SALT LAKE CITY (AP) -- A massive landslide that halted operations at a major Utah copper mine last week was one of the largest in the history of open pit mining, an expert said Tuesday.
The assessment came after Kennecott Utah Copper announced that 165 million tons of rock and dirt came crashing into one of the world's largest open pits in the April 10 landslide at Bingham Canyon Mine southwest of Salt Lake City.
That's the same volume as 735,000 school buses, making it one of the largest slides ever at an open pit mine, said Mike Nelson, associate professor and chair of mining engineering at the University of Utah.
"By almost any standard, this is a big landslide," Nelson said. "It's a large volume of material and it moved rather quickly ... Recovering from this will be a significant effort in terms of time and expense."
Two-thirds of the floor of the nearly mile-deep pit at Bingham Canyon Mine was covered by the dirt and rocks that partially buried several dump trucks.
Officials anticipated the slide and pulled 37 workers from the area. No one was injured.
Kennecott Utah Copper hasn't released an estimate of how much the slide will cost, but spokesman Kyle Bennett said copper production at the site is expected to reach only half of what had been expected this year. The mine is one of the biggest producers of copper in the country.
The company also doesn't know when the mine will reopen.
Kennecott Utah Copper has asked hundreds of mine workers to take voluntary vacation or unpaid leave. Workers who don't want to take time off will be assigned to jobs outside their usual roles at the mine, Bennett said.
About 800 people work at the mine, and there are a total of 2,500 employees within the entire operation, Bennett said.
Kennecott officials came up with the estimated number of tons in the landslide based on laser scanners and visual observations. Federal regulators have not yet allowed the company's geotechnical experts to go into the pit to further assess damage.
Officials do know that three of the company's 13 mechanical shovels and 14 of its 100 trucks were damaged along with some bulldozers and graders. Bennett did not know how much of the equipment is salvageable.
"We're optimistic that some of them will only require minor repair," Bennett said.
For more than a century, workers at the mine have whittled away at the mountaintop, digging the massive pit that measured three-fourths of a mile deep and 2.75 miles wide across the top. Miners have dug into a tooth-shaped volcanic core that has yielded millions of tons of copper and small amounts of gold and silver.
Kennecott — the latest in a succession of owners — is a wholly owned subsidiary of international mining conglomerate Rio Tinto.
Normally, Kennecott operates the mine all day, every day, with electric-powered shovels capable of scooping up 98 tons of crushed rock at a bite. The company continues to operate a copper concentrator, refinery and smelter at the site.
The landslide occurred not far from a visitor center on the rim of the pit that was closed earlier this month as a precaution.
Work resumed Sunday in an area not affected by the slide, but there's still a lot of work to be done before operations can resume elsewhere, Bennett said.
Company officials must assess damage to buildings and equipment then move rocks and dirt to recreate roads, he said.
"We are doing everything we can to get production back up and going," Bennett said. "We're less than a week removed from this major slide. We need to continue to make logical and methodical decisions about the operation."
Nelson said it's unlikely Rio Tinto will close the mine due to the value of the minerals at the site. The company had previously announced plans to extend the life of the mine.