Cerro Vanguardia, situated in the Santa Cruz province in Argentina, is unique among AngloGold Ashanti's surface mines because of the nature of the ore occurrence. The gold-bearing ore occurs in vertical veins up to 10 metres thick, which average 3.5 metres with near vertical dip. Separation of the multiple veins by barren country rock dictates that mining is carried out in multiple small open pits, with relatively steep walls: slope angles are around 65º.
To ensure that risk identification and management across the open-pit operations of AngloGold Ashanti are in line with world best practice, the company established Geotechnical Review Boards (GRBs) in 2003. These boards have a multi-disciplinary membership, including specialists from the disciplines of mining, rock mechanics, hydrology, safety as well as regional and site representatives. Their terms of reference cover all regions outside South Africa.
Dave Worrall, manager - surface mining (corporate technical group), was responsible for establishing the GRBs, whose mandate is to provide a tool to ensure best practice is implemented; and to perform detailed geotechnical risk assessments to quantify and manage risks for individual operations.
Carl Brechtel (corporate technical group) assisted in the Cerro Vanguardia risk assessment and comments, "The steep walls of the Cerro Vanguardia pits, and their relatively small size, means that there is limited room at the bottom of the pits, with consequent risk to people working there. We therefore carried out a detailed geotechnical risk assessment of Cerro Vanguardia operations, spanning a three-year period." The steep pit walls presented two main risks: major slope failure, and loose rocks falling into the pits.
"Through a rigorous computer simulation process, our geotechnical consultants modelled the risk of slope failure in terms of a number of variables," says Brechtel. "They applied block kinematics models to assess the risk of single rockfalls, taking into account gravity, velocity and range of movement on impact." Once quantitative risk levels have been established, overall risk is evaluated by assessing the effectiveness of measures in place to manage risk and mitigate the impacts of any event. This process examines five key areas, to establish:
- what systems are in place to monitor pit walls, in order to gain early warning of any instability;
- whether effective evacuation procedures are in place;
- the numbers of people present in at-risk areas at any given time, and the duration of such exposure;
- the differing exposure levels applicable to various occupations; and
- appropriate safety awareness and training.
A number of issues were highlighted at Cerro Vanguardia. "Catch-berms created in previous mining were below the specified width, leading to increased rockfall risk," explains Brechtel. Catch-berm construction designs and pit access ramps have been redesigned to reduce this risk. In addition, two-state-of-the-art electronic instruments - a laser imaging scanner and a digital stereo-photography camera system - have been purchased by the mine to improve reconciliation between design and actual geometry of the walls."
Ground water is a major component of pit stability, as the presence of water tends to reduce weight-bearing capacity for a given friction. John De Souza, a specialised hydrologist, has been seconded from the Africa region to supervise the installation of a de-watering system, which is planned to run for the life of mine. The system has been in operation since July 2004 and has lowered the ground water elevation by 14 metres in the largest pit, Osvaldo Diez. John McEndoo, group manager - safety, participated in the risk assessment and is assisting Cerro Vanguardia to develop pit evacuation procedures. "These will be fully integrated with the pit slope stability monitoring systems to ensure the safety of all personnel required to work in the pits at Cerro Vanguardia. The safety management programme is also being reviewed to ensure optimal employee awareness and training," comments McEndoo.