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Abstract:
Coalfields in North China encompassing than ten Provinces contain six to seven coal seams in the Permo-Carboniferous strata. The lower three seams, accounting for 37% of the total reserves , are threatened with karst water from the underlain Ordovician limestone. Hundreds of water inrush incidences have occurred in which a large amount of water suddenly flows into tunnels or working faces under high potentiometric pressure 20 years. Large-scale dewatering or depressurizing of the karst aquifer was considered essential to water inrushes and keep the mines safely operational. This practice has caused sinkholes, dry gs, water supply shortage, and groundwater Keywords Geological barrier contamination in the surrounding areas, which is environmentally not permitted. One of the alternative water control measures is to make full use of the layer between the coal seam and the karst er as a geological barrier. Similar to the application in the nuclear industry where a geological barrier of this application is considered a hydraulic barrier as well with the objective to prevent or constrain water flow from the underlying aquifer into mines. Its effectiveness to constrain water flow is described by a parameter referred to as hydrofracturing pressure (P-hf) When the water pressure in the underlying aquifer exceeds P-hf, a wedging effect takes place within the fractures of the geological barrier and, as a result, water inrush occurs. In-situ hydrofracturing tests were used to determine P-hf in bauxite and silty sandstone at tunnels. The P-hf in the silty sandstone is larger than that in the bauxite but they both vary with depth (distance from the bottom of the tunnel). Based on the test results, a new safety criterion for water inrush was derived for mines and it has been successfully applied to mining practices with the minium effort of dewatering in the karst aquifer. The same criterion can also be applied to tunneling and quarrying in areas with similar geological conditions