BOSTON – The legacy of the most expensive highway project in U.S. history is being defined not by the underground leaks that plagued it or the quintupling of its price tag to more than $14.6 billion.

No, it comes down to the hundreds of bolts, each weighing less than a pound, that were glued into the concrete tunnel roof to hold up the 4,600-pound concrete ceiling panels.

That’s the focus of legal wrangling over the death of a woman killed when several panels collapsed one night.

A review of records and interviews with investigators by The Associated Press has found that:

• The bolts, about five-eighths of an inch in diameter and 6 to 8 inches long, according to various sources, had not been inspected since the tunnel opened to traffic in January 2003.

• Inspections after the July 11 accident showed many of the bolts had failed.

• Pressure to finish work while keeping costs low may have clouded contractors’ judgment on which materials and methods to use.

Engineers who did not work on the project have criticized the use of bolts fastened with epoxy to secure heavy ceiling panels.

“In my opinion, that is a big ‘no,”‘ said Jai Kim, a civil engineering professor at Bucknell University.

Lee Mattis, principal engineer at CEL Consulting, a California firm that specializes in the design and testing of bolts used with epoxy, said they are usually used in sidewalks or walls. Crews rarely install them overhead, he said.

“The installation presents some real problems because you are trying to force material up into a hole, and because of gravity, it wants to fall out,” Mattis said.

Attorney General Tom Reilly said the project manager – Bechtel/Parsons Brinckerhoff -knew when the tunnel was being built that there were problems with the bolt-and-epoxy system but used that design anyway. Five bolts that had been tested during construction came loose after a ceiling panel was suspended from them.

After the collapse, investigators said 20 bolts had come out of drill holes. Inspectors then found more than 200 faulty bolts throughout the tunnel.

“The clock was ticking. The fuse was lit. It was just a matter of time until this tragedy occurred,” Reilly said.

What happened took place in a matter of seconds. Angel Del Valle, 46, and his wife, 39-year-old Milena Del Valle, were on their way to Logan International Airport to pick up his brother.

As they approached the Interstate 90 connector tunnel, Angel Del Valle saw a huge slab of concrete crashing down in front of them, then heard concrete panels hitting his car. He escaped with bumps and bruises, but the passenger side was flattened.

Review ordered

Within hours of the collapse, Gov. Mitt Romney ordered a “stem to stern” review of every tunnel. For the rest of the year, portions of the tunnels were shut down as repairs were made. Authorities questioned the project’s designs, construction and materials as they tried to determine whom to blame.

The Del Valle family has sued the state and the project’s contractors. The state attorney general sued 15 contractors accusing them of negligence and convened a grand jury to determine whether criminal charges are appropriate.

Those who have worked on the Big Dig and those trying to find out what led up to the accident agree there was enormous pressure to finish a project behind schedule and, eventually, $12 billion over its initial $2.6 billion budget.

‘A fine line’

“I think expediency became paramount,” said Thomas Trimarco, Romney’s top budget official, who was appointed to the Massachusetts Turnpike Authority shortly before the collapse.

“I’m not saying people deliberately ignored safety, but it’s a fine line when you’re under pressure and safety clearly is an issue, but also efficiency and expedience is an issue,” he said. “I think that’s where people probably got very close to the line, and I think expediency trumped best practices.”

After the accident, investigators discovered that the bolts holding up the ceiling panels were inspected only once, during installation in 1999. They weren’t examined again until after the accident, according to the state’s inspector general, Gregory Sullivan. “The Massachusetts Turnpike Authority and the Highway Department had legal responsibility to inspect those tunnels. They both failed,” Sullivan said.

Inspections after the collapse showed the bolts had pulled out of the drilled holes. Some of the bolts had no epoxy on them, while others had an uneven distribution of epoxy or epoxy that was discolored.

Representatives of Becthel/Parsons Brinckerhoff and Modern Continental Corp., the main construction contractor, rejected requests for interviews.

A representative of Gannett Fleming Inc., the ceiling designer, did not return repeated calls seeking comment.

“Supporting concrete ceiling panels by anchoring bolts to the roof with epoxy adhesive is widely and successfully used throughout the construction industry,” the company said in a news release several days after the collapse.

“We are confident that our work fully complied with the plans and specifications” set out by transportation officials, Modern Continental said in a news release shortly after the accident.

The Big Dig involved burying Interstate 93 beneath downtown Boston, building a bridge over the Charles River and connecting the Massachusetts Turnpike to the airport with a third tunnel beneath Boston Harbor.

Plagued from start

Almost from the day ground was broken in 1991, the project was plagued by construction problems and cost overruns. The tunnels have commonly had leaks that formed icicles on the walls during winter. In April 2005, dirt and gravel rained down on traffic inside a tunnel, damaging three cars and an ambulance.

The Big Dig’s last major piece of roadway opened in January 2006. Remaining work on surface streets and cleanup is to be finished in 2007.

Angel Del Valle said he believes the accident happened for a purpose, to expose Big Dig safety problems.

“I think that God chose Milena and I to go through this so another bigger tragedy could be avoided,” he said.

“Many people may be thinking about money, but at this point I don’t care about it at all,” Del Valle said. “In the accident, a person died. That was my wife, and that’s all I care about. That’s why I want justice done.”