Study on the Mechanical Behaviour of Unsaturated Silty Sand and Pavement Response Under Moisture Variations

Drainage has been widely adopted in pavement structures. Due to drainage, wetting and drying, subgrade soils may be subject to conditions ranging from saturation to partial saturation (unsaturated). Particularly, unsaturated soil exhibits matric suction in contrast to saturated soils. Matric suction can impact the mechanical response of subgrade soils. Thus, it is significant to clarify the effects of subgrade moisture variations on unsaturated soil behaviour and resulting pavement response. In this study, constant water content triaxial tests and the axis translation technique were employed to investigate the shear behaviour of unsaturated silty sand under single- and multistage loading. The multistage involved drying, wetting and free drainage stages. The shear behaviour and resilient response were demonstrated to reveal the effects of moisture variations in terms of shear strength and unloading modulus. The tests showed that peak shear strength generally demonstrated an increasing trend with a decrease in moisture content (about 20% at the confining pressure of 25 kPa), and the unloading modulus increased by about 126% at 25 kPa. Based on multi-layered elastic analysis, the effects of moisture variations on pavement response were also investigated as a representative. Maximum tensile/compressive strain, subgrade deformation and distress life were simulated. This study will help to extend the understanding of effects of dynamic moisture variations on unsaturated subgrade soils and support pavement drainage design.