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The plat doors at the surface are operated by pneumatic cylinders mounted in trunnion brackets.

The bolts securing a trunnion bracket failed. Evidence indicates there was a failure over a period of time and the bracket fell down the shaft. There was no damage in the shaft.

“Consider the installation of retaining and locating mechanisms that eliminate the risk of critical components falling down a shaft should they fail,” the MSU said in its latest Weekly Incident Summary.

In another incident a single 100 mm diameter rock fell a vertical distance of approximately 15 m onto a lower bench from rill material used to construct an open pit haul ramp windrow.

The final position of the rock was 1m short of a person who was dipping production holes but there were no injuries.

“Conduct detailed inspections and monitoring of high walls and associated windrows and bunds to identify and eliminate potential rock fall hazards,” the MSU said.

“Regular reviews of haul road and windrow construction is essential to assess compliance to design criteria.

“Consider potential rock falls when working adjacent to and below high walls. Controls such as modified work practices, stand-off distances, retention berms or physical protective barriers will mitigate or eliminate the risk.”