The effect of Water Retention Properties on the Shear Modulus of an Unsaturated Compacted Soil

Compacted soils used in earthworks are often unsaturated and frequently subjected to repeated loading from various sources, including traffic, wind action, and seismic activity. The shear modulus of soils is a fundamental dynamic parameter in the design and long-term stability of earthworks under such cyclic loading conditions, while their water retention properties play a crucial role in influencing shear stiffness. This study investigates the effect of water retention characteristics on the shear modulus of a compacted sandy loam, with specimens prepared at different water contents and tested under varying cyclic shear strain levels using a simple shear device. Results show that during the initial loading cycles, lower water content increased suction bonding, leading to a higher shear modulus. However, with continued loading, soil densification also became an important factor affecting shear stiffness. The samples with suction below the air entry value exhibited the lowest shear modulus due to minimal suction-induced stiffening. Beyond the air entry value, suction enhanced interparticle bonding and increased shear modulus; however, further increases in suction resulted in marginal stiffness gains, as the contribution of densification to shear modulus reduces with increasing suction. These findings underscore the importance of incorporating water retention properties into the assessment of compacted soil behaviour under repeated loading in earthwork applications.