Evolution of the superimposed mining induced stress-fissure field under extracting of close distance coal seam group

As the superimposition mining of the short distance coal seam group,the strata between get repeatedly per- turbation and destruction. Therefore the evolution characteristics of the superimposed mining induced stress-fissure field were different from the ones of the single coal seam. In order to obtain the evolution characteristics,the similar simulation was adopted in this passage. The study show that:the high stress relief length of the No. 3+4 coal was about 105 m and it continued to decrease. However,Its stress extent was 12 MPa which was 1. 5 times of the No. 2 coal seam and reached much higher degree. The stress relief extent of the survey line in the floor of No. 3+4 coal seam was still higher than the No. 2 coal seam,and the stress in the coal wall nearby the goaf dropped obviously,especially the No. 2 coal seam was mined,which the extent reached 30-40 m. Meanwhile,the extent dropped to 20-30 m,when the No. 3+4 coal was mined. The sum of the fissure increases apparently,and the frequency of the fissure character come to step growth. This passage proposed a model for the differentiation based on the characters of the mining induced stress-fis- sure field in the coal seam floor and the passage also give a conclusion about the evolution characters under the key time nodes,which provided experimental support for the judgement of the best gas drainage time.