Shear behaviors of expansive soils over a wide confining pressures range and their impacts on strength parameters

Understanding the shear behaviors of expansive soils under a wide range of confining pressures σ₃ is crucial for effectively managing their shallow hazards, especially at low σ₃ values. The shear behaviors of expansive soils under the low σ₃ values exhibit significant differences with those under the high σ₃ values, which have been inadequately addressed in existing research. This paper investigated the shear behaviors of two expansive soils across a wide range of σ₃ values (i.e., 15–400 kPa) through consolidated undrained triaxial tests. The results showed a significant nonlinear relationship between the shear strength and σ₃, particularly at the low σ₃ values. The nonlinearity of shear strength versus σ₃ in shallow expansive soils was described by a modified power function. As σ₃ decreased, both the brittleness index λ and curvature κ increased nonlinearly. Comparisons between the tangent method, based on the modified power function, and the K_f method, based on the Mohr-Coulomb criterion, suggested that neglecting the nonlinearity of shear strength versus σ₃ led to overestimations of the cohesion and underestimations of the internal friction angle at low σ₃ values, and the reverse trend was observed at high σ₃ values. The magnitude of these deviations depended on both the shear strength nonlinearity and the selected σ₃ values. The findings presented herein are helpful for the mitigation of shallow hazards in roadbeds, slopes, and foundation engineering associated with expansive soils.