Overburden failure and the prevention of water and sand inrush during coal mining under thin bedrock

Coal mining under thin bedrock or thick unconsolidated soil layers brings mining problems related to these special geological conditions. The meaning of the term “thin bedrock” is defined through the thickness statistics of the coal seam and the bedrock layer. The coal-bearing strata having thick, unconsolidated aquifers and thin bedrock located at the Taiping Coal Mine in Shandong province were taken as a geological prototype for subsequent study. The geological, hydro-geological and engineering characteristics of the thin bedrock were analyzed. An engineering geological model was than established. Overburden failure and the development of “Three Zones” were studied by physical model tests. The rupture pattern and rock failure were analyzed for mining conditions under thin bedrock. The height of the caving zone and the freely flowing water fractured zone of different mining thicknesses were separately calculated. The results show that a mining thickness greater than 3.5 m causes the height of the freely flowing water fractured zone to be sufficient to touch the weathered zone and the bottom of the Quaternary system aquifer, to various degrees. This, then, would lead to water and sand inrush into the working face. Measures to prevent water and sand flow inrush disasters by eliminating the power source are put foreword. A field dewatering scheme was designed and observational data were obtained. The dewatering project had an obvious effect and the water level at working face number 8309 dropped to a safe level. The average draw down of the groundwater was observed to be 7.86 m. This showed that the dewatering project played a role in decreasing the hydraulic pressure and ensuring safety mining.