The large-amplitude variation of the reservoir water level will easily lead to the concentration of incremental interstitial pressure

which will decrease the effective stress on the fracture surface

and increase the stress intensity factor of the crack tip. When the stress intensity factor reaches the critical value

the crack and fracture in the rock strata will transfix

expand and then damage; the continuous complex failure surface will shape; and the slope stability will decrease which will causes the destabilization of the bank slopes. Based on the previous contents

the impact of fissure water pressure on the crack intensity factor is analyzed from the point of fracture mechanics

and the expand of Ⅰ-Ⅱmixed mode crack is analyzed considering the impact of fissure water pressure. The results show that the crack growth angle of Ⅰ-Ⅱmixed mode crack change not only with the crack closure degree

oblique crack inclination angle and the magnitude of the biaxial stress

but also with the magnitude of the fissure water pressure and the friction coefficient of the crack edges. Moreover

in the same circumstances the non-closed crack growth angle is larger than the closed crack growth angle. For closed crack

the smaller the friction coefficient of the crack edges is

the larger the crack growth angle is. Finally

the rock mass fracture toughness KIc

KIIc considering the role of the fissure water pressure and the Ⅰ-Ⅱcompound fracture criterion under the compression-shear are conducted based on Mohr-Coulomb criterion. The relevant conclusions of our research provide an important reference for studying fractured rock mass failure under the water-rock interaction.