Effects of shear rate on cyclic loading shear behaviour of rock joints under constant normal stiffness conditions
The presence of joints and discontinuities within a rock mass can significantly affect its mechanical behaviour and therefore the stability of structures constructed at close proximity. Several studies have been carried out by previous researchers to understand the mechanical behaviour of joints under both constant normal load (CNL) conditions, in which the normal load remains unchanged during shearing, and constant normal stiffness (CNS) conditions to imitate the stiffness of the surrounding rock mass (Patton 1966; Ladanyi and Archambault 1969; Barton 1973, 1976; Seidel and Haberfield 1995; Indraratna and Haque 2000; Buzzi et al. 2008). The importance of CNS conditions to simulate the actual shear behaviour of rock joints in the field has been described by Johnston and Lam (1989), Skinas et al. (1990) and Indraratna et al. (1998). The above-mentioned studies focussed only on the monotonic loading shear behaviour of rock joints. The effects of cyclic loading on the shear behaviour of rock joints in earthquakes and blasting were investigated in detail by Plesha (1987), Hutson and Dowding (1990), Lee et al. (2001), Stupkiewicz and Mroz (2001), Grasselli and Egger (2003), Jafari et al. (2003) and Belem et al. (2009). As the shear rate might vary depending on the source of the load and rock media, Crawford and Curran (1981) carried out a series of experiments on artificial rock joints with various shear rates and normal stresses under CNL conditions. Based on the measured data, they concluded that the shear rate may influence the shear strength of hard and soft rock joints differently. In another study, Jafari et al. (2004) verified the results of Crawford and Curran (1981) for shear rates between 0.05 and 0.4 mm/min under monotonic loading. None of these researchers investigated the effects of shear rate on the cyclic loading shear behaviour of rock joints under CNS conditions, which is a critical issue in stability analysis of underground structures subjected to seismic events. Accordingly, three sets of cyclic loading shear tests with various shear rates and initial normal stresses were conducted on artificial triangular joints under CNS conditions. In this study, the experimental data are critically analysed.