Abstract: The instability characteristics of fractured rock masses under unloading conditions can be better understood depending on compression-shear failure experiments on fractured rock samples. In this paper, a series of normal unloading tests on rock specimen with a single fracture were conducted under pre-peak shear stress. The deformation of rock samples and energy evolution were analyzed. The results show that the normal stresses at failure under normal unloading condition are larger than those of direct shear loading test, and the shear resistance of fractures decreases. The shear displacement increases with the decrease in normal stress. The normal deformation ratios(K) under normal unloading conditions increase with the increase in the contour area ratios(Rs) of rock fracture surfaces. The total energy decreases at first, and then increases during normal unloading process. This phenomena can be used to predict the imminent unloading instability of fractured rock masses. The results in this paper have certain references for understanding the unloading instability failure of fractured rock masses.

Key words: penetrated fracture, normal unloading, deformation characteristics, energy evolution