Mechanical behaviour of expansive soils under three-dimensional stress

The coupled effects of intermediate principal stress and matric suction on the mechanical properties of expansive soils remain poorly understood. To address this, an improved suction-controlled true triaxial apparatus was used to conduct suction equilibration tests, constant-suction isotropic consolidation tests and drained shear tests with various b-values on intact medium expansive soil, investigating the coupled effects of matric suction and intermediate principal stress on the mechanical behaviour of expansive soils. The results show that the intact expansive soil exhibits elastic–plastic volumetric deformation during wetting and drying. The yield suction sp = 71 kPa: elastic swelling occurs for 0 kPa ≤ s ≤ 37 kPa, elastic shrinkage for 37 kPa < s ≤ 71 kPa and plastic shrinkage for s > 71 kPa. The isotropic compression yield stress increases with suction, confirming the existence of the loading-collapse yield line. Dilatancy increases with suction and decreases with increasing intermediate principal stress. Cohesion increases non-linearly with suction and b-value, while the internal friction angle decreases with increasing intermediate principal stress. These findings provide a reasonable basis for stability assessments of expansive soil engineering under three-dimensional stress conditions.