In water-rich geological formations, drainage systems play a vital role in controlling external water pressures and managing seepage flow in deeply buried tunnels. This study develops a hydro-mechanical coupling model that incorporates stress-dependent fracture permeability to quantify water pressure distributions on tunnel linings and characterize seepage behavior. The grouting–lining interface is modeled as a rock mass discontinuity, enabling three-dimensional analysis of fracture permeability under coupled stress–seepage conditions. The parametric study systematically examines three key design parameters: drainage hole number, drainage hole length, and groundwater head level. Results reveal a three-stage pattern of external water pressure reduction with increasing drainage hole quantity (reduced radial hole spacing): beginning with a gradual decline, followed by an accelerated decrease, and ultimately reaching a stabilized state. Seepage flow rates exhibit a progressive deceleration trend before stabilizing. While borehole penetration depth has limited impact on water pressure reduction, it significantly influences seepage flow rate within the drainage networks. The analysis reveals distinct spatial patterns in the distribution of lining pressure. It is observed that the maximum external water pressure on the tunnel lining demonstrates an approximately linear decrease as the groundwater level declines. A comparative analysis of external water pressure calculation results between 3D and 2D numerical models reveals that the 3D model predicts higher lining pressure values. These findings provide valuable guidance for optimizing drainage hole design in the construction of large-scale, deeply buried tunnels in water-rich strata.
Meng et al. (Sun,) studied this question.