Stability problems associated with an abandoned ironstone mine

The stability problems associated with a shallow, room and pillar ironstone mine have been analysed in detail using established, empirical approaches. Initial analysis centred on elastic beam theory and pillar strength formulae to establish the factor of safety against roof beam and pillar failure, respectively. The pillars have been shown to be inherently stable; even those that have been subsequently reduced in size as a result of pillar splitting have safety factors in excess of 3. The roof has been shown to be unstable using an elastic beam analogy, which is corroborated by the extensive surface damage. Confidence in the application of such approaches is shown to be difficult due to the lack of knowledge relating to the effects of time on mine stability; increased weathering of the mine structures may lead to weakening and ultimately failure at lower stresses than determined through the mine design approaches. Clearer definition of the rock mass behaviour is required in order to advance such techniques as valid approaches for the analysis of abandoned mines. However, the paper also highlights how such approaches are valid where failure mechanisms are well defined and understood. Probabilistic risk analysis has also been considered as an alternative approach. From a surface developer's perspective, such approaches are more useful.