Edelbro, C, Brummer, R, Pierce, M, Sandstrom, D & Sjoberg, J 2019, 'Raiseboring in difficult rock conditions', in J Hadjigeorgiou & M Hudyma (eds), Proceedings of the Ninth International Symposium on Ground Support in Mining and Underground Construction
, Australian Centre for Geomechanics, Perth, pp. 185-198.
Virtually all long ventilation shafts and orepasses in the Boliden mines in Sweden are excavated using raiseboring. With increasing mining depths and hence, increased virgin rock stresses, it has become difficult to keep the shafts intact after boring and until reinforcement or other stabilising measures have been implemented.
This paper contains a review of literature and case studies of raiseboring in difficult rock conditions. The focus is on design, pre-reinforcement, excavation methods, and final outcomes. The cases range from long ventilation raises to short stope slots. The data was used to further develop raisebore design charts relating to the ratio of maximum tangential stress and uniaxial compressive strength (UCS) to raisebore length and diameter. This enabled establishing limits for initiation of failure and problem-free boring that can be achieved using current technology.
For problematic shafts, remedial measures such as different types of pre-reinforcement or lining the raise while pulling out the muck are presented. Good experiences using rings of reinforced piles to allow raiseboring through the deep weathered surface were available from Australia. However, there were no case studies with pre-reinforcement in rock conditions similar to those of the Boliden mines. Nevertheless, vertically drilled and pre-reinforced perimeter holes around the eventual raisebore diameter is one suggested possible action. Based on industry experience, strategies to be considered for raiseboring in difficult rock condition are also presented.
Keywords: raise, shaft, pre-reinforcement