World Record Set Las Vegas Tunnelling Project Withstands Highest Pressurized Conditions
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 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. During the 14 years of drought, the water level has dropped by 35 metres. The two existing water intakes for the Las Vegas Valley are in danger of running dry, and Intake No.1 could be at risk next year. It is now only 24 feet (7 metres) 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 December 10, 2014 after three years of tunnelling the Herrenknecht tunnel boring machine (TBM) S-502 (Ø7.2 metre) reached its target on the lake bed. Starting from a 180-metre-deep shaft it has dug a 4.4-kilometre tunnel under the lake. During this time the machine was subject to water pressure of up to15 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, Illinois) 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 according to the geological conditions and has considerable additional equipment. Thus the 7.2-metre-diameter TBM is ideally designed for the extreme conditions of the project.
Three years of top performances
The Las Vegas machine launched in late 2011 from a 180 metre deep 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, a novelty in mechanized tunnelling. As a comparison: 15 bar is like diving at 150 metres or at a pressure of 15 kg per centimetre squared. With a diameter of over 7 metres and a length of 190 metres, immense forces were applied on the machine.
The TBM slaved away through abrasive geological formations, resulting in repeated downtimes to replace wear parts. Disks, 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 the multi-mode machine quickly ate its way through the rock at speeds of four or five centimetres per minute and up to more than a hundred metres a week.
Over the total distance the machine tunnelled about 40 percent in open hard rock mode and about 60 percent in closed slurrysupported mode.
Jim Nickerson, project manager at Vegas Tunnel Constructors: “We had planned a maximum of 30 percent of tunnelling in the more difficult closed mode; it ended up being almost 60 percent. That was a major technical challenge and required extensive maintenance.”
Special design of the multi-mode TBM for high groundwater pressures
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 centre, 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 120 seconds. In the latter the Multi-mode 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.
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.
Work on Intake No.3 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.