Evolution mechanism of tunnel instability and collapse in deep-buried marine soil-rock mixture strata

The instability and collapse mechanisms of tunnels in deep-buried marine soil-rock mixture (S-RM) strata remain poorly understood, posing significant challenges to engineering safety. This study employs a discrete element method (DEM) to establish an S-RM model, integrating ball particles and rblock blocks to simulate soil and rock, respectively. The deformation evolution, shear band formation, porosity variation, force chains, and anisotropy of S-RM under varying stress release rates are systematically investigated, with emphasis on rock content, water content, and rblock types (rubble and cobble). The results reveal that tunnel excavation reduces radial interparticle contact forces, inducing convergent squeezing deformation, while tangential forces increase, forming a soil arch dominated by horizontal force chains. Higher rock content enhances shear resistance and accelerates soil arch formation but intensifies dilatancy under high stress release, expanding collapse zones. Elevated water content increases lateral pressure coefficients, promoting earlier arch formation, yet reduces interparticle bond strength and rock anti-slip capacity, leading to premature shear failure. Cobbles, whose long axis tends to rotate in the slip direction, exhibit weaker shear resistance and lower dilatancy than rubble, thereby increasing soil arch instability. Crucially, shear band evolution and force chain fracture at side walls disrupt arch integrity, triggering progressive collapse. These micro-mechanisms elucidate the coupled effects of stress redistribution, particle interactions, and material heterogeneity on S-RM failure. Suggestions for construction control include minimizing excavation footage, implementing timely support, and reinforcing sidewalls with feet-lock bolts to stabilize soil arches. This work advances the theoretical framework for disaster mitigation in deep-buried S-RM strata, offering a DEM-based paradigm for predicting and controlling tunnel instability.