Assessing the effects of rock mass gradual deterioration on the long-term stability of abandoned mine workings and the mechanisms of post-mining subsidence – A case study of Castle Fields mine

Abstract

Post-mining subsidence is a serious problem in urban areas constructed above abandoned mine workings. Subsidence may severely damage the environment and threaten the stability of surface structures and infrastructure. This study aims at analysing the role of gradual weakening of rocks on chimney collapse and sinkhole subsidence formation, in shallow mine workings. The factors leading to the degradation of the rock masses mechanical parameters are explained. The long-term stability of mine workings at Castle Fields mine, in West Midland, UK, is investigated as a case study. Progressive chimney collapse resulted in a large sinkhole in this area, and damaged the Dudley cricket ground, in 1987. The modified Tributary Area Method, TAM, is used to assess the effects of deterioration on the long-term loading of pillars. The long-term strength of pillars is also assessed based on empirical and theoretical approaches. Then, three-dimensional (3D) numerical modelling is used to assess the role of deterioration on the mechanisms of chimney failure and sinkhole development. A deteriorating model is proposed to incorporate the effects of gradual deterioration of rock mass mechanical parameters. Results of this study indicate that gradual deterioration due to weathering plays a significant role in the propagation of cavity and the formation of sinkholes above abandoned mine workings. Ignoring the gradual softening and the effects of deterioration, in the stability analysis of mine workings, may result in catastrophic consequences.