The Boston Sunday Globe reported yesterday that the original design for the ceiling of the tunnel connecting I-90 to the Ted Williams Tunnel, a portion of which collapsed on July 10 and killed motorist Milena del Valle, called for twice as many anchor bolts to support the ceiling’s heavy concrete panels. (Scott Allen & Sean P. Murphy, “Designer proposed more bolts in Big Dig,” Boston Sunday Globe, Sep-17-06, p. 1) Big Dig construction manager Bechtel/Parsons Brinckerhoff changed the design from 4-bolt to 2-bolt anchors, and tested the bolts’ strength using a method “developed by an obscure Portugal-based group,” the International Society for Rock Mechanics, that was not intended for bolts anchored in concrete roofs. A Globe chart shows that the bolts, each of which carries a load of 2,600 pounds, were tested using a force of 3,250 pounds. A consulting engineer interviewed for the Globe article stated that the rule of thumb was to test bolts at twice their carrying load–in this case, 5,200 pounds. Bolts anchoring the ceiling in the tunnel’s high-occupancy vehicle lane, built at the same time, were tested using a force of 6,350 pounds.* Current standards would require testing the bolts using between 10,000-15,000 pounds of force. The Globe reports that five bolts that passed the 3,250-pound test “came loose shortly after the ceiling was hung from them.”
A grand jury is scheduled to be seated in October to pursue possible criminal charges in del Valle’s death. A central question will be whether Big Dig managers exercised the appropriate duty of care in designing the ceiling. Evidence that (1) the ceiling was originally designed to require more anchor bolts, (2) managers modified the original design because they believed “a 2 bolt anchor plate would be sufficient” to support the multi-ton concrete panels, (3) managers tested the bolts’ strength using standards less rigorous than industry rules-of-thumb, (4) some bolts passed these tests but failed when subjected to their working loads, and (5) managers knew of such failures but didn’t either redesign the ceiling’s anchoring system or more carefully supervise its installation could establish the basis for criminal culpability. Is Bechtel/Parsons Brinckerhoff worried? An unnamed company source (not authorized to discuss its legal strategy**) delivered this quote: “[From] everything I’ve heard from our lawyers, the genuine risk of criminal prosecution or criminal conviction is extraordinarily low.”
The article reports that the ceiling design firm ultimately agreed to the 2 bolt anchor design “assuming proper installation and quality of the product materials.” The article goes on to describe various problems that arose during installation including incorrect mixing of the epoxy components, failure to apply adequate epoxy material, and use of the wrong drill bits for epoxy bolts.
We’ll watch how this unfolds over the coming months. And years.
* One wonders why different safety tests were used on adjacent lanes in the same tunnel built at the same time.
** Of course, no one authorized to discuss legal strategy would produce such a juicy quote.