Modern-day engineering marvel brings water to parched Vegas
June 7, 2016
When the Hoover Dam was built in the 1930s, it was one of the 20th century’s most impressive feats of engineering. Now its reservoir, Lake Mead — the nation’s largest — is the site of a modern marvel designed to keep water flowing to Las Vegas during a historic drought.
Draining from the bottom of the tub
Over the last decade, the lake’s water level has dropped nearly 100ft, leaving it at an all-time low. Like a massive bathtub ring, chalky white rock circles the basin showing where water used to be. The Lake Mead Marina was once full of sailboats; it’s now dusty, dry land.
By 2007, the Southern Nevada Water Authority knew it faced a critical challenge. If the water dropped another 50ft, pipes that carried water to Las Vegas and other areas (providing drinking water for approximately 2 million people) would start sucking air.
Arup’s engineers worked with the authority and a joint venture of contractors Salini Impregilo and S.A. Healy to create a new solution — an intake at the bottom of the lake that could safeguard supplies even as the water level shrank in the drought.
“You can think of Lake Mead as a big bathtub, and it’s full of water,” said Luis Piek, an engineer in Arup’s San Francisco office and the project’s lead tunnel designer. “You’ve got two hoses that are draped over the side, and you suck the water out to supply the city of Las Vegas. But the water level keeps on dropping with the drought, and eventually when it drops below the end of the hoses you’re not going to be able to suck any more water out. So in an ambitious move, the water authority decided, well, let’s not put another hose over the side. Let’s put a drain in the bottom of the bathtub.”
This was an unprecedented engineering challenge — similar to creating a subway tunnel but in poor-quality rock, below a lake more than 500 feet deep, and under immense pressure from trillions of gallons of water lying above.
“You set out to build something at a depth and pressure that’s never been done before,” said Piek. “But how do you manage risk with something that’s never been completed at this magnitude? That was what we set out to accomplish.”
The basic outlines of the plan were clear from the start. A 23-foot-diameter tunnel boring machine (TBM) as long as two football fields would excavate and build a tunnel connecting the existing water treatment facility and intake system on Saddle Island to the deepest part of the lake, nearly 3 miles offshore.
Although every stage of this process presented extraordinary challenges, the final connection to the lake received the most attention.
The world’s most advanced bathtub plug
An early version of the design called for the water intake shaft to be drilled in the lake bottom and a vertical pipe dropped in and grouted into place. A giant domed steel door known as a bulkhead would be placed on top. In the meantime, the TBM would create a tunnel from the depths of Saddle Island to this spot.
The construction team would then need to connect the tunnel to the vertical pipe. Only when that was complete could a crane floating on the lake’s surface lift the bulkhead, allowing water to flow through the new tunnel.
After close review, the engineers concluded that this solution was too risky. It would require the ground around the connection area to be strengthened by grouting or freezing the fissures in the rocks — an extremely difficult task given the extreme water pressure and depth. But without such strengthening, a clean connection between the lake and tunnel would be very hard to achieve. Water could begin entering prematurely, which could lead to unwanted material flowing into the pipe — or even tunnel collapse.
Once problems of this nature begin, they’re practically impossible to correct. “It’s almost a point of no return if the water starts pouring in prematurely,” said Jon Hurt, a New York–based tunneling engineer and Arup’s project manager for the effort. “The massive investment that had gone into the project would be jeopardized.”
To prevent this, the team developed an innovative design that would utilize the TBM to maintain an engineered environment at all times, minimizing the chances of unplanned flooding. This approach involved creating an excavation in the lake bed with underwater explosives, then lowering a prefabricated intake structure resembling a giant concrete elbow into the hole, with the integrated bulkhead pointing upward. The surrounding cavity would then be filled with concrete to lock the structure into place. The TBM could mine directly into its side, creating a protective socket that water couldn’t penetrate prior to the lifting of the bulkhead.
“It was a carefully engineered structure. It had to be,” said Hurt. “We had to be able to build it floating on a barge and then move it out onto the lake and position it on the bottom. It had to be strong enough to have all the water pressure on the outside. And it had to be designed and positioned so that the tunnel boring machine could come and hit it within an inch or two of tolerance.”
A successful result
After three years of excavation, the TBM finally reached the intake structure and mined in directly on target, forming the link between the tunnel and intake. The bulkhead was then lifted from the top of the intake, securing Las Vegas’s water source for years to come.
The lessons learned from the project are already being used to inform other major tunnels around the world, like Hong Kong’s Tuen Mun–Chek Lap Kok Link.
Check out the latest issue of the Arup Journal to learn more about the technical details of this project, including geotechnical challenges, TBM assembly, and tunnel construction.