Shear strength of expansive soils under different moisture conditions: a comparative evaluation of laboratory and in-situ test methods for slope stability design

Expansive soils are prone to strength degradation after rainfall, yet conventional slope stability analyses often rely on laboratory-derived shear strength parameters that may overestimate in-situ conditions. This study evaluates and compares the shear strength of expansive soils using the borehole shear test (BST), laboratory direct shear test (DST), and consolidated undrained test (CUT) under natural and flooding conditions. Results show that shear stress- shear displacement (or axial strain) curves generally exhibit weak hardening characteristics. Significant reductions in cohesive strength occur after flooding. The cohesive force obtained by DST and BST decreases by 18.2 kPa and 13.1 kPa after flooding respectively, while the internal friction angle decreases slightly within 3°. Laboratory tests consistently yield higher strength parameters than BST, while flooding BST values closely align with those measured on actual slip surfaces. Numerical modeling of an unstable cut slope confirms that using flooding BST parameters produces more realistic safety factors. The findings highlight the importance of incorporating in-situ testing under representative moisture conditions for reliable slope stability design in expansive soils.