10. DESIGN OF GRAVEL AND LOW STANDARD ROADS 10.1 General Much of the information presented in this Section of the Pavement Design 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. 10.2 Design Principles 10.2.1 STEPS TO BE CONSIDERED IN THE DESIGN PROCESS 1. Traffic (Baseline flow and forecast) 2. Material and geotechnical information (Field survey and material properties) 3. Subgrade (Classification, foundation for expansive soils and material strength) 4. Thickness design (Gravel wearing coarse thickness) 5. Materials design 10.2.2 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). 10.2.3 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 211 in particular require materials that do not generate excessive dust in dry weather. Steep gradients place particular demands on gravel wearing course materials; this 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. 10.2.4 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. 10.3 Design Method The required gravel thickness shall be determined as follows: 1. Determine the minimum thickness necessary to avoid excessive compressive strain in the subgrade (D1). 2. Determine the extra thickness needed to compensate for the gravel loss under traffic during the period between regravelling operations (D2). 3. Determine the total gravel thickness required by adding the above two thicknesses (D1+ D2). 10.3.1 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 .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. 212 10.3.2 GRAVEL LOSS According to TRL Laboratory Report 673, an estimate of the annual gravel loss is given by the following equation: GL = f T2 / ( T2 + 50) ( 4.2 + 0.092 T + 3.50 R2 + 1.88V) Where GL = the annual gravel loss measured in mm T = the total traffic volume in the first year in both directions, 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 = 0.94 to 1.29 for lateritic gravels = 1.1 to 1.51 for quartizitic gravels = 0.7 to 0.96 for volcanic gravels (weathered lava or tuff) = 1.5 for coral gravels = 1.38 for sandstone gravels 10.3.3 TOTAL THICKNESS REQUIRED The wearing course of a new gravel road shall have a thickness D calculated from: D = D1 + N. GL Where D1 is the minimum thickness from Figure 11.3 N is the period between regravelling operations in years GL is the annual gravel loss Regravelling operations should be programmed to ensure that the actual gravel thickness never falls below the minimum thickness D1. 213 10.4 Pavement and Materials Depending on the CBRdesign of the sub grade, improved subgrade layers shall be constructed as required, on which the gravel wearing course is placed. 10.5 Crossfall and Drainage The crossfall of carriageway and shoulders for gravel roads shall be “ 4%” as indicated in ERA’s Geometric Design Manual - 2002. 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. Provision of drainage is extremely important for the performance of gravel roads. 10.6 Material Requirements 10.6.1 EXPERIENCE WITH LOCALMATERIALS Knowledge of past performance of locally occurring materials for gravel roads is essential. Material standards may be altered to take advantage of available gravel sources provided they have proved to give satisfactory performance under similar conditions. 10.6.2 MARGINALMATERIALS Figure 11-1 illustrates the performance characteristics to be expected of materials that do not meet the requirements for gravel wearing course. Refinements and amendments of the standard material specification may be necessary to overcome problem areas such as towns (dust nuisance) or steep hills (slipperiness). 10.6.3 IMPROVED SUBGRADE LAYER In General the use of improved subgrade layers has the following advantages: Provision of extra protection under heavy axle loads; Protection of underlying earthworks; Provides running surface for construction traffic; Assists compaction of upper pavement layers; 214 Provides homogenous subgrade strength; Acts as a drainage filter layer; More economical use of available materials. 10.6.4 SUBGRADE CBR All subgrade materials shall be brought to strength of at least a minimum CBR of 7% for minor gravel roads and at least a minimum CBR 25 % for major gravel roads. The different types of gravel roads are explained in Section 11.7. 10.6.5 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, remove the expansive soil to a minimum depth of 600 mm over the full width of the road, or b) Where the finished road level is designed to be greater than 2 metres above ground level, remove the expansive soil to a depth of 600 mm below the ground level under the unsurfaced area of the road structure, or c) Where the expansive soil does not exceed 1 meter in depth, remove it to its full depth. 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. iii) The excavation formed as directed in paragraph (i) should be backfilled with a plastic non-expansive soil of CBR value 3 - 4 or better, and compacted to a density of 95% modified AASHTO. iv) After the excavated material has been replaced with non-expansive material in 150mm lifts to 95% modified AASHTO density, bring the road to finished level in approved materials, with a side slope of 1:2, and ensure that pavement criteria 215 are complied with; the previously stockpiled expansive soil excavated as directed under (i) should then be spread over the slope. v) Do not construct side drains unless they are absolutely essential to stop ponding; where side drains are necessary, they should be as shallow as possible and located as far from the toe of the fill as possible. vi) Ideally, construction over expansive soil should be done when the in-situ moisture content is at its highest, i.e. at the end of rainy season 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 - 4 or better, and compacted to a density of 95% modified AASHTO in each 150mm layer; the subgrade material may be plastic but non-expansive. 10.6.6 MATERIAL CHARACTERISTICS Soils used for improved subgrade layers shall be non-expansive, non-dispersive and free from any deleterious matter. They shall comply with the requirements shown inTable 10-1 216 10.7 Gravel Wearing Course 10.7.1 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. Figure 10-1 shows the effect of the Shrinkage Product (SP) and Grading Coefficient (GC) on the expected performance of gravel wearing course materials. 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. For this reason the following two types of gravel wearing course material are recommended. 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.