Peter Kenyon, TunnelTalk
As northwest America moves into a fourteenth straight season of drought, latest data predicts water levels at Lake Mead in Nevada could plunge by further 21ft in the coming 12 months, reaching a critical trigger point of 1,062ft by April 2015. It is at this level that the aging Intake No.1, which currently draws much of the water needed for Las Vegas and the surrounding region, could fail. Peter Kenyon, for TunnelTalk, examines a much-needed current surge in TBM progress on excavation of the vital deep Intake No. 3 tunnel, the centerpiece of a delay-hit US$817 million project that may now not be completed until after the trigger point is reached.
- With water levels forecast to drop sharply at Lake Mead over the coming two years, more than ever before all eyes are on the tunneling effort that is now, at long last, making good progress on a final heading below the lake bed.
- Latest estimates anticipate TBM excavation will now be complete by Fall 2014, two years later than the October 2012 date that had been scheduled for final completion of the entire project when contracts were signed between the low-bidding Impregilo/SA Healy Vegas Tunnel Constructors (VTC) JV and Project Owner South Nevada Water Authority (SNWA) back in 2008. A revised commissioning date has not yet been established.
TBM has completed more than a third of the 3-mile excavation
- When TunnelTalk last reported on the project, in June last year (2012), progress on the US$447 million deep-level tunnel that will enable water to be drawn from the very bottom of the lake, was recorded at 750ft (228m). This included 500ft (152m) of already-excavated drill+blast tunnel along a new alignment forced upon the contractor as a result of inundation of the previous heading, followed by two further floods in the second half of 2010. Bronson Mack for South Nevada Water Authority reported Wednesday (October 9) that at 6,100ft (1.8km) the TBM is now more than a third of the way towards completion of its 3-mile (4.8km) drive, and, finally, on a heading that now sees it operating under the hydrostatic pressure of the lake above.
- This represents good progress on a previously stop-start drive that suffered a month-long delay following a fatality in June 2012 - a subsequent investigation revealed there were a unique set of events and that the accident could not have been easily prevented - followed in March this year by a 10-week maintenance stop for "required" repairs to the cutterhead and replacement of the disc cutters.
- Progress has been as exceptional since July (2013) as it was tortuous in the 20 months previous. In the last three months approximately 3,000ft (914m) of TBM advance has been made, compared to the 2,600ft (793m) of progress made between launch in January 2012 and July this year.
Lake Mead TBM readied for its drive under the lake
- "We did see improved ground conditions on the realigned starter tunnel back in 2011, but after that we encountered some areas of challenging ground conditions where the TBM had to be operated in high pressure closed mode," Mack told TunnelTalk from Las Vegas.
- "During this time the TBM was operating in high pressures up to 13.9 bar, for a distance of approximately 2,000 feet (610m). However, there have been no hyperbaric interventions to date although the contractor has made the necessary preparations for this and is prepared if it is deemed necessary for future maintenance work."
- High water inflows, too, have been a persistent problem throughout the TBM drive. "The contractor has encountered high water inflow, high water pressures, and unstable ground since launch of the TBM. They have had to grout to reduce the inflow and stabilize the ground. Total water intake from all underground areas has ranged mostly from 800 gal/min to 1,000 gal/min since TBM tunneling started, with a maximum pump out rate of 1,200 gal/min. That said, the TBM is currently in better ground that is producing less water."
Fig 1. Tunnel alignment with TBM current position
- This, and "more favorable" ground conditions encountered since July, has allowed the contractor to switch the TBM back to open mode, resulting in a much higher production rate, explained Mack, restoring optimism about a Fall 2014 date for the completion of tunneling.
- "The schedule is based on expected average production rates and the anticipated ground conditions. Based on the original production rate of approximately 35ft/day (10.6m/day), including planned maintenance stops, we believe that the Fall 2014 timeline is reasonable. There is always the potential that we could hit unstable ground or unforeseen conditions, but based on what we know, we are comfortable with the current schedule.
- "The contractor and our engineering team have overcome extremely difficult conditions. We knew this was a difficult and complex project from the start, so we are pleased with the progress thus far," said Mack (Fig 1).
'Tide' marks at Lake Mead are mineral deposits on rock that was once underwater
- Meanwhile, as progress underground finally gives reason for encouragement, above-ground signs of the looming crisis are clearly visible to the naked eye - the light colored bathtub-style 'tide marks' above the current waterline at 1,106ft (337m) elevation above sea level (masl) indicating just how rapid the fall in the water level has already been. The situation has become critical following a recent announcement that Lake Mead's top-up supply from the Hoover Dam, itself 45% depleted, will reach a record low allocation in 2014. Monthly data show water levels at Lake Mead have dropped 108ft (33m) since 2000, and 28ft (8.5m) since January 2012 alone.
- The race is well and truly on to bring Intake No.3 into service and begin pumping water from the bottom of the lake (from an elevation of 800ft (243.8m) amsl) to the two treatment plants currently served by the more highly elevated Intakes No.1 (1,060ft (323m) amsl) and No.2 (1,000ft (305m) amsl).
- But well-documented construction delays on the crucial Intake No.3 tunnel project mean that SNWA can no longer base its short-term water supply planning on an "if" scenario of levels plunging below the 1,065ft (324.6m) mark below which intake becomes impossible. It is now a question of "when" - and data released in the September report by the US Bureau of Reclamation puts that date at May 2015 (Fig 2). The Lake Mead No.3 Intake Project can only expect to complete by then assuming no further delays on a program that is already 30 months behind its original October 2012 delivery schedule.
- Once the level reaches 1,075ft (327.6m) masl water rationing is likely to begin in Las Vegas, which receives 90% of its supply from Lake Mead. This despite the fact that a program to raise awareness about the problem has resulted in significantly less annual water consumption. Other drastic water-saving measures have included a ban on new home developments having a front lawn.
- With all this in mind, an emergency temporary solution has been devised, one which is aimed at 'buying' 15ft (4.5m) of elevation and possibly one more year of time by taking advantage of the fact that the pumps serving Intake No.1 can operate to a slightly deeper elevation of 1,050ft (320m) if a way can be found to draw water into the system from Intake No.2. The solution involves excavation of a 110ft (33.5m) shaft that will link the existing tunnel that serves the under-threat Intake No.1 (and its associated pumping facilities) with a newly constructed drill+blast tunnel below it. The new half-mile long tunnel, recently completed as part of a US$52 million contract with Texas contractor Renda Pacific, has been excavated to complete the link between Intake No.2 and its existing tunnel and pumping facilities, and the new deep-level tunnel that will draw water from Intake No.3 (Fig 3). "It's really an action to avoid an emergency," said SNWA Deputy General Manager John Entsminger.
- "The new project includes construction of a 9ft x 21ft (2.7m x 6.4m) rectangular shaft, 110ft deep," Mack told TunnelTalk. "The shaft will be excavated using drill+blast to lower the existing shaft down to connect to an existing stub tunnel that was excavated as part of the connector tunnel work carried out by Renda Pacific. This shaft currently serves as a surge shaft for the Intake No.1 system.
Fig 4. No.1, No.2, and future No.3 Intake depths
- "The project also requires some marine work to close off the existing intake for the dewatering of the system to allow the underground work to commence.
- "It was critical to begin this work immediately in order to complete it during the low-water-demand period, as it will require a shutdown of Intake No.1, it's pumping station, and one of the treatment plants. SNWA already has provisions in place that will allow conveyance of water through the existing Intake No.2 during this period. This work is scheduled to be completed before the higher water demand period hits."
- Renda Pacific started excavation of the new US$12 million shaft last month (September 2013), with substantial completion scheduled for March 2014 and final completion two months later - a date that, critically, times completion at exactly the point in time when predicted water levels are expected to have fallen somewhere near the 1,065ft level at which Intake No.1 can no longer draw water into its system.
- "At elevation 1,065ft, we could possibly begin to lose the ability to draw water through Intake No.1; however, our pumps for Intake Pumping Station No.1 can still pump to a depth of elevation 1,050ft. Therefore, this emergency work will allow water to flow through Intake No.2, flood the connector tunnel, and fill the forebay for Intake Pumping Station No. 1. This essentially buys us another 15ft of operational elevation, maintains redundancy for our intake system, and allows us to continue to defer Intake Pumping Station No.3 (which was shelved in 2009 to save US$250 million of project cost). That 15ft could be the difference of a year or more, depending on future inflows into Lake Mead."
- Once elevation falls below 1,050ft, however, Intake No.1 will no longer be able to draw water.
Installing a steel set on the new starter tunnel heading (May 2011)
- Delays have plagued the project from the start. On June 28, 2010, the starter tunnel for the TBM became inundated 250ft into the 350ft drill+blast heading. This was followed by two less serious flood events on October 27, and then again on December 31. The flooding prompted a starter tunnel alignment change by the contractor team in a bid to circumvent the bad ground that was hampering efforts to get the TBM into the ground. By the time the new (and longer) starter tunnel was completed on the new heading, in July 2011, the Herrenknecht dual mode TBM had been standing on site for nearly two years. It was finally lowered into the ground in September 2011, and launched in January 2012.
Las Vegas Wash Channel passes close to tunnel crown
- It is to be hoped that the remaining predicted locations of future problem ground for the TBM team, both of which have been factored into the current schedule, do not cause longer-than-anticipated delays.
- "There are two main areas that may be challenging in the remaining work," said Mack. "They include the Las Vegas Wash crossing (a deep channel in the lake that passes close to the tunnel crown) and the intake riser connection. We anticipate operating the TBM in closed mode in both of these areas."
- TunnelTalk wishes contractor and owner the very best of luck, and will continue to monitor progress on this vital but difficult drive.
Fatal ring build accident at Lake Mead - TunnelTalk, June 2012
Long wait over for Lake Mead TBM - TunnelTalk, September 2011
Third inflow hits Lake Mead project - TunnelTalk, January 2011
Inundation at Lake Mead Intake No. 3 - TunnelTalk, July 2010
Lake Mead TBM design - TunnelTalk, November 2009
Funding woes affect water projects in Las Vegas - TunnelTalk, April 2009
Intake No. 3 tunnel contract award - TunnelTalk, June 2008
Lake Mead intake project design - TunnelTalk, September 2006
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