Laboratory evaluation and modelling of shear strength of infilled joints under constant normal stiffness (CNS) conditions

Infill materials found in natural rock joints may cause a reduction in joint shear strength, influencing rock mass stability. The shear strength of rock mass, already reduced by these discontinuities, will further diminish if they are filled with sediments, thereby posing significant concerns for any construction or excavation carried out in rock. These concerns invite accurate quantification of the shear strength of infilled joints and proper understanding of the basic mechanics of discontinua and the principles involved in their shear deformation. The practical application of any models developed through such studies will be of immense help to mining, tunnelling, and all other underground construction works. The geotechnical research work carried out by the University of Wollongong in the late 90’s included infilled joint modelling using hyperbolic techniques. A new shear strength model was developed in these studies for predicting unfilled and infilled joint strength based on the Fourier transform method, energy balance principle and the hyperbolic stress-strain simulation. Taking into account the field conditions frequently encountered, the diversity observed in joint shear response and the occasional inadequacy of data (for the estimation of Fourier coefficients and the hyperbolic constants), this study was undertaken to develop a semi-empirical methodology for predicting the shear strength of infilled joints. In this research study joint shear behaviour was studied under CNS and CNL conditions and also the effect of joint orientation and confinement. The study aimed to develop a methodology which includes joint surface characteristics, joint properties, and infill materials. A new model for predicting the shear strength of infilled joints based on a series of tests carried out on two types of model joint surfaces (with asperity angles of 9.50 and 18.50) is presented. Graphite, bentonite and clayey sand were 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 (ϕb) and the joint asperity angle (i) is pronounced, but diminishes with increasing t/a ratio so that the shear strength converges towards the infill alone. This decrease in shear strength with increasing t/a ratio is represented in a normalised manner by dividing the peak shear stress by the corresponding normal stress. Summation of two algebraic functions (A and B) that represent the joint and infill characteristics, correctly model the decay of normalised shear strength with increasing t/a ratio. The new model successfully describes the shear strength of the graphite, clay (bentonite) and clayey sand filled model joints