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Big machine is weeks away from digging under Lake Mead

If a snail took two years to go three miles, the other snails might laugh at him.

But considering where these particular three miles are, 23 months doesn’t seem all that slow.

At a construction site 600 feet beneath the shores of Lake Mead, workers have finally begun assembling the giant digger that will be used to excavate a new water intake pipe beneath the bottom of the nation’s largest man-made reservoir.

With an average speed of about 30 feet a day, the tunnel-boring machine is expected to complete its 3-mile journey through solid rock sometime in early 2014.

Marc Jensen, director of engineering for the Southern Nevada Water Authority, said getting the machine in the ground is a welcome milestone for one of the most complicated tunneling projects in the world.

"There’s a much happier attitude among the staff at the site," he said.

After all, it has taken a lot longer to get to this stage than originally thought.

Work on the so-called third intake was already about three months behind on July 1, 2010, when a crew excavating the starter tunnel for the boring machine hit a fault zone, causing water and debris to flood the work area.

The company hired to design and build the intake, Vegas Tunnel Constructors, spent weeks trying to stabilize the fault, but the area flooded twice more last year.

The contractor eventually abandoned that tunnel and excavated a new one in a different direction in hopes of skirting the troublesome fault line.

In February, the water authority board agreed to add another $39.5 million to what already was the single largest construction contract the agency ever issued.

The change order also extended the timeline for the project by 593 days to the summer of 2014.

Once the more than $800 million intake is complete, it will be able to draw from deep in Lake Mead and keep water flowing to Las Vegas even if the reservoir drops below the two existing intake pipes.

The valley depends on Lake Mead for 90 percent of its drinking water supply.

To reach the wet and muggy work area beneath the lake’s Saddle Island, miners have to descend to the bottom of a vertical access shaft roughly the same height as a 55-story building.

The front sections of the 1,500-ton tunnel-boring machine have been lowered down the same shaft piece by massive piece, a delicate and dangerous process that began Sept. 7 and continues today.

The machine is now being assembled underground. It will eventually stretch the length of two football fields and weigh more than three Boeing 747 jetliners.

It could start digging by year’s end.

The $25 million boring machine was built in Germany specifically for the third intake project. It is designed to operate at atmospheric pressures never before seen for a project of this kind. As a result, it comes equipped with a hyperbaric chamber such as the ones used by deep-sea divers, though workers in the tunnel won’t need to use it except in rare instances.

It takes a crew of about a dozen people to run the machine. The plan is for three shifts of workers to tunnel around the clock at least five days a week.

Some 360,000 cubic yards of rock will be excavated during construction of the tunnel. The finished, 20-foot diameter intake pipe will be built in sections behind the machine as it digs. Each time the machine advances, the concrete pipe will grow by 6 feet as a new ring is added just behind the machine’s shielded front section.

It will take roughly 2,500 rings, each weighing about 34,000 pounds, to line all 3 miles of tunnel.

About 130 people are now working on the third intake, with roughly 70 of them working to get the tunnel-boring machine up and running.

Meanwhile, at the opposite end of the project, a crew of about 20 people is busy blasting a hole in the bottom of Lake Mead from a floating barge.

The hole eventually will house a massive intake structure that will be lowered into it from the surface of the water.

The blasting work has taken longer than expected, but it should be finished by December. The 1,200-ton intake structure will be lowered to the lake bottom in January or February and secured in place with about 13,000 cubic yards of concrete.

The concrete will be pumped to the bottom of the lake in one continuous pour that is expected to last five days and involve about 1,300 loads from cement trucks that will be shuttled out to the site on barges.

As for the tunnel-boring machine, the upcoming two-year journey will be its last.

Though it will only have 3 miles on its odometer when the intake tunnel is finished, the machine is destined to be sold back to the manufacturer and stripped for spare parts.

Contact reporter Henry Brean at hbrean@reviewjournal.com or 702-383-0350.

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