WASHINGTON — The fatal Big Dig tunnel collapse in Boston could have been avoided if authorities had considered that the epoxy securing tons of ceiling panels could slowly pull away, federal investigators concluded Tuesday.
The National Transportation Safety Board approved a report saying the likely cause of the accident that killed a woman was "use of an epoxy anchor adhesive with poor creep resistance" that could not sustain long-term loads.
The board, meeting on the first anniversary of the accident, also issued a series of recommendations, including creation of mandatory tunnel inspection programs similar to those required for bridges and the development of protocols to test adhesive anchors used to hold tunnel ceiling panels.
Milena Del Valle, 39, was crushed to death on July 10, 2006, when 12 tons of concrete ceiling panels fell from the roof of the Interstate 90 connector tunnel as she and her husband drove toward Logan Airport.
The panels, weighing 4,600 pounds each, had been fastened to the concrete tunnel roof with bolts anchored in epoxy. The investigation found that 20 anchors had pulled away from the tunnel roof.
Similar adhesive anchors are used in tunnels in New York, including the Queens Midtown Tunnel, and Virginia, but they hold lighter panels that are less than 500 pounds, NTSB investigators said. They did not cite any concerns about those tunnels.
Portions of the Big Dig tunnels were shut down for months as workers methodically replaced all suspect bolts. The last remaining section of the tunnel reopened to traffic just over a month ago.
Modern Continental Construction Co., the company that constructed the I-90 connector ceiling, used "fast-set" epoxy supplied by Powers Fasteners Inc. to secure the anchor bolts in the tunnel, investigators said.
"The design is reasonable, it's the epoxy that's the issue," said Bruce Magladry, director of the NTSB's Office of Highway Safety.
Magladry described "epoxy creep" as the tendency of some epoxies to slowly give way under constant pressure. He said the epoxy used for the ceiling panels had "exceptionally poor" resistance to such creeping.
Magladry said there was no malice among those who built and oversaw the Big Dig.
"I don't think they understood creep at all," he said.
"Although the epoxy used in the tunnel had acceptable short term strength, it was incapable of supporting much lower loads over an extended period of time," Magladry said. "If any of the entities involved in the ceiling design and installation had considered creep as a possibility, a different epoxy or a different anchoring system would have been used."
The $14.8 billion Big Dig, the costliest highway project in U.S. history, was plagued by construction problems and multibillion-dollar cost overruns during its two decades of design and construction.
Last summer's accident led to tunnel shutdowns, extensive ceiling repairs, a wrongful death lawsuit and a wave of federal, state and criminal investigations, including the NTSB probe.
Del Valle's daughter, Raquel Ibarra Mora, said she was "extremely pleased" with the NTSB's thorough investigation.
"It's very difficult to relive this tragedy," Mora, 24, of Costa Rica, said through a translator.
Massachusetts Attorney General Martha Coakley is expected to announce soon whether she'll press criminal charges in connection with the accident. A spokeswoman said Coakley would not comment on the NTSB report because of her pending investigation.
Also pending is a wrongful death lawsuit filed by Del Valle's family against the Massachusetts Turnpike Authority, the agency overseeing the Big Dig, and several companies associated with design and construction of the project. The companies have said they stand behind their work.