Ground Control Monitoring in Backfilled Strip Mining under the Metropolitan District: Case Study

Ground subsidence and coal gangue induced by mining activities could significantly influence the nearby environment. To allow strict public scrutiny of mining under the metropolitan district, coal gangue backfill has been introduced together with the strip mining method to minimize ground subsidence and to reduce gangue disposal in Tangshan Mine in Hebei Province, China. Field investigation was conducted to evaluate the performances of the combined mining method. The results showed that fractures were primarily generated at distances ranging from 5 m to 85 m behind the longwall face. The majority of fractures occurred within the range of 30 m in the overlying roof strata. The roof strata over the goaf became stable at a distance of 78 m behind the longwall face. The cumulative roof displacement occurred from instrument installation to roof regaining stability was 1084.2 mm. The backfill stresses experienced a slow increase, with a maximum value reaching 0.4 MPa at a distance of 97 m behind the longwall face. The maximum ground subsidence reached 46 mm, which had little impact on nearby buildings. Based upon these findings, it can be concluded that the combination of the backfill technique in strip mining can effectively reduce ground subsidence.