The correct evaluation of shear behaviour of rock mass is important in the design of excavations in rocks, stability analysis of rock slopes, anchored rock slopes and design of rock socketed piles. The shear behavior of rock joints greatly influence by the boundary conditions. Constant normal load (CNL) boundary condition is suitable only for planar rock joints whereas for non planar rock joint Constant normal stiffness (CNS) boundary condition is suitable. Most of the equipment developed in the past, study the shear behavior under CNL boundary condition. Hence, a universal large scale direct shear testing machine for rock is developed and fabricated by authors to study the influence of boundary condition. To study the shear behavior of the rock joints a series of tests were conducted under CNL and CNS boundary conditions at optimum shearing rate, on the non planar modeled rock joint. The test results for the non planar joint indicates that CNS boundary conditions yield a higher peak shear stress than that under CNL boundary condition for the same initial normal stress. A detailed account of deformation behaviour is discussed in this study.
The correct evaluation of shear behaviour of rock mass is important in the design of excavations in rocks, stability analysis of rock slopes, anchored rock slopes and design of rock socketed piles. There are various factors which influence the shear behavior of rock out of that boundary condition is most important (1). The shear behavior of the planar rock joint is different than that of the non planar rock joint and their influence can be well understand by studying the shear behavior under the constant normal load (CNL) and constant normal stiffness (CNS) boundary condition. The shear behaviour of planar rock joints can be investigated in the laboratory by using a conventional direct shear apparatus where the normal load is kept constant (CNL) during the shearing process. This particular mode of shearing is suitable for situations where the surrounding rock freely allows the joint to shear without restricting the dilation, thereby keeping normal stress constant during shearing process. Shear testing under a constant normal load (CNL) boundary condition is only beneficial for cases such as non-reinforced rock slopes. However, for non planar discontinuities, shearing results in dilation as one asperity overrides another, and if the surrounding rock mass is unable to deform sufficiently, then an inevitable increase in the normal stress occurs during shearing. Therefore, shearing of rough joints under such circumstances no longer takes place under constant normal load (CNL), but rather under variable normal load where stiffness of the surrounding rock mass plays an important role in the shear behaviour. This particular mode of shearing is called as shearing under constant normal stiffness (CNS) boundary conditions. For underground opening or rock anchor-reinforced slopes, shear tests under CNL condition are not appropriate. A more representative behaviour of joints would be achieved if the shear tests were carried out under boundary conditions of constant normal stiffness (CNS).