Judgement criteria with utilization and grouting reconstruction of top Ordovician limestone and floor damage depth in North China coal field

As the mining depth of North China coal field increases year by year,most of the mines have been faced with the stage of coal mining from the upper coal group to the deep and lower coal groups. The threat of karst water disaster in the Ordovician limestone is on the rise,and the floor water inrush hazard increases year by year. However, although the floor water inrush coefficient method in the current code and regulation is widely used,it does not fully take into account the actual water blocking capacity of the Ordovician limestone top,and its evaluation method has some limitations. Therefore,based on the existing understanding of the geological and hydrogeological conditions on the top of Ordovician limestone,this paper puts forward the idea that the weathered backfill zone with good water resist- ance at the top of Ordovician limestone in some mining areas can be used as the aquifuge. The thickness of coal meas- ure aquifuge,the thickness of weathered backfill zone,the thickness of grouting reconstruction section and the destruc- tion depth of bottom plate disturbance are considered as the influencing factors of the discriminant of bottom plate wa- ter inrush coefficient,and the criteria for judging the thickness of the reconstruction of the strata section at the top of Ordovician limestone are established. The importance of the failure depth of the bottom plate in the discriminant crite- rion is discussed. And taking the mining of No. 11 coal seam in Sangshuping Coal Mine of Hancheng Mining Area as a typical case,through the 17 sets of numerical simulation schemes constructed,it is analyzed and concluded that the depth of coal seam mining has the greatest influence on the destruction depth of bottom plate disturbance,the mining thickness is the smallest,and the mining width has size effect;at the same time,combining the simulated data with the measured samples of similar mining areas in China,the prediction model of the influence of mining depth, mining width and mining height is given,and the verification calculation and applicability analysis are carried out. The results show that the error mean value,the percentage,variance and mean square deviation of the new model fitting equation are superior to the statistical formula,and have the characteristics of high prediction accuracy,small error and wide ap- plicability,which basically meets the practical needs of engineering. The research results can be used in the practical project of grouting in the water-bearing section of the Ordovician limestone top,and provide favorable technical guaran- tee for seat belt pressure mining of deep lower group coal.