Experimental study on the effect of stress and water content on resilient modulus of unsaturated subgrade soil: implications for pavement design

Subgrade soils possess essential mechanical properties that influence the load-bearing capacity and deformation behavior of pavements. This directly impacts pavement structural performance and longevity. This paper presents the experimental results on determining the factors affecting the resilient modulus (Mr) of a subgrade soil as determined by the triaxial loading machine. A silty sand subgrade soil, tested in its unsaturated form to represent site conditions, was subjected to characterization and cyclic loading tests. The variables considered for this research are deviatoric and constant stress, confining stress, and water content. The Mr is highly dependent on the deviatoric stress, confining pressure, and water content. The shear stress test result at the end of each cyclic loading shows that the soils at the optimum water content level show better shear resistance and elastic deformation below certain stress levels. In contrast, soils in the other moisture levels show complete plastic deformation. Moreover, while confining stress enhances Mr, deviatoric stress reduces it.