The Big Dig in Big Trouble

Boston's Big Dig project to put much of I-90 underground spanned parts of two centuries and cost more than any other single highway project in the United States. On July 11, when the project was mostly finished and people in Massachusetts thought they could begin to put the disruption and cost overruns behind them, a three-ton ceiling tile came loose in a connector tunnel and killed a newlywed woman. Further investigation has revealed that over a thousand fasteners used to hold up similar tiles are probably defective. What can we learn from all this?

The first lesson is an old one: nothing draws attention like death and destruction. According to a report by Sean Murphy and Raja Mishra in the July 18 Boston Globe, lab tests of the epoxy glue used to hold the fasteners in place were originally scheduled during construction. But officials of Bechtel/Parsons Brinckerhoff, the engineering firm in charge of the Big Dig, felt so confident in the epoxy that they canceled the tests. Now it looks like the tests would have been a good idea, because they might have revealed the kind of problems that ultimately led to the fatal ceiling collapse. But there was no immediate harm that resulted from skipping the tests, so the incident went by unnoticed.

The next lesson is one we hear starting in kindergarten: be sure to follow instructions. Engineering is a constant battle between expensive over-caution on the one hand, and reckless negligence on the other hand. Where lives are at stake, as in the construction of bridges and tunnels, laws require licensed engineers to sign off on plans and specifications. But all the licensed engineers in the world won't do any good if the contractors and builders don't carry out the engineers' instructions to the letter.

Speculation by experts centers on the possibility that the epoxy used to hold the concrete ceiling tiles up was either not prepared and applied correctly, or used with oily steel. Steel as it comes from the factory has a thin coating of oil on it, and unless this oil is cleaned off prior to use, adhesives such as epoxy cannot form a good bond. Even if the steel was clean, the widely varying temperatures at a Boston construction site may have interfered with the chemical changes that epoxy goes through in order to harden. Inadequately hardened plastic adhesives can "creep" under stress, moving a tiny fraction of an inch every month, until the entire joint fails. Whatever was done wrong, it appears to have been done wrong consistently, because Governor Mitt Romney has announced that over 1300 fasteners are suspect and will have to be removed or replaced.

Further investigations will eventually reveal what went wrong, and possibly who was responsible. Structural engineering is based mostly on physical science, and things don't generally fall down for no reason at all. But finding the physical cause gets us only part way toward preventing similar accidents in the future. Until the human organizations that let such things happen are repaired and kept in order, the same thing can happen again. In a way, it has.

The Boston tunnel collapse is strangely similar in some ways to a much more serious tragedy that happened twenty-five years ago this month. On July 11, 1981, several hundred people gathered on a suspended concrete walkway to watch a dance party in the newly opened Hyatt Hotel in Kansas City, Missouri. The walkway was held up by steel rods which should have been strong enough to support the weight of the crowd. If they had been installed according to the original engineering plan, everything would have been fine. But on the site, a contractor decided to make a subtle change in the way the rods were made and assembled. This change greatly weakened the structure and caused it to collapse that evening, killing 114 people and injuring 200. Again, we had heavy concrete slabs, dangerous to life, suspended by thin steel rods. Again, if the plans had been carried out to the letter, the disaster would not have occurred. This is not to say that nobody should ever suspend heavy concrete slabs with thin steel rods again, or that engineers never make mistakes. They do. But the point is that responsibility inheres not only in those who make plans, but in those who carry them out and those charged with making sure that the work agrees with the plans.

Everyone involved in a building project, from those who pay for it, to the architects and engineers, to the contractors, to inspectors, down to the lowliest laborer cleaning up afterwards, has to walk that same line between excessive over-caution and reckless carelessness. Since the vast majority of engineering projects work without major failures or loss of life, we can assume that most of these folks do their job well enough most of the time. But an accident like the Big Dig tunnel collapse reminds us of what has to happen at every step of the way, and what can go wrong if somebody doesn't pay enough attention to details that don't seem to matter at the time.

Sources: The Boston Globe articles cited are at http://www.boston.com/news/globe/city_region/breaking_news/2006/07/romney_number_o.html (Gov. Romney's announcement) and http://www.boston.com/news/traffic/bigdig/articles/2006/07/18/workers_doubted_ceiling_method/ (the neglected lab tests). A string of technical discussions on the general subject of epoxy ceiling fasteners and how they can fail is at the Engineering Tips website http://www.eng-tips.com/viewthread.cfm?qid=159632&page=1. The Wikipedia article about the Kansas City Hyatt Regency walkway collapse is at http://en.wikipedia.org/wiki/Hyatt_Regency_walkway_collapse.