DEM investigation of breakage-induced deformation and mesoscale damage evolution in rockfill under slow cyclic loading

Periodic water-level fluctuations in pumped-storage hydropower stations impose long-term slow cyclic loading on rockfill materials. This study investigates the macroscopic–mesoscopic evolution of rockfill materials under cyclic loading using the discrete element method (DEM). A mesoscopic DEM model was developed to capture both the particle-size effect on strength and cumulative cyclic degradation by introducing particle-size-dependent initial cracks into bonded structures and incorporating the Paris–Erdogan fatigue crack-growth law. The model was used to simulate one-dimensional compression tests under slow cyclic loading, and the associated deformation mechanisms were systematically analyzed. The results demonstrate that the proposed model reasonably reproduces the macroscopic mechanical responses of both individual particles and particle assemblies. Mesoscopic analysis reveals that the number of cracks increases rapidly during early loading and then approaches a stable level, with higher peak stresses and cyclic amplitudes promoting crack development. The evolution of crack number closely corresponds to the deformation response, indicating that accumulated deformation is primarily governed by particle breakage. Large particles are mainly crushed during initial loading, whereas subsequent breakage is suppressed as the particle-size distribution evolves toward a more stable state. A higher peak stress significantly expands the damaged region, whereas a larger cyclic amplitude mainly aggravates pre-existing localized damage. Displacement fields indicate that increasing cyclic amplitude causes a noticeable deflection in particle displacement directions. Coordination number and force-chain analysis reveal that peak stress controls the overall density of the load-bearing skeleton, whereas cyclic amplitude influences its stability via contact breakage and reorganization.