No Room to Gamble
The lure of fast money, glamorous shows, upscale shopping and fine dining draws thousands of tourists to Las Vegas each year and pumps millions of dollars into the local economy. But the region has a much more precious resource than gaming and tourism: water.
Surrounded by desert, Las Vegas Valley relies on the Colorado River and its two primary reservoirs, Lake Powell and Lake Mead, to provide enough of this “liquid gold” for its citizens. But as the region’s population has grown over the past decade, water resources have dwindled. A severe 11-year drought has caused the combined storage of both lakes to drop to 56 percent of capacity. By October 2010, Lake Mead alone had shrunk to 1,082 feet above sea level, just 39 percent of capacity. If the lake levels keep dropping, the situation could become dire. At 1,050 feet above sea level, Lake Mead would no longer be able to service one of its two intakes, creating a significant delivery shortage and affecting the Hoover Dam’s ability to generate electricity.
In May 2008, SNWA awarded six contracts totaling $817 million, including a $447-million design-build contract to Vegas Tunnel Constructors LLC--a joint-venture of S.A. Healy Co., Lombard, Ill., and Impregilo S.p.A., Sesto San Giovanni, Italy. The Vegas Tunnel Constructors project involves digging a 600-foot-deep tunnel access shaft on Saddle Island, running a 20-foot-diameter precast-concrete-lined intake tunnel 3 miles under the lake bed, and building a 16-foot-diameter, 60- to 80-foot-deep water intake shaft in Lake Mead. It’s a project that requires substantial skill and extensive teamwork--along with precise control and careful data management.
Tight TolerancesWhen Mike Adams, lead surveyor for Vegas Tunnel Constructors, joined the team in May 2008, he knew he was in for a challenge. With more than 18 years of experience on tunnel projects in his 34-year surveying career, Adams brought the expertise needed to ensure the control remained within tolerance. Still, it would be a professional milestone. “To my knowledge, no one has ever gone that distance at that depth under a lake before,” he says. “It has certainly been the most challenging project of my career so far.”
Initially, Adams was the sole surveyor for the Vegas Tunnel Constructors team. Using a Leica TCRA 1103 Plus Robotic Total Station, Adams set control for the construction roads and grading as the team began its work and then established control as the 600-foot-deep tunnel access shaft was dug on the west side of Saddle Island. That’s where the real challenge began. “The vertical tolerances on the shaft had to be very tight, but then there’s only a 30-foot backsight at the bottom before going three miles out. In other projects, I might have been able to go out and set out a backsight 500 or 1,000 feet away and then go with it. Here, everything I do has to be very, very tight. There is no room for error.”
The same holds true on the other end of the tunnel. With the water intake shaft 300 feet under water, closing the traverse isn’t possible. Yet all measurements have to be within 12 inches, and the connection to the intake has to be within 4 inches. Any mistakes would be extremely costly.
To maintain tight control, Adams works closely with the subcontractor who is mapping the lake bed as Vegas Tunnel Constructors does the blasting work. They use GPS on the water, but with a caveat. “The project relies on surface coordinates rather than state plane coordinates,” Adams says. “Near the beginning of the project, Clark County made a correction in its GPS and moved everything 6 feet to the east, so we’ve had to reposition ourselves using a localized program and ensure that we’re on the same coordinate system. Just about everything I’ve done out here, I’ve had to traverse.”
Data DeliveryOf course, collecting the data is only one part of Adams’ job. He then has to convey the coordinates to the construction team. Although Adams uses several different types of software in his work, he relies heavily on Traverse PC for his drawings. “I’ve been using Traverse PC since about 2002 in Alabama,” he says. “It’s very user friendly. I like the reduction it gives on my traverses, and it makes nice drawings. I’ve used it for four tunnels, and I’ve never been more than 1/10 inch off.”
Adams notes that ease of use was one of the main reasons he chose Traverse PC. “I don’t have a lot of time to go through all the steps required in other software programs,” he explains. “Traverse PC draws everything for me; all I have to do is put my points in. I can use it for slope staking and calculations, and I can lay my points over a map so everybody can see the data in a visual format. And it’s AutoCAD friendly, so I can make a drawing in Traverse PC and send it to my engineer, and he can make a 3D drawing out of it.”
Adams now has help with the surveying work. In January 2009, he hired an assistant, and later that year the crew added a third person, a registered land surveyor from California with nine years of tunnel experience. Still, even with a larger crew, there’s always plenty of work to do. “We’re traversing every week, checking for movement. We have to go back to the start every time to make sure nothing has moved,” Adams says. “We’ll soon be putting concrete segments in to control the water, and those can move. It’s a continuous schedule of traverse work. We have to keep it as tight as we can, and we have to be able to quickly convey the data to the rest of the team.”
The project was originally scheduled to be completed in July 2012, but the construction team hit a glitch in June 2010 when water flooded an assembly vault that was going to be used as a starter tunnel for the tunnel boring machine. Fortunately, no injuries occurred. Digging a new starter tunnel will add a number of months to the project timeline, moving the anticipated completion date to sometime in 2013. Other parts of the project remain on track, including a $42-million, 14-ft-wide by 16-ft-tall connector tunnel being built by Renda Pacific Contracting Inc., Roanoke, Texas. Another contractor, Barnard of Nevada, completed a $32 million modification project earlier this year that will connect the existing Intake No. 2 to Intake No. 3.
For his part, despite the highly challenging nature of the project, Adams doesn’t expect to encounter any problems. “I’ve been doing this type of work for most of my career,” he says. “I’m confident that we’ll be able to meet the project requirements.”
SNWA: Sustainable Water Management for Southern NevadaThe Southern Nevada Water Authority (SNWA) was formed in 1991 by a cooperative agreement among the following agencies in Southern Nevada:
- Big Bend Water District
- City of Boulder City
- City of Henderson
- City of Las Vegas
- City of North Las Vegas
- Clark County Water Reclamation District
- Las Vegas Valley Water District
Key Project Players for Intake No. 3Owner: Southern Nevada Water Authority
Construction Manager: Parsons
Design-Build Contractor: Vegas Tunnel Constructors LLC
Engineering Firms: Arup; Brierley Associates; CH2M Hill; MWH
Subcontractors: Crux Subsurface Inc.; Precast Management Corp.; Contri Construction
For more information about SNWA, visit www.snwa.com. S.A. Healy Co.’s Web site is www.sahealy.com, and Impregilo S.p.A. is at www.impregilo.it. Additional details about Leica equipment can be found at www.leica-geosystems.us. Information about Traverse PC software can be found at www.traverse-pc.com.