ABSTRACT:

The paper presents a case study for the application of the numerical method for stability assessment of the crown pillar in Chelopech underground mine. During the study a 3D model of the pillar was created by using specialized mining software Map3D, which is based on the Boundary Element Method. This model represents the sequence of extracting the crown pillar in 26 mining steps by applying the Long Hole Open Stope Mining Method with a backfill. The modeling was aimed at performing a detailed stress-strain state analysis of the crown pillar and the open stopes in it, detecting overstressed zones in which a rock mass failure can occur. Based upon this study, a comparative discussion for the application of Hoek-Brown and Mohr-Coulomb failure criteria with regard to the stress state of the rock mass has been performed.

1 INTRODUCTION

Numerical modeling plays a great role in the design and assessment of the rock mass behavior. The numerical modeling gives us an opportunity for detailed evaluation and analysis of various mining and engineering problems. The purpose of this research is to study and analyze the behavior and conditions of a part of the rock mass in Chelopech mine influenced by the mining works. To realize this study a 3D model of the crown pillar and the host rocks has been developed. This model is used for both evaluation of the general rock mass condition of the pillar before and during the mining works, as well as for assessing the behavior of the host rock mass. The factors that influence the rock mass stability and their mutual effect on the condition of the excavations and open stopes have been considered in the modeling. The result from the numerical analysis is based on the normal and shearing stress state distribution data, as well as the direction and magnitude of the rock mass displacements and deformations. The reliability of the model's prediction has been discussed. The extraction stages and zones of the pillar together with significant changes of the rock mass behavior and the parameters having an impact on its stability have been observed.

2 MATHEMATICAL MODEL FOR PERFORMING THE STABILITY ANALYSIS

For this research, a numerical model of the rock mass with the program MAP3D has been developed. This method is an attempt to mathematically simulate the way the rock mass responds to mining (Wiles 2005). This is done by re-creating the rock mass conditions such as the compression of the rock mass formed by the overburden and tectonic forces in the model. The geometry of the rock mass is modelled with a high level of accuracy. This gives us the opportunity for a reliable evaluation of direction and magnitude of induced stresses and their redistribution above the rock mass, which has to be extracted. In the model, the rock mass is seen as an elastic medium. This means that in general the deformations, if relatively small in magnitude, are mostly recoverable (Wiles 2005). Despite this condition, the obtained results from the modeling show the zones in which the induced stresses can exceed the strength of the rock mass and a failure can occur. Numerical modeling achieves this simulation by using certain physical constraints on how the rock mass can respond. Map3D is based on the very efficient Indirect Boundary Element Method and incorporates simultaneous use of both fictitious force and displacement discontinuity elements (Wiles 2005).

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