Investigating Reasonable Scale Of Pillars For Protecting Workings Lying Under Thrust Sheets

Xinji Mine Field is geologically characterized by the occurrence that the coal-bearing formations lie directly under tow thrust sheets consisting of gneiss from Antecambrian and limestone from Cambrian respectively. The coal resource exploration carried out in this mine field resulted in a prediction that these thrust sheets should have such hydraulic characters as heterogeneous and anisotropic aquifers. In order to avoid that the groundwater stored in the thrust sheets seepages through the fractures induced in the roof strata by mining into the working panel, a coal belt with a width of 80—100m from all major coal seams was designed as protection pillar. The aims of the present work are, based on analyzing the recharging conditions of extracted panels, the distribution of the hydrogeological parameters of the thrust sheets and the maximum height of mining induced fractures in the roof of the coal seam to extract, to take the working panel 1101 of Xinji Second Mine as an example and investigate the workability to mine a part of the coal in the designed protection pillars.The working panel 1101 is designed to mine the coal seam 11-2 and located at the western part of middle mining field of Xinji Second Mine. The vertical distance between the roof and the lowest thrust sheet varies from 17.5m to 75.8m. Experiences obtained while mining coal seam 8 and coal seam 11-2 suggests it brings no negative influences upon the mining activity to scale down the designed pillar size if the stratum overlying on the roof of the panel to be mined is not strong permeable aquifer or the fractured roof strata of the panel can not be directly recharged by a strong permeable aquifer within the thrust block.On the basis of performing amount of laboratory experiments and in situ tests and summarizing documents accumulated in geological exploration phase and former mining, the spatial distribution of thrust sheets, the recharging conditions of the mined panels driven into coal seam 11 -2 and the maximum height of the mining induced fractures in roof strata were analyzed or calculated by using geostatistic nonlinear methodology, dynamic flow rate progression and three-dimensional numerical simulation respectively. The conclusion resulted from present research is it would be guaranteed to extract safely the working panel 1101 when proper dewatering measures are implemented.