Modelling the Evolution of a Fracture Network under Excavation-Induced Unloading and Seepage Effects Based on a Fully Coupled Fluid-Solid Simulation

In order to assess the dynamic, coupled geomechanical and hydraulic response of a highly fractured rock during/after excavation, it is important to quantify the changes in the connectivity and conductivity of the fracture system. The objective of the work is to explore the seepage effect on fracture network evolution during a tunnel excavation in a highly fractured rock mass based on fully coupled hydro-geomechanical modelling. An empirical joint constitutive model (JCM) that can capture the normal and shear behavior of rough fractures has been implemented in the combined finite-discrete element method (FEMDEM) code ‘Solidity’. Solidity can simulate the deformability of matrix blocks, the interaction of pre-existing fractures, and the propagation of new cracks driven by mode I, mode II or mixed mode brittle failure using nonlinear fracture mechanics in terms of a ‘smeared crack model’. The ‘JCM-Solidity’ model is further coupled to the multiphase flow solver ‘IC-FERST’ which is based on a CV-FEM formulation. This is achieved using a conservative mesh-to-mesh interpolation between solids and fluids, while forces are balanced using the novel ‘immersed body method’ for full two-way coupling. Anisotropic adaptive mesh refinement is used in the coupled domain to capture the fracture characteristics throughout the simulation. Thus, geomechanically-constrained hydraulically equivalent fracture apertures inform the local permeability for often highly refined fluids mesh elements within fractured regions of the porous rock mass, leak-off being automatically accommodated. Fluid flow is solved over the entire domain which represents the fractured system, with underlying details from the solid model. Coupled deformation, fracturing and flow processes can thus be captured by such a fully coupled solution scheme