Safety Board Determines Cause Of Boston Big Dig Tunnel Ceiling Collapse Last Year

National Transportation and Safety Board, Aug 06, 2007

The National Transportation Safety Board today determined that the probable cause of the ceiling collapse in the D Street portal of Interstate 90 connector tunnel in Boston, Massachusetts, was inappropriate use of an epoxy anchor adhesive. Over time, the epoxy deformed and fractured until several ceiling support anchors pulled free and allowed a portion of the ceiling to collapse.

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. 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."

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 the car 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.

The Board states in its probable cause that the use of an inappropriate epoxy formulation resulted from the failure of Gannett Fleming, Inc., (Gannett Fleming) and Bechtel/Parsons Brinckerhoff (B/BP) 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 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.<!--