Adhesive Failure Analysis at the Big Dig

SpecialChem - Jul 18, 2007


In this week's editorial a recent and very well publicized possible adhesive failure, Boston's Big Dig Project, is examined. Potential reasons for failures are identified. As with most accidents there may be a multiplicity of events that contributed to the problem.

On July 10, 2006, a motorist driving through Boston's Ted Williams tunnel (known as the "Big Dig Project") was crushed and killed by several tons of falling concrete ceiling tiles. The epoxy-and-bolt system used to fasten the concrete slaps overhead failed, although the technique is used extensively in construction applications. An investigation into the cause of the failure is ongoing, and it is premature to draw conclusions about this tragedy. However, published reports have indicated that there are several suspicious areas that will be studied. It seems that most serious accidents are the cause of multiple unexpected occurrences rather than one event, and this case possibly falls into this class.

More than 1,100 of the tunnel's bolt support assemblies were installed with epoxy. It appears that the $14.6 billion USD Big Dig project used a limited number of bolts and as a result there was a very low margin of safety. It is suspected that one bolt failure may have resulted in additional stress on the remaining bolts that then precipitated other failures in a chain reaction.

Some bolts from the ceiling wreckage have shown indications of very little adhesive being applied. Thus, improper installation or errors in the mixing of the epoxy before application is also being investigated, and this could vindicate the tunnel's design. Investigators have subpoenaed construction documents to determine whether contractors dismissed installation protocol.

Management is also being suggested as a culprit. The chief complaints are1 that the state government lacked adequate supervision of private contractors and2 that the same company was involved in both the design and construction efforts - an arrangement that some observers say may have complicated the oversight. Several ceiling bolts apparently failed during construction of the tunnel in 1999, and although this was reported, action may have been insufficient to identify and solve the problem.

Also, Big Dig managers and designers eliminated half the ceiling bolts called for in the original ceiling design to save time. Ceiling bolts in the area of the collapse were safety-tested at the time of construction with a weight now regarded as too low, potentially allowing defective bolts to pass.

Several months ago it us believed that contractors may have used the wrong adhesive to install at least some bolts. Invoices showed that the construction company received and apparently used at least one case of fast setting epoxy to secure the ceiling bolts to the tunnel roof rather than the standard epoxy, which the ceiling designers had specified. The "fast-set" epoxy holds 25 percent less weight than standard epoxy and is not recommended for suspending heavy objects overhead.

And then on July 10 of this year, Federal investigators apparently singled out another possible cause - creep of the epoxy. The National Transportation Safety Board indicated that the collapse could have possibly been avoided if designers and construction crews had considered that the epoxy support anchors when under a load of 4600 pounds each could slowly pull away from the ceiling.

After the accident, the tunnel ceiling was permanently removed in the area of the accident, and elsewhere in the tunnel the ceiling was reinforced with additional bolts and brackets. Investigations into the cause are still underway and potential legal actions are being considered. However, the multiplicity of possible causes for the Big Dig failure is indicative of most adhesive failure settings. There are many reasons why adhesive bonds fail. There are generally four factors that contribute to failure of an adhesive bond.

  • Selection of materials
  • Joint design
  • Processing (e.g., mixing, application)
  • Service conditions

Close examination of failed adhesive joints can usually lead to an explanation of why the bond failure occurred. The investigator's "tool box" for determining the cause of adhesive failures generally includes both inexpensive methods as well as state-of-the-art instrumentation. But, perhaps, the most important tools to use in the investigation are simple common sense and a predetermined, coordinated approach. The current article in Omenxus4Adhesives, "The Simple Approach to Nondestructive Testing and Failure Analysis", describes several inexpensive techniques that can be used early to determine the cause of failure.

When a bond failure does occur, a wide variety of processes can be used in a failure analysis investigation. Not all of these, however, are always appropriate. Some are not relevant to a particular failure, and time and money are associated with each element of the analysis. Probing questions need to be asked to efficiently plan and develop the methodology required to analyze the failure.


References

  1. "Bolt Failure at Big Dig: An Anomaly?", Christian Science Monitor, July 20, 2006.
  2. "Big Dig Job May Have Used Wrong Epoxy", The Boston Globe, May 3, 2007.
  3. "Epoxy Creep Blamed in Big Dig Death", Associated Press, AP Online, July 11, 2007.

Should you have any comments or feedback, please contact me.

Edward M. Petrie

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