Abstract: In the conventional triaxial compression test, the stress-strain relationship of strong structured cohesive soils under a low confining pressure will show strain softening and is usually accompanied by plastic deformation. Generally, the internal structure damage of soil is the main cause of strain softening. Using classical plastic theory to describe the strain softening characteristics of materials may violate the Drucker's stability hypothesis, and cannot describe the plastic deformation during unloading. Based on the modified Cam-clay model and the plastic hardening rule proposed by Li and Meissner, an elastoplastic two-surface model is established. This model divides the stress-strain curve into strain hardening stage and strain softening stage at the peak stress point and the divided two stages are analyzed as independent loading events respectively. Meanwhile, a new structural parameter is proposed to characterize the degradation of plastic stiffness caused by soil structure damage during loading. Additionally, the comparisons between simulated and measured results of saturated structural clay in different consolidation states indicate that the model can describe the stress-strain curves and stress path curves with high accuracy and can give reasonable and good simulations of strain softening of structured clays in undrained condition.

Key words: structured soil, strain softening, plastic hardening rule, elastoplastic model, undrained shearing