The typical shear behaviour of rock joints has been studied under a constant normal load (CNL) or zero normal stiffness condition, but recent studies have shown that this boundary condition may not replicate more practical situations, and that constant normal stiffness (CNS) is a more appropriate boundary condition to describe the stress-strain response of field joints. In addition to the effect of boundary conditions, the shear behaviour of a rough joint also depends on its surface properties and the initial stress acting on its interface. Despite this, exactly how these parameters affect the shear behaviour of joints is not fully understood because the stress-strain response of joints is governed by non-uniform asperity damage and the resulting gouge that accumulates on their interfaces. Therefore, an attempt has been made in this study to predict the complete shear behaviour of rough joints incorporating the asperity deformation under CNS conditions. In order to validate this analytical model, a series of CNS shear tests were conducted on rough tensile (natural) joints and their replicas at a range of initial normal stresses that varied from 0.4 to 1.6 MPa. Comparisons between the predicted shear behaviour and the experimental results show close agreement.
An appropriate evaluation of the shear behaviour of rock joints is vital, for instance when analysing the stability of rock slopes, designing excavations in jointed rock, assessing the stability of concrete dam foundations, and designing rock socked piles. In conventional studies, the shear behaviour of a joint is usually investigated in the laboratory under constant normal load (CNL) boundary conditions where the normal stress remains constant and the surface of the joint dilates freely during shearing. However in engineering practice, the normal stress acting on the joint interface may vary during shearing, and the joint dilation may be constrained by the confined environment formed across the interface. This often represents a constant normal stiffness (CNS) condition. The practical implications of this are movements of unstable blocks in the roof or walls of an underground excavation, reinforced rock wedges sliding in a rock slope or foundation, and the vertical movement of rock-socketed concrete piles. Several researchers have emphasised the fact that a constant normal stiffness (CNS) boundary condition is more appropriate for many field situations [1-7].