Shear wave implications in wetting-induced collapse of sand–clay mixtures

Collapsible soils pose significant challenges to engineering projects due to their instability and sudden changes in volume upon variations in water content. In this study, the collapse potentials of specimens with sand–clay mixtures are experimentally investigated by considering factors, such as initial water content and clay fraction. The specimens are subjected to vertical stress up to σˊ_v = 440 kPa in an instrumented oedometer cell, including the wetting process at σˊ_v = 220 kPa. The settlement and shear wave velocity (V_s) are continuously measured during loading and wetting to propose a geophysical approach based on shear wave velocity measurements for better understanding of collapse mechanisms. The results show that the collapse potential of sand–clay mixtures increases with increasing clay fraction at a specific water content. However, the variation in the collapse potential can be the sole function of the clay water content (= water content/clay fraction). As the suction pressure decreases upon wetting, the V_s shows a time-dependent decrease during wetting, and the decreasing is proportional to the collapse potential, indicating the loss of small-strain stiffness. However, when the settlement ceases, a portion of V_s can be recovered over time because of the formation of a stable soil structure, leading to the ratio of V_s after collapse to V_s before collapse ranges from 0.85 to 1.05. Additionally, this study reveals that a relatively higher initial degree of saturation (> 50%) is necessary to recover stiffness/strength after soil collapse, emphasizing the importance of proper compaction in engineering projects involving collapsible soils.