Effects of multi-seam mining on surface subsidence at Cook Colliery

The impact of longwall operations on surface subsidence has been widely documented over the past 30 years. The mechanism of subsidence is widely understood and there are several different methods available to predict surface movements and optimise the longwall mining process. As coal reserves begin to exhaust the need to exploit multiple seam deposits is becoming increasingly important. This research project aims to identify the impacts of multiseam mining on surface subsidence. This will be achieved by modelling the subsidence profile of an extracted longwall panel that underlies existing bord and pillar workings. Numerical modelling has been undertaken based on the finite element method (FEM) and hybrid finite element method-discrete element method (FEM/DEM) using the Elfen software package. A number of models were generated for the investigation, namely, elastic, plastic and a FEM/DEM fracture model. The influence of inter-burden thickness on surface subsidence was also analysed by altering the distance between each coal seam. The vertical displacement and subsidence profile were the basis of the model comparison. The predicted subsidence for each of the respective models was compared to the measured site subsidence. This provided an indication of model performance and directed recommendations for the improvement of numerical modelling as a predictive technique. A discussion of the capabilities and limitations of numerical modelling was presented while  outlining its importance in addressing the growing complexities of mining geometries.