This paper presents the procedures and effects of pre-drainage of coal seam gas by long boreholes with large diameter. The investigation was carried out by onsite experimental measurements and laboratory numerical modeling. Onsite investigation results from the six roadways of Dingji mine show that the technology has dramatic effects of stress release and gas pre-drainage. For a single borehole, its effective influential radius reaches 2.3m after 7 days of gas drainage, with pre-drainage rate over 30%, residual gas contents between 2.35and 4.36m3/t and coal seam deformation over 1.1%. Boreholes were drilled according to a refined pattern of boreholes based on these parameters, which has highly increased the rate of roadway development to over 170m per month and the maximum to 212m per month. Of all routine inspections, only 4.48% indicates some degree of outburst potential after this technique is applied. The majority of inspections indicating outburst potential fall into locations where abnormal disturbances occurs by geological structures, indicating enhanced and intensified borehole drilling is necessary when development is close to geological structures.
Huainan Mining Group is a typical mining area that has seen the outburst situation in China, where there are 10 out of 13 mines with outburst hazard, with mining depths of 800–1000m. With high levels of crustal stress and gas content, combined with highly complex geological settings, problem of outbursts becomes more serious and complex. As a result, outburst accidents occurred on April 21, 2005 at Xizhuangzi mine and on January 5, 2006 at Wangfenggang mine with death tolls of 7 and 15 miners respectively. Under such situations, traditional outburst prevention and control methods can no longer give a satisfactory solution to the new problem [1], for example, using methane drainage by boreholes or blasting, the roadway development rate in most coal seams maybe as low as 20–30m per month [2]. Currently, rapid and safe roadway development is mainly dependent on drilling boreholes from roadways deployed in roof or preferably floor, but the costs of those roadways developed in advance are usually very high and unaffordable. Most importantly, the development of those roadways itself is time consuming, and the time for gas drainage cannot be guaranteed. Therefore, new methods need to be developed for secure and efficient outburst prevention and control. With the development of new drills of high performance, it is possible to drill long borehole with large diameter along coal seams, making large scale pre-drainage of coal seam gas as a promising technique for outburst elimination, which not only serves at driving headings but also within longwall mining panels.
Dingji mine is a newly constructed coal mine, and coal seam 11–2 is one of main coal seam to be mined. Its thickness is 0.44–6.05m, with average thickness of 2.49m. The mine's current mining scope lie in –790m and –900m contour lines, its gas contents are commonly in the range of 5–7m3/t, according to the data form geological exploration and underground measurement.
