Bechtel-Parsons Brinckerhoff, the joint venture program manager of the problem-plagued Boston Central Artery project, and project designers, have agreed to pay about $458 million to settle major claims that include tunnel leaks and a fatal tunnel plenum collapse. The deal with the state of Massachusetts and the U.S. Justice Dept. enables the firms to avoid possible prosecution and debarment from federal and state contracts.

The agreement was announced today in Boston by U.S. Attorney Michael J. Sullivan and Massachusetts Attorney General Martha Coakley. Under its terms, Bechtel Group Inc. and PB will pay a total of $407 million. Prosecutors say the settlement covers "defects in slurry-wall construction, use of out-of-specification concrete by certain contractors, failure to disclose financial information, and various cost recovery matters for deficient work."

Under the deal, Bechtel will pay $352 million and PB will owe $47.2 million.

About 24 project design firms, whose names were not disclosed, will pay $51 million. But the settlement notes that Bechtel/PB did not engage in criminal conduct or commit a fraud, and the firms did not admit liability.

Of the total to be paid, $415 million will be held by authorities in a special interest-bearing account that will fund future Central Artery "non routine" repairs, says Sullivan. He adds that the agreement "does not release" the B/PB joint venture from liability in the event of a future "catastrophic failure" of artery infrastructure that causes more than $50 million but it caps its financial responsibility at $100 million.

Coakley says that the agreement allows prosecutors to "resolve criminal and civil claims" against Big Dig firms. But it does not affect a lawsuit filed against Bechtel/PB by the family of Milena Del Valle, who was killed in 2006 by a falling concrete panel in the Interstate-90 tunnel.

"We have sufficient evidence against Bechtel-Parsons Brinckerhoff to indict them," Coakley nots, although she does not say that authorities intended to take such action. Powers Fasteners, the Brewster, N.Y.,supplier of the epoxy used to affix the panels that fell, was indicted by the state last November.

The firm was not part of the settlement announced today but it has already agreed to pay Del Valle's family $6 million. Powers could not be reached for comment.

Coakley says the state opted for the settlement because manslaughter charges against corporations carry penalties of only $1,000. "The amount of money to be paid now to Massachusetts far outweighs what we could have accomplished in a criminal case," she says. Coakley adds that the deal's "catastrophic reopener" provision in the case of future events provides "a safety valve."

"We entrusted Bechtel/PB to act as our eyes and ears on this project," says Sullivan. "They grossly failed to meet their responsibilities."

In a statement, Bechtel/PB Chairman John MacDonald, says that "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."

"Our companies have a long history of delivering safe, high-quality engineering and construction services," he adds. "Going forward, we will implement a number of specific measures to apply lessons learned to our future work, such as improving quality-management systems, more extensive and mandatory training for field engineers, and additional standardized specifications for design and construction."