Abstract: Using self-developed mesoscopic test equipment for coal and rock upon shear, taking sandstone as the research object, the research on rock shearing mesoscopic cracking propagation and coalescence mechanisms has been carried out under different conditions of normal stresses, and the sandstone shearing mesoscopic cracking propagation process and mesoscopic coalescence mechanisms have been analyzed, and the effects of saturated coefficient on mesoscopic cracking and crack morphology have been discussed. Results have shown that with the increase of normal stresses, the post-peak residual strength has an increasing trend, and the length of surface initiation crack has a decreasing trend. The time of crack initiation occurred before the peak since normal stresses have been applied. Rock bridges emerged in the process of middle crack formation and coalescence, constantly deflected in the process of the friction and slip, and finally lead off, forming local stripping. After the crack at the end of the specimen formed, because of the restriction of normal stresses and the stress concentration of the crack at the end, the secondary crack appeared at the end, which is connected with the original coalescence crack, then, the bifurcation formed, and caused the end partial peeling. Affected by normal stresses, many microcracks appeared between particles in the surrounding, which intensified the damage range in crack walls. With the increase of the normal stresses, the number of the macro discontinuous cracks appeared with the increasing trend during shearing, whose length was gradually decreasing, and both the number of secondary cracks and the range of crack development zone showed an increasing trend.

Key words: rock mechanics, shear failure, surface cracks, crack growth