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NTSB spreads blame for Big Dig tunnel ceiling collapse

WASHINGTON, D.C.—The National Transportation Safety Board (NTSB) reported in July that the probable cause of the ceiling collapse in the D Street portal of the Interstate 90 connector tunnel in Boston, Mass., part of the Big Dig project, was inappropriate use of an epoxy anchor adhesive. Over time, the epoxy, which had poor creep resistance, deformed and fractured until several ceiling support anchors pulled free and allowed a portion of the ceiling to collapse.

The board blamed project engineers, contractors, a supplier, and the project’s owner for failures that it said contributed to the tragedy. On July 10, 2006, a section of the tunnel’s suspended concrete ceiling became detached from the tunnel roof and fell onto a car, killing a passenger and injuring the driver.

According to the NTSB, use of an inappropriate epoxy formulation resulted from the failure of Gannett Fleming, Inc., and Bechtel/Parsons Brinckerhoff to identify potential creep in the anchor adhesive as a critical long-term failure mode and to account for possible anchor creep in the design, specifications, and approval process for the epoxy anchors used in the tunnel. The Board also noted that had Gannett Fleming specified the use of adhesive anchors with adequate creep resistance in the construction contract, the accident might have been prevented. As of press time, Gannett Fleming and Bechtel/Parsons Brinckerhoff had not publicly responded to the NTSB report.

Use of an inappropriate epoxy formulation also resulted from a general lack of understanding and knowledge in the construction community about creep in adhesive anchoring systems, the NTSB concluded, and that those responsible for overseeing the Central Artery/Tunnel project (CA/T)—including design and specifications for the tunnel’s ceiling—failed to account for the fact that polymer adhesives are susceptible to deformation (creep) under sustained load. In addition, it said, Powers Fasteners, Inc., failed to provide the CA/T project with sufficiently complete, accurate, and detailed information about the suitability of the company’s Fast Set epoxy for sustaining long-term tensile loads.

Contributing to the accident, the NTSB said, was the failure of Powers Fasteners to determine that the anchor displacement that was found in the high-occupancy vehicle tunnel in 1999 was a result of anchor creep because of the use of the company’s Fast Set epoxy, which was known by the company to have poor long-term load characteristics. The Board said that information provided by Powers Fasteners regarding its Power-Fast epoxy was inadequate and misleading. This resulted in Modern Continental Company using the Fast Set formulation of the epoxy for the adhesive anchors in the tunnel even though that formulation had been shown through testing to be subject to creep under sustained tension loading.

In response, Powers Fasteners said that "it would be an untenable conclusion if the federal investigators were to consider Powers Fasteners in any way responsible, since there is overwhelming evidence that the fault lies elsewhere." Powers Fasteners said that it was not involved in design of the ceiling or in the decision as to which epoxy to use, that it supplied a special order of Standard Set epoxy for use in the tunnel ceiling as requested by its distributor, and that the company did not know that Fast Set was used for the ceiling in place of the Standard Set epoxy.

Additionally, Powers Fastener said that in the fall of 1999 when the company was called in to inspect several anchors that had been installed but had slipped by approximately 1/2 inch, it assumed that Standard Set epoxy was used. At the time, project officials agreed with Powers Fasteners that the problem was caused by faulty installation, since the slippage was so spotty. Powers Fasteners said that it requested permission to perform ultimate load tests, but the contractor refused to allow these tests to be performed.

Also contributing to the accident, according to the NTSB, was the failure of Modern Continental and Bechtel/Parsons Brinckerhoff, subsequent to the 1999 anchor displacement, to continue to monitor anchor performance in light of the uncertainty as to the cause of the failures. "The Massachusetts Highway Department, Bechtel/Parsons Brinckerhoff, and Modern Continental should have instituted a program to monitor anchor performance to ensure that the actions taken in response to the displacement were effective," the Board said. "Had these organizations taken such action, they likely would have found that anchor creep was occurring, and they might have taken measures that would have prevented this accident."

The NTSB also blamed the Massachusetts Turnpike Authority (MTA) for failing to implement a timely tunnel inspection program. The NTSB concluded that had MTA, at regular intervals, inspected the area above the suspended ceilings in the D Street portal tunnels, the anchor creep that led to the accident would likely have been detected, and action could have been taken that would have prevented this accident.

As a result of its investigation, the NTSB made the following safety recommendations:

  • Develop standards and protocols for testing adhesive anchors to be used in sustained tensile-load overhead highway applications. These standards and protocols should consider site-specific ultimate strength values as well as the creep characteristics of the adhesive over the expected life of the structure.
  • Prohibit use of adhesive anchors in sustained tensile-load overhead highway applications where failure of the adhesive would result in a risk to the public until testing standards and protocols have been developed and implemented that ensure the safety of these applications.
  • Develop specific design, construction, and inspection guidance for tunnel finishes and incorporate that guidance into a tunnel design manual.
  • Review the use of adhesive anchors in highway construction and identify those sites where failure of the adhesive under sustained load could result in a risk to the public. Once those sites have been identified, implement an inspection and repair program to ensure that such failures do not occur.
  • Require creep testing for qualification of all anchor adhesives.
  • Disqualify for use in sustained tensile loading any adhesive that has not been tested for creep or that has failed such tests.
  • Use building codes, forums, educational materials, and publications to inform design and construction agencies of the potential for gradual deformation (creep) in anchor adhesives and to make them aware of the possible risks associated with using adhesive anchors in concrete under sustained tensile-load applications.

A synopsis of the NTSB report, including the probable cause and recommendations, is available online at www.ntsb.gov, under "Board Meetings." The Board’s full report will be available on the website in several weeks.