Background Information
Xstrata Copper – Mount Isa Mines commenced development of the N3500 ore body in April 2005, in preparation for work to begin on the extraction of ore resources in the near future. With operations expected to occur at depths of up to 2km, the N3500 area of Xtrata Copper - Mount Isa Mines will be the deepest mine in Australia.
Figure 1 shows a general overview of existing underground stopes in the S3500 orebody. The left-most heading (designated *) is in a very similar rock mass domain to that anticipated to be encountered in areas of the N3500 and is of particular interest. The high talc content in the rock lends itself towards weaker rock mass strength. Geological and geotechnical data indicates that the habit of the talc mineralogy in high talc areas is present on both structural surfaces as well as in the matrix of the rock. Preliminary excavation into the talc-rich rock has shown signs of significant large strain deformation and fracture which may be difficult to manage. This is displayed graphically in Figure 1 by the barrelling of the development heading in the last cut fired at the face, which is more oval in shape than the lower talc areas further away from the face.
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| Figure 1: Pictorial of stopes at Xstrata Copper – Mount Isa Mines S3500 ore-body extraction. The general direction of the N3500 orebody is shown. The figures on the right show a cross-cutting drive, which exhibited significant large strain deformation (change from square profile to oval shape) due presumably to increasing Talc mineralogy in the rock. |
The Project
The general aim of this project was to demonstrate the applicability of large strain deformation finite element procedures to modelling and developing a better understand of the plasticity of talcose rock at significant depths, such as those encountered in the N3500 orebody. This would hopefully result in more efficient and safe mining at extreme depths and improve the competitiveness of Queensland's mining industry. Particular aims of the project were to build models with inclusive stratigraphy and appropriate constitutive behaviour, in order to reproduce observable deformation behaviour in the competent rock and the incompetent talcose rock structures, and to demonstrate the effect of rock bolting strategies on deformation control.
Researchers at JCU were supplied rock specimen test data for competent (defined to have a low proportion of talc mineralogy) and incompetent rock samples (defined to have a critical proportion of talc mineralogy) by Xstrata Copper - Mt Isa Mines. As the rock samples were destroyed in the original laboratory tests, it was necessary for the researchers at JCU to develop computational models of the tests, and given the test data supplied, they were able to reverse analyse the pertinent rock properties relevant to multi-fracturing phenomena.
These properties were used to model the elasto-plastic large strain deformation of the Development Head in competent and incompetent rock. The excavation sequence through competent and incompetent rock was then modelled, in order to predict the global deformation behaviour. This modelling included elasto-plasticity and multi-fracturing. In particular it looked at assessing the deformation on circular and rectangular tunnel headings in response to excavation and geostatic stress conditions.
The research team also looked at the performance of stabilisation strategies such as retainment and reinforcement techniques in controlling the deformation that occurs in excavated tunnels areas of weak rock mass. In particular they looked at the effectiveness of rockbolting and concrete reinforcement. Their results suggested that rockbolts alone are insufficient to control excessive tunnel deformation. It was suggested that a suitably designed reinforced concrete support would be beneficial in stabilising a tunnel excavated in the poor rock conditions.
The results of this study were diffused to rock mechanics engineers, geologists and other Xstrata Copper technical services staff in Mt Isa.
Participants
Professor Jeff Loughran
School of Engineering, James Cook University
Industry Participant
Mr Nick Slade
Rock Mechanics Superintendent, Xstrata Copper – Mount Isa Mines
Reports
Final Report - October 2004 (1.64
MB PDF)
Project Proposal - June
2004 (1.18 MB PDF)