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The biggest factor in the collapse of Boston's Big Dig ceiling panel which killed a woman a year ago was "epoxy creep."
While National Transportation Safety Board members, in a hearing yesterday, said the project's design was reasonable, it was the epoxy used in the design that was in question. The lack of a timely tunnel inspection program by the state authority overseeing the project was also an important factor.
The NTSB determined the probable cause of the July 10, 2006 ceiling collapse in the D Street portal of the Interstate 90 connector tunnel in Boston was the use of an epoxy anchor adhesive with poor creep resistance - the epoxy formulation was not capable of sustaining long-term loads. The report released after the hours-long hearing yesterday said over time the epoxy deformed and fractured until several ceiling support anchors pulled free and allowed a portion of the ceiling to collapse. The use of the inappropriate epoxy formulation resulted from the failure of the contractors 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 inappropriate formulation also resulted from a general lack of understanding and knowledge in the construction community about creep in adhesive anchoring systems, according to the report.
Creep occurs when a force is continuously applied on a component, causing it to deform gradually. The rate at which polymers creep depends not only on the load, but also on temperature. In general, a loaded component creeps faster at higher temperatures, according to Politecnico di Torino.
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, according to the American Society of Civil Engineers.
The epoxy formulation used in the ceiling panels in question was a fast-set epoxy, rather than a standard-set epoxy.
According to a May 3, 2007 Boston Globe article, bolts secured with fast-set epoxy could safely carry 4,285 lbs each, rather than the 6,350 lbs the designers had planned on. Two independent engineers who reviewed the ceiling's specifications for the Globe estimated the ceiling's weight was close to 5,000 pounds per bolt, which is more than bolts secured with fast-set epoxy were designed to bear over the long term.
As a result of the July 10, 2006 accident, several portions of Boston's $14.6 billion Big Dig/Central Artery Tunnel Project were shut down. Ongoing investigations by federal and state officials following the accident uncovered additional problems almost on a daily basis.
The NTSB identified several safety issues during its investigation:
Insufficient understanding among designers and builders of the nature of adhesive anchoring systems;
Lack of standards for the testing of adhesive anchors in sustained tensile-load applications;
Inadequate regulatory requirements for tunnel inspections and
A lack of national standards for the design of tunnel finishes.
The NTSB report also said contributing to the accident was the failure of Modern Continental Construction Co. 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."
A December 1998 Inspector General's report reviewing the project's use of anchor bolts documents numerous problems with the bolts and glue used to secure the ceiling in the Ted Williams Tunnel, which opened to traffic in 1995. The I-90 connector connects the Massachusetts Turnpike with the Ted Williams Tunnel.
The Inspector General's Report also disclosed several findings that would indicate a problem could occur: poor design specifications, paying contractors to test improperly installed anchor bolts and lack of consultation with tunnel designers before allowing contractors to drill through steel reinforcements in the tunnel roof.
It also documented numerous problems with the bolts and glue used, including bolts that were too short and trouble with the epoxy used to glue the bolts into the concrete.
The NTSB is charging the Massachusetts Turnpike Authority also contributed to the accident by failing to implement a timely tunnel inspection program.