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Fine-Tuning Raise Bore Stability Assessments and Risk


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Author W A Peck, B Coombes and M F Lee


Raise boring is the preferred option for constructing ventilation shafts in Australia. The trend has been towards larger diameters and longer single-pass shafts, and if at all possible, reaming through to the surface. The risk of raise failures, due to missed, unforseen or latent ground conditions has therefore been increasing, along with the need for better, more confident stability assessments.

While the stability of specific weak zones and structures must be assessed individually, empirical rock mass classification schemes have been successfully used for about the last 20 years to assess general raise stability; eg the McCracken and Stacey method (M&S).

Unfortunately M&S, as originally documented in McCracken and Stacey (1989), did not include a database of raise diameter and rock mass conditions versus actual performance. Nor was there a procedure for assessing the risk of failure due to adverse ground conditions. Peck and Lee (2008) published an Australian database of raise bore performance. Raise diameter and ground conditions, in terms of a lower bound Qr value, were presented for 50 cases versus raise performance.

This paper updates the 2008 raise performance plot with more
and SRF-corrected data. McCracken and Stacey (1989) had previously recommended
that additional geotechnical data should be acquired and analysed when the
proposed raise plotted on the unstable side of raisebore stability ratio (RSR) = 1.3, but the Australian data used in this study also suggests that all sites with Qr <0.3 require additional scrutiny.

Contributions and sensitivities of various rock mass parameters (RQD, Jn, Ja, Jr, Jw, SRF, time-dependent weathering/deterioration) to raise stability and risk of failure are discussed. Numerous case studies are used to illustrate issues with stability assessments.

As raises can now be
remotely supported using sprayed fibrecrete, questions of time-dependency are
also discussed. In recent times, contiguous piles, ‘mini-piles’ and grouting
have also been successfully used to presupport or otherwise improve the
stability of near-surface sections of weathered, or poor quality rock masses.

Peck, W A,
Coombes, B and Lee, M F, 2011. Fine-tuning raise bore
stability assessments and risk, in Proceedings 11th AusIMM Underground Operators’

pp 215-226 (The Australasian Institute of Mining and
Metallurgy: Melbourne).