Abstract: Joints have a great influence on mechanical properties of rock under unloading condition. The stress-strain characteristics and the influences of rock mass strength, deformation characteristics, failure law, joint spacing and depth on the mechanical properties of rock mass are analyzed, based on the triaxial unloading destruction experiment of the rock mass with two prefabricated intermittent joints with different spaces. The results show as follows: Comparing to the completed rock mass, the axial strain of the jointed rock during the process of falling from the peak strength to residual strength in unloading failure is 3-4 times larger than the completed rock mass; and the ultimate strength is significantly lower than the completed rock mass; also the brittle feature is not obvious as the completed rock mass. During unloading failure, the deformation modulus of the jointed rock mass reduces significantly, which the reducing degree is 6-7 times larger than the completed rock mass; and the deformation modulus decreases faster with the increase of joint spacing. Comparing to the triaxial loading experiment results of the rock mass with prefabricated joints, the damage degree of the jointed rock mass under unloading condition is more intense; in addition to the shear fracture plane, the tensile cracks of different classes distributed along the direction of maximum principal stress are well developed; and the space of prefabricated joints has little effect on the rock mass failure mode.

Key words: prefabricated joints, unloading, mechanical property, experimental research