Shear Behaviour of Rock Joints Under CNL and CNS Boundary Conditions

Shear behaviour of rock joints can be studied under both constant normal load (CNL) and constant normal stiffness (CNS) boundary condition. CNS condition is suitable for non planar and reinforced rock joints whereas CNL condition is suitable for planar and non reinforced rock joints. In the present study shear behaviour of modelled rock joints with different asperity have been experimentally investigated under both CNL and CNS boundary conditions. Test results indicate that CNS boundary conditions gives higher shear strength as compare to CNL boundary condition when other parameter of testing is kept same. But, this effect tends to diminishes with increase in normal stress on the shearing plane and at high normal stress both CNL and CNS boundary conditions gives same shear strength. A new shear strength model is proposed for both the boundary condition and the proposed model is validated by comparing the predicted shear strength with present experimental results and results available in the literature for natural and artificial rock joints with different asperity and roughness. The new study and model will be useful for safe and economical design of underground openings in jointed rocks, stability analysis of rock slopes, design of foundation on rock and design of rock socketed piles.