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With the current trend of exploiting mines at increasing depths comes the challenge of managing increasingly high stress conditions. One tool at the disposal of mine engineers and designers is the numerical simulation of the stresses and deformations likely to be encountered ahead of the mining fronts for various mining scenarios. Although the benefits of numerical modelling are by now well accepted throughout the industry, the applicability of the various types of numerical modelling approaches is not always well understood and still the subject of much debate. This paper endeavours to explain the basic differences between the elastic and inelastic approaches, as well as provide practical guidelines concerning which one to use for various sets of circumstances. Case studies of high stress situations are provided that show where the simpler elastic approach was adequate, and where implementing an inelastic approach was unavoidable. Some insight is also provided into the type of practical information that can be extracted from advanced failure analyses (the exclusive domain of inelastic techniques), such as determining the degree of failure of a rock mass by examining the stress state of its failed zones, which indicates their position along the strain-softening post-peak response typical of hard rock masses.

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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.

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