On 10th July, 2006 a passenger car travelling 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, 26 tonnes 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.
There are still important lessons for engineers to learn from the ceiling collapse tragedy of Boston’s Interstate Connector tunnel, 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 Boston’s ceiling collapse:
• 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.
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 (NTSB) identified the probable cause as an inappropriate epoxy formulation and blamed engineers at Gannett Fleming 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.
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.”
Crosslinking
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 Fasteners 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 and Sika Corp. to revise product literature and packaging to clearly state that the fast-setting materials (the Fast Set formulation of Powers Fasteners’ Power-Fast epoxy injection gel and Sika’s Sikadur injection gel AnchorFix-3 epoxy, respectively) are approved for short-term loads only. Powers Fasteners has increased the safety factor on its fast-setting materials by a factor of four since the Big Dig collapse.
Ezrin also says that, further to 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 also wonders 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 International Code Council was urged to require creep testing for the qualification of all anchor adhesives. Click here and here.
The US attorney's office on 20th June accused Modern Continental, the Big Dig's largest construction contractor, of knowingly using the wrong epoxy to hold up concrete anchors that failed in the 2006 ceiling collapse. It also accused the company of knowing about poor workmanship on slurry walls in the I-93 Tip O'Neill tunnel before portions of the walls blew out in 2004.
Modern Continental was charged in federal court with making false statements, submitting false time and materials slips on contracts and wire fraud. If convicted, the company faces up to USD24.5 million in fines, as well as restitution payments.
The US attorney's office alleges that the company knew in December 1999 that the epoxy used to secure the ceiling's concrete anchors wasn't appropriate for long-term loads, but continued to use it anyway and then certified that work was properly done.
The company also allegedly certified that defective concrete panels in slurry walls in O'Neill tunnel were built to specifications, when it knew they weren't. A slurry wall blew out in September 2004, causing water to pour into the tunnel and a major traffic problem.
Federal prosecutors also say Modern Continental systematically overbilled the Big Dig in a scheme that totalled hundreds of thousands of dollars.
In a statement, Modern Continental called the charges "completely unfounded and without merit." The company said the charges were "an attempt after the fact to criminalize actions" that were approved by state officials.
In its statement, the company said its overbilling was a result of bookkeeping errors that were fixed. It said the company underbilled the project for other time and work, and ultimately charged less than it was owed.
Modern Continental said project officials knew certain slurry wall panels were built with slurry that didn't meet specifications, and in some cases specifically approved it. The company said there was no evidence that it affected the quality of the finished panels.
The company also pointed to the NTSB's investigation into the 2006 ceiling collapse, which found that Modern Continental didn't know the epoxy being used to secure the concrete anchors was unsuitable for that use. 27/08.
