Staff get concrete about geotechnical engineering

Reclaiming a mine site is very different from operating a mine, and the staff involved in the reclamation may have limited experience in this kind of work.

At the former Faro lead-zinc mine in Canada's Yukon, geotechnical training has been provided so that staff understand more about the purpose of their work and the kinds of specific problems to look out for.

Jim Cassie, a geotechnical engineer with BGC Engineering in Calgary, explained the training at the Northern Latitudes Reclamation Workshop in May.

Numerous owners before Anvil Range

Faro opened in the late 1960s and had numerous owners before coming under the management of Toronto-based Anvil Range Mining in 1996, but the company went bankrupt in 1998, with Deloitte & Touche as the interim receiver.

Deloitte & Touche oversees closure work at the mine on behalf of the federal government.

"We have a very spread-out site, we're also in different watersheds," Cassie said. "Approximately 50 million tonnes of acid-generating tailings are sitting in the bottom of the Rose Creek Valley."

Dams, dykes, berms, diversion channels, waste rock dumps, rock drains and ditches at the mine all need to be maintained.

"There are people at site that do the environmental monitoring and geotechnical surveillance program," Cassie said. "These people, of course, come from a variety of backgrounds, the ones specifically at Faro came from the processing, from the mill, some came from assaying, and some came from environmental monitoring. So if you were to look at their skill set on a piece of paper, they really have no geotechnical experience, no dam safety experience."

By contrast, at an active mine there are pit superintendents, underground rock mechanics engineers, mining engineers, geological engineers and grade control geologists who are familiar with earthworks, blasting and other geotechnical issues, Cassie said.

One of the objectives of the training was to give reclamation staff a basic understanding of geotechnical engineering jargon.

"In addition, they need to understand how earth structures perform, and deform, and move, and crack, so that they can start to understand minor issues versus big issues, and also how these things may in fact fail, if they are going to fail, so they can understand - one, things to look for, and two, maybe how they will solve problems as they occur," Cassie said.

Different training every year

A different training seminar for Faro staff was held each year, starting in 2001. The first one was an introduction to geotechnical issues, starting with the basics of soil mechanics, the parts of a dam, waste dumps, hydrotechnical concerns and geotechnical instrumentation.

"At Faro, for instance, we tend to deal a lot with cracks in dams," Cassie said. "Some are very minor cracks, for a variety of reasons, and other cracks are more significant, or require immediate attention."

The training enabled staff to distinguish between significant and insignificant cracks.

The seminar also taught staff about permafrost and ground ice. Although permafrost is not a big issue at Faro, it is an important concern at other northern sites.

Tailings dam failure modes were the focus of the 2002 training seminar.

The staff discussed emergency response plans and the kinds of equipment that would be used. They were shown cartoons, computer simulations and CNN clips of the different ways dams can fail.

In 2003, the seminar was about waste dump safety and monitoring, while the 2004 seminar discussed geotechnical instrumentation.

"As you can imagine, we have a rather significant suite of geotechnical instruments at the site," Cassie said. "We wanted to talk about how they work, what do they do, how do you read them, what can you do to monitor and maintain them." This year's seminar will be about seepage monitoring and weir flows.

The seminars have reduced the incidence of false alarms, because staff at Faro used to worry about every minor event.

"That's not to say that site staff are not watching, if you've met some of these site staff, they are extremely diligent about their monitoring instructions, and they have a very detailed log about every crack," Cassie said.

Staff have improved their judgment and have increased their ability to make decisions.

"Site staff really understand how important their job is, they really have a sense now ... I think we've become slightly more of a team, and that is good, that is probably the best benefit," he added.

Breaching the fresh water supply dam at Faro

A decision was made in 2002 to breach the fresh water supply dam at Faro. The dam was constructed in 1968 as part of the original mine development, to store up to approximately 5.6 million cubic meters of water for use by the mill in winter.

It was an earthfill structure, 410 meters long and 20 meters high, built on foundations of silty sand, glacial terrace gravel and till.

There was a rip rap cover on the face of the dam, and berms were added in 1969 and 1989.

The first berm was built after seepage problems were discovered and the second berm was built to improve the dam's seismic stability. It included a granular blanket to collect seepage.

The objectives of the breach included keeping the water level within the range of natural fluctuations, getting rid of the reservoir, controlling sediment releases and providing fish passage for up to a one-in-10-year flood.

The water level in the reservoir behind the dam was lowered by about 6 meters in summer 2003 and revegetation of the upper part of the reservoir was initiated.

Once Canadian and Yukon government agencies had issued approvals for the breach project, the excavation of the dam breach began on Nov. 11, 2003, and dam removal was completed on Dec. 16 that year.

Post-decommissioning monitoring has commenced and will continue for several years.

The Faro dam was the second-highest dam at the time that had ever been breached.

The cost was around $3 million. BGC Engineering designed the breach and Pelly Construction of Whitehorse was contracted to do the work.