Abstract: Consolidated undrained triaxial shear tests on undisturbed saturated Nanyang expansive clay have been performed in both reduced triaxial compression (RTC) and reduced triaxial extension (RTE) stress paths with three axial consolidation stresses (160 kPa, 240 kPa and 320 kPa) and three unloading rates (0.02 kPa/min, 0.2 kPa/min and 2 kPa/min) by GDS triaxial system. The influence of unloading rate on shear strength, pore-water pressure and effective stress path was analyzed. Test results showed that the stress-strain relationship was in hyperbolic under different unloading rates for RTC and RTE. The undrained shear strength increased with the increase of unloading rate from 0.02 kPa/min to 2 kPa/min. The pore pressure of the rate at 0.02 kPa/min was the minimum under two stress paths with the consolidation stress from 160 kPa to 320 kPa. Considering the change of the unloading rate parameter ρ0.02, the undrained shear strength increased 5.6% and 4.8% with every tenfold increase of the unloading rate. The rate effects on undrained shear strength were greater in reduced compression tests than those in reduced extension tests. The failure pattern of the undisturbed expansive soil sample was related to the shear rate and micro crack, demonstrating the coexistence of the main shear zone and the sub shear zones at low unloading rate. For the case of high unloading rate, the main shear zone was the only failure pattern.

Key words: undisturbed expansive clay, unloading rate, stress path, shear strength, shear band