Lake Mead No 3 intake tunnel awarded Jun 2008
Shani Wallis, Editor
Falling water levels in Lake Mead
- Design-build construction of the new Lake Mead raw water intake tunnel and shaft contract for the Southern Nevada Water Authority (SNWA) is awarded to Impregilo of Italy in JV with its US subsidiary SA Healy of Illinois. Lead designer for the JV is Arup with subconsultants, including Brierley Associate.
- The JV's $447 million proposal for the 3-mile (4.8km) long tunnel was 24% lower than the second bid of $588 million from the Traylor Bros/Obayashi Corp/Barnard JV with Halcrow as lead designer. A third qualified group invited to participate, of Kiewit, Vinci and Frontier-Kemper and lead designer Hatch Mott MacDonald, declined to submit a bid.
- Teams based their proposals on 10% design documents prepared for SNWA by the MWH/CH2M-Hill JV. The competing bidder received a $600,000 stipend for submitting their proposal.
Plan of the new Lake Mead intake tunnel
- With contract award in March 2008, Impregilo/Healy and its design team take on one of the most challenging projects of the moment. The third raw water intake will progress to an intake structure set at about 600ft (182m) below current lake level. Michael Feroz of Parsons Water Infrastructure Inc and Principal Construction Manager for SNWA said: "the new intake lies some 200ft (60m) deeper than the two existing intakes, built using drill & blast in the 1970s and the second in the late 1990s." The new tunnel will be a TBM operation starting in the open-mode and reverting to closed pressurized mode as the tunnel advances into softer formations where potential ground water pressures are equivalent to the 600ft (182m) hydrostatic head of the lake.
- Jim McDonald, Project Manager for the JV, moving to the project from the JV's successful pressurized TBM contract for the West Willamette River CSO abatement project for the Bureau of Environmental Protection in Portland, Oregon, confirmed that a 6m-diameter dual-mode Herrenknecht EPB/open mode TBM is on order for the job. From a 32ft (9.7m) diameter x 600ft (182m) deep access/pumping shaft on Saddle Island, the TBM will advance first through a block of hard Precambrian amphibolites and gneiss into softer rocks and the water bearing silts and sands of the Muddy Creek Formation to the 16ft diameter x 60-80ft deep (4.8m x 18-24m) intake shaft, erecting the six segments per ring in a one-pass bolted and gasketed concrete segmental lining as it goes.
Secion to the new intake structures
- The new deeper level intake in Lake Mead is needed to "sustain reliable water supply to the community of metropolitan Las Vegas", said Marc Jensen, SNWA Director of Engineering. Over the past eight years, water levels in the lake have fallen by more than 100ft (30m), compromising operation of the Authority's highest level intake. "Intake at the lower level will also draw better quality water from below the thermocline," said Jensen.
- Don Phillips, Design Manager for Arup, said, "we have delivered a design thatÉ..minimizes risk." The 1,571-day contract is scheduled to finish in July 2012. The overall $817 million project includes four associated design-bid-build contracts that are yet to bid for a 600-mgd pumping station; a connection tunnel to the second intake; a pipeline from the pumping station to the Alfred Merritt Smith Water Treatment Plant; and a large power substation and power lines. The full project is expected to be in service by early 2013.
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