Infill materials found in natural rock joints may cause a reduction in joint shear strength, influencing rock mass stability. This paper reports a study aimed at developing a semi-empirical methodology for predicting the shear
strength of infilled joints, taking into account joint surface characteristics and the properties of the joint and infill materials. A new model for predicting the shear strength of infilled joints is presented, on the basis of a series of tests carried out on two types of model joint surface having asperity angles of 9.58 and 18.58, with graphite and bentonite used as infill materials. All tests were carried out in a large-scale shear apparatus under constant normal stiffness (CNS) conditions. The results indicate that at low infill thickness to asperity height ratio (t/a), the combined effect of the basic friction angle (
b) and the joint asperity angle (i) is pronounced, but it diminishes with increasing t/a ratio so that the shear strength converges towards that of the infill alone. Summation of two algebraic functions (A and B) that represent the joint and infill characteristics correctly models
the decay of normalized shear strength with increasing t/a ratio. The new model successfully describes the observed shear strengths of the graphite and clay (bentonite) filled model joints.