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NSW Resources Regulator investigation report: collapse of shaft collar

Collapse of shaft collar
The blind boring drill rig operated by Australian Shaft Drilling Pty Ltd partially sunk in the collapsed shaft

The NSW Resources Regulator has published a causal investigation report into the catastrophic failure of an underground bulkhead seal that occurred at North Wambo Underground Coal Mine on 3 May 2020.

The report stated: A catastrophic failure of an underground bulkhead seal occurred at North Wambo Underground Coal Mine on 3 May 2020 at 8.15am.

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 The bulkhead seal location after the catastrophic failure
The bulkhead seal location after the catastrophic failure

This caused a rapid loss of drilling mud from a blind bore drilling shaft. An engineered underground bulkhead seal was installed directly beneath the blind bore drilling project, where the shaft was to intersect the roadway underground. The purpose of this bulkhead seal was to confine the drilling mud to within the shaft column for the duration of the drilling project, and to prevent interaction with the underground workings.

Within minutes of the drilling mud escaping from the shaft, the alluvial materials close to the surface began to collapse into the shaft void, due to the rapid loss of confinement pressure. A sinkhole rapidly developed and the drill rig personnel moved to a place of safety, before both the drilling rig and constructed drilling pad partially descended into the sinkhole.

Subsequently, an underground inspection was undertaken by mining supervisors, who confirmed that there was clear evidence the bulkhead had catastrophically failed, and that no further source of ground instability was identified.

 Looking inbye from the bulkhead seal site following the catastrophic failure
Looking inbye from the bulkhead seal site following the catastrophic failure

The mine had foreseen the risk of inrush and had documented this in its risk assessment. The risk control measure introduced to eliminate or minimise the risk was an inrush control zone underground while work was being undertaken to sink the shaft. The inrush control zone prevented workers from being exposed to the fatal hazard that ultimately occurred.

The key findings from the investigation were:

  • The incident occurred because of the catastrophic failure of the bulkhead seal.
  • The bulkhead seal had not attained design strength at the time of the failure. This was determined by a sampling program coordinated by the mine.
  • The risk assessments that were conducted for the project had not considered the impact of the rapid loss of confinement pressure created by the rapid loss of drilling mud on the stability of the upper section of the shaft being drilled.
  • The mine had deviated from its original plan of blind boring the shaft into solid coal, and had relocated the planned position of the shaft due to both underground and surface considerations.
  • Inadequate change management procedures were implemented when it was identified that the bulkhead had not attained its designed strength.

NSW Resources Regulator recommendations:

Mine operators and persons conducting a business or undertaking (PCBUs) have a duty to identify hazards and manage risks to health and safety associated with the operation of plant and equipment in accordance with the Work Health and Safety Act 2011 and the Work Health and Safety (Mines and Petroleum Sites) Act 2013 and Regulations.

It is recommended that mine operators, contract service providers and equipment suppliers implement strategies to:

  • eliminate or minimise risks associated with the rapid loss of drilling mud during blind boring activities
  • identify all foreseeable risks and nominate adequate controls during the planning phase of engineering projects
  • ensure that when deficiencies are identified mid-project, robust change management procedures are activated and additional controls implemented as appropriate.

Mine operators should also read the Post incident actions section in the report as it provides additional detail about what the mine has done, and intends to do in response to this incident.

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