A Grueling Subterranean Test of Brain, Brawn and Machine

When the Resolution Copper Co. hired Cementation USA to sink a 7,000-foot shaft in the Arizona desert, no one expected the project would be easy. But the miners had no idea how much the job would test their mettle and resolve. Nearly seven years after work began in earnest, the historic dig – which created the deepest single-lift shaft in the United States – is wrapping up, its success a testament to the workers’ ingenuity. One of their innovations involved a Cat® 279 Compact Track Loader, a machine rarely used in mine development, which cleared blast rubble off a small vertical inset and built a key tunnel. “We were working within some very tight constraints,” says Hector Denogean of Cementation, “and it really did the job.”

Hard rock mining – as the name suggests – is not a job for folks who give up easy.

Extracting the metals that have formed in the Earth’s crust over uncountable eons under unfathomable heat and pressure takes grit, testing the creativity, determination and endurance of the men and women who toil underground.

But some hard rock mining projects are harder than others. A lot harder. And some are harder than anyone ever imagined, as if Mother Nature herself is taunting the miners and demanding to know: “How much do you really want this?”

Randy Seppala, a project manager with Resolution Copper Co., a Rio Tinto-BHP Billiton joint venture, says his company knew from the start that its ambitious plan to mine a massive copper body located thousands of feet below the high desert town of Superior, Arizona was going to be on the tough side.

It was only after Resolution hired Cementation USA Inc., an underground engineering and mining contractor, and that company’s experienced team began work on the first exploratory shaft, however, that the magnitude of the challenge here become apparent.

For seven years, the project challenged the miners’ mettle and resolve, throwing up one seemingly insurmountable obstacle after another.

In the end, the shaft sinking was a success, proof that Resolution’s bold plan to extract the copper by approaching the ore body from below was feasible. But the veteran miners who worked the job say it’s one they’ll never forget.

“People who have been shaft-sinking and mining for generations and are ready to retire say they’ve never seen a place like this,” says Hector Denogean, the general superintendent and project manager for Cementation USA at the site.

“These conditions are some of the worst you can encounter. You can definitely say that. The worst. It’s a whole different world compared to a normal mine. This is a hostile environment.”

Workers enter the so-called “man bucket” that takes them down to Ten Shaft’s main lateral tunnel. The nearly 6,700-foot descent takes 15 minutes.

Hot Water Everywhere

The challenge – ironic given the desert setting – was water.

The shaft that Cementation was hired to sink was essentially a proof-of-concept, designed to show that Resolution could get down underneath the ore deposit, build a network of lateral tunnels capable of withstanding the rock stresses and pressure, and extract the copper using a technique known as block cave mining. So getting to the bottom was priority No. 1. Any lateral tunnels, known as “drifts,” would have to wait.

From the start, everyone understood heat was going to be an issue. “The deeper the mine, the hotter the rock,” Seppala says. “So we knew that going down 7,000 feet, we’d have 180 degree rock temperature.”

The water was “the big surprise.”

“Test drilling indicated we could expect about 80 gallons a minute,” Seppala says. “We ended up getting close to 500 gallons a minute.”

The constant deluge of water – some of it as hot as the rock – created conditions that overwhelmed the first chiller system installed onsite and brought the shaft sinking to a standstill more than once.

Merely pumping freezing air into the mine wasn’t enough. So Denogean’s team installed a larger exhaust system to pull the heat out and added an additional chilling system halfway down the shaft to re-cool the air.

Like everything else on the project, the six, 13,000-pound chiller units had to be slung down the hole. “Underground has its own set of inherent challenges and logistics is just one them,” Denogean says.  “You can’t just drive material to where it’s needed.”

Lifting lugs had to be engineered and weld on to the units and guides attached so they didn’t flop around in the shaft as they were lowered. And when they got to their destination, halfway down the open shaft, they had to be pulled in and installed on the tight inset.

Nothing was easy. Everything took much longer than it would topside. Equipment continually failed. “Nothing lasts down there as long as it would anywhere else,” Denogean says. “Not gear. Not electronics. Not monitoring devices. Nothing. You got the heat, you’ve got the water, you’re even dealing with a change in barometric pressure. So if we need something, we buy three or four of them. One’s not going to do it. One won’t last. “

Even with the additional chillers, conditions are “adverse,” Denogean says.  Arizonans don’t deny their state gets hot. But they insist it's more bearable than, say, South Florida in the summertime because "it's a dry heat.”

That may be the case in Phoenix and Tucson. But at the bottom of Ten Shaft, as the 7,000 feet hole Cementation built here was officially known, things are a lot stickier.

Down here, the ambient air temperature after cooling ranged between 85 and 90 degrees Fahrenheit all day and night and the humidity never, ever drops below 100 percent.

It's essentially South Florida.  In the summertime. On steroids. Hell on both men and machines.

 

The miners’ drenched work gear hangs in a “dry room” topside. Some 500 gallons of water a minute – some of it as hot as 180 degrees Fahrenheit -- rains down on the workers as they toil in the narrow shaft.

A New Challenge

In the fall of 2014, when Cementation reached the depth Resolution needed to hit to prove it could get underneath the copper body, the challenge changed.

Now Cementation had to begin so-called “breakout work,” blasting out the first of what will ultimately be the labyrinth of lateral tunnels that will honeycomb the site.

The first lateral tunnel had to be dug about 160 feet above the shaft’s floor to accommodate a large pump station and a fully enclosed electric room to power the pumps.

A tight horizontal inset had been blasted out on the way down. But mucking out the blasted rock and building the 16-foot-wide horizontal tunnel was going to require something other than the usual load-haul-dump scoop vehicle. Something small, nimble and versatile.

And so for that job, Cementation used an unusual tool for underground mining: a Cat 279 Compact Track Loader designed for work above ground.

The Compact Track Loader’s role was supposed to be limited to the initial mucking out. But after it was lowered down to the work site, more than 6,780 feet below ground, it proved to be far more versatile than anyone imagined. Without modifying the machine or its hydraulics, the miners were able to mount a jumbo drill to the front of the 279, which they used to bore holes for both explosives and ground support. A robotic shotcrete arm was hooked up – again without modifications – to shotcrete the tunnel’s walls and to cements its floors.

Keeping the 279 up and running in an environment as hostile as this required resourcefulness on the part of Cementation’s equipment maintenance team led by Bob Cruea. Cruea shared the team’s learnings and suggestions with Aaaron House, a mining account manager at Cat Dealer Empire Cat in Phoenix, who fed the input back to the product team at Caterpillar. But bottom line, the Cementation team was impressed by the Compact Track Loader’s performance.

“We’re going to keep one down there until we’re done,” Denogean says.  “It’s [a] quite good piece of gear.”

Resolution’s plan for the mine here calls for four more shafts as deep -- or deeper -- than the one Cementation has dug and miles and miles of lateral tunnels.

Denogean says that if Cementation gets the contract, he’ll use the Compact Track Loader again because of its performance on this job and because the conditions the miners confronted – and overcame – during the digging of Ten Shaft were an indication of the challenges ahead.

“It will be wet,” he says. “It will be hot. It’s not going to get easier.”

 

Hector Denogean and Bob Bob Cruea of Cementation USA go over the plan for Ten Shaft’s first big lateral tunnel or “drifting” located 6,700 below ground and 160 feet off the shaft floor.

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