Effect of Water Content and Grains Size Distribution on the Characteristic Resilient Young’s Modulus (Ec) Obtained Using Anisotropic Boyce Model on Gravelly Lateritic Soils from Tropical Africa (Burkina Faso and Senegal)

This research was carried out to determine the rheological parameters of lateritic soils in order to contribute to the improvement of the technical documents used for pavement design in tropical Africa. The study is based on the loading repeated of cyclic triaxial tests (LRT) performed at University Gustave Eiffel (formerly Institut Francais des Sciences et Technologies des Transports de l’Amenagement et des Reseaux (IFSTTAR)) in Nantes with the application of the European standard EN 13286-7: 2004 [1]. The tests were performed at constant confinement stress and using the stepwise method to determine the resilient axial () and radial () deformation as a function of the axial and radial stresses. Four gravel lateritic soils from different sites selected in Burkina Faso and Senegal were the subject of this research for the triaxial tests. These materials have a maximum diameter of 20 mm and a percentage of fines less than 20%. The LRT tests were carried out on samples compacted at three moisture contents (wopm - 2%, wopm and wopm + 2%) and at 95% and 100% of optimal dry density (γdopm). Test results showed that the characteristic resilient Young’s modulus (Ec) of gravelly laterites soils depends on the compacted water content and the variation of the grains size distribution (sand (o 20%), mortar and sand (Sindia and Lam-Lam) are more sensitive to variations in water content. The presence of water combined with the excess of fines leads to a decrease in modulus around 25% for Lam-Lam and 20.2% for Sindia. Materials containing a low percent of fines, mortar and sand (Badnogo and Dedougou) behave differently. And the resilient modulus increases about 225.67% for Badnogo and 312.24% for Dedougou with the rise of the water content for approximately unchanged the percentage of fines, mortar and sand. Granularity therefore has an indirect influence on the resilient modulus of the lateritic soils by controlling the effects of water on the entire system. Results of statistical analysis and coefficients of correlation (0.659 to 0.865) showed that the anisotropic Boyce’s model is suitable to predict the volumetric () and deviatoric strain () with stress path (Δq/Δp) of the lateritic soils. The predicted Er resilient Young’s modulus from anisotropic Boyce’s model varies according to the evolution of the bulk stress (). A correlation around 0.9 is obtained from the power law model.