Numerical Modelling of Rock Mass Stress Strain Changes Caused by Underground Excavation in the Bor Copper Mine

Abstract

Extensive investigations have brought up the fact that there are still considerable amounts of ore reserves of copper mine “Bor” (at Yugoslavia), located at the slope and under the abandoned open pit mine. This paper presents the methodology of modeling the process of inducing mass deficit, the process of caving and corresponding stress-strain changes in the surrounding rock mass. The rock mass of the investigated area is composed of: extrusive magmatic rocks, pyroclastites, volcanic sediment rocks (the Bor pelites) and conglomerates. The problem of determining parameters of a rock mass constitutive relations in a large scale has been analyzed. The finite element method (FEM) is adopted as a numerical procedure. The constitutive relations of the rock strata are modeled as elastic and plastic-brittle behaviour. The effects of strain softening around the ground openings are taken into account. Key problems of the model are caving moment criterion and size of the caved zone. Assuming that physical instability coincides with numerical divergence, the criterion of the moment of caving has been introduced as initial divergence of the solution of the stress-strain problem. The results show the development process of the zones of instability as a function of the progress of the exploitation. The proposed numerical procedure combined with in-situ investigation works has many advantages over the usual empirical procedures, and can be a valuable tool to mining engineers assessing the short and long term stability of rock mass caused by underground excavition at open pit slope.