Vegas' Sure Thing:
New Intake and Tunnel Will Provide Reliable Source of Drinking Water for Desert Valley
By Philip R. Snyder

The history of the U.S. West is closely linked with water. Cities like Los Angeles, Phoenix and Las Vegas – in fact, all cities, big and small – cannot exist without a carefully planned and sustainable water source. With the recent drought in the West, at no time have water resources been more important.

In the Las Vegas area, officials are taking steps to ensure that fresh water is available in the event of a prolonged drought. Lake Mead, the body of water formed by the impoundment of the Colorado River by the Hoover Dam, serves as the primary source of water for the area. The lake provides about 90 percent of the region’s freshwater needs.

As a result of the recent drought, however, the water level of the lake has dropped, so much so that officials fear that one of two existing intakes may not be usable. The Lake Mead Intake No. 3, now under construction, will serve to ensure that water will flow even if one of the intakes is knocked out of service.

Additionally, by tapping into water deeper in the lake and farther upstream, the Las Vegas area will get a higher quality water that will reduce treatment costs.

Managing Water Resources

Formed in 1991 to address the unique needs in the Las Vegas Valley, the Southern Nevada Water Authority is charged with managing the region’s water resources and developing solutions that will ensure future water supplies in the area. The Authority comprises seven member agencies: Big Bend Water District, Boulder City, Clark County Water Reclamation District, Henderson, Las Vegas, Las Vegas Valley Water District and North Las Vegas.

Upon its formation, the Authority assumed control of the Southern Nevada Water System infrastructure, which included a lake intake constructed in 1971. In 2002, construction of a new intake into the lake was completed at a deeper elevation. It was about that time, however, that the Las Vegas area began experiencing drought conditions that spurred officials to begin planning for a new intake.

“The drought started about 10 years ago, and it was in the early 2000s when the Colorado River was well below its average that water planners began to talk about a third intake,” recalled Marc Jensen, director of the Authority’s Division of Engineering. “In 2005, the board authorized us to include the intake into our capital plan.”

According to the National Park Service, Colorado River runoff has been down each year beginning in 1998. As a result, the water level in the lake – which is fed by snowmelt in the Upper Basin states of Colorado, Utah, New Mexico and Wyoming – is at its lowest point in more than 40 years.

Components of the new intake include an intake tunnel approximately 3 miles long and 20 ft in diameter, an underground pumping forebay, pumping station, electrical power connections and a discharge pipeline to the Alfred Merritt Smith Water Treatment Facility. The project is scheduled for completion in 2013.
Tapping into Experience

In order to access the lake from a deeper elevation, the tunnel needed to extend farther out into the lake than previously constructed intakes. In doing so, the alignment will pass through variable formations that create construction challenges that were not a factor in the construction of Intakes 1 and 2, which were completed primarily by drill-blast methods. Ground conditions include metamorphic, sedimentary and volcanic rocks.

In conducting geologic surveys of the area, the Authority discovered five areas along the alignment that potentially contain fractured rock with the connectivity to the lake that could have high water inflow. Additionally, one area passes under an old river wash area that could have sedimentary conditions. The Authority did numerous borings to determine where the bedrock was in that location.

Because of the challenging ground conditions, in addition to the tight timetable for completion, the Authority opted to use the design-build approach to contracting. Design-build, in which the owner hires a single entity for design and construction services, can accelerate the schedule by overlapping design and construction periods, and can save costs by allowing the contractor to have more input on the design.

In the case of Intake No. 3, Authority planners opted for design-build because they felt that it also provided the best means for risk mitigation. “We felt that the contractor was in the best position to define the means and methods,” Jensen said. “We felt it was the best way to manage risk and ensure the successful completion of the project.”

In 2003, a Nevada law was modified to allow design-build procurement. “We had a previous contract that we awarded design-build, and we felt that gave us enough experience to move forward with this one,” Jensen added.

As per state law, the contract for Intake No. 3 was awarded following a two-step process. After the evaluation of preliminary proposals and qualifications, the Authority requested final proposals from three of the four firms, offering stipends for those who submitted final proposals. Final proposals were evaluated considering a range of factors, including 30 percent based on price, with recommendations made to board following evaluations.

The contract was eventually awarded to Vegas Tunnel Contractors – a joint venture of Impregilo and Healy – with a bid of $447 million for the project. Design services for Vegas Tunnel Contractors are being performed by Arup and Brierley Associates. MWH and CH2M Hill were the engineers for the Authority for preliminary design. Parsons is performing construction management services for the owner.

Impregilo-Healy comprised the joint venture that completed the Portland West Side CSO contract on a cost reimbursable plus fixed fee basis. While not a design-build, that contract leaned on the experience of the contractor by bringing the firm on board before final design.

Additionally, the Intake No. 3 contract contains fixed prices for hyperbaric interventions for maintenance to the face of the tunnel boring machine, as well as for pre-excavation grouting of the shaft and tunnel. Because of the uncertainty associated with costly and time-consuming hyperbaric interventions and pre-excavation grouting, an allowance payment item is provided in the event that the amount exceeds reasonable estimates outlined in the proposals. Cost escalation for materials is also addressed. The Authority also opted to include a geotechnical baseline report and is using a dispute review board.

“The owner has shown that they recognize what it takes for a contractor to be in business and they are committed to being a partner so that the project is completed successfully,” said Jim McDonald, project manager for Vegas Tunnel Contractors.

Pushing the Limits

One of the most innovative elements of Intake No. 3 is the tunnel boring machine, which will need to negotiate variable ground under potentially high water pressure. The machine, being constructed by Herrenknecht, will combine the slurry approach used for the Portland East Side and West Side CSO projects and Seattle’s Brightwater project, with the hard rock design used for the Arrowhead East and West projects in Southern California.

“The machine is designed for extremely high pressure and can operate in slurry mode or in open mode with a screw auger that feeds a conveyor system,” McDonald said. “The screw auger is located at the center of the face, instead of at the bottom like a conventional EPB machine, which will help seal the face in the event of water inflow, as well as reduce wear.”

The machine is equipped with three Atlas Copco drills – two behind the cutterhead in an area that can be pressurized – for probing and pre-excavation grouting. A fourth drill can be mounted on the segment erector if needed. It is anticipated that the machine will operate in open mode from the start, but will switch to closed mode through a fault zone early in the alignment. Fractured basalt near the end of the alignment could also force the contractor to operate in closed mode.

While the machine is specified to operate in pressures up to 17 bar and perform maintenance under hyperbaric pressure, the contractor is hopeful that won’t be necessary. “We hope to do everything in atmosphere,” McDonald said. “By probing and pre-excavation grouting we feel that we should be able to find competent rock to stop in.”

McDonald said that Impregilo-Healy performed interventions of up to 4 bar in Portland, while Impregilo has performed interventions up to 6 bar in Russia.
The machine is being built in Herrenknecht’s factory in Schwanau, Germany. Acceptance testing was scheduled for June with delivery beginning in September. It is anticipated that mining will commence in April 2010 following chamber construction. The machine will install precast, gasketed segments which are being produced by Precast Management.

Making the Connection

Another tricky part of Intake No. 3 is the intake itself. Instead of drilling in the riser shafts and driving the tunnel to it as is common on shallow lake taps, the JV is planning to take an immersed tube type approach. The intake is to be constructed in sedimentary rock with the potential for water inflow.

To minimize the risk – and save time off the schedule – the JV is planning to build the intake structure and sink it into the lake bed, and then use the structure as an entry point for the TBM. The approach would also limit the man hours needed for divers to make tunnel connections 300 ft below the lake surface. Excavation for the intake is scheduled to begin in late summer.

Other design elements being incorporated by the JV are the use of plain concrete through the shaft to accommodate the top-down lining approach. Sprayed concrete and rockbolts in the adits and stub tunnels are being utilized to streamline the process, according to Arup’s Jon Hurt, design manager for the JV.

Currently, crews are sinking the 30-ft access shaft on Saddle Island. In late May the shaft was down 370 ft of 600 ft. Excavation is proceeding by drill-blast using a three-boom Tamrock shaft jumbo to drill blast holes. Because of the proximity to the lake and the nature of the ground, high water inflow is expected. To combat this, crews have adopted a regimen of probe drilling and pre-excavation grouting.

At the bottom of the shaft, a 200-lf, 46-ft wide by 34-ft high cavern will be built to serve as a launching chamber for the TBM. A stub tunnel will also be built 180 degrees from the launch chamber to allow for future expansion.

Also included in the shaft construction, about 400 ft from the surface, will be an 85-ft long by 20-ft wide by 20-ft high stub tunnel that will connect to an underground pump cavern that will be built under another contract. The stub tunnel will be used to house pumps for the slurry circuit and water discharge system during construction of the tunnel. Another short stub at about 200 ft below surface elevation will also be used for slurry and water pumps to reduce the amount of lift required on each pump.

Upon completion of the project, the Las Vegas area will have a reliable supply of water to meet its needs well into the future.

James W. Rush is editor of TBM.