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Kennecott wells aim to keep tailings in place

Published June 4, 2016 11:07 am

Pilot project • The plan is to remove water from billions of tons of tailings to prevent their movement across SR-201.
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When the long-feared big earthquake hits the Wasatch Front, Rio Tinto Kennecott wants to keep its massive tailings pile from liquefying and sliding across State Road 201.

Driven by a new seismic study that suggests a magnitude 7 earthquake could occur in the next 50 years, the mining company has launched a pilot project to remove water from the south side of the tailing impoundment, a stone's throw from the highway.

"We've been concerned about this slope since the late 1980s," said Paula Doughty, Kennecott's manager of tailings and water services, noting the goal of dewatering is to keep the impoundment in place if "the big one" strikes.



"We want to get [the tailings pile] dry to a point where nothing comes off of it," she added Friday in a news-media tour of the pile, which Kennecott built between 1906 and 2001 with more than 2 billion tons of tailings separated from valuable ores extracted from Bingham Canyon Mine.

The $2 million pilot project will involve drilling 12 dewatering wells along a bench of the tailings pile, roughly 60 feet above the Salt Lake Valley floor. These wells will be spaced 60 to 70 feet apart, extending 1.8 miles along the impoundment's south side.

Two dozen monitoring wells will be driven initially to check the impact of the work, Doughty said, before more holes are bored. Within a couple of years, 104 wells will be in place — at a cost of close to $27 million — to stabilize the tailings pile.

Even if they cannot immobilize the pile completely in the event of a major quake, she said that if this project functions as engineered, the tailings movement would not be "a tsunami" but more of a "pastelike material that would go out onto the road."

Twenty-five years ago, the tailings pile was a major concern for Magna residents.

Complaints about the potential health impacts of dust blowing off the pile prompted Kennecott to plant native vegetation on top of the 5,700-acre impoundment, which averages 230 feet in depth.

Fears mounted further when earthquake studies showed that the tailings pile could liquefy and spread quickly to the south, crossing SR-201 and spilling into some Magna neighborhoods.

Since 1998, Doughty said, Kennecott has striven to mitigate that threat by removing all surface water from the impoundment and drying its interior with a series of deep wells, intermediate "wick drains" and drainage pipes.

The main dewatering wells, 46 of them about 150 to 200 feet apart, have removed close to 5 billion gallons of water from deep in the tailings pile over the past quarter century, said Kennecott spokesman Kyle Bennett.

That should be sufficient to keep liquefaction action from reaching the Magna neighborhoods, or even going far into the Copper Golf Club course, which covers the ground between the pile and homes, he added.

But with new seismic studies projecting a 43 percent chance of a magnitude 6.7 or higher quake rattling the Wasatch Front in the next 50 years, Kennecott decided additional water-removal was necessary to pare back the slide zone even more.

"Everyone has known for 20 to 30 years that we're in a seismic zone, but we didn't think a big earthquake would happen so soon. That changed the risk profile," Doughty said. "That raised our attention. It wasn't a risk we wanted to take."

mikeg@sltrib.com

 

 

 

 

 

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