Experimental study on the approach to extracting high-level borehole draining gas in the fully mechanized extro-thick coal seam

This paper is to reportthe results of our experimental study on a renovated approach to extracting the high-level borehole draining gas in the case of fully mechanized extro-thick coal seam, which is regarded as the key to improving the extraction efficiency of such a case for this purpose, we have done field tests, through which we have found that it is necessary to divide the overlying strata crack evolution into six areas: the insignificant area of the roof movement, the weak roof movement area, the noticeable movement area, the badly affected area, the slightly affected area and the stable area. We have found that the ceiling of the exploiting area is about 45-55 m highwhereas that of the fissure area is 93.0-100.7 m high with the overburdened strata. Through testing and measurement, we have found that the vertical displacement is different on the coal seam roof with the roof subsidence decreasing upward along the seam floor. It has also been found that there has occurred vertical stress concentration and pressure relief in the overburdened strata, whereas the stress relief mainly appears in the separated strata. However, it would be unlikely for abscission to appear in the stress-concentrated area or outside of the coal pillar. It is for this reason that we recommend to use high-level borehole to control the gas draining in the similar conditions, and, what is more, high-level boreholes, that is, above 50 m above the seam roof, should be arranged in the place between the lower range of the caving zone and the upper range of the fractured zone. Thus, it can be concluded that high level boreholes should be laid in No. 3801 working face of Shuiliandong coal mine in a range of about 50 m from the coal seam roof, where the average gas extraction concentration should reach 42% and the average pure extraction eff-i ciency of the six boreholes should be getting to 5.9 m3/min. The above study results prove to be of great instructive significance to gas management of the working areas in similar circumstances.