A natural joint wall surface is an irregular rough non planar surface. The non planar surface consists of an heterogeneous distribution of asperities with various sizes and shapes over a flat smooth surface. Is the shear behavior dependent on the joint wall morphology? In order to give an answer to this question, we describe, analyze and compare joint wall morphology of sheared joints that were strictly identical before testing. Identical replicas of a natural fracture in a granite were molded from an original sample and shearing tests were performed under various normal stresses and stopped at defined shear displacements. During shear testing shear displacement rate was constant and the joint mean plane was kept horizontal. The analysis of the morphology of the joint wall sheared surface consists in the identification of the damaged areas using an image analyzer. The evolution of the size and location of the damaged areas are analyzed in relation with the normal stress for a given shear displacement.
Literature on rock joint mechanical behavior and their applications to various workings stability analysis is quite abundant over the last fifteen years; testing and modeling of rock joints become more and more important (Stephanson & Jing, 1995). Few works have been dedicated to the progressive degradation of the joint surface morphology during shear displacement and its influence on stress-strain-dilatancy. These mechanical parameters characterizing rock joint behavior depend on the contact areas between the joint walls, their morphology and the magnitude of the normal stress acting on them. The knowledge of the evolution of the joint wall morphology is a prerequisite to estimate and evaluate the parameters that we need for modeling their mechanical behavior. Thus this paper is a contribution to the characterization of damaged zones created during shearing experiments with various normal stress and shear displacements. This characterization is based on the acquisition of grey level images of the joints surfaces; after that, a region based segmentation enables to identify the damaged zones and to generate binary images. Finally measurements of geometrical characteristics of the damaged zones are performed.
A series of identical replicas of a natural fracture in a granite (Guéret, France) was realized using a cement mortar. The original sample of the fracture from which replicas were molded was drilled perpendicular to the fracture plane. The upper parts of the replicas were grey dyed while the lower parts were stained pink (Figure 1). Then, each part of the replicas was adjusted carefully in a steel box, to ensure their mutual position and orientation. The method of fabrication allows to create a great number of replicas so it is possible to perform shearing experiments keeping constant some parameters and varying the others. In this paper we present the evolution of the degradation of the joint surface morphology when increasing both the normal stress and the shear displacement, the rate of shearing and the direction of shearing remaining unchanged.
