Pavement Design ManualVolume I Flexible Pavements and Gravel Roads - 2002 Chapter 11 Gravel and Low Standard Roads 11. DESIGN OF GRAVEL AND LOW STANDARD ROADS 11.1 General Much of the information presented in this Section of the Pavement Design Manual is based on the "Pavement and Materials Design Manual" prepared by the United Republic of Tanzania Ministry of Works 1999, and on relevant ERA and TRL publications. Available information has been modified to provide a simple procedure to design gravel wearing courses and low standard roads, which is appropriate to Ethiopian conditions. Gravel road pavements are generally utilized for roads where design traffic flow Annual Average Daily Traffic (AADT) is less than 200. This Section sets out the standards for pavement design, and specifies the materials which may be used for gravel roads. 11.2 Design Principles 11.2.1 STEPS TO BE CONSIDERED IN THE DESIGN PROCESS 1. 2. 3. 4. 5. Traffic (Baseline flow and forecast) Material and geotechnical information (Field survey and material properties) Subgrade (Classification, foundation for expansive soils and material strength) Thickness design (Gravel wearing coarse thickness) Materials design 11.2.2 ALL-WEATHER ACCESS An essential consideration in the design of gravel roads is to ensure all-weather access. This requirement places particular emphasis on the need for sufficient bearing capacity of the pavement structure and provision of drainage and sufficient earthworks in flood or problem soil areas (e.g. black cotton). 11.2.3 SURFACE PERFORMANCE The performance of the gravel surface mainly depends on material quality, the location of the road, and the volume of traffic using the road. Gravel roads passing through populated areas in particular require materials that do not generate excessive dust in dry weather. Steep gradients place particular demands on gravel wearing course materials, which must not become slippery in wet weather or erode easily. Consideration should therefore be given to the type of gravel wearing course material to be used in particular locations such as towns or steep sections. Gravel loss rates of about 25-30mm thickness a year per 100 vehicles per day is expected, depending on rainfall and materials properties (particularly plasticity). Performance characteristics that will assist in identifying suitable material are shown in Figure 11-1. 11.2.4 MAINTENANCE The material requirements for the gravel wearing course include provision of a gravel surface that is effectively maintainable. Adherence to the limits on oversize particles in the material is of particular importance in this regard and will normally necessitate the use of crushing or screening equipment during material production activities. Ethiopian Roads Authority Page 11-1 96 for volcanic gravels (weathered lava or tuff) = 1. Determine the extra thickness needed to compensate for the gravel loss under traffic during the period between regravelling operations (D2).38 for sandstone gravels 11. 2.2 + 0.3.29 for lateritic gravels = 1.7 to 0. 11.3 Design Method The required gravel thickness shall be determined as follows: 1.3. Determine the minimum thickness necessary to avoid excessive compressive strain in the subgrade (D1).2 GRAVEL LOSS According to TRL Laboratory Report 673.50 R2 + 1.1 to 1. Determine the total gravel thickness required by adding the above two thicknesses (D1+ D2). 3. GL Where D1 is the minimum thickness from Figure 11.88V) Where GL = the annual gravel loss measured in mm T = the total traffic volume in the first year in both directions.94 to 1. 11. measured in thousands of vehicles R = the average annual rainfall measured in m V = the total (rise + fall ) as a percentage of the length of the road f = 0.092 T + 3. an estimate of the annual gravel loss is given by the following equation: GL = f T2 / ( T2 + 50) ( 4.5 for coral gravels = 1.Chapter 11 Gravel and Low Standard Roads Pavement Design Manual Volume I Flexible Pavements and Gravel Roads – 2002 11.Figure 3 gives the minimum gravel thickness required for each traffic category with the required thickness of improved subgrade materials for upper and lower subgrade layers.3.1 MINIMUM THICKNESS REQUIRED It is necessary to limit the compressive strain in the subgrade to prevent excessive permanent deformation at the surface of the road .3 N is the period between regravelling operations in years GL is the annual gravel loss Page 11-2 Ethiopian Roads Authority .3 TOTAL THICKNESS REQUIRED The wearing course of a new gravel road shall have a thickness D calculated from: D = D1 + N.51 for quartizitic gravels = 0. 425 mm) GC = ((% passing 28 mm) . 11. 11. 11. 11. Refinements and amendments of the standard material specification may be necessary to overcome problem areas such as towns (dust nuisance) or steep hills (slipperiness).4 Pavement and Materials Depending on the CBRdesign of the subgrade. This is to ensure that potholes do not develop by rapidly removing surface water and to ensure that excessive crossfall does not cause erosion of the surface.6. improved subgrade layers shall be constructed as required.1 EXPERIENCE WITH LOCAL MATERIALS Knowledge of past performance of locally occurring materials for gravel roads is essential.2002. on which the gravel wearing course is placed. CG Note: SP = (Linear Shrinkage) x (% passing 0.5 Crossfall and Drainage The crossfall of carriageway and shoulders for gravel roads shall be “ 4%” as indicated in ERA’s Geometric Design Manual . Shrinkage Product.6 Material Requirements 11.Pavement Design Manual Volume I Flexible Pavements and Gravel Roads .(% passing 2 mm)x (% passing 5 mm)/100 Figure 11-1. Expected Performance of Gravel Wearing Course Materials Ethiopian Roads Authority Page 11-3 .6.2002 Chapter 11 Gravel and Low Standard Roads Regravelling operations should be programmed to ensure that the actual gravel thickness never falls below the minimum thickness D1. SP 500 Slippery 400 300 Erodible Good but may be dusty materials Ravels Good 200 100 Ravels and corrugates 10 20 30 40 Grading Coefficient.2 MARGINAL MATERIALS Figure 11-1 illustrates the performance characteristics to be expected of materials that do not meet the requirements for gravel wearing course. Provision of drainage is extremely important for the performance of gravel roads. Material standards may be altered to take advantage of available gravel sources provided they have proved to give satisfactory performance under similar conditions. at the end of rainy season. the previously stockpiled expansive soil excavated as directed under (i) should then be spread over the slope. they should be as shallow as possible and located as far from the toe of the fill as possible. remove the expansive soil to a minimum depth of 600 mm over the full width of the road.e.4 or better. Assists compaction of upper pavement layers. remove it to its full depth.6. Acts as a drainage filter layer.5 TREATMENT OF EXPANSIVE FORMATIONS The following treatment operations should be applied on Expansive Formations for higher class roads of AADTdesign greater than 50: i) Removal of Expansive Soil a) Where the finished road level is designed to be less than 2 metres above ground level.7. Protection of underlying earthworks. The different types of gravel roads are explained in Section 11.4 SUBGRADE CBR All subgrade materials shall be brought to a strength of at least a minimum CBR of 7% for minor gravel roads and at least a minimum CBR 25 % for major gravel roads. v) Do not construct side drains unless they are absolutely essential to stop ponding.Chapter 11 Gravel and Low Standard Roads Pavement Design Manual Volume I Flexible Pavements and Gravel Roads – 2002 11. iii) The excavation formed as directed in paragraph (i) should be backfilled with a plastic non-expansive soil of CBR value 3 . i. iv) After the excavated material has been replaced with non-expansive material in 150mm lifts to 95% modified AASHTO density. with a side slope of 1:2. vi) Ideally.6. or b) Where the finished road level is designed to be greater than 2 metres above ground level. Page 11-4 Ethiopian Roads Authority . and compacted to a density of 95% modified AASHTO.3 IMPROVED SUBGRADE LAYER In General the use of improved subgrade layers has the following advantages: • • • • • • • Provision of extra protection under heavy axle loads. remove the expansive soil to a depth of 600 mm below the ground level under the unsurfaced area of the road structure. 11. construction over expansive soil should be done when the in-situ moisture content is at its highest. Provides running surface for construction traffic. where side drains are necessary. 11.6. bring the road to finished level in approved materials. or c) Where the expansive soil does not exceed 1 meter in depth. Provides homogenous subgrade strength. and ensure that pavement criteria are complied with. ii) Stockpile the excavated material on either side of the excavation for subsequent spreading on the fill slopes so as to produce as flat a slope as possible. More economical use of available materials. 11.Pavement Design Manual Volume I Flexible Pavements and Gravel Roads . the subgrade material may be plastic but non-expansive.6 MATERIAL CHARACTERISTICS Soils used for improved subgrade layers shall be non-expansive. Table 11-1 Material Properties G20 (Upper Layer) G7 (Lower Layer) CBR Dry Climatic Zones (See Note) Minimum 20 after 4 days Minimum 7 after 4 days soaking soaking CBR Wet Climatic Zones (See Note) Minimum 20 at OMC Minimum 7 at OMC Minimum 7 after 4 days Minimum 3 after 4 days soaking soaking PI [%] Maximum 25 Maximum 30 Compacted Density 95% of AASHTO T180 95% of AASHTO T180 Maximum particle size 2/3 of layer thickness 2/3 of layer thickness Compacted layer thickness Maximum 200 mm Maximum 250 mm Note: Climatic Zones for Ethiopia are described in Section 11. Ethiopian Roads Authority Page 11-5 .1 PERFORMANCE CHARACTERISTICS OF GRAVEL WEARING COURSE The materials for gravel wearing course should satisfy the following requirements that are often somewhat conflicting: a) They should have sufficient cohesion to prevent ravelling and corrugating (especially in dry conditions) b) The amount of fines (particularly plastic fines) should be limited to avoid a slippery surface under wet conditions.7 Gravel Wearing Course 11.6.4 or better. non-dispersive and free from any deleterious matter. and compacted to a density of 95% modified AASHTO in each 150mm layer.10.2002 Chapter 11 Gravel and Low Standard Roads The following treatment operations may be applied on Expansive Formations for light traffic class roads of AADTdesign less than 50: i) Remove 150mm of expansive topsoil and stockpile conveniently for subsequent use on shoulder slopes ii) Shape road bed and compact to 90% modified AASHTO iii) The excavation formed as directed in paragraph (i) should be backfilled with a plastic non-expansive soil of CBR value 3 . They shall comply with the requirements shown in Table 11-1.7. 11. Type 1 gravel wearing course which is one of the best material alternatives which shall be used on all roads which have AADTdesign greater than 50. 11. as determined by AASHTO T96. Page 11-6 Ethiopian Roads Authority . The material shall be free from vegetable matter and lumps or balls of clay. Type 2 & 3 These materials may be more rounded particles fulfilling the following: a) The Plasticity Index lies in a range of 5-12% in wet areas. The plasticity index should be not greater than 15 and not less than 8 for wet climatic zones and should be not greater than 20 and not less than 10 for dry climatic zones. The linear Shrinkage should be in a range of 3-10%. Trials should nevertheless be conducted to verify whether crushing occurs under traffic or whether crushing should be considered prior to use.2 GRAVEL WEARING COURSE MATERIAL SPECIFICATION Selected material shall consist of hard durable angular particles of fragments of stone or gravel. and in any case less than 16% in other areas b) The materials have the sanction of local experience Use of more rounded particles may allow the use of river gravel. For this reason the following two types of gravel wearing course material are recommended. The material shall have a percentage of wear of not more than 50 at 500 revolutions.Chapter 11 Gravel and Low Standard Roads Pavement Design Manual Volume I Flexible Pavements and Gravel Roads – 2002 Figure 11-1 shows the effect of the Shrinkage Product (SP) and Grading Coefficient (GC) on the expected performance of gravel wearing course materials.7. Type 1 The grading of the gravel after placing and compaction shall be a smooth curve within and approximately parallel to the envelopes detailed in Table 11-2. a minimum percentage by weight of particles with at least one fractured face of 40% may be considered. Excessive oversize material in the gravel wearing course affects the riding quality in service and makes effective shaping of the surface difficult at the time of maintenance. Subject to trials. This requirement may also be expressed in terms of crushing ratio. Type 1 or Type 4 gravel wearing course material may be used on new construction of roads having AADTdesign less than 50. The material shall be compacted to a minimum in-situ density of 95% of the maximum dry density determined in accordance with the requirements of AASHTO T 180. Type 1 material shall also be used for all routine and periodic maintenance activities for both major and minor gravel roads. Note that the above gradation and plasticity requirements are only to be used with angular particles and that crushing and screening are likely to be required in many instances for this purpose. Other alternatives are also specified in this chapter. 100 55 . the requirement may be relaxed to a CBR of 15. However. a Ethiopian Roads Authority Page 11-7 . They may be used if sanctioned by experience with plasticity characteristics as for material Type 1.100 65 . the CBR should be in excess of 20 after 4 days of soaking at 95% of maximum dry density under Heavy Compaction.100 - - - 20 80 .15 12-32 10-40 5-15 10-25 10-25 11. Test Sieve Size(mm) Table 11-2 Percent(%) by mass of total aggregate passing test sieve Type 1 Type 2 Type 3 Type 4 Type 5 Type 6 50 - - - 100 - - 37. Type 4 This material gradation allows for larger size material and corresponds to the gradation of a base course material.30 18 .100 45-65 80 .3 MAJOR GRAVEL ROADS (AADTDESIGN = 20 TO 200) Major gravel roads are roads which have a design AADT greater than 20 and less than 200.60 45 . Usually these roads are unsurfaced (earth roads). These roads generally fall within the design category of DS9 to DS10 (Refer to ERA Geometric Design Manual-2002).100 - - - 10 55 .85 35-90 30-50 60 -85 80-100 2.425 15 . 11.65 22-75 - - - 1 - 25-55 18-60 - - - 0.100 100 5 40 . For very low traffic.2002 Chapter 11 Gravel and Low Standard Roads Except for very low traffic (less than 15 vehicles per day).100 85-100 60-80 100 - 14 - 80-100 65 .075 5 .7.36 30 .5 100 - 100 80-100 - - 28 - 100 95 . It is recommended to use a gravel wearing course material of grading Type 1 in the new construction of roads having an AADT greater than 50 and for all routine and periodic maintenance activities.50 - - 20-40 45-70 50-80 2 - 30 .45 15-50 10-25 25-45 25-45 0. The use of this gradation of materials is subject to the local experience and shall be used with PIs in a range of 10-20. Type 5 & 6 These materials gradations are recommended for smaller size particles.Pavement Design Manual Volume I Flexible Pavements and Gravel Roads . which are constructed by labor-based methods.4 MINOR GRAVEL ROADS (AADTDESIGN < 20) Minor gravel roads are roads which have a design AADT (AADTdesign) less than 20. Chapter 2.7. They are normally community roads. These will generally fall within the design category of DS5 to DS8 (See ERA Geometric Design Manual -2002. Type 4 material may be used in the new construction of roads having an AADT less than 50.100 95 . for subgrade CBR values less than 5% and longitudinal gradients of greater than 6%. The CBR requirements may be reduced to 20% if other suitable material is not locally available. S15 Use lowest CBR value Yes Cutting? No No Minimum 5 CBR tests per section Special assessment of the section Yes Determine 90%-ile at the 0. 11.8.Chapter 11 Gravel and Low Standard Roads Pavement Design Manual Volume I Flexible Pavements and Gravel Roads – 2002 gravel wearing course is recommended. Demarcation of homogenous sections shall be reviewed and changed as required when the CBR test results of the centerline soil survey are available.2 HOMOGENOUS SECTIONS Identification of sections deemed to have homogenous subgrade conditions is carried out by desk studies of appropriate documents such as geological maps. The procedure to determine CBRdesign is shown in the flow chart in Figure 11-2. Site Reconnaissance Compilation of input data Demarcation homogenous sections Minimum 3 CBR tests per section of Yes Design improved subgrade as required CBRdesign: Classify S3. S4 or S2 for the purpose of pavement design. followed by site reconnaissance that includes excavation of inspection pits and initial indicator testing for confirmation of the site observations. Page 11-8 Ethiopian Roads Authority . for which the subgrade strength is classified into S5.8. Due regard for localized areas that require individual treatment is an essential part of the site reconnaissance. S7.1 GENERAL The CBRdesign is the CBR value of a homogenous section.8 Determination of CBRdesign 11. Materials for gravel wearing course shall comply with the requirements for Type 4 material for new construction and Type 1 for maintenance activities.1 X (n-1) point on the curve Plot values in ascending order Figure 11-2: Flow Chart for Design 11. The desired properties of the gravel wearing course material.2002 Chapter 11 Gravel and Low Standard Roads 11. are given in Section 11. 2. 3.8. The CBRdesign for cuttings is the lowest CBR value encountered for the homogenous section. Ethiopian Roads Authority Page 11-9 . where n = number of tests.Density relationship for the material. d is measured along the "x axis" and the CBRdesign is determined from the "y axis". The CBRdesign for sections that do not require special assessment or are not within cuttings are determined by the 90%-ile value of the CBR test results.7.4 LABORATORY TESTING Each CBR value shall be determined by laboratory measurement carried out for a minimum of three density values to give a CBR . 11.9. However. 11. This includes all design categories DS5.2 MINOR GRAVEL ROADS Pavement and improved subgrade for minor gravel roads shall also be constructed in accordance with Figure 11-3. a minimum in-situ density of 95% of the maximum dry density determined in accordance with the requirements of AASHTO T 180). This includes design categories DS9 and DS10 as defined in ERA Geometric Design Manual -2002.Pavement Design Manual Volume I Flexible Pavements and Gravel Roads .9.1 x (n-1). The 90%-ile value for a section of this type is the CBR value which 10% of the test results fall below. DS7 and DS8 as defined in ERA Geometric Design Manual -2002.e. and the LA abrasion value may be increased to 55% for minor roads. Calculate d = 0. 11. CBR values are plotted in ascending order (number of tests on the "x axis" and the CBR test result values on the "y axis"). the CBR may be reduced to 20%. The following example shows how this is calculated. The CBR value is determined at the normal field density specified for the respective operation (i.1 MAJOR GRAVEL ROADS Pavement and improved subgrade for major gravel roads shall be constructed in accordance with Figure 11-3.9 Improved Subgrade and Pavement Design 11.8. if better quality material is not locally available. 1. DS6. GW.3 STATISTICAL ANALYSIS The flow chart in Figure 2 shows the procedure to determine CBRdesign. Ethiopia is divided into two climatic zones. in many arid areas.1 ZONES For the purposes of gravel wearing course design. All places with elevations over 2. engineering judgement should be made for individual projects as to which category the design falls. However.000 meters (average rainfall 80mm/month) are considered to be wet zones and all places with elevations 2. Page 11-10 Ethiopian Roads Authority .50 150mm 50 .10.10. unlike drainage design techniques.000 meters or less (average rainfall 20mm/month) are considered to be dry zones.2 ARID AREAS It is acknowledged that. rates of rainfall may be extremely high over short durations.Chapter 11 Gravel and Low Standard Roads Pavement Design Manual Volume I Flexible Pavements and Gravel Roads – 2002 AADTdesign <20 S15 20 . Pavement design techniques.10 Climatic Zones 11. 11. do not take this into account as they are based on annual rates of rainfall.200 175mm GW 200mm GW GW 150mm GW 175mm GW 100mm G20 100mm G20 S7 200mm GW S3 Dry Zones Wet Zones Dry Zones mm mm mm 150 150 150 150 GW G7 GW 150 300 GW G20 Wet Zones mm 150 150 G7 GW G20 mm 200 200 200 G7 Dry Zones GW 150 G20 200 G7 Wet Zones mm 200 G20 200 250 G7 GW G7 Figure 11-3: Pavement and Improved Subgrade for Gravel Roads for ADDTs < 200 11.