Abstract: Under the effect of repeated lifting and lowering of the reservoir water level, the rock mass in the dissipated zone is in the soaking-air drying circulation state, which deteriorates its mechanical properties significantly. Thus, the deformation and stability of the bank slope are directly affected. Considering the soaking-air drying cycle and water pressure changes, the water-rock interaction tests were carried out on the sandstone of the bank slope in the Three Gorges Reservoir area. Scanning electron microscope (SEM) was used to analyse the effect of water-rock interaction on the sandstone microstructure. The degradation process of sandstone under the action of the water-rock was simulated using PFC2D discrete element software. The results showed that the degradation effect of water-rock on sandstone mechanics was significant. It was found that the elastic modulus and uniaxial compressive strength generally showed a steep and slow deterioration trend, and the deterioration effect was particularly obvious in the first six times of immersion-air drying cycle. Under the action of water-rock, the microstructure of sandstone gradually changed from a dense pore-type cementation structure to the irregular cellular structure, which caused the deformation of sandstone and the degradation of strength characteristics macroscopically. Through the analysis of PFC2D discrete element simulation, it was found that under the action of water-rock, the strength of internal particles and the bond strength between the particles of rock decreased gradually, the heterogeneity of rock increased, strain localisation features under load increased progressively, and the overall bearing capacity reduced slowly. Moreover, the failure mode gradually transformed from the typical tension damage into a shear failure.

Key words: water-rock interaction, circulating soaking-drying, degradation, microstructure, strain localisation