Deformation and permeability of fractured rocks using fluid-solid coupling under loading-unloading conditions

Deep underground excavation causes considerable unloading effects, leading to a pronounced bias pressure phenomenon. The deformation and seepage characteristics of rock masses under different gas and confining pressures were investigated via triaxial loading and unloading seepage tests. When the influential coefficient of effective confining pressure ( β ) is less than 0.065, the seepage force considerably weakens the strength of fractured rock masses. Conversely, when β is greater than 0.065, the opposite is true. Moreover, the increase in the axial load leads to an increase in the precast fracture volumetric strain, which is the main reason for the increase in fracture permeability. This effect is particularly significant during the unloading stage. Based on the test results, a method for calculating the dynamic seepage evolution of rock masses, considering the effects of rock mass damage and fracture deformation, is introduced, and the effectiveness of the calculation is validated. The entire description of the seepage under loading and unloading was accomplished. The equivalent relationship between the lateral and normal stresses on fracture surfaces ranges from 0.001 to 0.1, showing an exponential variation between the lateral stress influence coefficient on normal deformation (χ) and seepage pressure. Before the failure of the rock mass, the seepage in the fractures was in a linear laminar flow state. However, after the failure, when the gas pressure reached 2 MPa, the flow state in the fractures transitioned to nonlinear laminar flow. The results are important for predicting hazardous gas leaks during deep underground engineering excavation.