Nonlinear Constitutive Model of Rock Joint in Geological Structure and Application of Implicit Return Mapping Algorithm

As a common geological structure, rock joints have a great influence on the mechanical properties and stability of rock mass engineering. The shear constitutive model of the rock joint and its calculation methods have played an important role in theoretical and experimental research. Most of the existing elastic–plastic shear constitutive models of the rock joint are simplified to linear form or only considering hardening behavior, while the shear test results show that the post-peak nonlinear softening curve is the most common form for shear curves. Therefore, based on Barton shear strength criterion, the shear stress softening is characterized by the change of the post-peak joint roughness coefficient (JRC), and an increment elastic–plastic constitutive model reflecting the shear stress softening is established. According to the constraint condition of constant normal stress, the increment model of the normal deformation with shear deformation is obtained. Based on principle of the implicit return mapping algorithm, an algorithm for the shear softening elastic–plastic constitutive model in the stress space is established. The applicability and rationality of the model and algorithm are verified by a large number of the joint shear experiment results under different conditions. The established elastic–plastic constitutive model provides an effective reference for correctly evaluating the shear deformation characteristics of the joint.