Shear characteristics and mesoscopic damage mechanism of long time soaking red sandstone under loading and unloading conditions

The bank slope rock mass below the flood control restricted water level (145 m) in the Three Gorges Reservoir area has experienced long-term immersion in the process of reservoir water lifting and lowering; the water level changes lead to the rock mass being subjected to two working conditions, tangential loading shearing and normal unloading shearing. The difference in rock shearing characteristics directly affects the stability evaluation of bank slopes in different reservoir operation stages. Tangential loading and normal unloading shear tests were carried out on the typical feldspar quartz sandstone after different soaking days; then the characteristics changes of sandstone shear under the two stress conditions were obtained. The microscopic mechanism of the differences was revealed by solution test, SEM test, and nuclear magnetic resonance test. The results show that: (1) compared with the initial sample, after soaking for 80 days, the cohesion loss of the sample is greater than the loss of internal friction angle. The cohesion of the sample under tangential loading is reduced by 40.5%, and the internal friction Angle is only reduced by 2%, while the cohesion of the sample under normal unloading shear is reduced by 31%, and the internal friction Angle is reduced by 8%. (2) The long-term immersion of the sample results in the dissolution of the cement minerals, the gradual development and penetration of the secondary pores, and the increase of porosity. After 60 days of immersion, basically, the water content, porosity, and pore structure of the sample reach a stable state; the particle skeleton overcoming the shear action is almost no longer affected by the soaking water. This is the reason why the shear properties of the samples with long-term water saturation gradually weaken and become stable. (3) Under normal unloading shear conditions, the deviation between the main crack surface and the theoretical shear surface increases; the fracture surface is more inclined to form “S” and “M” types. The increase of the actual shear plane increases the peak shear stress that the rock can bear. Because the biggest contribution to the rock is the skeleton particle, the internal friction angle is larger, and the filling cementing material that provides cohesion makes less contribution to the tensile shear failure. The cohesion obtained by normal unloading is also lower. This study can provide basic information for the stability evaluation of wading slope in reservoir area with the fluctuation of water level and the selection of test method considering the actual working conditions.