Alert no. 408 | 25 May 2022 | Version 1

Impact of flooding on slope stability at mines and quarries

What happened?

Several mines and quarries have recently reported incidents associated with slope instability and failures in ground integrity to the MMQ Inspectorate.

How did it happen?

Extreme rainfall on the east coast of Australia this year has created one of the worst flood disasters on record for Queensland. The flooding caused the ground across parts of Queensland to become saturated and vulnerable to even small amounts of rain.

Key issues

Groundwater has a detrimental effect on slope stability. An industry-wide awareness of the likelihood of slope instability due to current groundwater conditions is critical. Saturated conditions leading to standing water within the pit can result in unsafe working conditions.

Recommendations

Given the current conditions, pit inspections should be conducted with diligence on a more frequent than usual basis. These inspections should be conducted by a competent person to ensure that geotechnical hazards are identified, and suitable controls implemented.

Legislation calls for a person, who has an obligation under the Act to manage risk in relation to ground control at a mine or quarry, to ensure appropriate measures are taken to prevent or control ground failures. The obligation holder must consider water inflow, drainage patterns, groundwater regimes, mine dewatering procedures and their influence on slope stability over time.

Consideration must be given to the following items to determine appropriate ground control measures:

• Strategies to divert water away from the slope crest .
• If the topography is suitable, construction of gravity diversion ditches is often beneficial for minimising the amount of water reaching the slope.
• Diversion ditches could be lined if there is a risk of infiltration of water into the material above the crest.
• Collection sumps and the ability to store runoff from high-intensity storm events must be included in the mine design.

If the ground conditions are determined to be unacceptable you should withdraw workers, establish an exclusion zone, and follow the risk management process in consultation with a person with appropriate geotechnical competencies.