Boston, July 11, 2006—four concrete panels weighing about 3 tons each fell from the ceiling of a Big Dig tunnel, crushing a woman in a car to death. The accident occurred in a 200-foot section that connects the Massachusetts Turnpike to the Ted Williams Tunnel. Said the Modern Continental Company, the contractor for that section of the project, “We are confident that our work fully complied with the plans and specifications provided by the Central Artery Tunnel Project. In addition, the work was inspected and approved by the project manager.”
The panels, installed in 1999, are held with metal trays secured to the tunnel ceiling with epoxy and bolts. The epoxy–bolt system is a tried-and-true method: holes are drilled into the concrete ceiling, cleaned, and filled with high-strength epoxy; a bolt is screwed into the hole; as the epoxy cures it bonds to the bolt. “That technique is used extensively,” said an engineering professor at the Massachusetts Institute of Technology. For work like the Big Dig’s ceiling, he said, safety “redundancies” are added; that is, enough epoxy-and-bolt anchors that would hold the ceiling panels even if some failed. But in the connector tunnel, he contended, too few anchors were used. “They didn’t have enough to carry the load. There was no room for error.” He added, however, the evidence was preliminary, and such a conclusion would be premature.
Some of the bolts in the ceiling wreckage had very little epoxy, and three of them had none. State Attorney General Thomas Reilly’s investigation is focusing on whether the epoxy used failed, or construction workers who installed the bolts misused or omitted the epoxy. An accident caused by improper installation or errors in mixing the epoxy, he said, would implicate the tunnel’s design and designers. (Epoxy requires on-site mixing before use.) He added that some documents reflected a “substantial dispute” among engineers over the anchor system’s adequacy to support the weight of the ceiling panels.
Seven years before the accident, Safety Officer John Keaveney wrote a memo to one of his superiors at contractor Modern Continental Construction Co. saying he could not “comprehend how this structure can withhold the test of time.” He said his superiors at Modern Continental and representatives from Big Dig project manager Bechtel/Parsons Brinckerhoff (B/PB) assured him the system had been tested and proven. Keaveney told the Boston Globe he began to worry about the ceiling panels after a third-grade class toured the Big Dig in 1999. While showing the class some concrete ceiling panels and pointing to bolts in the ceiling, a girl asked, “Will those things hold up the concrete?” “I said, ‘Yes, it will hold,’ but then I thought about it.”
Some have argued that the investigation should look at the tunnel’s design: Why were the concrete panels so heavy, weighing 2½ to 3 tons apiece? Why were they there at all? And why did the failure of a single steel hanger send 6 to 10 of the panels crashing down? Eyewitness report indicate the accident began with a loud snap as a steel hanger gave way, which set off a chain reaction that caused other hangers holding up a 40-foot steel bar to fail and send 12 tons of concrete smashing below. Were the bars under-designed to handle the weight?
Investigators are also looking at whether the use of the wrong epoxy may have played a role. Invoices from 1999 show that at least one case of a quick-drying epoxy was used to secure ceiling bolts rather than the standard epoxy specified by the designers. The epoxy holds 25 percent less weight than standard epoxy.
Additional issues raised during the investigation include the following:
• Design changes that resulted in the use of concrete ceiling panels in the connector tunnel which are heavier than panels used in the Ted Williams Tunnel
• The lack of steel supports in sections of the connector tunnel ceiling to which bolts holding the concrete panels could have been connected
• Possible tunnel damage caused by blast vibrations from nearby construction of an office tower
• Use of diamond-tipped drill bits, instead of carbide bits, in drilling holes for the bolts (epoxy may not hold as well in smoother holes drilled with diamond bits)
• Impact of cold weather during installation of the epoxy–bolt system.
B/PB, the project management contractor, said in a statement: “Determining the causes of this specific failure will require a thorough forensic analysis of design, methods, materials, procedures, and documentation.” As investigators scrutinize the history of the $14 billion project, criticism is reviving that Massachusetts lacked adequate supervision of private contractors. B/PB was involved in both the design and construction efforts—an arrangement some say may have compromised oversight. “There was no one checking the checkers,” said one US Representative. Wrote one blogger: “I wouldn’t want to be the registered engineer whose signature is on the design. It will be his fault if the materials and workmanship are found not to be up to specifications. But who knows if it is his fault. This is a huge mess and the whole bunch of them, engineers, managers, inspectors, and testers, should be investigated.”
QUESTIONS
1. With 20–20 hindsight, draw a CE (fishbone, Ishikawa) diagram to illustrate possible causes and effects. Include the possible causes mentioned in the case. The diagram should have been developed before construction; therefore, also indicate other possible failure modes and other causes you can think of. How would the diagram (developed after the accident) be of value during litigation?
2. List the characteristics that should have been classified as critical.
3. Propose guidelines for a process to ensure that the epoxy would provide sufficient bonding to the concrete ceiling.
4. Explain the role that configuration management should have played in preventing the accident.
5. What role could modeling/prototyping, laboratory tests, checklists, and training have played?
6. Explain how someone within B/PB would be accountable regardless of the findings of a forensic investigation. Would B/PB be off the hook if a subcontractor were found guilty?
7. What would the implications have been if the engineer who signed off on a specific design was an engineer-in-training instead of a registered engineer?