A bulking model to simulate stope convergence in deep tabular excavations

Abstract

This paper investigates the use of a novel time-dependent bulking model in a displacement discontinuity code to simulate the convergence between the roof and the floor of deep tabular stopes. This study is important as the current layout design criteria that are based on elastic theory have become outdated. This approach ignores rock failure and fails to predict the actual magnitude and the time-dependent nature of the stope convergence. This has implications for support design and for estimating the stress acting on remnants in mature mines. A rock mass modulus reduction strategy, often used to circumvent this problem, reflects the expected lower effective rock modulus for near-stope fractured material, but must be applied for the entire rock mass region in the displacement discontinuity solution procedure for tabular layout problems. As an alternative, it is proposed in this paper that the rock bulking can be expressed as a function of the elastic closure by using a constitutive rule for the reef-normal compressive reaction stress that is expressed as a function of the compaction strain. The proposed model was tested using convergence data collected from a shaft pillar extraction in a deep gold mine. The bulking model, in conjunction with a limit equilibrium model to simulate the face crushing, seems capable of replicating the underground behaviour. As the stresses on remnants and pillars can be more accurately simulated, this approach holds promise of improving current design criteria. Calibration of the model nevertheless remains a challenge and a programme of routine convergence measurements will have to be implemented on the mines.