Abstract:Based on the theory of linear-elastic fracture mechanics, the influence of vertical seismic action on seismic dynamic failure of jointed rock mass is discussed. When a single peak acceleration loads on rock mass, the severest accelerated direction is horizontal slip-outward, and for acceleration with two directions, the severest ground motions are the combination of horizontal slip-outward with upward or downward according to that its failure mechanism is tenso-shear or com- presso-shear crach. The amplitude, direction and combination of horizontal and vertical accelera- tions can change the failure mechanism of rock mass and the changing is catastrophic and irreversible. The dynamic stress factor from two lower accelerations of horizontal and vertical might cause more serious failure than that from higher single acceleration. For given jointed rock mass and static stress field, the amplitude and direction combination of first ground motion accelera- tions causing failure in rock mass will determine the unique development direction, mechanism and final characteristics of rock mass failure, therefore, the seismic dynamic failure of rock mass is much more complex than static failure. The influence of vertical seismic action on seismic dynamic failure of jointed rock mass should be considered comprehensively.