Assessing risk of pillar instabilities in abandoned mine workings through probabilistic modelling: A case study in the West Midlands, UK

The long-term stability of abandoned underground mine workings presents significant geotechnical challenges, particularly in relation to post-mining subsidence and associated ground movements. These processes can lead to environmental degradation, infrastructure damage, and safety hazards, posing substantial risks when repurposing undermined land. A comprehensive understanding of the geological and geotechnical properties of the rock mass, especially its time-dependent behaviour, including creep and moisture-induced softening, is crucial for assessing stability risks and informing sustainable land-use strategies. However, incorporating these complex, time-dependent mechanisms into numerical models remains computationally demanding. Existing elastic-visco-plastic constitutive models often require parameters that are difficult to obtain, limiting their practical application in engineering practice. Subsidence above abandoned mining excavations is influenced not only by structural instability of the excavated voids, but also by progressive weathering and mechanical degradation of rock materials forming the roof, floor, and pillars. These processes may result from complex, coupled thermo–hydro–chemo–mechanical (THCM) interactions that evolve over extended timescales and are inherently difficult to quantify or represent accurately within conventional engineering assessment frameworks. This study develops simplified yet practical analytical models for rapid stability assessments, particularly for early-stage risk evaluations. The assessment of abandoned mine stability is further complicated by uncertainties related to altered rock mass properties, unreliable historical records, and evolving excavation geometries. To address these challenges, we integrate analytical models with probabilistic methods, such as Monte Carlo simulations, to systematically account for variability in rock mass characteristics and mine geometries. Due to data availability, this study focuses on a case study of the Cow Pasture abandoned limestone mine (West Midlands, UK), highlighting the critical role of moisture-sensitive roof strata in the long-term stability of mine workings and illustrating a potentially plausible instability scenario. Results indicate that a combination of inadequate pillar design and the progressive softening of roof materials due to moisture infiltration likely contributed to pillar foundation failure and punching, which may have ultimately led to overburden collapse and surface subsidence. These findings underscore the critical importance of adopting probabilistic methodologies in the stability assessment of abandoned mine workings, enabling a rigorous consideration of uncertainties related to excavation conditions and inherent rock mass properties, and providing essential guidance for the development of informed risk mitigation strategies. • Subsidence mechanisms over abandoned room-and-pillar mines are investigated using a probabilistic framework and through a real case study. • Temporal deterioration of rock mass parameters, including weathering effects, is incorporated into the analyses. • Analytical models are enhanced with Monte Carlo simulations to quantify the effect of uncertainties on the stability of roof, floor, and pillars. • A practical method is proposed to account for weakening due to weathering and moisture softening for the long-term stability analysis of old mining excavations.