Collingwood Park – Mine Subsidence

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Mueller College – Perfoming Arts Centre
February 12, 2015
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Collingwood Park – Mine Subsidence

NJA Consulting undertook one of our largest individual investigations for the Queensland Department of Mines in relation to a high profile mine subsidence event that occurred in April 2008. Part of the Ipswich suburb of Collingwood Park is underlain by two decommissioned underground coal mines – Westfalen Number 3 and New Redbank Collieries. These mines used the “bord and pillar” method of mining at depths from approximately 60m to 140m. Bord and pillar mining at the collieries created numerous underground pillars ranging up to 11m in height in some areas of Westfalen Number 3, and with various dimensions and shapes left behind to support the load from the rock above the mine after coal was extracted.

A individual pillar may fail, triggering overload of neighbouring pillars, leading to roof failure and caving of rock above pillars into the previously excavated roadways or mine voids, and ultimately ground subsidence which may penetrate to the surface. In December 1988, a subsidence event occurred near Lawrie Drive, Milgate Street and Rush Court in Collingwood Park. In April 2008, a second subsidence event occurred, resulting in surface movement and damage to houses within an area near the intersections of Duncan, McInnerney, McLaughlin and Moloney Streets. Both subsidence events occurred as a result of pillar failure within the Westfalen No. 3 Colliery.

Subsequent to the 2008 subsidence event NJA Consulting structural and forensic engineering staff undertook detailed surveys and building inspections to over 400 individual houses. The damage to each dwelling varied in accordance with the position of the dwelling relative to the epicentre of the subsidence event. Houses at the epicentre, where the slope of the supporting ground had effectively remained unchanged, suffered very little damage. House towards the out edge of the subsidence event, where the slope of the supporting ground was badly affected, suffered the most damage. The most badly affected houses suffered differential movement in excess of 200mm over the footprint of the building. These differential movements resulted in both tilting and curvature to the floor of the dwellings. As a result of the ground subsidence significant damage in the form of cracking and other related defects were observed. In some cases crack widths of up to 25 mm were observed. NJA provided detailed investigation reports to the Department of Mines. Each report provided specific information in relation to each dwelling including the extent of damage and the recommended future course of action for the subject dwelling.