Document Type

Conference Paper

Publication Date


Publication Details

Luke Clarkson, Effect of Punch Longwall Retreat on Highwall Stability, in Naj Aziz and Bob Kininmonth (eds.), Proceedings of the 16th Coal Operators' Conference, Mining Engineering, University of Wollongong, 10-12 February 2016, 509-521.


Punch longwall mining takes advantage of the final highwall exposure in an open cut coal mine, driving gateroads straight into the targeted coal seam then retreating the longwall back to just short of the highwall. A designed barrier pillar remains between the final longwall position and the highwall, which is subject to redistributed ground stresses. Material is strained to magnitudes unlike those typically measured in open-cut mining, and unlike typical longwall ground behaviour, the highwall was observed to strain in an opposite direction to the longwall caved and subsidence zone. Mine personnel and equipment may become exposed to the unstable highwall rockfall hazards, highlighting the importance of understanding the mechanism and implementing appropriate controls. This paper describes the assessments undertaken through radar, survey prism and Light Detecting and Ranging (LiDAR) monitoring, as well as geotechnical inspection and analysis. Initial results show highwall movement is directly correlated with longwall goafing and any delayed ground movements are more related to the rate of retreat, the type of goafing behaviour and the influence of strata deteriorating in other locations along the face of the highwall. This paper also describes climatic conditions as the primary limitation with radar monitoring. In addition, diurnal steel mesh movement was measured with changing atmospheric temperature. Steel mesh is usually installed as ground support above portal entries to the underground workings. By filtering out any measured mesh movement, true trends of strata deformation in these areas can be identified. Recommendations are made for efficient and reliable radar data acquisition. Further recommendations are made to restrict people and plant access to a safe standoff from the highwall as the longwall approaches the final retreat position within the panel. Effective monitoring of highwall performance throughout the longwall retreat to establish stable trends will enable continued, safe access to these locations.