Tunneling Under Pressure
Herrenknecht TBM S-502 in the intake structure with the bulkhead separating the lake and the tunnel. The project encountered water pressures up to 15 bar, which combined with heterogeneous soils, made mechanized tunneling extremely challenging from a technical standpoint.
Las Vegas, the glittering city of luck with its fascinating fountains, draws 90 percent of its water from Lake Mead. The largest reservoir in the United States, Lake Mead is fed by the Colorado River. The areas around Las Vegas, however, are taking more from the lake than flows into it as snow melt and rain water from the Rocky Mountains.
Over the past 14 years of prolonged drought, the water level has dropped by 115 ft. As a result, the two existing water intakes for the Las Vegas Valley are in danger of running dry, and Intake No. 1 could be at risk as early as next year – it is now only 24 ft below the lake level.
The construction of a new, deeper water intake commissioned by the Southern Nevada Water Authority (SNWA) is a race against time. But now the crucial section has been mastered: on Dec. 10, 2014, after three years of tunneling, the Herrenknecht tunnel boring machine (TBM) S-502 (7.2-m/23.6-ft OD) reached its target on the lake bed. Starting from a 600-ft deep shaft, it has dug a 2.7-mile tunnel under the lake. During this time the machine was subject to water pressure of up to 15 bar – a new world record. The previous record of 11 bar had been held by a Herrenknecht multi-mode TBM for the Swedish tunnel project Hallandsås, completed in 2013.
Herrenknecht developed and built the Las Vegas machine in close cooperation with the customer, Vegas Tunnel Constructors. The joint venture of Salini Impregilo (Milan, Italy) and S.A. Healy Co. (Chicago) is constructing the intake tunnel for the SNWA. The tunnel passes through the so-called Muddy Creek Formation, with alternating layers of hard rock and conglomerates as well as fault zones. The fault zones full of secondary minerals are partly filled with water from Lake Mead.
The machine is a multi-mode TBM, which can be operated in open or closed mode depending on the geological conditions and has considerable additional equipment. Thus, the 7.2-m diameter TBM is ideally designed for the extreme conditions of the project.
The Las Vegas machine launched in late 2011 from a shaft at the lakeside. For months it struggled through shattered rock and clay with full hydrostatic pressure caused by water from the lake. In doing so, it had to withstand pressures of up to 15 bar, an extreme challenge in mechanized tunneling. As a comparison: 15 bar is like diving at 500 ft or at a pressure of 200 psi. With a diameter of over 20 ft and a length of 600 ft, immense forces were applied on the machine.
Herrenknecht S-502 is a multi-mode TBM, which can be operated in open or closed mode according to the geological conditions. Thus, the 7.2-m diameter TBM is ideally designed for the extreme conditions of the Lake Mead project.
Forward Progress
The TBM chewed away through abrasive geological formations, resulting in repeated downtimes to replace wear parts. Discs, parts of the cutting wheel and the entire bearing seal were replaced. As soon as the machine reached good, stable conditions, the site crew changed from closed slurry-supported mode to open hard rock mode. In this mode, the machine quickly ate its way through the rock at speeds of 1.5 to 2 in. per minute and up to 1,000 ft a week.
Over the total distance, the machine tunneled about 40% in open hard rock mode and about 60% in closed slurry-supported mode. Tunneling was interrupted at regular intervals for maintenance and repairs. The work was carried out proactively by Vegas Tunnel Constructors in close collaboration with Herrenknecht.
“Constructing a project of this magnitude certainly brings unique challenges,” said, Erika Moonin, SNWA project manager. “This is a significant accomplishment for the entire project team. The field crews deserve special recognition for their hard work and dedication; they work in difficult conditions and consistently bring their best each day.”
Jim Nickerson, Project Manager at Vegas Tunnel Constructors, added: “We had planned a maximum of 30% of tunneling in the more difficult closed mode; it ended up being almost 60%. That was a major technical challenge and required extensive maintenance. The fact that we were able to drive the tunnel successfully despite the adverse conditions is the result of very good cooperation with Herrenknecht.”
On Dec. 10, 2014, with centimeter accuracy, the Herrenknecht multi-mode TBM reached its target at the bottom of Lake Mead. Starting from a 180-m deep shaft, within three years it dug a 4.4-km tunnel under the largest reservoir in the United States.
Multi-Mode Tunneling
Adapting to the geological conditions, the Herrenknecht TBM can operate in open and closed mode. In open mode, the excavated material is fed to the cutterhead center, where the central screw conveyor removes it onto belt conveyors. In the event of water inrush, the main chamber is sealed by closing the rear discharge gate. In this way, the change from open to closed mode can be done in just 2 minutes. In the closed mode, the TBM is driven completely in the slurry-supported mode with a cushion of air to control the support pressure.
Three drills installed in the shield area offer comprehensive advance drilling possibilities for probing and grouting. If required, a fourth drill can be installed on the erector. They allow stabilization of the tunnel face both for the drive as well as for inspections and maintenance. This increases the possibility of working under atmospheric conditions in the excavation chamber or even at the tunnel face. If the water pressure is too great, work needs to be carried out in a compressed-air atmosphere. For this case, the TBM is equipped with a special, newly developed lock system that allows normal compressed air entries to about 4.5 bar. For use at even higher pressures or for longer work intervals, saturation divers can get to the machine and back again by transfer shuttle under constant pressure.
“In close cooperation with the customer, we have configured the Las Vegas machine in such a way that we are well prepared for all possible situations,” says Werner Burger, chief engineer at Herrenknecht. “Even though Vegas Tunnel Constructors did not have to pull out all the stops, it was good to know that they could have. With such demanding projects, in addition to Plan A you also need a Plan B or even C.”
The Herrenknecht TBM cutterhead with the last of the intake front showing the disc cutter marks and kerf spacing.
Sustainable Future
Lake Mead Intake No. 3 substantially increases the reliability and flexibility of Southern Nevada’s water treatment and delivery system. Combined with the low lake level pumping station that is currently under consideration, the new intake enhances the SNWA’s existing intake system, enabling Southern Nevada to continue accessing its water resources in Lake Mead, even if lake elevations drop too low for Hoover Dam to release water to downstream users.
“This is one of most complex and challenging tunneling operations ever attempted; therefore, today is a significant milestone for this historic project.” said Southern Nevada Water Authority (SNWA) General Manager John Entsminger. “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.”
Work on Intake No. 3, which was initiated in 2008, is due for completion in the summer of 2015. The new extraction tunnel will then take the lake water via Intake No .2 to a drinking water treatment plant, from where it will be supplied to households and businesses.
Information for this report was supplied by Herrenknecht and the Southern Nevada Water Authority. Photos are from Herrenknecht.
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