Improvement of squeezing ground prediction and monitoring capabilities for Mount Isa Mines Northern 3500 orebody

Abstract

This paper investigates readily incorporable improvements to the identification of squeezing ground conditions and the determination of the depth of movement for Mount Isa Copper Operations’ Northern 3500 (MICO N3500) orebody. Numerical modelling using the boundary element method (BEM) indicated that the current use of elastic material properties to calculate the rockwall condition factor and predict tunnel squeezing is appropriate, as no apparent benefit was gained by employing elastoplastic material properties or including the tunnel void geometry. Further analysis using the finite element method indicated that complex, large-scale models would be required to make meaningful improvement on the elastic BEM model. However, due to large time and computational overheads such models are not practical within the site’s engineering workflow. In conjunction with numerical modelling, a prototype monitoring device was also developed to address the problem of rigid instrument shear and inspection camera obstruction when determining the depth of movement in a squeezing rockmass. The reverse downhole probe device will allow distance measurement and a live video feed to be returned to the excavation surface by taking advantage of stable ground beyond the excavation zone of influence and using a thin, flexible cable to transmit power and data.

Keywords:

• Squeezing ground
• Tunnel convergence
• Deep hard rock mining
• Instrumentation

Acknowledgements

The authors would like to thank the MICO technical services team, particularly Geoffrey Potgieter and Cornelius Moller, rock mechanics engineers and geotechnicians for their various contributions to the project. The FEM-DEM package Elfen was made available by Rockfield Software Limited for use in this study, and this is greatly appreciated. The intellectual property contained in this paper concerning the prototype device is covered by Australian provisional patent application number 2014903801.