Knowing shear mechanism of discontinuities under monotonic and cyclic loading conditions is a key factor in stability analysis of rock slopes and rock foundations. In this paper, the effect of cyclic loads on the shear mechanism of joint asperities is described. For this purpose, a fresh tension-split joint from a granite block was used as a reference and thirty five mortar replicas were fabricated. Cyclic shear tests were conducted on replicas under low values of constant normal load (CNL) using a servo-controlled loading system. Tests were performed in two consecutive steps; load-controlled and then displacement controlled. In the load controlled part, after initial increase of shear stress to 0.25 MPa, the test was stopped for 2 minutes, then increased to 0.5 MPa, and finally decreased to 0.25 MPa. This process was repeated ten times. At the end, the test was continued up to 10 mm shear displacement under displacement-controlled loading in order to pass the peak shear strength and reach the residual strength. Characterizing joint surface and modeling of involved asperities during the tests was undertaken using a Matlab code developed with new mathematical expressions. The algorithm used in the code is based on three-dimensional coordinates of the scanned joints' surfaces before and after the test. It was found that a high percentage of degradation happens during the first cycle of loading and during the other cycles, degradation occurs gradually. While comparing tests conducted in monotonic and cyclic loading conditions, it was observed that under low number of cycles, the pronounced mechanism is consolidation of the joint surfaces. This consolidation increases contact areas and, therefore, shear strength parameters.
According to the significant influence of pre-existing discontinuities on the instabilities of geotechnical structures, knowing the shear mechanism of discontinuities under real loading conditions is the key to designing of geotechnical projects. Two main loading conditions that must be considered in the design of these projects are static (monotonic) loads and cyclic (dynamic) loads. Most of the previous experiments have focused on determining shear mechanism and shear strength parameters of rock joints under monotonic loading (Barton 1977; Jing et al 1992; Grasselli 2001; Moradian et al 2010; Asadollahi 2011; Barton 2013). Some of them have conducted displacement-controlled cyclic loading conditions (Hutson et al. 1990; Huang et al. 1993; Jing et al. 1993; Homand et al. 1999; Lee et al. 2001; Jafari et al. 2003). The shear strength of rock joints under load-controlled cyclic loading conditions has also been studied (Jafari et al. 2004; Tsubota et al. 2013). Jafari et al. (2004) investigated the effect of the number, the frequency, and the stress amplitudes of the cycles on the peak and residual shear strength of rock joints. They reported that shear strength decreases with increasing the number, frequency, and stress amplitudes of the cycles. Tsubota et al. (2013) showed that shear strength decreases with increasing the number of cycles, while the frequency of loading (0.1 and 1 Hz) has no significant impact on the shear strength. They also stated that the dynamic shear strength is equal to or higher than the static one, which is on the opposite of the results obtained by Jafari et al. (2004). Although the mentioned studies have provided some insight into shear strength parameters of rock joints under cyclic loading conditions, the shear mechanism of rock joints is still not well understood. Therefore, owing to the influence of various parameters on the cyclic properties of rock joints, further studies are necessary.
