Theoretical Study of the Evolution Characteristics of the Plastic Deformation Zone of Type I–II Composite Fractured Rock under Osmotic Pressure

The coupled seepage–stress action has a significant deterioration effect on the structural face of the hydraulic tunnel enclosure, which intensifies the shear rupture tendency of the deteriorated structural face of the rock mass. The plastic deformation of a typical I–II composite fissure was taken as the research object, the characteristics of the tip plastic zone of the composite fissure seepage rock were explored, and the influence law of osmotic pressure and fissure rock parameters (fissure dip angle, Poisson’s ratio, and fissure length) on the radius of the tip plastic zone was analyzed. Based on the Drucker–Prager yield criterion and the stress intensity factor of the composite fracture, the theoretical analytical formula of the fracture plastic zone radius under the action of high and low osmotic pressure was established, and the fracture rock plastic zone radius was significantly correlated with the fracture parameters. The radius of the plastic zone of fracture under low osmotic pressure evolves in a trend of decreasing–increasing–decreasing with the increase in fracture dip angle, and the peak radius of the plastic zone appears at 45°. Poisson’s ratio and fracture length have less influence on the radius of the plastic zone. The radius of the plastic zone of fracture under high osmotic pressure grows in an incremental nonlinear curve, and the peak radius of the plastic zone appears at 90°, being positively correlated with the length of fracture. This study can provide theoretical reference for the analysis of the stability of the surrounding rock in hydraulic tunnels.