Evolution mechanism of water inrush in filling structures of deep-buried tunnels under thermo-hydro-mechanical coupling

Deep-buried tunnels traversing complex hydrogeological zones with clay-sand-filled structures are highly susceptible to water inrush hazards. High ground temperature, high in-situ stress, and high-water pressure render these events a complex thermo-hydro-mechanical coupling problem. However, current research on water inrush often insufficiently investigates the multi-field coupled instability mechanisms within highly permeable filling media and frequently neglects the influence of temperature. This study aims to investigate the evolutionary mechanism of seepage instability in filling structures that trigger water inrush hazards under the complex conditions of deep-buried tunnels. Laboratory tests were conducted using a large-scale triaxial thermo-hydro-mechanical system, and a DEM-CFD coupled model was established to numerically simulate the seepage process. The influences of temperature, particle size distribution, and confining pressure were analyzed on the seepage characteristics of the filling media. By examining the variations in water inflow rate, discharged clay-sand particle mass, porosity and permeability, we analyzed the entire process of seepage behavior and instability evolution under the thermo-hydro-mechanical coupling effect. The results show that: (1) Temperature significantly affects water inflow, discharged particle mass, porosity, and permeability. Higher temperatures remarkably increase porosity and permeability, with the maximum permeability coefficient of filling media at 90℃ being 1.6 times that at 45℃. (2) The Talbol power index exhibits a positive correlation with water inflow rate and discharged particle mass, while confining pressure is negatively correlated with water inflow rate. (3) For filling materials dominated by clay-sand particles or with favorable gradation, the seepage instability process exhibits distinct phase characteristics, with different stages reflected in changes in water inflow, porosity, and permeability. The experimental results are consistent with the numerical simulation results. (4) In high ground temperature environments, temperature enhances convective heat transfer and energy exchange between water and filling media, thereby accelerating the process of water inrush caused by seepage instability. The findings provide scientific support for risk assessment, early warning, and prevention of water inrush hazards in deep-buried tunnels crossing clay-sand-filled structures.