The effect of cyclic normal load on the direct shear response of planar joints has been investigated using a dynamic shear box device GS-1000. The shear tests were investigated under the influence of different slip velocities, vertical cyclic frequencies, vertical cyclic force amplitudes and constant normal load levels. Laboratory test results show that, the shear force and the dynamic friction coefficient show cyclic behavior under cyclic normal load conditions. A significant time shift between peak shear force and peak normal force with peak shear force lagging is observed, where the time shift is mainly caused by the shear stiffness of the contact surface. The relative time shift decreases with increasing normal load and vertical cyclic force amplitude. The peak value of the friction coefficient is nearly identical with the static friction coefficient, while, the minimum value of the dynamic friction coefficient changes with changing in slip velocity and vertical cyclic force amplitude. Finally, a shear strength criterion is proposed, which can predict the shear strength of planar joints under constant shear velocity and cyclic normal force conditions.
Discontinuities, joints and anisotropy are the main features of rocks and rock masses in geotechnical engineering projects, where the frictional behavior of joints plays a central role in the stability and hazard evaluation, e.g. for surface and underground excavations, slopes, dam foundations or geothermal reservoirs (Hoek and Brown, 1980; Hoek and Bray, 1981; Babanouri et al., 2011, Du et al., 2016). In order to gain a deeper understanding of the shear behavior of joints, direct shear box tests under constant normal load and constant normal stiffness conditions are becoming increasingly popular (e.g. Barton and Choubey, 1977; Lee et al., 2014; Nguyen et al., 2013 and 2014; Dang et al., 2016a). Due to blasting, explosions or earthquake excitation, rock masses suffer dynamic loadings in addition to static loads. Therefore, dynamic effects on rock masses need to be considered and several researchers investigated the rock, rock mass and joint behavior, respectively, under dynamic loading conditions (e.g. Crawford and Curran, 1981; Kana et al., 1996; Lee et al., 2001; Jafari et al., 2003; Bagde and Pertros, 2005; Belem et al., 2007; Guo et al., 2011; Liu et al., 2011, 2012; Konietzky et al., 2012; Cabalar et al., 2013; Nguyen et al., 2014; Zhou et al., 2015; Dang, 2016b).
