The Western Australian Department of Mines, Industry Regulation and Safety (DMIRS) have commenced a safety investigation into the event which caused a large sinkhole to engulf mine vehicles at an undisclosed WA mine site in October 2018.
In their ‘significant’ incident report, the DMIRS states that drill and blast activities were occurring nearby when a working pit floor subsided into a backfilled stope, forming the 13-metre-deep sinkhole.
The report does not state where the event occurred but explains that the stope had previously self-mined to near surface and had been backfilled in stages since 2016.
The event resulted in the loss of an integrated tool carrier and an explosives truck that was parked on the blast pattern. The vehicles were unoccupied at the time of the collapse, but four people were working nearby and watched the event unfold. Several charged blast holes were also engulfed in the sinkhole.
The DMIRS believes that the event was primarily caused by backfilled underground workings not being treated as a void, in which work was undertaken above an area of unknown stability.
The DMIRS also identified several contributing causes to the event including a risk assessment in December 2017 which identified the stope void as a hazard, yet the recommended controls were not adhered to and a decision was made in July 2018 to move from strict void management protocols to standard mining practices on the (incorrect) assumption that the pre-2018 backfilled section of the stope void was tight-filled.
While managing mining above underground workings, the DMIRS suggests that risk conditions within backfilled stopes be appropriately assessed and for managers to factor in the potential for change over time.
The DMIRS’ investigation is ongoing and any information is based upon a Significant Incident Report which was approved for release on 11 January 2019.