The Navachab gold mine, the only gold mine in Namibia, sets the trend in adopting sophisticated engineering techniques to achieve an optimal risk-reward relationship in open-pit mining. The Anglogold Ashanti mine was the first open-pit mining operation in Africa to adopt a “Design with Risk” mining approach developed by SRK Consulting.
Located about 150 km east of Swakopmund, near the town of Karibib on the Trans Kalahari Highway, Navachab has produced gold from a disseminated orebody yielding 1.8 gram per metric ton (g/mt). The philosophy has proved itself at Navachab, explained Peter Terbrugge, principal engineering geologist and a corporate consultant to SRK, and is subsequently being used in several mines around the world, even as far as the Chuquicamata mine in Chile. Terbrugge, who has been involved with Navachab since the initial feasibility study was completed in the mid-1980s, reviews the pit slope designs and is responsible for the geotechnical program at the mine.
The philosophy supporting the “Design with Risk” approach is to quantify the geotechnical risk of a slope angle and compare it to the resulting financial benefits, with the intention of achieving the optimal risk-reward relationship, within a mine’s acceptable risk profile. “This so-called aggressive slope management involves multiple slope monitoring systems which can identify potential slope instability and effectively communicate this to the onsite geotechnical team,” Terbrugge said. “It also involves proactive measures such as detailed in-pit mapping to identify potential failures, monitoring of blasting practice to prevent damage to the slope and ensuring the dewatering program continues to depressurize the slope.
“The most important aspect of the slope management program is to ensure the work force is aware of the geotechnical risks and hazards in the pit and has well rehearsed evacuation plans in order to safely evacuate the pit when required,” Terbrugge said. “Navachab runs a continuous Hazard Awareness Campaign that informs the workforce of the potential geotechnical hazards, so that every employee forms a part of the slope management program. Regular pit evacuation drills are also conducted,” he said. The slope management program had to integrate with the risk profile designed into the slope so risks identified in the design can be constantly monitored.
“The biggest contribution to success at Navachab has been the forward thinking management and their excellent relationship with the geotechnical team, allowing us to experiment with approaches and systems so that the mine can gain the greatest benefit from the Slope Management Program,” Terbrugge said.
Since the start of mining in 1990, SRK has been involved in on-going slope risk management for the mine, and has developed and improved the design for the Navachab mine. The original life of mine was planned to 2003, to a depth of 190 m. SRK’s ability to optimize the slope design contributed to the feasibility of the current pushback, which will take the mine to 2011 and to a depth of 240 m. This has obvious profitability implications with further exploitation of the resource without compromising the safety of miners.
Currently, SRK is assisting Navachab with a major pit expansion feasibility study, which will encompass an expansion to the north, east and west of the main pit, increasing the size of the main pit considerably. Conceptually it will effectively double the dimensions of the main pit taking it to a depth being in excess of 350 m, with an extension of the life of mine to beyond 2020 anticipated. Life of mine and production schedules will be finalized during the study.
With assistance from SRK, Navachab has been at the forefront of developments in open-pit slope management in Africa. It was the first site to have an integrated early warning system connected to the slope monitoring system, thereby allowing for instantaneous evacuation in the event of any potential geotechnical risk; this is the first site in Africa to install a micro-seismic monitoring system for an open-pit; the first site in Africa to apply a Geobrugg Catch Fence in open-pit mining, to manage the exposure of personnel and equipment to rock fall risks; one of the first sites to implement continuous robotic total station slope monitoring; and one of the first to have incorporated a real-time slope monitoring radar into the slope management program.
Discussing on-site operations, Robert Armstrong explained how SRK had assisted Navachab through challenging geotechnical conditions. As an example, he referred to mining the bottom 50 m of the Cut 2 design that was completed to a depth of 190 m in 2006. Armstrong, a senior engineering geologist with SRK, conducts the day-to-day assistance provided to Navachab as well as the necessary slope designs.
“Typically, mining the bottom of an optimally designed and executed open-pit can present geotechnical challenges,” Armstrong said. “The rock mass is very sensitive to the mining operation, and every blast could trigger a slope failure. It was therefore necessary to monitor the rock mass after every blast and to decide whether it was safe to continue mining or not, we assisted the mine in this regard and the mine was able to complete the cut.
“Groundwater can also have a serious effect on the stability of an open-pit mine,” he said. “It reduces the effective strength of a rock mass by applying a dilating pressure on open discontinuities thereby increasing their ability to fail. SRK have done extensive ground water studies on the site, with the assistance of AGA specialists. This has allowed us to develop a dewatering program in conjunction with the mine, which has shown positive results in the stability of the pit.”
According to Armstrong, some of the main features of the design by SRK include: the Cut3/Eastern Pushback to a depth of 230 m, mining down the eastern/footwall slope; the north extensions of the main pit—a 180-m deep circular pit 500 m across just north of the main pit; and Grid A—a small 400-m long satellite pit 3.5 km north of the main pit, on the side of a hill resulting in steep slope angles in what is a competent rock mass, although affected by structures.