Discontinuities such as fault planes, joints and bedding planes in a rock mass may be filled with different types of fine-grained material that are either transported or accumulated as gouge due to weathering or joint shearing . Filling materials are of the most important geotechnical parameters of discontinuities that have great effect on the shear strength of the joints. This research tries to find a logical relation between uniaxial compressive strength (UCS) of the filling materials with shear strength of the filled discontinuities. For this purpose, joints are made artificially in laboratory scale and connected to each other with different combination of gypsum & clay mortar as the filling materials in dry condition.
According to the conducted tests and analysis of the relevant diagrams, it is concluded that joints with higher UCS of filling materials have higher shear strength so that they have greater value in cohesion (C) and smaller value in friction angel (Φ).
Generally rock masses present in nature are characterized by discontinuities such as joints, fractures and other planes of weakness. Discontinuities that are infilled with fine-grained material which is either transported or appears as a result of weathering or joint shearing, will adversely affect the behaviour of the rock mass . These fine infill materials may drastically reduce the shear strength of the rock joints compared to an unfilled or clean joint, because they may prevent the walls of the rock joint from coming into contact during shear. Hence, the investigation of shear behavior of the joints is of prime importance .
The shear strength of a filled joint is often assumed to be that of the infill material alone, if the infill thickness is higher than a certain critical value. for smaller values of infill thickness, the rock-to-rock contact influence becomes increasingly prominent.
In this study, a series of laboratory test carried out on artificial and idealized models of rock joints in order to determine the relation between the uniaxial compressive strength of the filling material with cohesion and frictional properties of joints using Mohr-Coulomb criteria. The tests carried out in dry condition and joints have no roughness (smooth joints).