New York — Arup and Salini-Impregilo/SA Healy JV received the Global Tunnelling Project of the Year (over $500 million) at the NCE Tunnelling and Underground Space Awards 2015. The project was selected from a strong shortlist, which included three other Arup projects – the National Grid London Power Tunnels, C901 Admiralty Station in Hong Kong, and the Crossrail Running Tunnels in the UK, which was awarded the Judges Supreme Award.
Arup led the engineering design of the vital new intake tunnel for Lake Mead, the largest man-made reservoir in the United States, which provides water to Las Vegas residents. It will safeguard future water quality and supply amid declining lake levels and ensure that Lake Mead can provide water to Las Vegas in case of severe drought. The 4.6-km-long, 6.8-m-diameter Tunnel Boring Machine (TBM) Tunnel at Intake No. 3 is the deepest sub-aqueous tunnel in the world. Arup worked directly for Vegas Tunneling Contractors, a joint venture of Salini-Impregilo and S.A. Healy. The project is owned by Southern Nevada Water Authority, and has been in operation since September.
At 28 million acre-feet of water, Lake Mead covers 247 square miles and has a depth of 532 feet at full capacity. Before the Tunnel Boring Machine (TBM) Tunnel at Intake No. 3 was completed, the lake had two intake shafts. These shafts risked becoming inoperable as the lake's water level continued to drop, potentially shutting off water supply from the lake to Las Vegas residents. The new intake is more than 200 feet below the current lake level.
The three-mile-long TBM tunnel connects the 600-foot-deep access shaft onshore to the lake intake 330 feet below the surface. Due to the depth of the tunneling beneath the lakebed, the TBM was configured to work with high water pressures up to the current limits of the TBM technology; water pressures of up to 15 bar were encountered during tunneling.
"The integrated design of this significant and technically advanced assignment involved a team of global Arup engineers, and called on the firm's expertise in tunneling, geotechnics, hydraulics, site development and offshore design," said Don Phillips, principal and design manager for Arup. "The technical complexity and sheer risk to human health and viability make this project one of the most critical undertakings in the United States at the moment."
The tunnel passes through extremely challenging ground and groundwater conditions and required the expertise of Arup's materials and hydraulics specialists to design the project's deep water lake intake. Arup designed the concept for the intake structure in the lake bed during the bid stage to significantly reduce risk, cost and schedule — vital considerations for all stakeholders.
Arup provided structural, geotechnical, highway, maritime, and water engineering, as well as site supervision services and tunnel design. With extensive experience in major tunneling projects around the world, Arup has been involved in design and construction of 200 km of tunnels over the last decade, including 60 transport tunnels and more than 30 for water and various other services.
Created by the construction of Hoover Dam in the 1930s, the 110-mile lake lies on the Nevada-Arizona border about 30 miles (48km) southeast of Las Vegas, and is supplied by the Colorado River. The River provides water to more than 25 million people in seven states, including the residents of Las Vegas, Los Angeles and Phoenix. In its current state, it is estimated the reservoir has only 25 more years of water capacity before drying up. It has not reached full capacity since 1983.
The project team included:
- Owner: Southern Nevada Water Authority (SNW)
- Design-build contractor: Vegas Tunnel Constructors (VTC), a joint venture of S.A. Healy and Salini Impregilo
- Engineer-of-Record: Arup
- TBM Manufacturer: Herrenknecht
- Geotechnical engineer: Brierley Associates