Statistics were used and analysed in order to determine the overburden movement by using nine borehole extensometers and twenty-seven monitoring anchors. The numerical computer model was established to simulate movement of overburden in shallow coal seam with great panel width, through which the movement features of overlying strata were observed in different face advancing interval. It is proved that fracture zone and curve subsidence zone can be precisely distinguished and the separating deformation between two zones can clearly be seen. The overburden movement continued for three weeks and the mining influencing range of overburden varied from 330 metres to 420 metres. The separation between strata with different hardness has often been formed in the overburden movement.
Longwall mining method is a highly productive underground mining method in which a panel or a block of coal is completely extracted When a longwall panel of sufficient width and length is excavated [1, 2], the overburden roof strata are disturbed in order of severity from the immediate roof toward the surface, or even the aquifers [3, 4], which can lead to serious mine-flooding accidents and increased damages of the ecological environments [5, 6]. In order to describe the specific overburden strata movement, a numerical model was established to simulate the process of mining by using the FLAC3D.
2. MINING CONDITION OF STUDY AREA
The longwall panel studied in this paper is located in the Appalachia Coalfield, United States. The longwall panels were Panels B5 and B6. (Fig. 1). The overburden depth varied from 600 ft to 900 ft.(180-270m). The average mining height was 7 ft(2m), Panel Length was 12,000 ft(3600m) for panel B5 and 5,700 ft(1430m) for panel B6. The panel width for both panels B5 and B6 was 1,433 ft(435m). The headgate and tailgate entry width was 16 ft (5m). The chain pillar system between Panels B5 and B6 was 200 ft(60m) wide. The average longwall face retreat rate was 30-50 ft/day(9-15m/day) during the longwall face mining under the study area.