Soil constitutive models are widely investigated and applied in soil mechanical behaviors simulation; however, the damage evolution process of soil with various shear deformation behaviors was rarely studied. This study introduces a novel shear constitutive model for slip zone soil considering its damage evolution process. Firstly, an innovative method for determining the shear stiffness is proposed to assess the damage degree of slip zone soil during shear deformation. Further, a damage evolution model based on the log-logistic function is derived to characterize the damage evolution process of slip zone soil, and a new shear constitutive model based on the damage evolution process is subsequently proposed. Both the damage evolution model and the shear constitutive model are verified by the ring shear test data of the slip zone soil from the Outang landslide in the Three Gorges Reservoir area of China. Compared to the traditional peak-solving constitutive model based on the Weibull distribution, the proposed shear constitutive model has the distinct advantage of describing not only the brittle (strain softening) mechanical behavior but also the ductile and plastic hardening mechanical behavior of soil. In summary, this method offers a rapid determination of the damage evolution process and the shear behavior constitutive relationship of slip zone soil in landslides.
