Numerical study of water inflow into tunnels in stratified rock masses with a dual permeability model

Stratification planes form an anisotropic structural setting in the stratified rock mass, and strongly influence the water inflow into tunnels. However in the numerical simulation studies, the equivalent porous model pays little regard for the anisotropic structural setting and the discrete fracture network model costs too much computing power. In this study, a dual permeability model is utilized to calculate the water inflow into tunnels and the anisotropic flow behavior with finite element numerical simulations. The dual permeability model takes the stratification planes into artery fractures, and consider the rock matrix with other fractures as an equivalent isotropic medium. Parameters including the piezometric head above the tunnel center, the artery fracture angle, and the artery fracture spacing are studied and discussed. It is shown that both the piezometric head above the tunnel center and the artery fracture spacing affect the water inflow a lot. In this research, the influence of the artery fracture spacing on water inflow is larger than that of the piezometric head above the tunnel center when the artery fracture spacing<4.62 m, and it is opposite when the artery fracture spacing>4.62 m. What is more, the artery fracture angle affects the water inflow slightly but it controls the flow direction. The water inflow case of the Huangjiagou Tunnel is used to verify the proposed model. The results show that the dual permeability model could give good predictions to the Huangjiagou Tunnel and describe the anisotropic flow behavior in stratified rock mass tunnels.