May 2010

Engineering Exchange

Engineering for change: New Afton requires flexible design and planning

By H. Ednie


New Afton cold storage warehouse installation nearing completion in February 2009

The New Afton Mine, owned by New Gold Inc., is currently under construction, with a planned production start-up in mid-2012. Once built, New Afton will produce 11,000 tonnes per day of primarily gold and copper ore with an average grade of 0.95 per cent copper and 0.69 gpt gold.

Located west of Kamloops, British Columbia, on the site of the original open pit Teck Afton Mine, the New Afton Mine will be an underground block caving operation. This mining method, not typically done in Canada involves approaching from the underside of the ore body and mining from below, basically collapsing the ore on itself. The block caving method will result in some surface subsidence and potential damage to surface infrastructure. The effects of this subsidence will need to be monitored throughout the life of the mine so that remedial actions can be planned to minimize the extent of any damage. 

“Over the 15 years of the mine life, the surface above the ore body will continue to slump inwards effectively creating a surface depression instead of a large open pit on surface,” explains Sean Lynn, project manager for AMEC, which was contracted for engineering, procurement and some construction management services for the project. As part of their contract, AMEC is responsible for the design of the surface facilities and infrastructure as well as the underground infrastructure. Block cave mine design is being provided by AMC Consultants.

The challenge posed by the block caving method is the necessity to delineate the surface area that will move as mining activities continue as it is possible that some movement could be seen in areas around the mill building. An estimate of the amount of subsidence that can be expected was prepared by AMC Consultants, with input from geotechnical experts. 

Given its size and importance, the mill building has required some special design measures to mitigate potential damage to building components as a result of movement, including the incorporation of lateral subsidence joints in four places across its width, dividing it into five roughly equal sections.  This is intended to concentrate, as much as possible, any subsidence damage into movement in the joint material. 

Particular attention will be paid to the subsidence joints in the crane runways of the building.  Regular inspection of the joints and the alignment of the rail and connecting parts will be necessary.  “Periodic, controlled crane test runs are also advised,” Lynn adds.  If subsidence movement occurs, the short lengths of crane rail located directly over the subsidence joints may need to be replaced to reduce excessive crane wheel wear and prolong the serviceability of the cranes.

In addition to the subsidence effects on surface infrastructure, there are piping installations running the length of the mill building that required special attention to ensure they can handle the effects of potential building movement. AMEC carried out an extensive review of the pipes that would be affected by building movement and incorporated numerous expansion joints into the design.

Changing with the times

AMEC was awarded the engineering and procurement contract for the New Afton project in May 2007. By September 2007, New Gold rolled AMEC’s contract into a full EPCM agreement, and work was getting started on procurement and field activities. At the time, AMEC had 60 people working on the project.

In February 2008, AMEC mobilized the construction management team to the field and launched the earthworks and concrete foundation construction. The mill building contract was awarded as well as those for other on-site buildings. However, in November 2008, New Gold announced a slowdown in the project’s progression, due to market conditions and other activities across the company. Much of the team was demobilized from site. 

AMEC’s role became one of negotiator, helping New Gold cancel many of the procurement contracts already in place. Although much of the major mill equipment — SAG mill, ball mill, regrind mill, flotation cells, electrical transformers —  was acquired and either installed or placed in long-term storage, most of the smaller equipment orders were delayed or cancelled.

“The decision to slow down the project was perfectly logical to us,” Lynn explains. “The challenge became to get out of the commitments in a cost-effective way.” Instead of cancelling orders outright, many commitments to purchase were maintained but delayed, and AMEC was able to get the certified engineering drawings to use in planning. As a result, the lengthy procurement process will not have to be endured again upon restart.

With the slowdown in late 2008, the plan was to carry on with underground development, but no other work until January 2011. Earlier this year, however, New Gold realized an opportunity to proceed more swiftly with certain elements of the construction program and revisited that decision. AMEC has helped develop the 2010 construction program, which includes all buried services installation, phase 1 of the underground infrastructure installation (including conveyors and a temporary crusher), completion of the crushed ore stockpile area and the reclaim tunnel and slab, and the surface installation of the head end of one of the conveyors, which discharges directly to the ore stockpile. 

The plan is to have the underground conveyors and temporary crushing circuit up and running by March 2011, to begin stockpiling ore on surface. Planned development ore stockpile capacity is 400,000 tonnes.

“Currently, we are just ramping up to complete the engineering necessary to support the 2010 construction program,” Lynn says. Although the project construction schedule has changed in response to market conditions, New Afton is still well on its way to becoming a new producer of gold, copper and silver in British Columbia.

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