Measurement of unsaturated soil shear strength through drained-vented triaxial tests

This study presents an in-depth investigation into the shear strength characteristics of unsaturated soils, focusing on the influence of shear rate and initial water saturation ( S r0 ). Utilizing the drained-vented (DV) triaxial test method, the present study investigated the shear strength behavior of silty clay under various shear rates and water saturation levels, and compared the outcomes with traditional suction-controlled (SC) and constant water content (CW) tests. The findings highlight the pivotal role of excess pore water pressure dissipation during shearing, which significantly affects the shear strength of both saturated and unsaturated soils. Notably, for soils with high initial water saturation, a decrease in shear strength is observed with an increase in shear rate, which is attributed to the rise in pore water pressure. Conversely, for soils with low initial water saturation, the shear rate exhibits minimal impact on shear strength due to negligible water drainage. The research delineates the optimal shear rates for DV tests based on the initial water saturation: a slower rate of 0.0028 mm/min for samples with high water saturation ( S r0 >66%) and a faster rate of 0.8 mm/min for samples with low water saturation ( S r0 ≤66%). A novel testing methodology for determining unsaturated soil shear strength under DV conditions is introduced, streamlining the measurement process and significantly reducing testing time. This method not only promises substantial cost savings but also aligns closely with natural engineering conditions, offering valuable guidance for geotechnical applications.