Mechanical mechanism of overlying strata breaking and development of fractured zone during close-distance coal seam group mining

This study mainly investigates the mechanical mechanism of overlying strata breaking and the development of fractured zones during close-distance coal seam group mining in the Gaojialiang coal mine. First, a mechanical model for the second “activation” of broken overlying strata is established, and the related mechanical “activation” conditions are obtained. A recursive formula for calculating the separation distance of overlying strata is deduced. Second, a height determining method for predicting the height of fractured zones during close-distance coal seam group mining is proposed based on two values, namely, the separation distance and ultimate subsidence value of overlying strata. This method is applied to calculate the fractured zone heights in nos. 20107 and 20307 mining faces. The calculated results are almost equal to the field observation results. Third, a modified formula for calculating the height of a water-flowing fractured zone is proposed. A comparison of the calculated and observed results shows that the errors are small. The height determining method and modified formula not only build a theoretical foundation for water conservation mining at the Gaojialiang coal mine, but also provide a reference for estimating the height of water-flowing fractured zones in other coal mines with similar conditions.