The appropriate assessment of the strength of a jointed rock mass is a fundamental requirement for the successful design of structures built in or on rock. Due to the complexity of the rock mass, a large number of empirical methods are developed, for its estimation. Its analytical treatment has been tackled mainly by the plane of weakness theory. In this study, an extended plane of weakness theory is applied to a well documented jointed rock physical model. For the failure mechanisms observed in the experiment, the non linear strength envelope provided by this extended plane of weakness theory fits well to the experimental data.
Rock mass strength estimation belongs to the problems that are too complex to be tackled easily with analytical methods. Complexity is mainly due to fracturing, anisotropy inhomogeneity, and the variety of pertinent modes of failure. Therefore, empirical correlations, that do not need theoretical treatment, are usually employed. They come from the systematic observation of the factors that affect the strength. Analytical methods for the estimation of jointed rock strength have been developed, for the case of sparsely jointed rock masses. The most widely known analytical method for such a purpose is the plane of weakness theory, presented by Jaeger [1]. This method has been extended in order to take into account the roughness of joint surfaces. This extended method is presented and then, both the original and extended theories of weakness plane are applied to a jointed rock physical model.
The original theory of weakness plane (Jaeger, [1]) allows for the analytical evaluation of the anisotropic or equivalent isotropic strength of jointed rock. This theory uses the Mohr - Coulomb failure criterion for the joints. However, this linear criterion is unable to describe adequately the shearing behaviour of rock joints, which are rarely smooth, and their strength is a non linear function of the existing normal stress. Experiments ondiscontinuities have shown that for low normal stresses, the surface roughness causes expansion with shear movement, while for higher normal stresses there is failure of asperities and suppression of any expansion.
Various researchers, such as for example, Goldstein et al. [5], Hayashi [6], Lama [7], Brown [8], Einstein and Hirschfield [9], experimented on artificial specimens made by blocks, usually of plaster, arranged in such a way to form a jointed structure. By this procedure, the jointed structure of the specimen is considered to represent the rock mass. For low values of confining stress, shear failures of joints or in intact material, were observed. For higher lateral stresses, the failure was due to the formation of many almost parallel shear planes, mainly within the intact material. The strength of the specimens depended on joint dip, except from the case of high lateral pressures, where the strength of the specimen was nearly equal to that of the intact material. The behaviour of specimens was brittle for low lateral pressures and ductile for higher.
