Abstract: To study the mechanical characteristics of unloading rate damage effect for Nanyang undisturbed expansive soil, the unloading-reloading undrained triaxial shear test was conducted with three unloading rates (0.02 kPa/min, 0.2 kPa/min and 2 kPa/min) through the GDS triaxial test system. The effect of over-consolidation ratio and consolidation state have also been studied. Test results showed that the shear mechanical properties of expansive soil are related to the initial unloading rate. Under the same axial strain, the deviatoric stress is smaller with lower initial unloading rate. The slope of stress-strain curve changed significantly before and after the principal stress direction rotation. Under the same consolidation state and over-consolidation ratio, pore water pressure increased first and then decreased. Peak pore water pressure strain decreases as the unloading rate increases. As the unloading rate decreases, the tangential modulus of the undisturbed expansive soil decreases monotonously. The higher the initial unloading rate, the greater the undrained shear strength. After the initial unloading damage of expansive soil sample, the shear strength obtained by reloading conventional triaxial elongation test was lower than that of non-damage soil sample under both isotropic consolidation and K0 consolidation. The damage degree SD obtained by the failure strength qf of expansive soil greatly underestimated the damage degree of the damage rate to expansive soil. It is recommended to use the strength of peak pore water pressure qumax to design and calculate the slope of expansive soil. The mechanical properties evolution of undisturbed expansive soil with unloading damage rate is influenced by the axial strain and cracking character.

Key words: expansive soil, unloading rate, unloading damage, shear strength, triaxial shear test