The Big Dig is the most expensive highway construction project ever undertaken in the United States.

A joint venture between Bechtel Infrastructure Corp. of San Francisco and Parsons Brinckerhoff (PB) Inc. of New York City reached a settlement with the Commonwealth of Massachusetts and the U.S. Attorney for the District of Massachusetts in late January regarding its work as a management consultant for the 15-year-long Boston Central Artery/Tunnel project, also known as the Big Dig.

The Big Dig is the most expensive highway construction project ever undertaken in the United States. Although it was estimated at $2.8 billion in 1985, more than $14.6 billion had been spent in federal and state tax dollars as of 2006.

The project rerouted the Central Artery of Interstate 93, the chief controlled-access highway through the heart of Boston, into a 3.5-mile tunnel under the city. The project also included the construction of the Ted Williams Tunnel, which extended Interstate 90 to Logan International Airport; the Zakim Bunker Hill Bridge over the Charles River; and the Rose Kennedy Greenway in the space vacated by the previous I-93 elevated roadway. Bechtel and PB said they will pay $407 million to settle a government lawsuit – thus avoiding criminal charges – over the highway tunnel project, part of which collapsed and killed a motorist in 2006. Bechtel will contribute $352 million; PB will pay $47.2 million to total $399.2 million.

Roughly $8 million in interest has accrued since it was set aside in January, and it will be included in the settlement. The agreement also includes other participating companies who will pay $51 million collectively, bringing the settlement’s total value to $458 million.

In 2006, concrete ceiling panels in the I-90 tunnel to Logan International Airport collapsed, killing 39-year-old Milena Del Valle. Her family sued Bechtel, PB and other companies relating to the incident. While the case is still pending, state officials say the settlement between the joint venture and the state could protect other motorists for years to come. Approximately $415 million will go into a new, state-administered fund to pay for future repairs in the Big Dig. In addition to the ceiling collapse, the tunnels have also suffered several leaks, one of them big enough to flood the freeway and shut down traffic.

“We have always said that we take responsibility for our work,” said John MacDonald, chairman of the Bechtel/PB joint venture. “We understand and acknowledge with this resolution that our performance did not meet our commitment to the public or our own expectations. Above all, we deeply regret the tragic death of Milena Del Valle in the I-90 tunnel.”