rig. Comparison of CPTUs from Standard Push and EAPS Wireline Deployed Systems. Figure 107. (Farrington. sampling. and CPT works. the use of a jackup rig may be warranted. and (b) The Explorer. Figure 108 shows two jackup-type platforms in use for drilling. In swampy coastal areas with shallow water. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 106 . 2C and 2D).Comparative studies of the EAPS deployment and normal direct-push technology for CPTs have been made by Applied Research Associates. If water depths are greater than 15 m. Vessels for Offshore CPTs: (a) Markab.(a) (b) Figure 109. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 107 . Australia (b) Bucentaur. Brasil (courtesy Fugro Geosciences) NCHRP Project 20-05. 37% of the respondents indicated no use of the CPT whatsoever in their state/province. (b) correction of tip resistances in clays and silts (qt). With a total 56 DOTs responding. Therefore. At a constant rate of penetration of 2 cm/s. quick. The CPT capabilities include the direct assessments on the geostratigraphy with detailed demarcation on the numbers. The data also lend themselves to use in direct CPT methods that output solutions for foundation capacity and displacement calculations. analytical. Yet. the CPT outperforms the normal and conventional rotary drilling and sampling operations in the field and the associated laboratory testing that can take weeks to produce results. thereby preventing penetration. The most common obstacles to CPT use include the presence of very hard ground. The digital data can also be post-processed to provide evaluations on geotechnical parameters related to soil strength. time rate of dissipation (t50). a mechanical CPT system obtains similar information but at a coarser 20-cm depth interval. the test can be completed to 30 m depth in only 1 to 2 hours. and flow characteristics. the CPT soundings obtain continuous logging of the soil layers and stratigraphy. A review of 15 available methods to tackle hard ground conditions is presented to aid the DOTs in selecting an approach or suite of techniques to overcome these problems. penetration porewater pressure (u). in fact. the two methods can be done complementary. and relative hardness of the various strata in the subsurface environment. However. Those DOTs using the CPT have found value in its ability to post-process the multiple readings in assessing questions related to the design and performance of embankments. The basic electric cone penetrometer obtains readings of tip stress (qc) and sleeve friction (fs) at 1-cm to 5-cm vertical intervals. and downhole shear wave velocity (Vs). These soundings are recorded and stored digitally. the 64% of the respondent DOTs did foresee an increase in probable use of this technology on future highway projects. cemented layers. Results of a questionnaire were distributed to the 52 US and 12 Canadian DOTs to survey the state-ofpractice in highway site investigations relevant to use of the cone penetration test. groundwater table(s). The piezocone collects a third reading of penetration porewater pressures (u) that is particularly useful for the following conditions: (a) saturated soils below the groundwater table. On a positive note. and lab testing program. seismic piezocone testing with dissipation phases (SCPTù) is a particularly attractive in-situ test for modern day highway projects since it offers up to five independent readings on soil behavioral response within a single sounding (Mayne & Campanella. and the analysis of shallow and deep foundations. NCHRP Project 20-05. depths. In hard abrasive ground. sampling. From a practical stance. This provides an optimal means for data collection and parameter identification. The digital CPT data can be post-processed to provide input soil parameters for empirical. and/or numerical simulations of geotechnical problems. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 108 . and thicknesses of soil layers. the CPT appears to be underutilized at present for highway projects in the USA and Canada. slopes. or dense geomaterials. thus can be quickly manipulated to create cross-sections and subsurface profiles of the general ground conditions. and reliable manner. In many cases.CHAPTER 13 – CONCLUSIONS AND RECOMMENDED FUTURE RESEARCH Conclusions Cone penetration technology (CPT) can assist the geotechnical highway engineer in the collection of sitespecific soils information in a cost effective. Moreover. presence of interbedded lenses. sleeve friction (fs). including: cone tip stress (qt). stiffness. 2005). 63% indicated they were using the CPT on their projects to some degree. ground improvement studies. and (c) conducting piezodissipation tests to evaluate soil permeability and coefficient of consolidation. with the CPT providing immediate profiling of the subsurface conditions and follow-up confirmation and select verification by the boring. stress history. g. the data can be conveyed wireless via telecom transmission back to the DOT geotechnical engineer in the office. in fact. O-cell). Therefore. drilling and sampling.g. as soon as the sounding is completed on-site. the post-processing of soil engineering parameters can commence "on-the-fly" with assessments of undrained shear strength (su). laboratory testing (e. Today. consolidation). Integrated Approach to Site Investigation and Evaluation of Geotechnical Parameters On large and critical DOT projects... the proper approach to geotechnical characterization will require a combination of geophysics. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 109 . Instant feedback by text-messaging or a simple cell phone call back to the CPT crew and field engineer can request a piezodissipation test immediately. Real-time decisions can be made by senior project engineers or the chief engineer.Figure 110. constitutents.g. and/or full-scale load tests (e. As the data are available immediately. laboratory testing. grain size. NCHRP Project 20-05. or to advance the sounding deeper than originally specified. Future Directions The growth of cone penetrometer technology is guaranteed in futuristic site characterization and geotechnical investigations because of its direct tie to computerization for data acquisition and postprocessing of digital data records. ages. in-situ soundings. fabrics. mineralogies. vane). such as alternate in-situ tests (e. and axial pile capacity (Qu) produced on-the-spot as the engineer watches the CPT sounding being advanced. and environmental histories. parameter values interpreted from the cone penetration test may need verification with other means. preconsolidation stress (σp'). The wide diversity and complexities of natural geomaterials makes for a challenging task because of their many geologic origins. and engineeringa analyses (Figure 110).. triaxial shear. NCHRP Project 20-05.. stiffness.g. Both devices are now readily available across the USA and Canada. residual soils. some of the desired needs of the DOT community include improved software capabilities for handling and post-processing the large amounts of CPT data that are generated (Figure 111). however either have not yet been fully-developed for practice or else not made known to the DOTs for implementation. Full Suite Soil Parameters Needs in CPT Software Entire Pile Load-Displacement-Capacity Interpret NonTextbook Geomaterials Other (Software for Ftgs & Pilings) 0 5 10 26 Respondents 15 20 25 QUESTION 54 Number of Replies Figure 111. static-dynamic CPT). as well as new directions of research and applications of CPT (Figure 112). Current available software packages have been listed in Appendix B with their website information. In the case of several of the research needs listed in Figure 112. Research Needs in CPT Continuous Soil Sampling Correlations with CPT Miniaturized CPT System for Pavement Subgrades Static-Dynamic Penetrometer for Hard Ground Vision Cone to See Soil Layers & Contaminants Continuous Shear Wave Profiling Vibrocone for Site-Specific Soil Liquefaction Other (organics. continuous Vs. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 110 . DOT Survey Results Indicating CPT Software Needs. saprolites. With regard to the top priority (continuous soil sampling correlations with the CPT).g. silts.. collapsible soils.From the survey results. software) Dielectric Readings for Water Contents Automatic Wireless Transmission of CPT Data 27 Respondents 0 5 10 15 20 QUESTION 56 Number of Replies Figure 112. and intermediate geomaterials. Needs related to pavement investigations appear to show an excellent area for CPT growth and use. several of these topics have been initially addressed by universities (e. It is likely that some of these developers will introduce new software that may address the issues of CPT interpretation in nontextbook type geomaterials such as peats. this is now very possible and perhaps best achieved by local site calibrations in a particular geologic region using side-by-side comparisons of CPTu soundings with geoprobe samples. VisCPT) and manufacturers (e. especially since the readings can be effectively scaled down to shallow depths using miniature size probes and sensors. Research Needs in Cone Penetrometer Technology Identified by DOT Survey. u1. with the tip. centrifuge modeling. Vs. The methodology has been used to provide approximate nonlinear stress-strain-strength curves for both sands and clays (Mayne. thus allowing as many channels as possible. and pH. The importance and applicability of the shear wave velocity (Vs) in providing the fundamental soil stiffness (Gmax = ρTVs2) has been shown herein with examples applied to both full-scale shallow and deep foundation systems. resistivity (Ω). The importance of Gmax in pavements is also fundamental and can be integrally related to the more common resilient modulus (MR) for proper analyses (Brown. and porewater readings reported earlier in Figure 15. 2006). as well as discussed use for obtaining saturated soil unit weights and application for evaluating seismic ground hazards. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 111 . and therefore could be used as such for any and all depths.The newest electronic models of penetrometers in production collect the data directly in digital format. chamber testing..e. fs. It may soon become possible to take as many as ten readings continuously with depth. 1996). and numerical simulations with advanced constitutive soil modeling will permit a more reliable and defensible interpretative framework for evaluation of the varied and complex soil parameters needed for design. Results of Special Close-Interval Shear Wave Profiling at NWU Campus. five standard series of SCPTu soundings were advanced. Towards improving the state-of-practice in collecting Vs by seismic cone testing. including: qt. dielectric (ζ). Tip Resistance Shear Wave Velocity qT (MPa) 0 0 2 4 6 8 2 4 6 8 10 12 14 0 2 4 6 8 10 Vs (m/s) 0 100 200 300 400 500 SCPT Downhole (pseudointerval) Depth (m) 10 12 14 16 18 20 22 fcpt01 fcpt02 fcpt03 fcpt04 fcpt05 12 14 16 18 20 22 Frequent True-Interval Probe Figure 113. NCHRP Project 20-05. every 20-cm). lateral stress (σh). means of making continuous Vs measurements are underway. Figure 113 shows results from the national test site at Northwestern University using a special probe to capture Vs measurements at close frequency intervals (i. sleeve. u3. In addition. u2. The results from the special frequent-interval downhole testing are seen to be "well-behaved" and much finer resolution and profiling Vs than the standard coarser 1-m intervals. Developmental research with laboratory experiments. and porewater readings. u2. Continuous Seismic Piezocone Testing AutoSeis CPT deployed from rig with continuous qt. et al. In this regard.An improvement to the SCPTu would be the capability for continuous shear wave measurements without the current practice of stopping every 1-m to conduct a standard downhole test. and Vs readings with depth qT (MPa) 0 0 5 10 15 20 0 ub 500 (kPa) 1000 1500 0 FR = fs/qt (%) 2 4 6 8 10 0 100 Vs 200 (m/s) 300 400 500 5 10 15 Depth (m) 20 20 mm/s geophones 25 30 35 In-String Source Special DeCoupling Rod 40 geophones Figure 114. 1986). then switch to downhole geophysics test for shear wave determination (Campanella. Both concepts are depicted in Figure 114. fs. sleeve. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 112 . the geophones in fact reside at all times within the penetrometer. it is merely that the practice continues to promulgate the original concept of oscillating between continuous CPT for the tip. or (2) an instring source-receiver unit which would "talk" to each other at a set distance and provide continuous Pand S-wave readings with depth. 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Symbols ac amax an c c' cu cv cvh d d dc de fp fs ft g g h hp hs k kc kE q qb qc qe qt qt1 qult rd s su t t tj t50 u0 u1 u2 u3 wt z ze = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = radius of cone penetrometer PGA = maximum (horizontal) ground acceleration (during an earthquake) net area ratio (see ASTM D 5778) length of rectangular foundation effective cohesion intercept su = undrained shear strength coefficient of consolidation coefficient of (vertical and horizontal) consolidation diameter of pile foundation width of rectangular foundation diameter of cone penetrometer = 2ac equivalent diameter pile side friction measured cone sleeve friction total cone sleeve friction gravitation constant ( = 9.2 ft/s2) exponent term in modified hyperbola for modulus reduction depth to incompressible layer for shallow foundations drainage path thickness (during consolidation) height of friction sleeve hydraulic conductivity (cm/s). Task 37-14: Synthesis on Cone Penetration Test (February 2007) . or ubt) porewater pressures measured behind the sleeve displacement at pile top depth (below ground surface) foundation embedment depth Page 134 NCHRP Project 20-05. also coefficient of permeability reduction factor from LCPT direct CPT method ΔEs/Δz = rate of soil modulus increase with depth applied stress by shallow foundation end bearing resistance for deep foundation measured cone tip resistance effective cone resistance = qt . technical nomenclature. and ACRONYMS This section lists the common symbols.8 m/s2 = 32. SYMBOLS. and acronyms that are used in the synthesis.u2 total cone resistance (correction per ASTM D 5778) stress-normalized cone tip resistance ultimate bearing stress for foundation system stress reduction factor (for seismic ground analyses) displacement of foundation cu = undrained shear strength time foundation thickness thickness of friction sleeve time for dissipation to reach 50% completion hydrostatic (porewater) pressure porewater pressures measured midface of cone tip porewater pressures measured at shoulder position (behind the tip.GLOSSARY OF ABBREVIATIONS. 5 for undrained) mass density = γT/g.0 kN/m3 = 64 pcf) Poisson's ratio (ν' = 0.7 psi) effective preconsolidation stress ( = Pc' = σvmax') effective vertical stress total vertical (overburden) stress effective vertical (overburden) stress effective lateral stress effective geostatic lateral stress shear stress cyclic shear stress shear strength (maximum shear stress).ν)/[(1+ ν)(1−2 ν)] relative density of sand foundation Young's modulus equivalent (Young's) soil modulus (E' for drained and Eu for undrained) normalized sleeve friction parameter factor of safety Rf = friction ratio = fs/qt (%) shear modulus = E/[2(1+ ν)] G0 = ρT Vs2 = small-strain shear modulus embedment depth of foundation CPT soil classification index footing displacement modifier for embedment footing modifier for relative rigidity displacement influence factor for foundation on Gibson soil NCHRP Project 20-05. saltwater: γw = 10. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 135 .2 for drained and νu = 0.8 kN/m3 = 62.zw α αc αG βG βp φ' γs γd γT γsat γw ν ρT σatm σp' σv' σvo σvo' σh' σho' τ τcyc τmax AF Ab As B Bq Cc CK CM Cs Cse Cte D' DR Efdn Es F FS FR G Gmax He Ic IE IF IGH = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = depth to groundwater table ratio of soil modulus to cone tip resistance: α = Es/qc ratio of constrained modulus to net cone resistance: αc = D'/(qt-σvo) ratio of constrained modulus to small-strain shear modulus: αc = D'/Gmax Eso/(kE·d) = dimensionless Gibson soil parameter angle of plastification (for NTH piezocone method) effective friction angle shear strain dry unit weight total unit weight saturated unit weight unit weight of water (freshwater: γw = 9. For undrained conditions: τmax = cu = su area of shallow foundation base area at pile foundation base perimetric area on sides of pile foundation width of foundation normalized porewater pressure parameter virgin compression index modifier for pile installation (beta side friction) modifier for pile material (beta side friction) swelling or rebound index side friction coefficent (Unicone method) tip coefficient (Unicone method) constrained modulus = E (1. where g = gravitational acceleration constant atmospheric pressure (1 atm = 1 bar = 100 kPa ≈ 1 tsf ≈ 14.4 pcf. IH I0 Ip Iρ IR Kc KF K0 KP L M N PI PL Q Q Qb Qs Qt Qult Rk RF St Tv T* U U' U* Vs Vs1 W = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = displacement influence factor for shallow foundation on homogeneous soil displacement influence factor for rigid pile plasticity index (also PI) elastic displacement influence factor for pile foundation G/τmax = rigidity index of the soil = ratio of shear modulus to shear strength fines correction factor for CPT in soil liquefaction analysis foundation flexibility factor σho'/σvo' = lateral stress coefficient passive stress coefficient length of pile foundation 6sinφ'/(3-sinφ') = critical state frictional parameter for strength envelope penetration resistance or "blow counts" from SPT plasticity index probability of liquefaction normalized cone tip resistance applied vertical force to foundation base capacity (at tip or toe of pile foundation) shaft capacity along sides of pile foundation applied top load on pile ultimate axial capacity (force) of the foundation bearing factor term for foundations on clay resistance factor sensitivity (of fine-grained soils) time factor for one-dimensional (vertical) consolidation modified time factor for radial dissipation (for piezocone) energy density from dynamic compaction operations Δu/σvo' = normalized excess porewater pressure to effective overburden Δu/Δui = normalized excess porewater pressures for dissipation testing shear wave velocity stress-normalized shear wave velocity weight of the foundation NCHRP Project 20-05. Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 136 . Task 37-14: Synthesis on Cone Penetration Test (February 2007) Page 137 . Trondheim overconsolidation ratio = σp'/σvo' amax = peak ground acceleration pressuremeter test resistitivity cone penetration test soil behavioral type (used in soil classification by CPT) seismic cone penetration test screw plate load test standard penetration test Transportation Research Board total stress cells (spade cells) NCHRP Project 20-05.Acronyms ASCE ASTM BRE CCT CPT CPTu CPTù CRR CSR DOT ISSMGE LCPC NCHRP NGES NGI NTNU OCR PGA PMT RCPT SBT SCPT SPLT SPT TRB TSC = = = = = = = = = = = = = = = = = = = = = = = = = = American Society of Civil Engineers American Society for Testing and Materials Building Research Establishment. UK calibration chamber tests cone penetration test piezocone test (cone penetration test with porewater pressures) piezocone test with dissipation readings with time. cyclic resistance ratio cyclic stress ratio = τcyc/σvo' Department of Transportation International Society of Soil Mechanics and Geotechnical Engineering Laboratoire Central des Ponts et Chaussées National Cooperative Highway Research Program National Geotechnical Experimentation Site Norwegian Geotechnical Institute. Oslo Norwegian University of Science & Technology.