The dust has settled from the Big Dig tunnel collapse in Boston two years ago, but there are still important lessons for engineers to learn from the fatal tragedy, which was easily avoidable.
“The message still hasn’t adequately penetrated enough that when engineers are dealing with new materials, they should use caution with the sources that they rely on,” says Myer Ezrin, a failure analysis expert and former researcher at the University of Connecticut’s Institute of Material Science. “Engineers working with material they have little or no experience with — particularly if it is a life and death matter as it was in the Big Dig — have to investigate the choices and then confirm that investigation.”
In Ezrin’s view, these are the engineering errors made in the ceiling of Boston’s Interstate Connector Tunnel:
The wrong material was chosen as the adhesive to hold up the concrete panels as a suspended ceiling.
There was a communication breakdown between the construction engineers and the resin suppliers’ engineers.
Engineers failed to adequately investigate why anchor bolts using the same adhesive in another tunnel failed in 1999.
Use of a suspended ceiling, particularly one made from concrete, was a mistake in the first place.
On July 10, 2006 a passenger car traveling to Boston’s Logan Airport passed through the D Street portal of the Interstate 90 connector tunnel in Boston, part of a project often referred to as the “Big Dig.” As the car approached the end of the tunnel around 11 p.m., 26 tons of concrete panels fell, killing a passenger. The panels were part of a suspended ceiling anchored to the concrete roof with threaded bolts in an epoxy-filled hole that had been drilled. Massachusetts Attorney General Martha Coakley charged epoxy supplier Powers Fasteners of Brewster, NY, with one count of involuntary manslaughter, which carries a maximum fine of $1,000. Other contractors avoided possible criminal charges with a $450 million settlement with state and federal officials.
The initial, and most serious error, was the use of a fast-setting adhesive supplied by Powers. In a report issued a year ago, the National Transportation Safety Board identified the probable cause as an inappropriate epoxy formulation and blamed engineers at Gannett Fleming Inc. and Bechtel/Parsons Brinckerhoff for failing to identify potential creep in the anchor adhesive as a critical long-term failure mode. The board noted that Gannett Fleming specified the use of adhesive anchors with adequate creep resistance in the contract. Selection of a better adhesive could have prevented the accident.
“The accident was due in part to a lack of knowledge and understanding of the chemistry and technology of polymers by engineers responsible for the design and construction of the collapsed ceiling,” says Ezrin. “Why after more than 50 years of polymers being used in accident-critical installations is there still a gulf between the engineers that use polymers and those that make polymers.”
The specific problem was failure to understand the chemistry of epoxy adhesives. The ability of epoxy adhesives to withstand heavy, sustained loads depends on the level of crosslinking in their molecular chains, according to Ezrin. “Crosslinking bridges individual chains with covalent bonds, effectively moving the polymer in the direction of infinite molecular weight,” he says. “The result is reduced dimensional change under load.”
The crosslinking in two-part thermoset systems, such as epoxy adhesives, is achieved through a second chemical called a hardener. Its chemical composition determines the amount of crosslinking and ability to resist creep under load.
According to the NTSB, Powers Fasteners failed to provide Big Dig engineers with sufficiently complete and accurate information about the suitability of its Fast Set epoxy for sustaining long-term tensile loads.
In Ezrin’s view Big Dig engineers should have conducted an investigation of the Fast Set adhesive and not just simply accepted the supplier’s recommendation.
The problem was exacerbated by installation problems. In some cases, the threaded bolts were not covered adequately with the epoxy adhesive. “Part of the difficulty is that adhesive is injected upside down vertically,” says Ezrin. “Another problem of the epoxy is that it may not bond well to the concrete roof.” Adding to the confusion, Powers also maintained that in some instances, installers used the wrong grade of epoxy.
The NTSB found the adhesive suppliers at fault and ordered Powers Fasteners, a distributor and its supplier Sika Corp., to revise product literature and packaging to clearly state that the fast-setting materials (Power-Fast Epoxy Injection Gel Fast Set and Sikadur Injection Gel AnchorFix-3 epoxy, respectively) are approved for short-term loads only. Powers Fasteners also supplies a Standard Set, which could have been adequate for the Big Dig application. Powers has increased the safety factor on its fast-setting materials by a factor of four since the Big Dig collapse.
Ezrin says the Big Dig failures were particularly maddening because there had been a similar failure in another tunnel in 1999. “The engineers involved just assumed the failure was due to faulty installation and did not explore the potential of a creep-related failure,” he says.
Ezrin is also puzzled why the suspended ceiling was made from concrete. “A lightweight ceiling made, for example from foam, is a very common type,” he says. After the fatal collapse in 2006, engineers decided, in fact, that a suspended ceiling was not required after all and ordered all of the ceiling modules be removed.
In its final report, the NTSB recommended federal and state highway authorities develop standards and protocols for the testing of adhesive anchors used in sustained tensile load overhead highway applications. The standards should consider the creep characteristics of polymers, the NTSB said. A mandatory tunnel inspection is also in order, the board said. The International Code Council was urged to require creep testing for the qualification of all anchor adhesives.
|• Make sure you understand the basics of potential polymer failure, such as susceptibility to creep under long-term tensile loads.
• Don’t simply accept a supplier’s certification of the capability
of its materials.
• Establish clear and ongoing lines of communication with
engineers throughout the supply chain.
• Conduct follow-up tests and inspections to ensure system