A cavern that took two years to excavate has unexpectedly filled with water, halting underground work on the new water intake pipeline under construction at Lake Mead.
Project officials were assessing the damage Wednesday, but the setback could delay completion of the $700 million intake by several months.
"It's too early to know," said J.C. Davis, spokesman for the Southern Nevada Water Authority. "We built some contingency time into the project with the idea that there were going to be some problems. We just didn't know what those problems would be."
The project's manager, Jim McDonald, said the flood occurred when workers in the cavern 60 stories underground hit a water-filled fault, allowing it to empty into the chamber.
He called the incident "unexpected," but said it was not a life-threatening emergency that required a hasty evacuation.
McDonald said the fault was breached on June 28 and the cavern gradually filled over the next four days.
By Friday morning, workers could no longer access the site, where they were building a mansion-sized assembly area and a 100-foot-long starter shaft for the massive machine that eventually will be used to mine a 3-mile tunnel beneath Lake Mead.
The water eventually filled the entire space and climbed partway up the 600-foot access shaft before the flow was brought under control by pumps removing 300 to 400 gallons per minute.
No workers were injured in the flood, but several large pieces of excavation equipment were left behind and are now under water.
"It's a geo-technical issue," McDonald said. "It's not unusual in deep underground tunnel work to find conditions that are adverse."
Before the underground work can resume, he said, crews on the surface will have to drill down into the fault and try to shore it up by injecting it with grout. Once the leak is plugged, the water will be pumped out and the work area inspected to make sure it is safe for excavation to resume.
McDonald said he has "every confidence" that the cavern remains structurally sound. "It's designed to be under water," he said.
McDonald works for Vegas Tunnel Constructors, a joint venture of the Italy-based Impregilo group and its U.S. subsidiary, S.A. Healy Co.
Impregilo is one of the biggest contractors in the world, with projects that include flood control gates in Venice, Italy, new locks at the Panama Canal, and the world's longest railway tunnel through the Alps of Italy and Switzerland.
Vegas Tunnel Constructors won the construction contract in March 2008 and began excavating in June of that year. Almost immediately, their progress was slowed by an unexpectedly large amount of groundwater seeping into the work area.
Even before last week's flood, the excavation work was running several months behind schedule.
So far, however, Davis said the water authority has no concerns about the contractor's performance or its approach to worker safety.
Others do not share that sentiment.
State safety regulators are investigating complaints against the massive construction project, but it is unclear where the complaints came from or whether they concern the recent mishap underground.
Elisabeth Daniels, spokeswoman for the Nevada Occupational Safety and Health Administration, said the agency does not comment about ongoing investigations.
Davis and McDonald said they couldn't comment either, because they were unfamiliar with the complaints.
Any delay in the project will be unwelcome news to water authority chief Pat Mulroy, who has repeatedly said her agency is in a "race against time" to finish the so-called "third straw" to keep water flowing to Las Vegas despite drought and shortages on the Colorado River.
Roughly 90 percent of the valley's drinking water is drawn from Lake Mead through two existing intake pipes.
Intake No. 3, as it is officially known, will reach deeper into the reservoir to protect the valley's water supply should the lake shrink low enough to shut down one of the two shallower straws.
A decade of drought and overuse on the Colorado already has caused the surface of Lake Mead to fall 120 vertical feet.
Contact reporter Henry Brean at email@example.com or 702-383-0350.