The shear behaviour of soft rock joints is investigated in laboratory under both Constant Normal Load (CNL) and Constant Normal Stiffness (CNS) conditions. The laboratory behaviour is modelled numerically using the Universal Distinct Element Code (UDEC). The predicted shear stress, normal stress and dilation behaviour with shear displacements are compared with the laboratory results. It is observed that UDEC can predict the peak shear stress of unfilled joints under CNS, however, it overestimates the joint dilation as well as the normal stress. The maximum peak shear stress in UDEC is attained at a greater shear displacement in contrast to the laboratory observations. The UDEC predictions are generally in good agreement with the laboratory data under CNL condition, where the asperity degradation is found to be less significant.
The correct evaluation of the shear strength of rock joints is imperative in the design of excavations in jointed rocks, and in the stability analysis of jointed/bedded rock slopes. Currently, the shear strength of rock joints is mainly assessed in the laboratory using the conventional direct shear apparatus, where the shear load is varied while keeping the normal load constant (ie. CNL method). However, for non-planar or rough discontinuities, shearing causes dilation of the joints as asperities ride over each other (Ohnishi & Dharmaratne, 1990; Kodikara & Johnston, 1994; Haberfield & Seidel, 1998 and Indraratna et al. 1999). If the surrounding rock mass is unable to deform sufficiently, then an inevitable increase in the normal stress occurs during shearing. The stiffness of the rock normal to the joint, however, can be assumed to be constant (CNS), even though the normal stress changes. Therefore, the CNL condition is often unrealistic in circumstances where the normal stress in the field changes considerably during the shearing process.