Monitoring and analysis of ground movement from multi-seam mining

Abstract

Two surface deep-hole extensometers , each consisting of 11 anchors, were installed above a longwall panel operated approximately 70 m underneath a previously mined seam to investigate ground movement behaviour. The active longwall was 400 m wide and obliquely orientated with the previously mined longwalls. One instrument hole was drilled through the central part of an old goaf, and the other was drilled through the edge of the old goaf near a remnant pillar, representing two distinct mining regions. The interburden strata between the two worked seams at the two monitoring locations were found to undergo similar goafing processes of delamination , re-compaction and consolidation; however, the strata above the upper mined seam behaved differently. The strata above the central part of the goaf underwent re-compaction whereas the strata near the remnant pillar experienced significant shear displacement. Such strata deformation behaviour resulted in an irregular bottom of the subsidence profile over the active longwall goaf. Based on the observed average strain, the caved and highly fractured zones were estimated to be approximately 4 and 18 times the mining thickness, respectively. Possible failure scenarios of the instrument boreholes and the associated extensometer wire movement were also analysed; it is estimated that approximately 37–67% of the total monitored displacement was attributed to the shear movement of strata around the boreholes. The monitoring results were informative to optimising goaf gas drainage system by identifying gas emission sources, characterising fracture connection in the surrounding strata and assessing failure scenarios of vertical goaf wells. • Overburden movement above a multi-seam longwall was monitored with extensometers. • Longwall caving dynamics and parameters were quantified from the monitored data. • Strata above the previously mined goaf further recompacted. • 37–67% of total extensometer reading was attributed to strata lateral movement. • Gas emission sources and flow pathways were characterised from strata movement behaviour.