Elastoplastic model for saturated rock based on mixture theory

In order to avoid the difficulties in evaluating the Biot's coefficient value of Skempton's effective stress used to formulate nonlinear constitutive model, the engineering mixture theory is chosen to build the elastoplastic model for saturated rock. Firstly, according to the principle of homogeneous response in the engineering mixture theory, the constitutive laws of saturated porous media are revealed as follows: "The solid matrix bulk strain determines solid matrix pressure, the skeleton elastic and plastic strains determine Terzaghi's effective stress and dissipate Terzaghi's effective stress, and the fluid matrix bulk strain determines pore pressure". Secondly, according to the Hoek-Brown yielding criterion and the non-associated flow rule, the saturated rock elastoplastic model is provided on the basis of the existing rock damage model. Finally, the proposed saturated rock elastoplastic model is validated by the triaxial drained and undrained shear test results. The researches show that the saturated rock elastoplastic model based on the engineering mixture theory can fairly accurately simulate the macroscopic mechanical behaviors of the overall stress-strain curve of rock including elastic stage, elastoplastic stage and descending stage, and illustrate the changing rule in the triaxial undrained shear tests that the pore pressure increases first and then decreases with the exteral shear stress. The engineering mixture theory does not use the Skempton's effective stress to build model, as a result, it can overcome the difficulties in determining the formula for Biot's coefficients in Biot's nonlinear model and is more convenient to establish the elastoplastic model for saturated rock.