IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _________________________________________________________________________________________ Volume: 03 Issue: 04 | Apr-2014, Available @ http://www.ijret.org 190 LABORATORY MODELLING OF ROCK JOINTS UNDER SHEAR AND CONSTANT NORMAL LOADING G Budi 1 , K.U.M. Rao 2 , Debasis Deb 3 1 Assistant professor, Department of Mining Engineering, Indian School of Mines, Dhanbad, Jharkhand, India 2 Professor, Department of Mining Engineering, Indian Institute of Technology Kharagpur, West Bengal, India 3 Professor, Department of Mining Engineering, Indian Institute of Technology Kharagpur, West Bengal, India Abstract In this paper, the shear behaviour of artificial rock joints under constant normal loading conditions is studied. Idealised joint surfaces were prepared using a developed molding method with special mortar and shear tests were performed on these samples under CNL conditions. Different levels of normal load and shear displacement were applied on the samples to study the shear behaviour of saw- tooth shaped joints before and during considerable relative shear displacement. Nine types of saw-tooth joints have been selected for simplicity of modelling to quantify the effect of CNL conditions on joint shear behaviour. It was found that the shear strength of joints is related to rate of shear displacement, joint roughness and applied normal stress condition. Finally, based on the experimental investigations and observations made of sheared joint samples, four conceptual models of shear stress-shear displacements have been developed. These four models summarize the entire experimental results based on elastic, dilation and residual zone defined a long the shear displacement axis. The findings of this study expand current state of knowledge of joint shear strength which may be of significance for further research and for understanding the shear behaviour of rock joints for a stability analysis of the designed structures in surface and underground rock engineering. Keywords: shear strength, artificial rock joints, direct shear test, constant normal loading ----------------------------------------------------------------------***------------------------------------------------------------------------ 1. INTRODUCTION The performance of surface and underground excavations with respect to its design objectives is controlled by the stresses and displacements that develop during and after excavation under the prevailing in situ stresses, induced stresses, ground water pressure distribution and causes stress concentration around the opening. The spatial distribution of rock mass strength and deformability and especially the intersecting discontinuities such as joints, fractures, faults, and bedding planes make the stress distribution and associated deformation highly discontinuous. Being generally weak, these discontinuities will directly influence rock mass deformation and stability of the underground structures under various loading conditions [1]; [2]. The stability of underground mine excavations, tunnels, open pit slopes and others in a jointed rock mass is influenced by the shear-strength characteristics of the joints. Surface and underground openings are created for many purposes. The openings such as tunnels, large underground caverns for hydro-electric projects, underground storage facility for cryogenic purposes (storage of very low temperature material - LNG) and lastly the nuclear waste repositories (very high temperature material) have no time limit. The stability requirements are for indeed for an infinite time. Contrary to this, the underground excavations for mineral extraction have a defined life, until the complete extraction of the mineral resource. The design and construction of these engineering structures in rock mass encounter unique problems mainly due to the nature and the variability in the strength of the rock mass and also due to the pre-existing stresses in the ground. Movements along rock joints in surface or underground excavations such as underground openings, foundation of dams, tunnels, and slopes can occur in any direction, depending on kinematic constraints and the external forces (including water pressures, and forces induced by earthquakes) acting on the structure. Therefore, it is imperative to understand the variation of shear strength of rock joints for the stability analysis of these rock structures. The preliminary condition to be considered explicitly in assessing the performance of underground excavations is to study the rock joints response under static normal loading conditions. Majority of works are based on the laboratory investigations using the conventional direct shear apparatus, where the shear load is varied while keeping the normal load constant (CNL method, where in the normal stress acting on the joint interface is considered to be constant throughout the shearing process). The extensive literature has revealed that there are many aspects which influence the shear characteristics of rock joints. One of the fundamental issues is the rock joint surface roughness. Much of the earlier works of underground opening stability studies were focused on the static stability where the initial