Degradation laws of joint strength and micro-morphology under repeated shear tests

In order to analyze the degradation laws of shear mechanics properties and micro-morphology of joint planes under repeated shear tests, single joint sandstone specimens are prepared and six times of repeated shear tests are carried out respectively considering four kinds of normal stresses. The results show that: (1) During the repeated shearing process, the shear stress-shear deformation curve changes obviously. After 2 or 3 times of shear tests, there is no obvious peak shear strength and the shear hardening phenomenon is obvious, and the residual shear strength is gradually stabilized. (2) With the increase times of repeated shearing, the shear strength of the joint planes are deteriorated obviously. The deterioration trend is fast in the first place and then become slow. The degradation rate of the first two times of shear tests is more than 65% of the total deterioration value. In addition, the greater the stress, the greater and fast the shear strength degradation rate. (3) Under the repeated shearing effect, the occlusive micro-embossment and depression begin to incise, wear and fill with each other on the joint plane, and the concavo-convex fluctuation of the joint plane gradually decreases, thus makes the morphological parameters and roughness coefficient gradually decrease, and then it leads to the gradual deterioration of shear mechanics properties. (4) The deterioration equation for roughness coefficient under repeated shearing is established. Combining with N. Barton's empirical formula for shear strength of structural planes, the formula for calculating the shear strength considering the repeated shearing effect is established. The results show that the calculated values agree well with the experimental ones.