Lake Mead inflows - TunnelTalk
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Lake Mead suffers another setback Jan 2011
Paula Wallis, TunnelTalk
A third inflow of water and material on the Lake Mead Intake No 3 project has the owner and contractor reviewing more aggressive options on a way forward.
Another inflow of muck on New Year's Eve (31 Dec) forced workers on the graveyard shift to pull back and take remedial measures. According to Marc Jensen, Engineering Director for the Southern Nevada Water Authority (SNWA), the crew was drilling drain holes in the TBM starter tunnel and preparing for additional grouting in the ongoing recovery operation from previous inflows.
Southern Nevada Water Authority Intakes 1, 2 and 3

Southern Nevada Water Authority Intakes 1, 2 and 3

Jim McDonald, Project Manager for the design-build contractor, VTC, declined to discuss the situation when contacted.
Speaking for the Owner, Jensen said: "This most recent inflow was a much smaller event of maybe 100 to 150yds3 (91m3 - 137m3) of material. But it is the third event. The first occurred over a four day period at the end of June when inflow of more than 400yds3 (366m3) of water and material caused a full retreat."
That initial event happened about 250ft (76m) into the 350ft (106m) long starter tunnel and flooded the 200ft (60m) long TBM launch chamber and rose about 150ft (46m) of the 600ft (182m) deep access shaft. It was during recovery of that inflow when the second event occurred on October 27.
"The contractor had almost recovered and was close to resuming excavation at the original tunnel face when our hopes were dashed and the several hundred cubic yards of the second incident flowed in," said Jensen. "Crews were able to install a bulkhead about 40ft (12m) back from the incursion area to stop the muck entering the TBM assembly chamber. As they were recovering from that event the third inflow occurred on New Year's Eve. So it has been a very troubling and difficult situation. The geology has not been cooperative."
Following the third inflow crews installed a second bulkhead behind the first as a precaution, said Jensen, who added that the bulkhead does not have a full flow of material against it.
"The chamber is still relatively clear," said Jensen. "We have not had any significant amount of material flow into the TBM assembly chamber so the integrity of the chamber has been preserved. All of this is still confined to the starter tunnel."
VTC, The Vegas Tunnel Constructors, is the joint venture between Impregilo of Italy and its US subsidiary SA Healy of Chicago that won the $447 million design-build contract in March 2008. Arup is the contractor's lead designer and Brierley Associates is geotechnical engineer and designer of the conventional excavations. MWH/CH2M-Hill JV prepared the GBR and Parsons is providing Program Construction Management. The tunnel is part of a $700 million new and deeper water intake program by SNWA to secure supply of water from Lake Mead to Las Vegas and surrounding region.
The 7.18m diameter Herrenknecht Mixshield is on site and ready for launch

The 7.18m diameter Herrenknecht Mixshield is on site and ready for launch

The GBR identified several faults in the area, but Jensen said, where they are, how wide they are, how far they extend and their orientation was the subject of considerable speculation. It is a geological feature that is common to all three of the intake tunnels. There were some surprises during excavation of the first two intakes in regards to facture zones where additional water came in, but Jensen said, the rock was relatively more stable and more cohesive.
The size of the initial void is still unknown. "It is hard to say for sure," said Jensen. "We know there was 400yds3 (366m3) of cementitious material pumped into the void that was encountered above the inflow zone, but whether we completely filled that void is hard to say. Probably not because more than 400yds3 (366m3) of material flowed into the chamber. Following the second event in October nobody tried to do any kind of probing from the surface to identify another void."
As recovery of the starter tunnel continues, work progresses elsewhere on the project. This includes construction of the prefabricated intake riser. Crews are still blasting the bottom of the lake and mucking out the rock to lower the nearly completed riser structure into the excavation.
Fabrication of the tunnel segments is also ongoing, as well as some work to prepare the assembly chamber for the TBM. The Herrenknecht Mixsheild TBM arrived on site October 2009 and has been preassembled on the surface ready for launch as soon as the starter tunnel is finished.
The series of setbacks has crushed the construction timeline, pushing the initial launch of the TBM from summer last year (2010) to an undetermined date and throwing off target a 2013 inauguration of the new intake.
"A lot depends on where we go from here," said Jensen. "VTC is identifying alternatives for either proceeding or doing something different and they are still debating and discussing that with us. Until we have a decision, I'm not comfortable sharing the information, but we are getting close to making a determination."
When asked if a change of alignment was one of the options on the table, Jensen said: "yes", but would not elaborate. Another option floated earlier was possibly launching the TBM in a shorter started tunnel.
It is too soon to say how long the work will be stalled or how much the delays will cost. Jensen said loses will certainly run into tens of millions. Exactly who will pick up the tab for the overruns is a topic for future discussions between SNWA and VTC.
"We are still talking with each other very civilly and working together as partners to find the best solution," said Jensen. "Everybody has the same goal, they want to get this project completed successfully, as quickly and as safely as it can be. These setbacks have been difficult for all of us."
References
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
New Lake Mead intake for Las Vegas - TunnelTalk, Sept 2006

           

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