Long wait over for Lake Mead TBM Sep 2011
Paula Wallis, TunnelTalk
- Following a series of setbacks, the TBM is finally going into the ground on the Lake Mead Intake No 3 project outside Las Vegas, Nevada. The shield and the cutterhead of the Herrenknecht Hybrid Mixshield were lowered into the access shaft last week (7 Sept) after sitting on site for almost two years.
- The original 350ft (106m) starter tunnel was abandoned after inflows of muck and water derailed excavation on three separate occasions.
- "We are feeling better about the project," said Jim McDonald, Project Manager for the Vegas Tunnel Constructors JV of Impregilo and its US subsidiary, SA Healy. "The new alignment for the starter tunnel turned out exceptionally well. The ground was just a world away from the conditions we had encountered on the original alignment."
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Starter tunnel construction
(Video by SNWA)
- The problems started at the end of June 2010. Excavation was about 150ft (46m) into the tunnel when an inflow of more than 6,500yds3 (5,000m3) of water and material flooded the 200ft (60m) long TBM launch chamber and rose about 150ft (46m) in the 600ft (182m) deep access shaft. Another several hundred cubic yards flowed in on October 27, 2010 during the recovery operation and a third event happened on New Years Eve. Two bulkheads were installed to prevent muck of the second and third incursions entering the TBM chamber.
- In January 2011, a decision was made to change the alignment for better ground. "The original alignment was plugged with grout and concrete, back to a bulkhead about 50ft (15.2m) from the TBM chamber, and a new alignment veered off at a 23° angle to the right to navigate around the unstable area," said Marc Jensen, Engineer for the Owner, Southern Nevada Water Authority.
- Excavation on the new starter tunnel, which is at the same depth as the original, began in February this year and was completed in late July.
- "Excavation was all drill+blast, but we did install a pipe canopy pre-support system for about two third of the 370ft (113m) length because of the uncertainty," said McDonald. "It is harder to start the canopy installation when it is too late so we started early as a precaution."
- "It was a blocky fractured ground, but it did not have the same loose unstable material encountered on the original alignment," said Jensen. "Progress was slow but steady, and on July 26 the contractor reached the end of the starter tunnel, which is about 100ft longer than the original, to accommodate the complication of having to assemble and launch the machine on the angle off the assembly chamber."
- With the starter tunnel complete the Herrenknecht TBM, that arrived on site October 2009, will take over excavation of the 3 mile (4.8km) tunnel to an intake shaft on the bottom of Lake Mead about 300ft (91m) below water level. Capable of converting rapidly from an open-faced system to a closed pressurized system, it will be asked to withstand potential hydrostatic pressures of up to 17 bar.
- The TBM will excavate several thousand feet before rejoining the original tunnel alignment for a target of the same position and depth.
Lake Mead Intake No 3 alignment
- "It sure was a tough start, but we are optimistic that the worst is behind us," said McDonald. "We have probed ahead of the starter tunnel end wall with another horizontal core hole and we should have a pretty good area for launching the machine."
- Construction of the $447 million design-build contract, with Arup as lead designer, began in March 2008 with the access shaft on Saddle Island. Brierley Associates designed the conventional excavations, MWH/CH2M-Hill JV prepared the GBR, and Parsons is providing Program Construction Management.
- "The realignment and the associated differing site condition costs totaled about $44 million, some of which we will recover from our builders risk insurance policy," said Jensen. "The contractor did agree to some responsibility and some risk and that was all included in the agreed amount." Despite all this Jensen said the relationship between the Owner and Contractor remains strong and possibly better than ever.
- The contractor estimates it will take about three months to assemble the TBM for an expected launch date at about the first of the year. "Because the shaft is so deep, we are using a strand jack system to lower the TBM and its components so it is a little slower than some, but it is steady," said McDonald.
- Slow and steady may be a welcome change considering the tumultuous start that has pushed substantial completion out to May 2014, and a challenging drive still to come.
Lake Mead suffers another setback - TunnelTalk, Jan 2011
Inundation at Lake Mead Intake No 3 - TunnelTalk, July 2010
Lake Mead No 3 intake tunnel awarded - TunnelTalk, June 2008
Lake Mead TBM designed for the extreme - TunnelTalk, Nov 2009
By J McDonald, Vegas Tunnel Constructors and W Burger, Herrenknecht AG
New Lake Mead intake for Las Vegas - TunnelTalk, Sept 2006
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