Seepage stability analysis of a deep-buried tunnel in fractured rocks based on a non-Darcy hydro-mechanical coupled method

Non-Darcy seepage commonly occurs in fractured rocks with an adequate groundwater system around deepburied tunnels. This study develops a numerical method for capturing the non-linear hydro-mechanical coupled processes in a rock mass system comprised of a tunnel lining, grouting circle, and surrounding rocks with discrete fracture networks. The proposed model is validated by comparing it with analytical solutions and field data obtained from a diversion tunnel of the Jinping II Hydropower Station on the Yalong River. The investigation of the non-Darcy flow regions around the diversion tunnel indicates that fractures with relatively higher conductivities dominate the non-Darcy effect, and the highest Forchheimer number reaches 0.5 during the period of torrential rain. The seepage stability of the diversion tunnel lining under three different cases (failure of the seepage control capacity of the grouting circle, potential karst pipeline penetrating the diversion tunnel, and torrential rain attacking the area) is further studied. It is found that the pore pressure at the external edge of the lining increases with the enhancement of the grouting circle permeability, and reaches 2,472.4 kPa when the karst pipeline is connected. Additionally, it is observed that the pore pressures obtained based on the Darcian and non-Darcian assumptions differ by up to 468.8 kPa in Case I. However, the non-Darcy effect gradually decreases as the lining permeability decreases. The results of this study provide practically useful suggestions for the design and operation of diversion tunnels subjected to high water pressure.