Slide Show Photo courtesy of Southern Nevada Water Authority. The first section of the TBM was hoisted into the drop shaft on Sept. 7, 2011. Related Links: Big Drop in Lake Mead Curbs Water, Power
Southern Nevada's newest piece of mega-hardware—a custom $25-million Herrenknecht tunneling-boring machine—will make its long-awaited underground debut later this year.
The machine works like a giant mechanical earthworm, gnawing through dirt, rock and muck while forming a protective tunnel that will eventually channel raw Colorado River water onto nearby treatment plants before the water is pumped to homes and businesses throughout the Las Vegas valley.
The 1,800-ton, 600-ft-long TBM is the workhorse of a $526.6-million third raw-water intake tunnel project at Lake Mead, 30 miles southeast of Las Vegas. A joint venture of S.A. Healy Co., Lombard, Ill., and Impreglio S.p.A., Sesto San Giovanni, Italy, known as Vegas Tunnel Constructors (VTC) LLC, is the design-build contractor. The additional intake is needed because the lake has dipped 100 feet since 2000 amid a decade of drought and boom growth.
The TBM will carve out a three-mile-long, 20-ft-dia reinforced tunnel under Lake Mead that draws water at 860 feet. That depth is deeper than the other two intakes, which may soon become inoperable if water levels continue to fall. The drill head, which rotates in 16 different positions, is already underground. The five-piece TBM should be fully assembled by early December, with commissioning to follow.
It has been a long, hard road to reach this point.
Herrenknecht’s Schwanau factory in southern Germany took 17 months to design and manufacture the machine, which was shipped in five large pieces to California’s Long Beach Harbor and then transported to the jobsite outside of Boulder City, Nev., in 61 truckloads. The transportation feat took months of planning and coordination, mapping out the route and securing special heavy-haul permits.
In 2010, an intake starter tunnel flooded three times in six months, prompting the construction team to abandon its original alignment and drill in a drier direction, roughly 23° northeast from the problematic tunnel along a ½° uphill slope. The move increased project costs by 15% and pushed the completion date back two years, to 2014.
As it stands, the TBM is being lowered piecemeal down a 30-ft-dia, 600-ft-deep access tunnel using a head-frame-gantry system with dual 200-ton strand jacks. It took crews 32 hours to lower the 315-ton drill head.
"There was only three to four inches clearance on either side," explains Jim Nickerson, VTC deputy project manager.