Award of Merit: Antamina Tailings Facility

Category: Natural Resources, Mining and Industry

GOLDER ASSOCIATES

Beginning in 1998, Golder Associates’ Burnaby, B.C. office joined a team of Colombian and Chilean consultants to provide design and engineering services for the tailings facility at the Antamina copper-zinc open pit mine. BHP-Billiton, Noranda, Teck Cominco and Mitsubishi developed the mine in a high-risk seismic zone, 4,000 metres above sea level in the Andes Mountains of Peru. At a cost of US $2.3 billion and with reserves of 559 million tonnes, the mine is an important stimulus to the Peruvian economy.

Golder Associates’ prime involvement was with the tailings impoundment facility, providing site investigation, design and construction quality assurance. The facility’s main element is a 130-metre high, concrete-faced, rockfill starter dam — the first tailings dam to be built using this method of construction. More conventional approaches for tailings dams use earthcore or a plastic liner.

The structures were designed for an earthquake event generating a peak acceleration of 0.48 g (gravity). A rockfill dam was selected because it is a forgiving structuring during large earthquakes. The dam will be raised in stages over the 26-year life of the facility. It will eventually be 240 metres high, one of the highest tailings dams in the world.

Golder carried out all the detailed site investigations, including drilling and testing 40 diamond core and reverse circulation holes to depths up to 180 metres.

The tailings dam was constructed using the mine trucks and rockfill from the open pit. By using 240-tonne haul trucks the fill was placed twice as fast as previously constructed rockfill dams. Other elements of the tailings facility are a 650-metre long tunnel to divert the Ayash River around the tailings dam, and a 1,300-metre long decant tunnel and 100 metre shaft to carry decant water and riparian diversion water. A hydraulic model was constructed to confirm the performance of the vortex developed in the drop shaft of the decant tunnel. A plug in the diversion tunnel has to withstand a 220-metre depth of tailings and water.

There are 14 kilometres of channels to divert water around the tailings facility and seven kilometres of tailings pipelines will be installed eventually. As well the engineers designed a 29-metre high, 270-metre long earth and rockfill embankment water supply dam.

During construction of the tailings dam Golder had to adjust the design to accommodate the subsurface geology conditions, including karst in the upper right abutment. Grouting the dam foundation involved 38,400 metres of drilling and the injection of 2.9 million litres of cement grout. A 0.6-metre thick concrete plinth founded entirely on bedrock provides continuity between the grouted foundation and the concrete face. Innovative aspects in the design of the dam include a concrete curb system to support and protect the steep upstream slope of the filter, and a composite stop-log to close the sidehill decant structure.

A project office was set up in Vancouver to coordinate the engineering efforts of up to 60 team members working in Vancouver, Bogota, Santiago annd on site. Quality assurance in the field was done with a staff of 40 people from six different countries.

Construction of the tailings impoundment facility was completed on budget at a cost of US $100 million, two months ahead of schedule. The first concentrate from the mine was shipped in July 2001.

Name of project: Antamina Tailings Impoundment Facility

Other key players: Ingetec (dam and tunnel engineering); Geotecnica Consultores (tailings pipeline and decant structure engineering)