SURFACE SUBSIDENCE FROM UNDERGROUND COAL MINING IMPACTING RESIDENTIAL HOUSING: A CASE STUDY OF RISK ANALYSIS, MITIGATION PROPOSAL AND ONGOING MONITORING

Two subsidence events twenty years apart resulted in damage and destruction of residential housing near Ipswich in the state of Queensland, Australia. Led by the Australian governments, Commonwealth Scientific and Industrial Research Organisation, a research program was undertaken to determine the cause of subsidence, identify areas at risk, propose a stabilising technology and monitor the site. Site investigation including surface to void drilling, three-dimensional seismic survey, evaluation of historical mining data and interviews with ex-mine site personnel identified the most likely cause of subsidence as the over-stressing and failure of inadequately sized remanent coal pillars. It was concluded that water from the ongoing flooding of workings may have impacted pillar stability. A factor-of-safety evaluation of over 1,100 remanent coal pillars together with risk analysis of future surface subsidence was undertaken and identified another panel of the abandoned colliery that placed housing at unacceptable risk of future damage. An evaluation of bulk backfill identified a mitigation technology to ameliorate that risk. Continuous monitoring for over seven years by an instrument array of geophones, extensometers and piezometers has evaluated and reported the stability of strata overlying the colliery. This paper describes (1) the novel technique developed for evaluation of risk of surface subsidence for many hundreds of coal pillars accounting for the unique spatial and geometric attributes of every individual pillar. Included in the risk analysis study is water and time impacts on each pillars Factor of Safety (FoS); (2) mitigation technology developed for the stabilisation of pillars; and (3) the results of over seven years of strata monitoring.