Design change to again delay Lake Mead project

After two previous flooding incidents in the project’s starter tunnel, the cavern 600 feet underground once again filled with water and muck during the final work shift on Dec. 31.

The contractor on the project now plans to abandon the starter tunnel and excavate a new one in a different direction in hopes of skirting a fault line filled with water and fractured earth that has caused the trouble so far.

Marc Jensen, director of engineering for the water authority, said the design change could put the project behind schedule by several more months, but he could not be any more specific.

"We’ve already lost almost a year," Jensen said.

When the water authority launched the project, in the middle of an extended drought, officials were pushing for a 2013 completion date to ensure that the system would have sufficient capacity to meet the Las Vegas Valley’s peak demand in the summer. The drought has continued since then, but water use has declined and new water banking agreements have been reached. Concerns about capacity have been extended several years.

The valley depends on two intake pipes to draw roughly 90 percent of its drinking water from Lake Mead. The third intake will pull from deeper in the reservoir, allowing the flow of water to continue even if the reservoir shrinks enough to shut down one of the two existing straws.

Officials knew from the beginning that building a new intake would be no easy task. The project involves mining a 23-foot tunnel through 3 miles of solid rock beneath the bottom of Lake Mead and connecting it to an intake structure that will stick up from the lake bed at one of the deepest spots in the reservoir’s Boulder Basin.

The tunnel under the lake will be excavated using a massive tunnel boring machine that will be lowered in pieces down the 600-foot vertical access shaft and put together underground. But that can’t happen until the contractor can figure out a way to keep that assembly area and starter tunnel from flooding.

The project was already about three months behind on July 1, when workers excavating the cavern hit a fault zone, causing water and debris to pour into the work area. Efforts to seal off the fault proved unsuccessful, and the entire cavern filled with muck within five days.

"We were fortunate no one was injured, but the inflow did bury some equipment and make a big mess in the starter tunnel," Jensen said.

Vegas Tunnel Constructors, the project’s general contractor, spent weeks trying to stabilize the fault by drilling down from the surface and injecting grout into the fracture zone. Once that was done, workers went back underground to drain and clean out the cavern.

They had just reached the original spot where the July 1 mishap occurred when more water, rock and clay flowed into the cavern on Oct. 27.

Jensen said the latest mishap occurred on the last day of 2010, "during the last shift before the holiday weekend."

He declined to estimate how much the project’s final price tag might go up as a result of the tunneling problems.

Authority officials are currently in talks with the contractor to determine how much the design change will cost and who should pay for it. Jensen said he expects to have an answer to those questions within the next two weeks.

He added that some of the problems encountered so far might be covered by insurance. The potential effect on water ratepayers, if any, is not yet known.

Jensen said authority engineers are still reviewing the specifics of the proposed design change, but they are confident the new strategy will work.

Vegas Tunnel Constructors plans to excavate a new starter tunnel from the same vertical access shaft at about a 20-degree angle from the current tunnel.

The company is a joint venture of S.A. Healy Co. and its Italy-based parent company, the Impregilo group, which is one of the biggest construction firms in the world. Impregilo’s projects 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.

The third intake project, which Vegas Tunnel Constructors won in March 2008, is the single largest construction contract ever awarded by the water authority.

Contact reporter Henry Brean at hbrean@review journal.com or 702-383-0350.