A numerical study of stress changes in barrier pillars and a border area in a longwall coal mine

Abstract In longwall mining, stope voids are filled by caved-in rock materials. Cave-in and fracturing of roof strata cause severe disturbance to and stress changes in the host rocks. The study site is the Svea Nord coal mine. Longwall mining method was implemented to extract coal at the study site. It is intended that the coal in the border area on one side of the longwall panels will be mined after completion of the longwall mining. There is a concern about how the longwall mining affects the stress state in the border area and how the stress changes would affect future mining in the border area. 3D numerical modelling was conducted to consider the above concerns. Two models were constructed in the study. The first one is a local model that includes only two panels to study how the stresses in the barrier pillars are changed during panel mining. The second one is a mine-scale global model that includes all the panels with the aim of studying the disturbance to the border area by the longwall mining. The simulations show that the stresses in the barrier pillars fluctuate up and down during mining because of periodic cave-in events behind the longwall face. A failure zone of about 12 m exists in the wall of the barrier pillar. A large portion of the barrier pillar is still intact and is thus capable of protecting the border area. A disturbed zone of about 20 m is found to have developed in the wall of the main tunnels on the side of the border area after all the longwall panels have been mined out. The stress state in the remaining portion of the border area remains unchanged. Therefore, by considering a pillar with width of 25–35 m into the border area along the access tunnels, it will be possible to mine the rest of the border area with room-and-pillar method in the future.