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Big Dig Epoxy: Who Blew It?
Incorrect use of epoxy is blamed for the Big Dig tunnel ceiling collapse, but that is all that’s clear from the NTSB report
Doug Smock, Contributing Editor -- Design News, July 11, 2007
Visit our Big Dig special report page for full coverage and photos of the tunnel collapse.
Powers Fasteners of Brewster, NY, is strongly defending its epoxies that were blamed Tuesday by the National Transportation Safety Board (NTSB) for a ceiling collapse one year ago that caused one death in the Boston highway construction project called “the Big Dig.”
On July 10, 2006 a ceiling collapse in one of Boston’s Big Dig tunnels killed a 39-year-old woman when the car she was riding in was crushed. NTSB members issued their final report yesterday, following a year-long investigation, saying the likely cause of the accident was “use of an epoxy anchor adhesive with poor creep resistance, an epoxy formulation that was not capable of sustaining long-term loads. Over time, the epoxy deformed and fractured until several ceiling support anchors pulled free and allowed a portion of the ceiling to collapse.” The board unanimously approved the report.
Powers provided standard epoxy for the ceiling assembly and fast-setting epoxy for less demanding applications such as ceiling tiles, says Karen Schwartzman, a company spokesperson in an interview with Design News. The distinction is important because, if true, it lifts the cloud of suspicion hanging over epoxies for use in very high-load structural applications. Powers’ position, however, was confusing and was criticized by the NTSB.
“We filed all of the ICBO reports showing the correct use for each type of material,” said Schartzman. ICBO (International Conference of Building Officials) sets the standards for construction materials. “Specifically, as found by the NTSB, Powers supplied to project officials the ICBO evaluation material which stated that the Fast Set was permitted only to be used for short-term loads,” Schwartzman says. The stated limitations were included in the final specifications for the ceiling, according to Powers, which supplied 120 units of Standard Set epoxy. According to Schwartzman, that was the correct amount for adhesion of the bolts holding the ceiling panels. Powers supplied $1,297 worth of Power-Fast Standard Set epoxy for the connector tunnel ceiling. The total project cost was $15 billion.
An official Powers statement says: “The specifications were simply ignored by those responsible for construction who were under pressure to complete the project.”
According to a report released Tuesday by the NTSB, Powers failed to notify the contractors on the Big Dig of the problems associated with the fast-setting materials. “It was difficult to find — it was in the fine print,” commented Bruce Magladry, director of the Office of Highway Safety for the NTSB. “But even if they did find it, I’m not sure they would have understood it.”
Schwartzman said Powers Fasteners had no idea the wrong type of epoxy had been used until Tuesday when the report was released.
Also under scrutiny is the role of Gannett Fleming, which designed the ceiling in the Big Dig connector tunnel. “They set a performance-based standard for the epoxy, that it had to be an epoxy of certain strength,” said Magladry. “They did not specify an epoxy to use, but rather one that performs in a certain way.
Doug Bailey, a spokesman for Gannett Fleming, said the designer “only approved the use of an epoxy that could have been the standard set epoxy.” He said that Gannett Fleming also only learned that the fast setting epoxy was used on Tuesday. And a spokesman for another major player, Modern Continental, said it applied the materials “in conformance with contract plans and specifications.”
Powers Fasteners’ own position, however, is confusing. Schwartzman stated that Powers Fasteners clearly stated that Fast Set could only be used for short-term loads. However, at the end of its official statement was this puzzling conclusion: “While based on our experience with the product, we think the Fast Set would have held properly had it been installed in accordance with Powers’ installation instructions and safety recommendations, Powers will review and fully evaluate the basis for the NTSB’s conclusions. In the interim, we will revise our literature and packaging to recommend use of Standard Set rather than Fast Set for sustained long-term loads.”
Muddying the water further was significant discussion from the NSTB about failure of Gannett Fleming as well as project coordinator Bechtel/Parsons Brinckerhoff to consider long-term creep in its specifications. It would appear based on publicly available information that the issue of creep only applied if Fast Set epoxy had been approved and supplied for the ceiling application.
The NTSB report found numerous problems in the construction of the ceiling. There were voids, or insufficient epoxy, in 19 of the 20 failed anchors. That factor alone reduced load capacity by 40 percent. Also, plastic caps were placed on every bolt, further reducing load-bearing capacity by 10 percent.
The Power-Fast system that Powers says it supplied for the connector tunnel ceiling consists of threaded bolts, rods or reinforcing steel that are put into pre-drilled holes that are filled with epoxy. The epoxy is a two-component material dispensed from a handheld tool. Epoxy is sold in cartridges that are placed in the tool and mixed before being dispensed. Components are mixed in a one-to-one ratio.
The NTSB is recommending that the use of adhesive systems in highway construction projects be reviewed, particularly where a sustained load could cause materials to creep over time.
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