Authors: Sexton, M; Mikula, PA; Lee, MF

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Sexton, M, Mikula, PA & Lee, MF 2008, 'Trident Mine Raisebore — A Bored Pile Case Study', in Y Potvin, J Carter, A Dyskin & R Jeffrey (eds), Proceedings of the First Southern Hemisphere International Rock Mechanics Symposium, Australian Centre for Geomechanics, Perth, pp. 137-148.

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Abstract:
As part of bringing their new Trident gold mine into production, Avoca Resources Ltd had to construct two ventilation shafts (three and four metre diameters) through to the surface. Several zones of extremely weathered material and damp clays were present at both proposed shaft locations, including five metres of soft clay at one of the sites. Single-pass conventional raiseboring was the most desirable method of construction (safest, quickest and cheapest), but this meant back-reaming through the weathered ground. Stability assessments suggested that the proposed raise diameters were unlikely to be suitably stable after reaming, even for a short period of time, prior to stabilising the freshly exposed weathered walls by remotely sprayed fibrecrete. Conventional raiseboring was therefore considered risky. The risk of overbreak, failure and loss of the raises was minimised by installing a ring of non-contiguous, cable-reinforced grouted mini-piles through the problem zones before back reaming. Regardless of this preparation, there was a high awareness of the risk of failure while the raises were back-reamed through the weathered zones. The weathered sections were fibrecreted as soon as the raiseboring was completed. The paper documents key aspects of the project, in particular design and operational aspects of the piles.

References:
Barton, N., Lien, R. and Lunde, J. (1974) Engineering Classification of Rock Masses for the Design of Tunnel Support, Rock Mechanics, Vol. 6, No. 4, May 1974, pp. 189–236.
McCracken, A. and Stacey, T.R. (1989) Geotechnical Risk Assessment of Large Diameter Raise-bored Shafts, Shaft Engineering Conference, Inst Min Met, pp. 309–316.
Peck, W.A. and Lee, M.F. (2008) Stability of Raise Bored Shafts in Australian Mines, 13th Australian Tunnelling Conference, Melbourne, 4–7 May.




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