Techniques for prevention and control of Ordovician limestone water disasters based on modified water inrush coefficient method

There are technical problems in preventing and controlling Ordovician limestone water disasters during mining above ultra-thin aquifuges, such as those of the lower coal group in Weibei Coalfield. At Sangshuping Coal Mine, horizontal directional drilling technology was used to develop water prevention and control ideas of Ordovician limestone top utilization and grouting transformation. Firstly, considering the loss of water resistance in the floor failure zone, the water inrush coefficient method was used to determine the critical thickness of Ordovician limestone top utilization and grouting transformation. The directional drilling exploration was carried out by drilling from the two opposite ends of a mining face. Combined with the results of borehole water pressure tests, the borehole grouting technology and parameters were determined, and then the grouting effect was analyzed. Secondly, three-dimensional seismic and ground transient electromagnetic testing were used to detect low-resistivity abnormal areas and structural development in the mining area. The direct current and transient electromagnetic methods were used to detect water-rich areas in front of a roadway. After the roadway system was formed, DC electric sounding and audio-frequency electric methods were used to explore the water-rich areas under the roadway and the floor of the mining face. A radio wave method was used to detect the structure of the mining face and changes in coal thickness. Finally, according to the geophysical and directional borehole exploration results, inspection technology was implemented to evaluate the feasibility of mining under pressure comprehensively. The study shows that the top of the Ordovician limestone was not water-rich and could be used as a relative aquifuge. The water inrush coefficient was reduced to below 0.073 MPa/m, which provides a solid basis for further improving the estimation of critical water inrush coefficients in Detailed Rules for Water Disaster Prevention and Control of Coal Mines. This will ensure the safe mining of the working face and forms a prevention and control system of Ordovician limestone water disasters comprising a modified water inrush coefficient estimation method combined with an “exploration-grouting-inspection” approach. It extends the lower limit of safe mining in the mining area.