Macro-Meso Damage Mechanism of Sandstone Under Wet-Dry Cycles: A Study Based on Nuclear Magnetic Resonance Technology

Water level fluctuations in reservoir areas subject bank slopes to intense wet-dry cycles (WDCs), compromising rock mass stability. This study investigates the macro-meso damage evolution of yellow sandstone from the Wudongde Reservoir. Specimens subjected to 0-20 WDCs were analyzed using nuclear magnetic resonance (NMR) alongside Brazilian splitting, uniaxial, and triaxial compression tests. Results indicate that porosity increases linearly with WDC, rising from 6.12% to 17.61% after 20 cycles, driven by the transformation of micropores into macropores. Macroscopic mechanical parameters, particularly tensile strength and cohesion, exhibit significant exponential and sharp decay, respectively, while the internal friction angle remains relatively stable. Notably, increasing confining pressure effectively mitigates WDC-induced deterioration by inhibiting microcrack propagation. The damage mechanism is primarily attributed to the dissolution of clay binder and uneven mineral swelling/shrinkage, whereas the rigid mineral skeleton remains largely intact. These findings provide a theoretical basis for quantifying rock damage and predicting slope stability in complex hydrological environments.