Abstract: Due to the influence of cycle water pressure permeation, the physical and mechanical properties of sandstone in the fluctuation zone deteriorate with time, which leads to a decrease in bank slope stability. Undrained triaxial compression tests were conducted on sandstone subjected to varying cycles of pore water pressure. The evolution of the deformation, strength, and brittleness index of sandstone were investigated, and the mechanisms of microstructural deterioration were discussed. The results show that the peak stress decreases with the increase of osmotic water pressure action cycle, and the strength degradation gradually decreases with the increase of confining pressure. With an increase in the number of cycles of pore water pressure, both cohesion and internal friction angle decrease exponentially and linearly, respectively, indicating that the decrease in cohesion is the primary cause of strength deterioration. The elastic modulus, failure strain and Tarasov brittleness index decrease gradually with the increase of pore water pressure action period, however there is a significant enhancement effect with increasing confining pressure. Moreover, the failure mode has evolved from single slope shear failure to conjugate shear failure, with the fracture angle decreasing from 67.2° to 62.05°. The stress concentration and stress fatigue caused by cyclical pore water pressure at the crack tip exacerbate the internal structural degradation of sandstone. Additionally, the increase in pore water pressure during undrained compression exacerbates the damage and degradation of the sandstone, while the confining pressure causes the closure of some seepage channels, reducing the seepage damage of sandstone.

Key words: reservoir slope, sandstone, stress-seepage coupling, cyclical pore water pressure, brittleness index, damage degradation