Evaluation of mechanisms contributing to valley closure subsidence effects under irregular topographic conditions

Valley closure subsidence has been observed for decades in Australia and overseas where underground extractions have occurred beneath or in close proximity to valleys and other forms of irregular topographies. Valley closure is referred to as the inward movements of the valley sides towards the valley centreline. Upsidence is the reduction in subsidence or relative upward movement at the bottom of the valley. Due to the complexity of the local geology and interplay between several geological, topographical and mining factors, the underlying mechanisms that actually cause this behaviour are not fully understood. Numerical investigations are conducted to investigate stress related failure mechanisms that may contribute the observed valley closure subsidence. Numerical models are developed using the Distinct Element Code, UDEC and 3DEC, based on assessments of the geological conditions in the Southern Coalfield, New South Wales, Australia. The numerical models are then systematically calibrated and validated against field observations and empirical incremental profiles. The positive correlations indicate that the models are capable of replicating the features of mining induced subsidence and horizontal movements. The calibrated models are subsequently extended to develop several hypothetical scenarios (two-dimensional and three-dimensional) to quantify the effects of the geological, topographical and mining factors on valley closure subsidence and their inter-relationships. The numerical modelling predictions of valley closure subsidence are consistent with field observations. The results of this research indicate that the major influences on valley closure subsidence are longwall locations with respect to valley and the horizontal compressive stress. A stress arching concept is proposed and the redistribution of the horizontal compressive stress within the valley results in a strong pushing effect on valley walls, leading to pronounced bedding plane shearing that contributes to greater closure values. The roles of valley geometric factors, depth of cover and geological features around valley have also been quantitatively identified in this study. The results have applications in the study of both the underlying mechanisms that lead to this non-conventional subsidence behaviour, and how these should be incorporated in future valley closure subsidence prediction and mitigation of its impacts on natural features