For a long time, mining coal under buildings, water bodies and railways ("under three" for short) mainly apply technology of strip mining in China, whose effect of ground subsidence control is good, but coal recovery is low (about 40%). As coal resources has been decreasing, whether a large of coal left in strip-pillar area can be recycled or not has become one of the most urgent and difficult technical problems. This paper studied the lows of ground movement and the characteristics of rock destruction through numerical simulation. It also analyzed problems existed in the course of the secondary mining of strip-pillar area: hidden danger for failure instability of underground strip pillar and the severe damage of surface buildings. We cite an example that the strip-pillar area in a coal mine has been designed to the secondary mining and the preliminary practice has taken good effect. So we hope it can be used to provide experience in similar conditions for the future of strip-pillar area mining.
Strip mining is a technology widely used in "under three" coal mining in china at the end of the last century. This technology designed the coal mining area to many strip shape, and then mining a strip, leaving one. It use the leaving pillars to support the overlying rocks, so as to effectively control the surface subsidence, and be beneficial to protect buildings on the ground. However, due to the low coal mining rate and large resources loss, this technology is a technology mining coal under villages in China during the special period. Because of the improved efficiency of the coal industry and the shortage of coal resources, the secondary mining of strip-pillar area is more and more noticed.
Recently there are a few researches on wide strip pillar[1~5] in China, but the secondary mining of strip-pillar area is little researched and no mining experience used, the law of rock and surface movement about it is lack. Therefore, the research on the secondary mining of strip-pillar area has theoretic and application value.
This article use the way of discrete element method to built the model of strip-pillar mining under a village. The mining coal seam is level coal seam, the thickness of mining is 3 m, the depth of mining is 550 m, using strip mining and mining four strips, mine 50 m, leave 90 m. The model will simulate to mine strip pillar, in order to study the character of surface movement and rock destruction in secondary mining of strip-pillar area. It provides the theoretical support for further the secondary mining of strip-pillar area.
Figure 1 shows the configuration of rock destruction due to strip mining by numerical simulation. After mining the strip coal seam, the immediate roof above goaf breaks down, and forms the trapezoid collapse zone whose top is narrow, underside is wide.