Safety Board Investigation Determines Cause of Fatal Tunnel Ceiling Collapse
The National Transportation Safety Board investigation has determined that the probable cause of the ceiling collapse in the D Street portal of Interstate 90 connector tunnel in Boston, Mass., was inappropriate use of an epoxy anchor adhesive. Over time, NTSB said, the epoxy deformed and fractured until several ceiling support anchors pulled free and allowed a portion of the ceiling to collapse.
On July 10, 2006, a passenger car was traveling eastbound in the D Street portal of the Interstate 90 (I-90) connector tunnel in Boston, Massachusetts, en route to Logan International Airport. As it approached the end of the I-90 connector tunnel, a section of the tunnel's suspended concrete ceiling became detached from the tunnel roof and fell onto the vehicle. Concrete panels from the ceiling crushed the right side of the vehicle roof as the car came to rest against the north wall of the tunnel. The passenger was fatally injured and the driver had minor injuries. A total of about 26 tons of concrete and associated suspension hardware fell onto the vehicle and the roadway.
Epoxy is a polymer and its stiffness is time and temperature dependent. If a load is applied suddenly, the epoxy responds like a hard solid. But if the load is then held constant, the molecules within the polymer may begin to rearrange and slide past one another, causing the epoxy to gradually deform in a process called "creep." CSB notes that those responsible for overseeing the Central Artery/Tunnel project, in design and specifications for the tunnel's ceiling, failed to account for this. The epoxy used in the tunnel had poor creep resistance.
"This has been a multi-faceted and complicated investigation for the Safety Board. We expended many of our resources to find out what happened and to prevent similar accidents," said NTSB Chairman Mark V. Rosenker. "It is very clear that had this accident occurred at a different time of day it would have been more catastrophic, although one fatality is one too many."
Among its findings, CSB states in its probable cause that the 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. CSB also notes that had Gannett Fleming specified the use of adhesive anchors with adequate creep resistance in the construction contract, a different anchor adhesive could have been chosen, and the accident might have been prevented.
In addition, Powers Fasteners Inc. (Powers) failed to provide the project with sufficiently complete, accurate, and detailed information about the suitability of the company's Fast Set epoxy for sustaining long-term tensile loads, which was known by the company to have poor long-term load characteristics.
Contributing to the accident was the failure of Powers to determine that the anchor displacement that was found in the high-occupancy vehicle tunnel in 1999 was a result of anchor creep due to the use of the company's Fast Set epoxy. This resulted in Modern Continental Co. 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, CSB found.
A synopsis of the report, including the probable cause and recommendations, is available at Web site, www.ntsb.gov/events/Boardmeeting.htm. The full report will be available in several weeks.