Experimental investigations and modelling of shearing of calcite healed discontinuities of granodiorite under typical stresses

The hydromechanical properties of the rock masses in the upper part of the Earth's crust are strongly governed by the presence of rock discontinuities. For this reason, experimental investigations have been made on calcite-healed discontinuities of a granodiorite massif located in the center of France where an engineering project is planned. Mechanical laboratory tests of the joints have been performed using a new 3-D direct shear box (BCR3D) designed for high quality measurements. The performances planned and those realised in the thermo-hydromechanical field by the BCR3D are presented. The natural joints are of various shapes and thicknesses as shown by the thin sections made on the intact material. Their behaviour depends on the thickness and on the mineralogy of the infill material, which is mainly calcite, and of a lesser extent chlorite. Normal cyclic loading and shearing (at constant normal stress and at prescribed normal stiffness) have been applied at low shearing rate. The initial normal stress ranges from 5 to 20 MPa. At low normal stress, the cyclic behaviour classically shows an asymmetry of the mobilised shear stress that is supposed to be linked with a mechanical anisotropy and exhibits a small normal relative displacement, alternatively contraction and dilation. At high initial normal stress, a strong damage of the joint itself and of the neighbouring rock matrix takes place during the shearing. Thin sections show the development of new fractures slightly inclined on the existing one and the reactivation of pre-existing thin cracks. At the macroscopic scale, the relative amplitude of the peak of shear stress observed during the first shearing decreases with the stress level, especially for very thickly filled joints. Subsequently, only a strong contraction of the joint is observed in this case. Two comments referring to the interpretation of the tests on rock joints are made, The first one deals with the mean plane of the joint for a non-planar joint and how a test should be interpreted, taking into account a possible inclination of the mean plane of the joint on the plane of shearing. The second is related to the edge effects during the test of a rock joint sample. The second part of the study deals with some simulations of the experimental tests using a mechanical constitutive model that is able to reproduce some observations of the joints loaded by normal stress and shearing. The model is not presented in details. It is based on the incremental non-linear framework and has several hierarchical versions of complexity depending on the number of allowed constitutive parameters. Several preliminary model tests are presented, highlighting the influence of these parameters and the features modelled as a function of the complexity of the model. (C) 2002 Elsevier Science B.V. All rights reserved.