Abstract: Under lateral unloading conditions, the failure mode of deep excavations is significant different from that of under loading conditions. In the deep excavation supporting design and foundation construction, the paramount important issue is the safety. A series of undrained triaxial shear tests on undisturbed soil samples under K₀ conditions were performed to evaluate the lateral deformation properties of cohesive soils. The test results indicate that, under the unloading stress path, the undisturbed soil samples were under axial compression at the failure state with a maximum axial strain of 2.6%, which is much less that at of in the conventional triaxial shear test on the same soil type. The critical unloading ratios of different soil types are similar, and the ratios demonstrate the normal distribution between 0.14 and 0.27. It is also found that the soil sample demonstrates a good relationship with the average consolidation pressures.

Key words: Deep excavations, cohesive soils, silty clay, critical unloading ratio