Construction crews hit water June 30 while tunneling 600-ft below Southern Nevada's drought-racked Lake Mead to create a third straw. The incident required a hasty evacuation, and could delay project progress by months.
A 200-ft-long by 37-ft-high vault breach caused four days of gradual water seepage that eventually made it no longer accessible. Workers were building a 28-ft-dia, 100-ft-long horseshoe-shaped starter tunnel before using a $25 million Herrenknecht 1,500-ton tunnel boring machine when the incident occurred.
Vegas Tunnel Constructors LLC – a joint-venture of S.A. Healy Co., Lombard, Ill. and Impreglio S.p.A., Sesto San Giovanni, Italy – won the $447-million design-build contract in March 2008 to build a 24-ft-dia, 3-mile intake tunnel beneath Lake Mead that will draw water deeper than its counterparts in case drought conditions persist. A 115-ft drop in water level since 2000 has left the lake half full.
“We always know in underground construction there are some unexpected risks and certain conditions,” says Marc Jenson, Southern Nevada Water Authority’s engineering director. “We are working together to remedy the situation. We have no expectation that this devastating to the project. This is just a minor setback.”
The flooded cavern was brought under control by two portable pumps that removed up to 400 gallons per minute. No one was injured; some excavation equipment, however, was left behind underwater. Underground TBM assembly had yet to occur.
On July 12, Spokane, Wa.-based subcontractor Crux Subsurface Inc. started drilling north of the 32-ft-dia., 600-ft-deep access shaft along Saddle Island's western shore line. The firm will locate and reinforce voids in the rock with grout to stabilize geotechnical conditions. Water will then be pumped-out, clearing the cavern, so TBM staging and assembly can commence; the cavern will become a forebay upon completion.
The project, SNWA’s largest to date, had a 1,571-day schedule and expected finish date of July 2012. It's too early to determine the delay length and repair cost, and SNWA has yet to rule on the accident.
“There are contingency measures built into the schedule that identify areas for adjustment for different conditions,” says Jensen. “We have a change order allowance.”