Seepage-induced simulation of excavation and support strategies for tunnels in weak surrounding rock

Under groundwater seepage, excavation disturbances in tunnels within weak surrounding rock easily trigger collapse accidents. This study, based on an actual tunnel project and considering fluid–solid coupling, establishes a three-dimensional numerical model of tunnel construction in weak surrounding rock, investigates the instability mechanisms of tunnel excavation under groundwater seepage, and optimizes tunnel support strategies. The results show that when fluid–solid coupling is considered, the deformation zone and stress concentration zone of the surrounding rock expand significantly, and peak stresses increase; initial support stresses generally increase, with the crown-bottom area being the most affected; a bedding dip of 22° causes a pronounced asymmetric distribution of deformation and stress in the surrounding rock, a feature further intensified under fluid–solid coupling; for circular excavation with a reserved core soil method, the optimal construction and support scheme is rock bolts 5 m long with a spacing (circumferential × longitudinal) of 0.9 × 0.85 m, forepoling microtubes 5.5 m long with a spacing (circumferential × longitudinal) of 0.6 × 2 m, and double-layer angles of 10° and 30°; after optimization, excavation disturbance is reduced, effectively limiting crown heave and lowering stresses in the waist surrounding rock. This study provides a reference for excavation control and support optimization for tunnels under similar geological conditions.