Safety margin for tunnel was smaller than most, US says
Preliminary report issued
The Big Dig tunnel ceiling that collapsed in July was designed with a smaller margin of safety than other tunnel ceilings around the country, leaving nothing to prevent heavy concrete slabs from falling on a passing car when ceiling bolts fell out, according to a preliminary report by federal investigators obtained by the Globe.
The Interstate 90 connector’s drop ceiling was held up by steel hangers, which were suspended from bolts that had been glued into the tunnel roof. But there were no beams attaching the ceiling to the walls, and the ceiling was constructed with half as many ceiling bolts as in the original design.
‘‘No redundancy was built into the ceiling in the event the hangers failed,’’ the National Transportation Safety Board states in its report. ‘‘The NTSB has researched other tunnels throughout the country and has found that significant redundancy is built into the ceiling design’’ so that the ceilings would not collapse when bolts fall out.
The report does not reach conclusions about the cause of the ceiling collapse, because the investigation is not finished. But it shows that the federal agency is looking into the adequacy of tests done to ensure the strength of the ceiling bolts and says that an employee of the testing contractor told investigators that testing differed from what was required by the construction contract.
The accident, which killed Milena Del Valle as she rode in a car to Logan International Airport with her husband late on July 10, already has triggered federal and state criminal investigations, a wrongful death lawsuit filed by Del Valle’s family, and the resignation of the chief of the Massachusetts Turnpike Authority. But the investigation by the NTSB will carry enormous weight in assessing what went wrong and who should be punished, because of the agency’s expertise in reconstructing disasters.
The preliminary report, sent to key federal officials yesterday, describes significant lapses by both the state government and private companies involved in the $14.6 billion Big Dig project. From the tunnel’s original design in the mid-1990s to the moment of the accident, people involved in the project made decisions or mistakes that increased the risk of disaster, the report suggests.
For instance, the report said, officials from Bechtel/Parsons Brinckerhoff, the joint venture overseeing the Big Dig, decided to use epoxy bolts to suspend the ceiling from the tunnel’s roof, even after they switched from a lightweight material used for the ceiling in the adjacent Ted 8Williams Tunnel to much heavier concrete ceiling panels to save money. 8Epoxy bolts, which are held in place by a kind of super glue, are rarely used to suspend such heavy 8objects over people’s heads, 8according to interviews with engineers who specialize in the low-cost, but somewhat delicate bolts.
Then in 1997, the NTSB found, Bechtel/9Parsons Brinckerhoff managers directed the company designing the ceiling to follow a design that ‘‘provides little or no redundancy’’ if more than one hanger, each held up by two bolts, comes loose. NTSB researchers found that other US tunnels commonly feature side supports or other methods that back up the primary suspension system.
By contrast, the connector tunnel had no side supports, and Bechtel/Parsons Brinckerhoff persuaded the designers at Gannett Fleming to cut in half the overall number of ceiling bolts holding up the connector ceiling, according to a 1998 memo previously reported by the Globe. After the accident, state investigators found that ceiling panels in the connector tunnel could collapse if as few as four ceiling bolts fail.
The NTSB report also suggests that flaws in construction of the ceiling, carried out by Modern Continental Construction Co., may have made the design more hazardous. While offering few details, the report said FBI and State 8Police interviews with Modern Continental workers point to 8‘‘potential problems in the procedures used during installation.’’ In addition, epoxy samples taken from parts of the tunnel away from the accident site showed ‘‘dark brown discoloration,’’ a sign that the epoxy was mixed improperly, diminishing its strength.
The NTSB also said safety tests carried out to prove the ceiling bolts were securely in place may have damaged some bolts. An 8inspector from the bolt-testing company, ConAm Testing, told NTSB investigators that some bolts were tested while the concrete ceiling panels hung from them, which required workers to lift the weight off the bolt being tested and shift it onto nearby bolts. That extra weight could have weakened the bolts not under8going the test, say engineers not connected to the Big Dig or the investigation.
The NTSB investigators also are looking at whether the bolt safety tests were rigorous enough. The Globe has reported that during construction in 1999, at least five bolts came loose soon after the connector ceiling was suspended from them, even though they had passed earlier safety tests. At the time, Bechtel/Parsons Brinckerhoff officials required Modern Continental to retest bolts in parts of the connector using stricter safety standards, but bolts in the eastbound lane, where the ceiling caved in, were not retested.
Federal investigators have found no evidence that the Turnpike Authority rechecked the ceiling bolts after the connector opened in 2003, the report said. If they had, the report finds, they would have seen some bolts showing ‘‘extreme displacement.’’ Some of the 5Æ-inch-long bolts had come more than an inch out of their holes, the report said.
Murphy can be reached at smurphy@globe.com; Allen at allen@globe.com. 