Studying the behavior of rock joints surface asperities in direct shear test

In most of the criteria and constitutive models presented for the shear behavior of rock fractures، only the two mechanisms of shearing through the surface asperities، and sliding along the asperities have been considered. However، the deformation of asperities before and during the shearing process is an important mechanism influencing the behavior of rock joints. The contribution of each mechanism to the shear behavior of rock joints varies depending on the morphological، mechanical، and stress conditions. In this research، a large number of concrete and plaster replicas of natural rock joints in seven groups of mechanical properties and in four degrees of surface roughness were constructed. In order to study the behavior of surface asperities، the replicas experienced CNL direct shear tests under four different values of normal stress. The results of 110 direct shear tests showed that under low normal stresses، the joints with lower wall compressive strength mobilize higher shear strength. While the previous studies of the joint shear behavior had supposed that for all the values of the normal stress، the shear strength increases as the wall compressive strength increases. The observed phenomenon، which is in contrast with the previous findings، can be described considering the contribution of different mechanisms in the direct shear tests. The effect of parameters of joint surface roughness، joint wall compressive strength، normal stress، deformation modulus، and basic friction angle on the damaged area of rock joints were then investigated. Finally، based on the statistical analyses of the test results، equations were derived to predict the damaged area.