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.