A fully coupled hydraulic-mechanical solution of a circular tunnel in strain-softening rock masses

This paper presents a fully coupled hydraulic-mechanical strain-softening model considering Biot's effective stress. Both the hydraulic and mechanical parameters are considered as functions of the confining pressure and plastic shear strain. Using the proposed fully coupled hydraulic-mechanical model, the displacement, stress and pore-water pressure around a circular tunnel in both Mohr-Coulomb and Hoek-Brown strain-softening rock masses are derived using a non-associated flow rule. The proposed solutions are validated by the conventional hydraulic-mechanical elasto-brittle-plastic and strain softening rock mass using analytical and numerical methods, and the examples of the fully coupled hydraulic-mechanical strain-softening rock mass are investigated. The results show that the seepage force enlarges the displacement, residual and plastic radii. With the increase in the initial pore-water pressure, the displacement, water inflow, residual and plastic radii increase in an approximately linear manner. The increasing bulk modulus of the solid constituent increases the above four variables, which reach the approximate limit values, corresponding to the Darcy's effective stress solutions, when K-s = 10 K-0. The increasing permeability of the plastic rock mass reduces the increasing rate of the pore-water pressure but increases the water inflow. For the proposed fully coupled examples, both the hydraulic parameters of permeability and Biot's coefficient show a non-linear decrease with the increasing radius.