Coal Inspectorate | Alert | No.435 V 1 | 27 September 2023

Support system failure and strata fall in main drift at an underground coal mine

What happened?

On 21 May 2023 Coal Mine Workers (CMWs) were conducting remedial strata control work in the drift which serviced both the M&M (dolly car) and the conveyor. The CMWs observed the roof strata dribbling after undertaking scaling of the area.  The CMWs waited to assess the area before moving in to bolt the location. A fall of ground occurred. No injuries were sustained.

The fall blocked the primary means of egress leaving only one escapeway from the mine available for use. The fall dimensions were approximately 3 metres long, 3 metres high and the full width of the roadway (refer Figure 1).

All personnel inbye of the fall were withdrawn from the mine via the man winding shaft.

How did it happen?

An independent Geotechnical Engineer provided advice to the Site Senior Executive (SSE) and Underground Mine Manager (UMM) to determine the cause of the support failure and the subsequent strata fall as well as to develop an appropriate staged plan for the recovery of the fall (refer Figures 2 & 3).

The area where the fall occurred had been heavily reinforced at the time that the drift was driven, some 50 years prior. Over time, in this case several decades, the reinforcing steel liner and steel set had rusted to the point of failure.

Key issues

Immediately following the fall, the underground part of the coal mine was placed under a Directive, per section 167 of the Coal Mining Safety and Health Act 1999, to suspend operations for an unacceptable level of risk. Limited access was permitted to the inbye side of the fall via the man winding shaft to allow for statutory inspections to continue. Limited access was also permitted in the drift to allow for a specialist crew to recover the fall.

Recommendations

The recommendations contained in this alert are general in nature, relating to the management of strata hazards in underground coal mines, and are not direct findings relating to the cause of this incident.

The Mine Operator should ensure that the SSE and UMM at their underground coal mines review their Strata Control Principal Hazard Management Plan (PHMP) to ensure this is effective to prevent any potential for a similar incident. Consideration should be given to the following:

• Having sufficient geological and geotechnical information to adequately assess the requirements for the strata at your mine.
• The design of the strata support system is to be based on relevant geological and geotechnical data to provide support appropriate to the life cycle of the excavation.
• The minimum design considerations of the support system must consider strata properties, the properties of the support system and potential failure surfaces.
• Strata support has been installed as per design specifications and detailed in the approved support plan.
• The strata monitoring system is adequate to provide effective monitoring to ensure the integrity of strata support over time in all circumstances.
• Mine management must develop and implement a suitable quality control process to ensure that the standard of installation meets that required by the design criteria for ground conditions at the mine.
• The integrity of the strata support system has not been compromised due to the life span of the components originally installed for providing effective strata support and monitoring.
• The changes of any other factors, particularly relating to but not limited to, shallow depth underground excavations associated with mine entries with a long life cycle that may impact on the installed strata support such as changes in surface topography, water control and catchment, erosion, weathering, and the potential for water ingress.
• Ensure that the assessment and review of the Strata Control PHMP is conducted by a suitably qualified, independent and competent geotechnical engineer.