Report reveals cause of Big Dig tunnel collapse

The use of epoxy anchor adhesive with poor creep resistance was the cause of a fatal roof collapse in a tunnel in Boston’s Big Dig on July 10, 2006, according to a report from the National Transportation Safety Board (NTSB). Safety issues cited in the July 10 report included “insufficient knowledge among designers and builders [on the project] of the nature of adhesive anchoring systems,” lack of standards for testing those adhesive anchoring systems and for designing tunnel finishes, and inadequate tunnel inspection requirements.

An adhesive anchoring system used in the Big Dig tunnel to hold several large concrete slabs on the roof of the tunnel failed, and one of the slabs fell onto a passing car and killed a female passenger. The epoxy used in the system deformed and cracked with time, allowing the anchors on several slabs to pull free, according to the report.

According to the American Society of Civil Engineers (ASCE), the NTSB asks ASCE to use the Big Dig accident “to emphasize to your members through your publications, Web site, and conferences, as appropriate, the need to assess the creep characteristics of adhesive anchors before those anchors are used in sustained tensile-load applications.” The entire report is available on the ASCE Web site at http://www.asce.org.