Abstract: The asperity of sliding zone in rock is an important factor which the nonuniformity of sliding resistance and the mode of instability depend A traditional criterion accepted extensively up to now for judging whether a sudden instability will occur is to compare the stiffness of testing machine with the contact stiffness of sliding zone.However,the experimental investigations on rocks by using triaxial compression apparatus show that the high-pressure and low-pressure sudden instabilities are different from each other in mechanism and criterion.The former takes place along a sliding zone with uniform-resistance and is caused mainly by the difference between potential energies at peak strength and residual strength;the latter along that with nonuniformsresistance and is caused by the difference between static and kinetic frictions.According to the solutions of the equations of motion corresponding to two kinds of sudden instabilities respectively,their criteria were obtained as follows:
K_EE′ < (K₁sin²θ-K₃cos²θ)/ζ
for high-pressure sudden instability,and
K_EE′ >K₁sin²θ+K₃cos²θ
for low-pressure sudden instability,where K₁ and K₃ is the stiffness of axial and confining pressure system respectively,K_EE′ is the contact stiffness of sliding zone,i.e.the slope of descending segment of stress-strain curve,θ is the angle between the directions of the specimen axis and the normal line of sliding zone,and is the nonuniform resistance coefficient with a positive value less than 1.0.It is emphasised in the paper that the strength condition is only a necessary but not sufficient condition for strong earthquake and there are a series of necessary conditions such as those of environmental stiffness,nonuniform-resistance,etc.,apart from strength condition.