Experimental and Mesoscale Simulation Study on the Seepage–Stress Coupling Behavior of Cemented Sand and Gravel Materials

Cemented sand and gravel (CSG), widely used in dams and cofferdams, is susceptible to fracture under coupled seepage and mechanical loading. This study combines wedge-splitting experiments and mesoscale numerical simulations to investigate the hydro-mechanical fracture behavior of CSG under crack-face water pressures of 0, 0.02, 0.05, 0.10, 0.15, and 0.20 MPa. A coupled model was developed by integrating the lattice discrete particle model (LDPM) with discrete poromechanics. In this framework, deformation and fracture of the solid phase are resolved through interactions among polyhedral cells, while water transport is represented by a dual-lattice seepage network and coupled to the solid response through the effective stress principle. After calibration using triaxial compression tests, the model was used to simulate pore-pressure evolution, crack initiation, and crack propagation. The results show that increasing hydraulic pressure accelerates crack propagation and localizes the fracture process zone, leading to more brittle failure. The peak load, initial fracture energy, and effective process-zone length all decrease with increasing water pressure. The simulated mean peak loads agree well with the experimental results, with relative errors of 0.26–6.58%, while the internal water-pressure histories recorded by three embedded sensors are reproduced with root-mean-square errors of 3.4–11.2%. The size-effect analysis further shows that the nominal strength follows Bažant’s size-effect law, confirming the quasibrittle nature of CSG fracture under seepage coupling. These results provide a mesoscale basis for evaluating fracture safety and optimizing seepage-control measures in CSG dams and cofferdams.