Mining Thick Coal Seams Under Thin Bedrock–Deformation and Failure of Overlying Strata and Alluvium

In underground coal mines, the failure of overlying strata can have disastrous effects where the working face is overlain by thin bedrock covered with thick alluvium. Roof failure under these conditions can cause a massive water and sand inrush. This paper presents a case study for a design to prevent such disasters in the Baodian mine, China. First, the engineering geological and hydrogeological conditions of the overlying lithified strata and the alluvium were obtained from field and laboratory studies. Numerical models were then built with different bedrock thicknesses using distinct-element modelling software. The deformation, failure, and subsidence of the overlying strata during simulated coal mining were studied using these computer models. Finally, the results of the model studies were combined with the geological data to design a reasonable layout for the longwall panel to be mined in the Baodian mine. Initial results showed that the alluvium was somewhat impervious and water-poor. The models showed that the first caving and weighting intervals of the roof decreased with decreasing bedrock thickness, and decreasing bedrock thickness also increased maximum subsidence of the alluvium. The maximum height of the caving zone and the minimum height of the sand-prevention coal and rock pillars were 34 m and 46 m, respectively. Knowing this allowed a somewhat shorter (204 m) but safe working face to be designed. This research provides a good background for the design of safe mines under similar conditions.