Fundamentals of rock joint deformation

This paper describes laboratory investigations of the deformation characteristics of rock joints under normal and shear loading. Normal deformability was studied by conducting loading/unloading and repeated load cycling tests on a wide variety of fresh and weathered joints in five different rock types. The data invariably showed non-linear behaviour, irrespective of the rock and joint type. A hyperbolic function is suggested to describe the stress-closure/opening curves of joints. Quantitative relations between normal deformability and relevant joint parameters (aperture, wall strength and roughness) are developed. Tentative conclusions on the changes in normal stiffness during shearing are also presented. The behaviour of dislocated (mismatching) joints is studied qualitatively and analytically. Shear deformability was studied by performing direct shear tests under normal stresses in the range of engineering interest. It is shown that behaviout in the pre-peak range is invariably non-linear depending on the joint type, and can be adequately described by easily measured parameters and hyperbolic functions.