The Shear strength properties of rock discontinuities depend upon whether they are clean and closed, or open and filled with various infill materials. The most obvious effect of a filling material is to separate the discontinuity walls and reduce the rock-torock contact, which influences the joint shear strength. The aim of this paper is to investigate the influence of infill materials on the shear strength of natural rock joints. For this purpose, laboratory tests have been carried out in constant normal load (CNL) conditions and the output of the analyses has been conducted to propose an empirical criterion for prediction of the shear strength of the natural infilled rock joints. Finally, this empirical criterion has been validated based on the experimental method. The results show that this criterion can perform the predictions with an acceptable confidence level.
Rock mass is characterized by joints, fractures and other planes of weakness that reduce the shear strength [1,2]. When an excavation is carried out, primary rock movements take place along the existing joints due to stress relief and associated stress re-distribution. Therefore, it is important to quantify the shear strength of discontinuities in the design and construction of surface and underground rock structures as well as in mining operations. Over many years, fine sediments resulting from weathering and other surface processes could subsequently ingress to rock joints, reducing the overall shear strength of the joint surface. The jointed rock mass often fails due to these infilled joints because they are often the weakest planes initiating sliding . Despite their frequent natural occurrence, filled discontinuities have been studied much less systematically, perhaps because of the difficulties arising from the increased number of variable parameters. The most important effect of filling material is to separate the discontinuity walls and thereby reduce rock-rock contact, but shear strength will also be influenced by the nature of the filling material itself the characteristics of the wall-fill interfaces. Because of the lack of reliable and realistic theoretical or empirical relations and the difficulties in obtaining and testing representative samples, engineers generally rely on judgment, often considering the shear strength of the infill itself to be conservative. In critical cases, in situ tests may be carried out to provide site specific design criteria, but invariably amount of testing that can be undertaken precludes the establishment of fundamental relations. Besides, all of the experimental studies performed in the literature, have used modeled joints or replicas for numerous testing programs. In this paper, a comprehensive statistical analysis has been applied on a series of data obtained from comprehensive test program on natural infilled rock joints. The output of this analysis has been conducted to propose an empirical criterion for prediction of the shear strength of the natural infilled rock joints.