Boddington Gold Mine
Western Australia: The Boddington Gold Mine is a large open cut mining operation that produces more than 230,000 ounces of gold per year.
To control costs, especially in times of fluctuating gold prices, engineers review the design of the pit to determine if its walls can be steepened. This is an attractive option, since it minimizes waste removal and maximizes recovery of ore. The tradeoff is a decreased margin of safety, since steeper walls can compromise stability.
Boddington engineers had safety in mind when they reviewed the design of two pits that had previously experienced slope failures. They determined that modifying the angle of the pit walls would be possible, but wanted a simple, reliable alarm system to ensure safety. The system would provide equipment operators with an early warning of impending slope failure.
Pit One: An earlier slope failure, shown in the photograph, revealed the potential for much larger failure nearby. Subsequent monitoring with optical survey prisms indicated significant movement along a large joint adjacent to the right of the excavator. Continued mining in this area would require an alarm system and modification of some mining practices.
EL tiltmeters were installed in critical locations and connected to a data logger that monitored tilt continuously. The data logger compared tilt readings to an alarm threshold. If an alarm condition occurred, the logger would trigger a siren and strobe light that were positioned close to the production area. With the installation of this system, equipment operators were briefed, and the result was a high level of confidence for continued mining in the area.
Pit Two: Located in an area of saturated soils and jointed rock, the steeper walls of the redesigned pit raised concerns over the final stability of the pit walls. Tiltmeters were placed within the pit perimeter along lithological contacts where failure was deemed likely.
The tiltmeter plot shows forty hours of the final production cut. From hour 30, wall movement accelerated, indicating a potential failure. This kind of information makes it possible to maximize ore recovery while ensuring operator safety.
[Editor's note: The tiltmeter readings are given in mm per 2 m because the tiltmeters were mounted on rigid 2-meter long vertical beams. The beam makes it possible to convert the angular measurement from the tiltmeter to a lateral distance, from which movement can be plotted. For more information, refer to EL Beam Sensors.]
In summary, Boddington Mine's monitoring system of prisms and tiltmeters, combined with alarms and some modification to mining practices, allowed mining to continue through to a safe and successful completion of the pits.
Thanks to Rob Parker, Engineering Geologist at Boddington Gold Mine, for providing this story. He can be contacted at Boddington Gold Mine, PO Box 48, Boddington, Western Australia, 6390.