Abstract: During the gestation and development of landslides, due to the uneven effect of external conditions, local stress concentration occurs in the landslide. Once the concentrated shear stress is greater than the peak shear strength of sliding zone, the strain-softening behavior of sliding zone appears. The redundant shear stress will be transferred and be borne by the adjacent sliding zone, then the progressive failure in slope will happen. The progressive failure process of the thrust-type gently inclined shallow landslide is divided into five stages considering the back edge thrust. The progressive failure mechanical models for different stages are established based on the coordination of mechanical balance and deformation. The critical loads and criteria for different progressive failure stages are given by the comprehensive analysis of the progressive failure process of the landslide. The shear strength of sliding zone in different stress states is re-characterized, the stability coefficient of each progressive failure stage of the landslide is given, which makes the physical meaning more clear. Finally, the established mechanical model is applied to a simple shallow landslide, and the quantitative relationship among the back edge thrust, slope displacement, shear stress distribution and stability coefficient is achieved. The model well reflects the space-time characteristics of the mobilized shear strength of sliding zone in the progressive failure process of landslides. By comparing with the calculation results of unbalanced thrust method, results show that it is dangerous to adopt peak strength in engineering design, but uneconomical to adopt residual strength. It is necessary to use progressive failure theory in engineering design of landslide.

Key words: strain-softening, shallow landslide, progressive failure, stability analysis