【文章內(nèi)容簡介】 of the coal seam. The position of the hole opening was located at the top of the drilling field and the final hole position ended m into the roof of the C13 coal seam. Its design is shown in . The total length of the draining holes was 8879 m,of which 1460 m was drilled through the C13 coal seam. The space between holes was 10 m in the non pressure relief area.5 Pressure relief levelAfter mining the protective layer, this layer moved and deformed, which caused ground stress to reduce,the permeability coefficient to increase and pressure relief occurred. Pressure relief levels are related to the space of the layer, rock lithology, mining height and so on. The levels are higher and the pressure relief effect is better in the protected layer. The change of ground stress can visibly reflect the pressure relief level, but the stress is hard to measure in the field, so we adopted the extent of coal deformation and the coal seam permeability coefficient to reflect the level indirectly. Relative deformationWe adopted a base point method to investigate the deformation of the C13 coal seam. To start, we drilled a deep hole, and then installed test points in the roof and floor of the C13 coal seam, where the deformation was determined by the relative displacement of the two test points. The measured results are shown in . We can see that, during mining of the B11 coal seam, the C13 coal seam was first pressed, and then expanded, the maximum pression deformation was 27 mm and the maximum expansion deformation mm, so that the maximum relative pression deformation was %, and the maximum relative expanding deformation %. The bigger the expansion deformation, the better the effect of pressure relief. Variation of permeability coefficientThe orieinal coal seam nermeabilitv coefficient was only ().After mining the B11 coal seam, the pressure was relieved from the C13 coal seam and the permeability coefficient clearly increased. By using the amounts of residual gas and the borehole gas inrush, we calculated that the permeability coefficient had increased to m2(), an increase of almost 2880 times, which indicated that the C13 coal seam was now in the condition where gas has been drained.6 Drainage effect of pressure relief gas Amount and rate of gas drainage1) Analysis of drainage of pressure relief gasWhen the protective working face advanced 40 m over the suction drilling field, the amount of gas drained rapidly increased. shows the variation in the amount of gas drainage in the drilling yard of the Huainan coal mine. The original 20 days was the period of increased gas drainage, enhanced pressure releasing activity and the amount of gas drainage increased. The period between 2080 days was the active period of gas drainage, the pressure release activity became regular and permeability reached a maximum. Gas drainage was regular and the amount of gas drained from a single hole was over m3/min. After day 80 the attenuation period set in, where the coal seam became gradually pacted, permeability reduced, the residual pressure became small and the amount of gas drainage reduced exponentially. According to this research, a highly effective drainage period was 2 months, the length of the affected area was 160 m and the number of active drainage holes was 16. On average, the amount of gas drained from a single hole was about m3/min Aftter 4 months of continuous gas drainage ,the rate of gas drainage reached over 60%.2) Total amount of gas drainage and amount of residual gasThe total amount of gas drainage includes the amount drained from penetration boreholes and wind emission in the floor roadway. The period of working face 21213 for which statistics were obtained was from , 2000 to Jan. 31, 2001, 365 days in accumulated amount of gas drainage by penetralion boreholes was and the amount by wind emission Mm3.The reserves of the protected layer were Mm3 and the total amount of gas drainage ,so that the rate of gas drainage reached 60%. The amount of gas inrush and amount of gas flowing into workout areas of the protective layer were not inccluded in our calculation, so that the ac