1 Copyright © 1997 Elsevier Science Ltd Int. J. Rock Mech. & Min. Sci. Vol. 34, No. 3-4, 1997 ISSN 0148-9062 To cite this paper: Int. J. Rock Mech. & Min. Sci. 34:3-4, Paper No. 130 FRACTAL ANALYSIS OF SHEAR JOINT ROUGHNESS T. Belem; F. Homand-Etienne; M. Souley Laboratoire Environnement Géomécanique & Ouvrages, Ecole Nationale Supérieure de Géologie, B.P. 40, Vandoeuvre-lès-Nancy Cedex, F-54 501, France ABSTRACT The morphology of mortar replicas of two natural joints (a granite and a schist) made from the silicone rubber casts was characterized using a laser sensor profilograph to create three-dimensional topographic maps before and after each shear test. Three successive direct constant normal load shear tests under three normal stress levels have been carried out on the joint replicas: σ n = 0.4, 0.8, 1.2MPa and σ n = 0.4, 0.8, 1.4MPa for the schist and the granite joint replicas, respectively. Based on geostatistical methods, the variation in surface roughness in the course of shear tests is described using the fractal dimension D ƒ computation. A roughness parameter SR s and a degradation parameter D w were also defined and computed. SR s and D w quantify respectively the degree of joint surfaces roughness and the degree of degradation of sheared joint surfaces. The variation of D ƒ as a function of σ n have been compared to the variation of SR s as a function of σ n in the course of shear tests. Results indicate a good correlation between D ƒ and SR s . Copyright © 1997 Elsevier Science Ltd KEYWORDS Fractals • Geostatistics • Rock Joints • Roughness • Shear Tests • Surface Degradation INTRODUCTION The mechanical properties of rock masses in the upper part of the earth crust are strongly dependent on the presence of rock discontinuities (cracks, joints, schistosity planes, faults, etc.). The frictional forces produced during shear displacement on these contacting surfaces affect the state of stress in the upper part of the earth crust. Discontinuities in rock masses by their capability to modify the thermo-hydro- mechanical characteristics of rock masses, influence the stability of rock engineering structures which have become a field of interest in relation to radioactive waste disposal projects. Most of the laboratory test results show that joint roughness is of paramount importance in the mechanical behavior of rock joints. In particular joint roughness has a fundamental influence on the development of dilatancy. Consequently, the shear strength of joint during relative shear displacement is joint roughness dependent (Patton, 1966; Ladanyi & Archambault, 1969; Barton & Choubey, 1977). Furthermore, it is well demonstrated that the joint surface roughness decreases with the accumulation of shear displacements due to the failure of asperities, and this is in turn called surface roughness degradation. In order to develop realistic constitutive models of rock joints, numerous experimental and numerical studies have been allowed to characterize the surface morphology of discontinuities and to relate the surface morphology parameters to their thermo-hydro-mechanical properties. A wide variety of techniques