GEOTECHNICAL DESIGN MANUALCHAPTER 10 SLOPE STABILITY ANALYSIS NYSDOT Geotechnical Design Manual Page 10-1 January 21, 2014 2014 .(Intentionally left blank) NYSDOT Geotechnical Design Manual Page 10-2 January 21. ............... 10-13 10............... 10-11 10................ 10-11 10...................................1 OVERVIEW ....................................... 10-6 10....3...... 10-14 10...3......... 10-9 10............5 Comparison of Methods .......................2 Slope Geometry..................................................................3................................8 Computer Programs .............................2 REQUIRED INPUT DATA FOR SLOPE STABILITY ANALYSIS .................................................................................................................................3 Soil Shear Strength ..3..............2 Preliminary Methods ....................3. 10-14 10................................3..............................2.......................................................3.........................................1 Soil Profile ..............................................9 Required Factor of Safety .......................................2........................................4 Pore Water Pressure .6 Back Analysis ............... 2014 ......................................3................................4 REFERENCES .....................3 METHODS OF STABILITY ANALYSIS .................................................................................................................3 Method of Slices .1 Limit Equilibrium Equations ...... 10-5 10...... 10-10 10...... 10-5 10...................................................................................................................... 10-6 10...............................2................. 10-8 10.....................2...................................... 10-14 10.......................7 Evaluation of Remedial Methods................... 10-4 10......... 10-9 10..................Table of Contents 10................................................................................................ 10-14 10.3.........................4 Methods Derived from Method of Slices............................................ 10-15 NYSDOT Geotechnical Design Manual Page 10-3 January 21.................. remediation methods. Table 10-1 Slope Stability Analysis – Geotechnical Problems An investigation should be made of all proposed fills supporting highway facilities and cuts that support important adjacent lands or structures. Detailed stability assessment of landslides is described in NYSDOT GDM Chapter 16. geotechnical designers are directed to FHWA/NHI 05-123 Soil Slope and Embankment Design. the stability of a slope may be assessed using a limit-equilibrium analysis.CHAPTER 10 Slope Stability Analysis 10. Fill Slopes Determination of stable fill slopes. other than highly fractured rock masses that can in effect be treated as soil. but not limited to: Problems Slope Stability Analysis Natural Slopes Evaluation of existing natural slopes. This chapter provides a basic outline of slope stability analysis. Deep/Shallow Foundations Assessment of overall stability for structures located on slopes. This type of analysis is also utilized to determine stability conditions of existing natural or constructed slopes and evaluate the influence of proposed remediation methods if required. Slope stability analysis is used in a wide variety of problems including. Cut Slopes Determination of stable cut slopes. Landslides Assessment of landslides. Retaining Walls Assessment of overall stability (global and compound). For detailed design guidance and reference. & back-analysis. NYSDOT Geotechnical Design Manual Page 10-4 January 21. are described in NYSDOT GDM Chapter 15. 2014 . with appropriate drainage conditions and shear strengths. It is also essential that locations of existing landslides be identified before new highway alignment is fixed. Stability analysis techniques specific to rock slopes. Once slope geometry has been modeled and subsurface conditions have been determined.1 OVERVIEW Slope stability analysis should be used to determine whether a proposed slope meets the required safety and performance criteria during design. ) as these fine details of the stratigraphy could control the stability of the slope in question. The number of borings required longitudinally depends on the continuity of the soil conditions and the extent of the possible problems as discussed in NYSDOT GDM Chapter 4.2 REQUIRED INPUT DATA FOR SLOPE STABILITY ANALYSIS It is expected that geotechnical designers performing slope stability analysis on NYSDOT projects include the following basic data in the stability model: Parameter Basic Input Data Soil Profile Detailed Assessment of Soil Stratigraphy Slope Geometry Derived from Topos. the presence of silt-clay varves. (i. NYSDOT Geotechnical Design Manual Page 10-5 January 21.e. Photogrammetry and Survey Soil Shear Strength Total and Effective Stress Analysis Pore Water Pressure Determination of Water Regime Table 10-2 Basic Input Data The input data needed for slope stability analysis is also described in: • NYSDOT GDM Chapter 2 Project Geotechnical Planning • NYSDOT GDM Chapter 6 Engineering Properties of Soil and Rock • NYSDOT GDM Chapter 13 Soil Cut Design • NYSDOT GDM Chapter 15 Rock Cut Design • NYSDOT GDM Chapter 16 Landslide Analysis and Mitigation 10.CHAPTER 10 Slope Stability Analysis 10. It is important to define any thin. Seismic investigation can be a useful supplement if the rock surface is a major influencing factor. In a potential slide area.1 Soil Profile Detailed assessment of soil stratigraphy is critical so that a representative soil profile can be modeled. etc. weak layers present. Knowledge of the geologic nature of the units present at a site. borings should be taken at the top and bottom of existing or proposed slopes if possible. Borings should be progressed in sufficient quantities to adequately define subsurface materials.2. is necessary for appropriate determination of ground conditions and the development of a realistic geotechnical model. The location and characteristics of the critical soil strata are usually obtained by taking continuous split spoon or thin-walled tube samples. 2014 . including past performance. ie.2. The effective stress can only be calculated indirectly if the pore water pressures at the state of failure are known. structures. and should include accurate locations of streams. construction loading).2. followed by waiting period). Surfaces needed to reflect slope geometry can be derived from photogrammetric mapping. If a watercourse occurs at the bottom of the slope then the stream bed should be surveyed or estimated. long-term (very slow loading condition). Cracks caused by earth movement should be mapped to provide a realistic failure model. This test also provides a range of undrained shear strengths at various confining stresses levels. 10. The rate of load application will determine the selection of soil shear strength parameters used as input in the analysis as follows: NYSDOT Geotechnical Design Manual Page 10-6 January 21. both in the laboratory and in the field. 2014 .CHAPTER 10 Slope Stability Analysis 10.3 Soil Shear Strength The shear strength of the soil along the failure surface is a function of the effective stress at failure. and utilities.2 Slope Geometry Representative slope geometry is required for slope stability analysis. Consolidated undrained triaxial tests with pore pressure (CU/p) measurements are commonly performed by NYSDOT to determine the drained friction angle of the critical soil. impermeable soils will experience loading and at what rate of load will be applied – short-term (rapid loading condition. field surveys. For side hill conditions. or published topographic maps. cross sections should be carried above the existing road or to a controlling feature such as rock outcrop and down to the bottom of the natural slope. or a multi-stage loading (placement of a portion of load. roads. See NYSDOT GDM Chapter 8 for guidance on the development of shear strength parameters. The geotechnical designer should determine if subject slope consisting of cohesive. 2014 . NYSDOT Geotechnical Design Manual Page 10-7 January 21. if the slope is relatively free to deform after the cut is made or is otherwise unloaded. residual shear strength parameters should be obtained and used for the stability analysis. 1992) For assessing the stability of large landslides that have moved over a period of years or for cuts in highly overconsolidated clays. Pore Pressure and Unit Weights for Slope Stability Analysis (after Duncan. Drainage Condition.CHAPTER 10 Slope Stability Analysis (Table 5-2 FHWA-NHI-05-123) Table 10-3 Shear Strengths. Separate piezometers sealed into multiple soil or rock deposits may be necessary to obtain a better estimate of the groundwater conditions. NYSDOT Geotechnical Design Manual Page 10-8 January 21. 2014 . An understanding of pore water pressure and groundwater regime at the study site is critical for establishing an accurate model for slope stability analysis. or canal either following a long period of rainfall accumulation. Figure 10-1 Types of Aquifers (US Department of the Interior. depending on the geologic complexity of the stratigraphy and groundwater conditions. Sudden drawdown often occurs in clayey slopes in which the excess pore water pressures do not dissipate as the water recedes. river. thereby keeping the overall shear strength low.2. This condition can occur on the slopes adjacent to a reservoir. Sudden drawdown is a rapid lowering in the level of water standing against a slope. planned lowering of water through control structures. 1981) Detailed piezometric data at multiple locations and depths within and below the slope might be required. or failure of water impoundment structure. The use of piezometers (vibrating-wire or open-well) should be considered if the existing or future groundwater conditions cannot be readily determined by field observation and these conditions would have a detrimental effect on the stability of the slope or cut based on the nature of the subsurface materials or geometry of the site.CHAPTER 10 Slope Stability Analysis 10.4 Pore Water Pressure A change in effective stress caused by pore water pressure is a major contributor of slope failures because the pressures tend to alter the shear strength of the soil along the shear zone. A slope with a FS =1 is theoretically marginally stable.1 Limit Equilibrium A soil slope is commonly analyzed based on Limit (or force or moment) Equilibrium methods that measure its stability by a factor of safety (FS) (ie.CHAPTER 10 Slope Stability Analysis 10. The shear strength (limit) required along a failure surface is calculated to just maintain stability and is then compared with the available shear strength to provide the overall FS for the slope. FHWA/NHI 05-123 Soil Slope and Embankment Design. the ratio of resisting force to driving force).3. Figure 10-2 Various Definitions of Factor of Safety (FS) (FHWA. or just ready to move.3 METHODS OF STABILITY ANALYSIS Methods of stability analysis are detailed in Publication No. 2014 . 10. 1994) NYSDOT Geotechnical Design Manual Page 10-9 January 21. Infinite Slope Planar Circular.CHAPTER 10 Slope Stability Analysis 10. Log Spiral Block Figure 10-3 Limit Equilibrium Methods NYSDOT Geotechnical Design Manual Page 10-10 January 21.2 Preliminary Methods The following figure shows limit equilibrium methods that can be readily solved by hand or spreadsheet and do not depend on the distribution of the effective normal stress along the failure surface. Published design charts and methods shown in following figure should be only used for preliminary evaluation of slopes and are described in detail in FHWA/NHI 05-123. 2014 .3. Assumptions are generally involved to render the problem determinate. This condition is usually analyzed by sectioning the potential failure mass into small vertical slices and treating each as a unique sliding block. Figure 10-4 Method of Slices 10. Force equilibrium methods. This analysis is known as the Method of Slices. As a result. 2014 . While originally calculated by hand.4 Methods Derived from Method of Slices A number of limit equilibrium methods are based on the Method of Slices.3. Moment and force equilibrium methods. and 3. these more rigorous methods were converted to computerized versions which are commercially available to geotechnical designers doing work for NYSDOT. The following table lists commonly available methods of analysis and the conditions of static equilibrium that are satisfied in determining the factor of safety. based on the number of equilibrium equations to be satisfied: 1. with each method of analysis using a different assumption. 2.3. NYSDOT Geotechnical Design Manual Page 10-11 January 21.3 Method of Slices If mobilized strength for a soil is to be calculated. Slope stability problems are usually statically indeterminate.CHAPTER 10 Slope Stability Analysis 10. the distribution of the effective normal stresses along the failure must be known. Overall moment equilibrium methods. The methods are generally divided into three categories. the computed factor of safety varies between the different methods. CHAPTER 10 Slope Stability Analysis Table 10-4 Static Equilibrium Conditions Satisfied by Limit Equilibrium Methods NYSDOT Geotechnical Design Manual Page 10-12 January 21. 2014 . The final selection of computerized method is left to the geotechnical designer. 2014 . Figure 10-5 Comparison of Methods (Fredlund and Krahn. NYSDOT Geotechnical Design Manual Page 10-13 January 21. NYSDOT does not specify a slope stability method required for analysis of slopes. and understanding of the limitations of the various methods. based on their experience.CHAPTER 10 Slope Stability Analysis 10.5 Comparison of Methods The chart below shows a presentation of Fredlund and Krahn (1977) study comparing limit equilibrium methods using a representative slope. accuracy of input data.3. 1977) A detailed comparison of limit equilibrium methods is discussed in FHWA/NHI 05-123. 10. the soil strengths and pore pressures can be adjusted until the calculated FS is equal to 1. etc.3.CHAPTER 10 Slope Stability Analysis 10.3.75 *Load Factor of 1.25 - Bridge or Wall Structure on Slope* 1.6 Back Analysis After a slope failure. slope geometry. impacts to public. Table 10-5 Minimum Required Factors of Safety – Slope Stability Analysis NYSDOT Geotechnical Design Manual Page 10-14 January 21.7 Evaluation of Remedial Methods Once the failure surface is identified by back analysis. pore pressure or other conditions at the time of failure.8 Computer Programs The Geotechnical Engineering Bureau currently uses SLOPE/W (2012) to perform slope stability analysis.9 Factor of Safety The following table provides minimum NYSDOT required Factors of Safety if a geotechnical designer has enough information to adequately define soil profile.3.30 . Mitigation measures for slope failures are described in NYSDOT GDM Chapter 16 and Chapter 22.25 - Embankment End slopes 1. environmental.65 Minor Wall with Minimal Impact on Slope* 1. Based on incremental improvement of FS and other constraints (economical. By using a known or assumed failure surface.30 - Cut Slopes 1.50 - Landslide Remediation 1. some form of back analysis can be performed in order to estimate the material shear strength. The chosen soil strengths and pore pressures can then be used to evaluate repair methods and determine relative improvement to the FS. 10. XSTABL is also used to calculate the factors of safety according to the Simplified Bishop and Simplified Janbu methods of analysis. 10.) the geotechnical designer can then recommend appropriate remedial actions.50 . an analysis is usually performed in an effort to identify probable causes.3. 2014 . soil shear strength and pore water pressure in a slope stability model: Geotechnical Element FS LRFD Embankment Side Slopes 1. real estate.0. T.4 REFERENCES Abramson.. Geotechnical Design Procedure GDP-14.gov/divisions/engineering/technical-services/technical-servicesrepository/GDP-14b. Publication No. https://www.. Highway Research Record. Publication No..G. FHWA-SA-94-005. L. S. Lee.. Design Procedure for Launched Soil Nail Shallow Slough Treatment. Leshchinsky.pdf McGuffey. Number 457.. 2014 . Collin. V. D.. Geotechnical Engineering Bureau. James G. Earth Cut Slope Design in New York State. FHWA/NHI-05-123. 1994. and Sharma. G. New York State Department of Transportation. Advanced Technology for Soil Slope Stability. C. Volume 1: Slope Stability Manual. Office of Technical Services. Soil Slope and Embankment Design. and Hung.ny. Boyce.dot. NYSDOT Geotechnical Design Manual Page 10-15 January 21.. 1973.CHAPTER 10 Slope Stability Analysis 10.