In October 2018, a working pit floor subsided into a backfilled stope, forming a 13 metre deep sinkhole. The stope had previously self-mined to near surface and had been backfilled in stages since 2016. At the time of the incident, surface drill and blast activities were occurring in the vicinity. The subsidence event resulted in the loss of an integrated tool carrier and an explosives truck that were parked on the blast pattern. A number of charged blast holes were also engulfed in the sinkhole.
The vehicles were unoccupied at the time of collapse; however, four persons were working in close proximity.
Direct causes
- At the time of subsidence, the backfilled underground workings were not treated as a void allowing work to be undertaken above an area of unknown stability.
Contributory causes
- A risk assessment in December 2017 identified the stope void as a hazard; however,
- recommended controls were not adhered to.
- The 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.
- Inadequate change management associated with the decision to move to normal operational practices.
- The exact cause of subsidence was undetermined due to lack of access, records and uncertainty about the void shape and fill material.