Analysis of the stability of shallow abandoned colliery workings

Abstract

Subsidence related to shallow abandoned mine workings are considered an inherent threat to safety, particularly of the transport infrastructure. In order to better constrain the conditions that can lead to subsidence, a study taking the form of a parametric analysis to investigate the features and properties significant in the causes of abandoned mine working instability has been undertaken within the numerical modelling code FLAC3D. The effect on stability of variations in excavation geometry along with variations of the in-situ stresses related to the overburden loading have also been investigated. Use has been made of interface elements to allow the modelling of coal measures rocks as a horizontally laminated assemblage of strata whereby discrete or discontinuous behaviour representative of the failure of layered rock masses over abandoned mine workings can be modelled. The modelling of variably dipping discontinuity features commonly present in rock masses has also been undertaken using an anisotropic constitutive model. The effects of fluctuations in the groundwater table are modelled as variations in pore water pressures on the stability of excavations have also been investigated. The modelling results indicate that the friction angle of the rock mass (which may be considered a composite of the intact rock and discontinuity friction angles) is a key parameter in controlling the geometry of failure of the strata forming the excavation roof. Pore water pressures are also shown to be significant in causing the initiation of instability. Empirical tools to allow the estimation of the height of collapse before arching or choking of the void occurs are also introduced as well as a numerical based methodology to allow the modelling of collapse propagation in rock masses above excavations. Numerical modelling was also undertaken of the subsidence event that occurred at Dolphingstone in Scotland which suggests that the increase in the ground water table may have been responsible for the collapse.