In October 2019, two workers at an underground mine were approaching a paste retaining wall during paste filling, when the wall catastrophically failed. An inrush of fluidised paste entered the drive inundating the workers, who escaped by climbing up the wall mesh.
One of the workers called in the emergency on a handheld radio. They were rescued approximately 15 minutes later by climbing into the bucket of a bogger.
Direct causes
- The wall failed due to excessive hydraulic pressure being exerted from the paste fill.
- The workers were inside an exclusion zone at the time of the incident.
Contributory causes
- A paste wall was constructed in a drive at the top of the stope with the paste fill pipe through the wall. This fill method was used, as there was a blind portion of the stope above the drive and a continuation of the stope underneath the drive to the level below.
- The stope was blocked with substantial rock falls from the backs and walls, which prevented a cavity monitoring survey being carried out.
- The rock falls prevented the paste from percolating into the stope and this led to a build-up of hydraulic pressure behind the paste wall.
- No pressure monitoring devices or cameras were in place to assess the fill conditions from a remote location.