Experimental investigation on the damage and deterioration of sandstone subjected to cycling pore water pressure

The nuclear magnetic resonance (NMR), ultrasonic testing, and triaxial compression tests were employed to investigate the deterioration laws of multiple physical and mechanical properties of sandstone under different pore water pressure cycles, and Micro- and Meso- deterioration mechanisms were analyzed via scanning electron microscopy (SEM). The critical stresses exhibited different variation trends concerning the cycling pore water pressure, which reflecting the difference in damage sensitivity to critical stresses. Observing the transverse relaxation time (T₂) spectrum areas and SEM images, the number of internal pores and the crack size developed with the increase of pore water pressure cycles. Furthermore, the NMR images indicated that the sandstone pores under cycling pore water pressure gradually expanded from the bottom action end to the entire sample. The wave velocity of sample was linearly negatively correlated with the number of pore water pressure cycles and increasing porosity. The micro- and mesoscopic deterioration mainly resulted from the coupling hydraulic-mechanical effect, leading to the porosity increase and an increase in pore water content at the micro-level, which in turn resulted in the deterioration of the physical and mechanical properties of sandstone at the macro level. Damage characterized by the strength, deformation, acoustic parameters, and porosity indicating that the cycling pore water pressure have a cumulative damage effect on sandstone. However, the sandstone in the same damage state has a more significant effect on the deformation than the strength. The results can be adopted as a reference for the safety and security of reservoir slope.