Feds blame epoxy for Big Dig ills

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.