Multiscale mechanical degradation of layered sandstone under different water–rock interactions

Previous studies on the water-induced deterioration of layered rock masses have focused primarily on hydrostatic immersion, whereas the mechanism by which water flow decreases the mechanical properties of minerals and induces rock failure remains unclear. In this study, layered sandstone specimens are subjected to air drying, hydrostatic immersion, or hydrodynamic scouring. Macro-mechanical properties, failure modes, bedding plane slip, and micro-mechanical properties are jointly analyzed using uniaxial compression tests, digital image correlation, three-dimensional surface scanning, and nanoindentation. Compared with the dried specimens, the uniaxial compressive strength and elastic modulus of layered sandstone are reduced by 40.96% and 35.05%, respectively, due to hydrodynamic scouring. The fracture surface roughness is also reduced by 43.5% and the relative slipping displacement between the two sides of the bedding plane is increased by 240.1% by it before the onset of instability. Based on micromechanical parameters from nanoindentation tests and combined with the heterogeneity coefficient, it is confirmed that water–rock interaction intensifies the mechanical disparities between the bedding planes and the matrix, offering a micro-mechanical interpretation for the evolution of fracture surface roughness. The findings provide a new perspective for multiscale analysis and mechanistic interpretation in rock mechanics and a theoretical basis for assessing rock mass stability under flowing water conditions.