Fine-Tuning Raisebore Stability Assessments and Risk

CIM Montreal 2011
Benjamin Matthew Coombes, Warren Arthur Peck, Max Frank Lee,
Abstract In Australia, raiseboring is the preferred option for constructing ventilation shafts. The trend has been to larger diameter, longer single-pass shafts reamed through to the surface, if possible. Hence the risk of failure due to latent ground conditions has been increasing, as has the need for more reliable raise stability assessments.
The McCracken and Stacey (1989) method of assessing geotechnical risk for large-diameter raisebored shafts by assigning a Raise Stability Ratio (RSR) has been the basis for most Australian pre-construction raisebore feasibility studies for the past 20 years. McCracken and Stacey (M&S) provided a chart that quantifies geotechnical risk in terms of proposed shaft diameter and a raisebore rock mass quality index Qr.
Peck and Lee (2007) published an initial Australian database of raise performance for a range of geological settings including porphyry copper, high grade metamorphic and sediment hosted base metal deposits. This paper examines raise diameter and ground conditions versus raise performance for 50 cases. When RSR = 3.0, M&S considered that there was a probability of failure of 25%. The Australian database suggests that, for an RSR of 2.7 to 3.5, 25% of raises have, in fact, failed. A further 25% displayed significant overbreak, 8% required support such as remotely sprayed fibrecrete and 42% were stable.
This paper investigates the reasons for this variable raise performance for similar RSR values. For all raises, three key geotechnical parameters, RQD/Jn, Jr/Ja and Jw/SRF were investigated to assess their impact on raise performance. Seven representative case histories are presented to illustrate the papers conclusions.
Keywords: Stability, Raisebore, Risk, geotechnical
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