After the impoundment of a high dam reservoir

the water pressure environment of the rock masses in dam base and reservoir bank changes

which may easily induce engineering problems such as bank slope instability and dam collapse. In order to investigate the influences of different constant water pressures on the rock mass of dam base

triaxial compression tests were conducted on sandstone with initial damage under different high constant porewater pressures

and the multidirectional fracture mechanism was analyzed by combining computed tomography (CT) and scanning electron microscopy (SEM). The test results show that: (1) Under the confining pressure of 80 MPa

the greater the pore water pressure

the more brittle the sandstone

the lower the peak strength

and the smaller the volume expansion stress. The pore water pressure increases from 10 MPa to 50 MPa

and the peak strength decreases by 33%. (2) Under different pore water pressures

there are significant differences in sandstone internal deterioration range and deterioration effect as the fracture surfaces of sandstone specimens have various forms and directions. Due to CT scaning results

with the increase in pore water pressure

the deterioration effect spreads from the middle of the specimen to both ends. When the ratio of the water pressure to the confining pressure is less than 25.0%

the deterioration of pore water pressure is mainly concentrated in the middle of the specimen. When the ratio of the water pressure to the confining pressure is larger than 62.5%

the pore water pressure has obvious deterioration effect on the whole specimen. (3) The SEM test reveals that with the increase in pore water pressure

the microgranular structure of sandstone changes from shear slip failure to shear fracture failure

and the microcrystalline structure of sandstone changes from cauliflower to rice granules. The macroscopic failure mode changes from plastic failure to brittle failure

and the multidirectional fracture plane is formed

which is related to the migration of fine particles and the fracture of large particles in the meso-particle structure under pore water pressure. The formation of the multidirectional fracture plane is directly related to the uneven accumulation of the microscopic crystal structure.