One of the primary objectives of this work is to improve the understanding of the frictional behaviour of rough rock joints under shear loads, and to relate its shear strength to the "shape" of the joint interface (roughness). Discontinuities have, indeed, an important influence on the deformation al behaviour of rock systems. The choice of a general criterion to determine the shear strength of rough rock joints is a problem that has been investigated for many years. Numerous shear models have been proposed to relate shear-strength to measurable joint parameters, but their limitations have to be recognized. The main problem is how to measure and quantify the roughness in order to introduce the morphological aspect of the joint into a shear strength criterion. The first part of this work focuses on the measurement and description of how roughness influences the size and distribution of contact areas during shearing. It has been found that the variation of the contact area can be expressed as a function of the local dip of the surface, measured along the shear direction. The close agreement between this empirical description of the potential contact area and surface measurements permits one to predict the real contact area involved in the phenomenon. In the second part of the work, a new shear strength criterion is proposed to model the shear resistance of joints under constant normal load conditions. It is based on the proposed empirical estimation of rock joint roughness, and on the results from more than rift)' constant-normal-load direct-shear tests performed on replicas of tensile joints and on induced tensile fractures for seven rock types. The proposed model is able to describe experimental shear tests conducted in the laboratory, and the required parameters can be easily measured through standard laboratory tests.
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