The demanding TBM drive out under the bed of Lake Mead is over. The 7.18m diameter hybrid Herrenknecht TBM operated by crews of the Vegas Tunnel Constructors (VTC) JV of Salini Impregilo and SA Healy Co, completed its mission on Wednesday 10 December 2014 with breakthrough into the project’s intake structure to record yet another technical achievement for the Lake Mead No 3 Intake Tunnel project.
Working under an operating face pressure of 8.9 bar, the Herrenknecht mutli-mode Mixshield TBM bored through the soft eye of the previously installed intake structure to complete one of the world’s most technically demanding tunnel drives to date. At breakthrough, the underwater intake structure was open to the lake and exposed the TBM to a hydrostatic head of 110m or 323ft of water at invert level.
Once the TBM had passed through the soft eye, VTC crews, working from a barge anchored in the lake, installed a hemispherical bulkhead into the throat of the intake structure, setting the steel bulkhead onto a corbel at about 100m below the lake surface.
On reducing the pressure of the air bubble in the TBM’s plenum, the water in the intake structure was drained through the machine’s slurry system, which allowed crews to enter the TBM chamber in free air and through the cutterhead into the riser for the breakthrough celebration photos. With ventilation services established, crews will begin sealing the tunnel to the intake structure and start dismantling the TBM for withdrawal of components back through the completed tunnel and its 182m (600ft) deep access shaft to the surface, an operation that is expected to take about two months.
In explaining the hole-through process, Jim McDonald of the VTC JV said, “there is less than 20 gal/min (1.5 l/sec) of water seeping into the working area through the annulus around the TBM shield.” This will be sealed off with the finishing works and the installation of M&E works to complete the new water Intake No 3.
McDonald also added thanks from the JV to its client, Southern Nevada Water Authority, and to Herrenknecht, “for the partnership approach that is bringing this complex project to a successful conclusion”.
The new intake project is designed to secure adequate supplies of potable drinking water from Lake Mead to customers in the Las Vegas service area. As the TBM progressed, accurate guidance by the machine's VMT guidance system was used to ensure a spot-on junction with the soft eye in the preinstalled intake structure that was completed in 2012.
The intake structure consists of a lower concrete cast and placed section with a prefabricated stainless steel riser and top portion.
“To build the intake structure, we first had to excavate the lake bed,” explained VTC JV Project Manager Jim Nickerson. “The soil overburden was excavated using a clamshell and a crane on the barge plus an airlift. The rock was blasted using shape charges, which were designed specifically for our job,” he continued. “We blasted three times a day with 49 charges/blast and in total, we ended up detonating 23,000 charges.”
Once the foundation was prepared, more than 11,200yd3 (8,500m3) of tremie concrete was placed around the lower precast structure. “This was placed working continuously during a 12-day period,” said Nickerson. “The rock in the area is basalt, which is weathered and blocky, and it was actually through the tremie concrete that the TBM mined into the eye of the intake structure.”
To complete the project, carrier pipes will be backfilled into the segmentally lined tunnel.
In its news release statement, SNWA (Southern Nevada Water Authority) confirmed conclusion of the 3-mile (4.8km) journey by the Herrenknecht TBM that began in 2011. The 23.29ft (7.18m) diameter TBM was used to excavate the $817 million intake and ended with hole-through into the 100ft (30m) tall intake riser that was secured to the bottom of the lake in 2012.
“This is one of the most complex and challenging tunneling operations ever attempted,” said SNWA General Manager John Entsminger, “and the breakthrough is a significant milestone for this historic project. Coupled with a future pumping station capable of withdrawing water, even at extremely low lake levels, the new intake system helps assure a dependable supply of water for future generations of Southern Nevadans.”
VTC was contracted to complete the design-build contract on behalf of the SNWA in March 2008 with lead designer Arup and supported by Brierley Associates.
Through its construction, the project has been hit by a series of setbacks. These included a series of groundwater inundations that flooded the drill+blast starter tunnel and access shaft that added more than two years and almost $40 million to the project’s cost, and a fatal accident in the ring build area of the TBM drive that claimed the life of one of the workers. About 2,400 rings of five segments and a key line the 6m i.d. TBM tunnel. Extensive wear to the TBM, a damaged main bearing seal and failed pinion gear bearings then caused additional delays and required extraordinary measures to gain access for repair and replacements, measures reported in detail by TunnelTalk News Editor Peter Kenyon following his interview with VTC Project Manager Nickerson and SNWA Project Manager Erika Moonin in September 2014.
The project is acknowledged as a record-setting undertaking, encountering the highest operating water pressures of any tunneling operation to date. "Constructing a project of this magnitude certainly brings unique challenges,” said Moonin on the occasion of the breakthrough. “This is a significant accomplishment for the entire project team and the field crews deserve special recognition for their hard work and dedication. They work in difficult conditions and consistently bring their best each day.”
The Lake Mead Intake No 3 project is a race against time to secure reliability and flexibility of Southern Nevada’s water treatment and delivery system. The new intake enables Southern Nevada to continue accessing its water resources in Lake Mead, even if lake levels drop too low for Hoover Dam to release water to downstream users.
Construction of the lower level Intake No 3 facility was initiated in 2008 and the project is scheduled for final completion in 2015.