Surface subsidence due to abandoned coal mines in NSW

John Johnston

Subsidence due to abandoned coal mine workings is a significant hazard to surface infrastructure in the Coalfields of NSW. This is due predominantly to large areas of land, initially exploited for the extraction of coal, being progressively developed following extraction of available coal reserves. As a result of the extensive utilisation of the bord and pillar mining method, an ongoing potential for surface subsidence due to both a progressive roof collapse (forming a sinkhole) or a failure of the pillar system (forming a trough) remains decades or even centuries following the cessation of mining.

The compilation of a useable or complete databases containing measured subsidence effects from which future predictions can be made has been historically challenging, resulting in a large variety of engineering assessment methods used to estimate both likelihood of occurrence and the potential surface effects of a failure, generally based on rules of thumb or through the modification of methods developed for use in mine planning.  This presentation covers two recently developed empirically derived assessment methods intended for use in the assessment of subsidence risk over abandoned coal mines. Both methods rely on large databases of historical pillar failures and sinkhole occurrences within the Newcastle, Tomago, Greta, Hunter and Lithgow Coal Measures of New South Wales.

About the speaker

John Johnston Senior Mine Safety Officer (Subsidence), Department of Regional NSW

John graduated from the University of Newcastle with a Bachelor’s degree in Civil Engineering in 2013 and is employed at the NSW Resource Regulator in the Mine Safety Inspectorates Subsidence Engineering section. Beginning his career as a consulting geotechnical engineer in investigation, design and construction, he joined Subsidence Advisory in 2017, where he spent three years as a Senior Engineer and four years as Engineering Manager, predominantly focused on the assessment and remediation of abandoned mine workings. He has recently returned to Newcastle University to complete a research degree focused on unplanned subsidence under the supervision of A/Professor Klaus Thoeni and Dr Stephen Fityus.

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