An Underground Inrush Incident

Two sub-contractor underground miners were bogging out vent rise reamer cuttings.

Bogger

They sensed something was wrong and started walking away from the reamer stockpile area and back down the drive.

Both personnel were hit by a large inrush of water, mud, reamer cuttings and flung into parked machinery, resulting in serious injuries to both  employees.

The inrush occurred when a volume of water trapped in a ventilation rise overcame a blockage in the rise and suddenly flowed down into the working area. The stockpile of reamer cuttings located under ventilation rise was being removed by a bogger  just before the incident.

The blockage in the ventilation rise was caused by the stockpile of cuttings from reaming building up in the working area and closing off the ventilation rise brow and acting like a plug.

Water build up in the ventilation rise came predominantly from a two hundred litre/minute aquifer encountered during reaming and clean-up.

Bogging of the area stockpile was done in unventilated conditions immediately prior to the incident.

Immediate Causes

  • The volume of reamer cuttings and wash down material and the volumetric capacity of the  stockpile were not reconciled allowing cuttings to build up on the stockpile, closing off the ventilation rise.
  • The decision to regulate the ventilation through ventilation rise by “cracking the brow” was not defined and subject to misinterpretation. The term cracking the brow implies that the brow may have been closed and is “cracked” by the bogger taking away sufficient material to allow some ventilation flow through the area cuddy
  • The failure to recognise warning signals of no water flow from ventilation rise and no ventilation in the bogger working area
  • Failure to determine the brow was open

Contributing Factors

  • Failure to follow the risk assessment actions  which required  reconciliation of  the volume of reamer cuttings produced to volume of cuttings removed by bogging
  • The risk assessment did not take into account the requirements of ventilation in  the bogger working area level prior to and after the rise breakthrough
  • The risk assessment assumed that an open brow meant that ventilation was open and did not allow for the fact that the operator may have bogged out a closed brow at some stage – thus giving the appearance of an open brow but leaving a hang up further up ventilation rise.
  • It was assumed the brow would be left open at all times and there was no other positive test in place to assess if this was true other than visual observation of the brow.
  • Acceptance by the bogger operator and shift superintendent to work in unventilated headings and failure to spot warning signals prior to starting work.
  •  Inadequate inspections of the brow to determine if it was always opened sufficiently to avoid a hang up in ventilation rise
  • Shift boss not conducting his statutory role as shift superintendent gave direct orders to the workforce.
  • Shift boss not ensuring the bogger operator was aware of the hazards of bogging vertical development work
  • Poor visibility in the cuddly – very humid/misty atmosphere

Root Causes

  • Inadequate coordination of ventilation  activities and failing to follow the risk assessment requirements
  • Inadequate checking by the principal (Mine Operator) that the mining contractor was following the requirements of the risk assessments and associated procedures
  • No re-evaluation of  the risk assessment after introducing the requirement to “Crack the Brow” to control ventilation 
  • Misunderstood roles and  accountabilities of Contractor and the Mine Operator shift superintendents
  • Significant errors of judgement by made experienced personnel
  •  Review process not in place for changes to an existing risk assessment 
  •  Inadequate controls specified in the risk assessment by relying on brow inspections to determine that ventilation rise was open
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