The ‘design with risk’ approach to openpit mining, developed by South African-founded SRK Consulting and first used in Africa at Namibia’s Navachab gold mine, is now being used at several mines around the world – even as far afield as the Chuquicamata copper mine, in Chile.
Located about 150 km east of Swakopmund, near the town of Karibib, Navachab has produced gold from a disseminated orebody yielding 1,8 g/t.
SRK Consulting principal engineering geologist and corporate consultant Peter Terbrugge explains that the ‘design with risk’ approach entails quantifying the geotechnical risk of a slope angle and comparing it to the resulting financial benefits, with a view to achieving the optimal risk-reward relationship within the client’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 on-site geotechnical team. 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 programme continues to depressurise the slope,” says Terbrugge.
He adds that the most important aspect of the slope management programme is to ensure the workforce is aware of the geotechnical risks and hazards in the pit and has well-rehearsed evacuation plans to safely evacuate the pit when required.
“Navachab runs a continuous hazard awareness campaign that informs the workforce of the potential geotechnical hazards so that every employee is part of the slope management programme. Regular pit evacuation drills are also conducted,” he says
The slope management programme had to integrate with the risk profile designed into the slope, so that risks identified in the design could be constantly monitored.
He says that the biggest contribution to success at Navachab has been the forward thinking management and their relationship with the geotechnical team.
“This allows SRK to experiment with approaches and systems so that the mine can gain the greatest benefit from the slope management programme”, he says.
Since the start of mining in 1990, SRK has been involved in ongoing slope risk management for the mine, which has improved and developed the design for the mine.
“The original life-of-mine was planned to 2003, to a depth of 190 m. SRK’s ability to optimise 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 mineworkers,” he says
Currently, SRK is assisting Navachab with a major pit expansion feasibility study for the mine, which will entail 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 in excess of 350 m, with an anticipated extension of the life-of-mine to beyond 2020. Life-of-mine and production schedules will be finalised during the study.
As a result of the ongoing work at Navachab, the mine is regarded as being at the forefront of developments in openpit slope management in Africa.
Navachab was the first site to have an integrated early-warning system connected to the slope monitoring system, allowing for instantaneous evacuation in the event of any potential geotechnical risk. The mine was the first site in Africa to install a microseismic monitoring system for an openpit, the first site in Africa to apply a Geobrugg catch fence in openpit mining, and to manage the exposure of personnel and equipment to rockfall risks; it was 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 programme.
“Having assisted Navachab with implementing these systems, and being closely involved with the results, SRK is in a position where it can recommend practical and useful systems to other mines, with first-hand knowledge of how the systems work and the results that can be expected,” he concluded.
Terbrugge has been with SRK for more than 35 years, and has been involved with Navachab since the initial feasibility study was completed in the mid-1980s. He reviews the pit slope designs and is responsible for the geotechnical programme at the mine.