Experimental study on pore variation and meso-damage of saturated sandstone under unloading condition

The degradation of rock mass strength is macroscopically manifested as a reduction in cohesion and an increase in the internal friction angle. Microscopically, it manifests as the propagation of internal fractures, which is also the fundamental cause of rock mass damage and deterioration. The complex mesoscopic fracture structure within the rock mass directly influences its macroscopic mechanical properties and failure characteristics. To more accurately understand the mechanical behavior of rock masses under unloading conditions, it is essential to investigate the internal mesoscopic fracture structure of the rock and its impact on the overall mechanical properties.

To study the crack propagation and meso-damage evolution of saturated sandstone under unloading (unloading confining pressure), triaxial unloading confining pressure tests were designed and conducted on sandstone samples under different initial axial pressures (70%, 80%, and 90% of the triaxial compressive strength, TCS). The results indicate that samples with higher initial axial pressure exhibit larger axial strain and smaller radial strain at unloading failure. As the unloading confining pressure ratio increases, the elastic modulus gradually decreases, while Poisson's ratio and strain gradually increase.

Using ¹H Nuclear Magnetic Resonance (NMR) technology, the variations in rock porosity and T₂ spectrum curves were analyzed. The T₂ spectral peaks show that pore size increases with the unloading confining pressure ratio, and a higher initial axial pressure leads to more significant pore size growth. Porosity increases exponentially with the unloading confining pressure ratio. Within this trend, the number of micropores continuously increases, whereas the numbers of mesopores and macropores first decrease and then increase. The initial axial pressure promotes the development and expansion of pores.

The fractal characteristics of the T₂ spectrum were analyzed, and the relationship between the degree of damage and the unloading confining pressure ratio was established. The variation trends of rock pore characteristics, energy, and damage degree are generally consistent. Finally, based on damage mechanics theory, a damage constitutive model for rock under loading and unloading conditions was developed. The overall correspondence between the theoretical model predictions and the experimental curves is satisfactory.