Fracturing above longwall panels and the associated changes in hydraulic conductivity

Information from conditions observed when mining overlying seams have been combined with interpretations of surface subsidence surveys and physical models to develop a fracture model for subsidence above longwall panels. This model proposes zones of high hydraulic conductivity around the sides of collapsed goaf as an alternative to the existing layer models. In the absence of a thick spanning unit, fracturing can extend upwards from the longwall extraction to a height equal to twice the width of each longwall panel. An indication of whether fracturing has extended to the surface is if the relative deflection across the extraction exceeds about 0.4-0.6m. A simple method is proposed to estimate hydraulic conductivity values from measured vertical subsidence and the spacing of joints opened by the subsidence. The method produces hydraulic conductivity values that are consistent with reports of mine water inflows and impacts on shallow aquifers.