Upshot of strata movement during underground mining of a thick coal seam below hilly terrain

Underpinning-based simultaneous extraction was successfully used to extract more than 90% of thick and contiguous sections of a coal seam at Chirimiri mine, lying below a hill cap with a rapid change in depth of cover and is placed above the surrounding ground level. The last few panels of the mine encountered strata control problems during the depillaring. Underground instrumentation and observations showed that the last phase depillaring of the property near the free surface of the hill slope experienced shearing of overlying strata along two incompetent layers. Field investigations revealed that large amount of lateral movement due to the hilly nature of the overlying strata is the main source of the problem. A simple two-dimensional study on a simulated model also demonstrated the occurrence of the problem during working below an unconfined rock mass of hilly nature. Different evidences were collected from the site to identify the main horizons and the direction of lateral movement and, accordingly, two different strata management plans, one for each section, were successfully executed to overcome the encountered strata control problems. Field investigations in the rest of the panels of the site showed favourable impact of the adopted strata management plan.