Fatal ring-build accident at Lake Mead Jun 2012
Shani Wallis and Armand van Wijck, TunnelTalk
An accident in the ring build area of the Intake No. 3 tunnel at Lake Mead in Nevada has claimed the life of one worker and injured another. The accident happened at 4.30pm on Monday this week (June 11).
During the ring build process, the key segment in the crown was said to have "slipped forward" opening a gap of about 4in wide by 2ft long. This allowed annular grout, being injected at the time, to burst through, bringing with it about half a cubic yard of ground.
Ring-build area, like this, was site of fatal accident

Ring-build area, like this, was site of fatal accident

Segmental lining above the Lake Mead TBM backup

Segmental lining above the Lake Mead TBM backup

Clearance for the resumption of tunneling has now been approved by the Nevada Occupational Safety and Health Administration (OSHA) following safety discussions between its inspectors and VTC officials, he said.
Davis added: "Those discussions were between OSHA and VTC but I was informed that the primary focus was on providing additional and more robust supports for the concrete rings during the assembly process, to avoid slippage."
Beth Moore, the Public Information Corordinator for the project's owner, the Southern Nevada Water Authority (SNWA), told TunnelTalk that annular grout at about 200psi (14 bar) pressure, and pieces of rock and gravel hit and killed one of the three men in the ring-building crew and injured another. "The most critically injured worker died of head injuries in the tunnel," said Moore, "before being taken to the surface by emergency services."
This is the first fatality on the project, which has suffered serious setbacks since work started in 2008. Two inundations of the drill+blast starter tunnel at the base of the 600ft (182m) deep access shaft caused serious delays. The TBM for the 4.8km long tunnel was left on the surface in its crates for more than two years until the original heading was abandoned and a new starter tunnel was excavated on a new alignment. The TBM started excavation earlier this year (2012) and is currently about 133 rings or 240m, about 750ft, into the drive.
Fig 1. New tunnel out under Lake Mead

Fig 1. New tunnel out under Lake Mead

The TBM drive is lined to 6m i.d. with a universal segmental ring comprising five segments and a key in each 1.8m wide x 356mm thick bolted and gasketed ring. Vegas Tunnel Constructors (VTC), the contractor, led by Impregilo of Italy and with its USA subsidiary S.A. Healy of Chicago, procured a high-specification Herrenknecht Mixshield to excavate the 7.22m o.d. tunnel. The multi-mode machine is designed to cope with demanding geological conditions and to withstand the potential 17 bar hydrostatic pressure of the maximum water depth in the lake above the alignment. Tunneling is progressing on a 24hr/day, 7day/week production schedule and a total 2,500 rings are to be installed as the machine advances towards an intake structure on the bed of the lake. The TBM is still under Saddle Island (Fig 1) and is yet to pass out under the water of the reservoir.
Lake Mead TBM readied for its drive under the lake

Lake Mead TBM readied for its drive under the lake

Details of the situation in the heading were reported by a special VTC rescue team that entered the tunnel on Monday evening. Once the VTC team and OSHA officials determined that the tunnel was safe to enter, work began on securing and sealing the gap in the lining and cleaning the work area so the joint investigation can move forward. It was reported that the concrete segment did not fall into the tunnel, that no further material had come into the tunnel, and that other segments installed were stable.
The SNWA, along with OSHA and Vegas Tunnel Constructors, have announced a full investigation into the accident to find out how the one segment slipped forward and how to prevent the same happening again. Tunneling activities will not resume until that determination is complete.
In its statement, SNWA extended condolences to the family of the deceased worker and expressed support of the tunneling crews working on the project. In the meantime all work on the jobsite is once again at a standstill before tunneling can resume towards a revised project completion date of Summer 2014.
Long wait over for Lake Mead TBM - TunnelTalk, September 2011
Lake Mead TBM designed for the extreme - TunnelTalk, December 2009


Detail is not provided but from the read it seems the thrust ring was retracted to be ready for the next set. If so, the TBM was not moving forward so the tapered segment would not have been pulled.

Might the taper have been the source of the movement? The exterior pressures were reported to be quite high. The resulting squeeze might have been sufficient (depending on joint friction) to cause the movement. Sort of like squeezing a watermelon seed in your fingers.

Carl Linden, P.E., Senior Construction Manager, URS Corporation (Los Angeles office)

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