Mechanical Failure of Sandstone with Directional Heterogeneous Water Distribution Under Uniaxial Compression

Water is a key factor affecting the mechanical properties and stability of rock masses in underground engineering. In practical engineering settings, water distribution is commonly spatially heterogeneous, and the relative orientation between water distribution and the stress direction may further complicate the mechanical response and failure behavior of rocks. To investigate this issue under controlled laboratory conditions, Linyi red sandstone was selected, and four groups of specimens with distinct water-bearing states (oven-dried, fully saturated, axially semi-saturated, and radially semi-saturated) were prepared using tailored immersion protocols. Laboratory uniaxial compression tests and simplified discrete element simulations were combined to examine the macroscopic mechanical response, failure localization, and mesoscopic damage evolution of sandstone under directional heterogeneous water distribution. The results indicate that the water-bearing state strongly affects the uniaxial compressive strength and apparent deformation modulus of sandstone; compared with oven-dried specimens, fully saturated specimens show an approximately 40–60% reduction in these parameters, whereas semi-saturated specimens exhibit intermediate values. The relative orientation between the water distribution and loading direction further influences the failure pattern of semi-saturated specimens. Failure in semi-saturated specimens tends to initiate or localize in water-affected regions, while the multi-stage post-peak response of radially semi-saturated specimens can be interpreted as a sequential load-transfer process between saturated and dry regions. Heterogeneous water distribution also affects microcrack development and force-chain redistribution, with the idealized dry–wet transition region acting as a sensitive zone for crack initiation and stress redistribution. This study clarifies the first-order influence of directional heterogeneous water distribution on the mechanical behavior of sandstone and provides support for stability assessment and disaster mitigation in underground rock engineering under complex water-bearing conditions.