Fastener Applications Scrutinized in Boston’s Big Dig Tunnel Collapse

Investigators in the fatal collapse of concrete ceiling panels in a Boston tunnel are focusing on the potential faulty application of the epoxy used to anchor the bolts. There has been no indication that a “bolt failure” occurred, as some TV broadcasts dubbed it.
During a televised press conference, Mass. Gov. Mitt Romney said the anchors were “letting go at lower pressures than they were designed to handle and that suggests that this epoxy system is not working the way it had been designed and engineered to work.”
Initial bolt requirements outlined in 1999 required the fasteners to pass pull tests for 6,350 pounds, more than twice the weight of the panels, the Boston Globe reported. Following the collapse, pull tests conducted by the Massachusetts Highway Department caused Romney to declare that all 1,454 epoxy joints throughout the tunnel were “unreliable” and must be reinforced.

The Globe reported that “glue on bolts removed from the tunnel roof near the accident site was brittle and cracked, not like smooth glass as it should have been,” indicating that the epoxy didn’t appear to have sufficient compressive strength, possibly from improper application.
While stressing that he has no first-hand knowledge of the fastening system used in the Boston tunnels, Carmen Vertullo, owner of Simply Better Inc. and an instructor with the Los Angeles Fastener Association’s Certified Fastener Specialist training program, explained to FastenerNews.com that anchor manufacturers use various rigs to ensure that the epoxy is properly mixed, but they cannot control other critical factors such as hole size, cleanliness, timeliness of use, quantity used or condition of the substrate.
“I would expect that it is not a specification or strength issue. Considering the application, there is probably high safety factor in this design,” Vertullo explained.
Evidence released so far indicates the problem most likely occurred during application. In the past few days a Big Dig technician claimed to have witnessed critical bolt failures in the tunnels during the mid 1990s, the Boston Herald reports. The technician allegedly showed the Herald photographs depicting Big Dig workers “using methods to install epoxy bolts that were flagged as problematic by inspectors.” The photographs allegedly show workers applying epoxy to a screen that was then inserted into the bolt hole.
A memo written by the oversight firm Bechtel/Parsons Brinckerhoff in 1995 ordered tunnel workers to “stop installation with wire screens,” according to the Herald. The memo emphasized that “this method is not for solid concrete, but for hollow masonry walls.” Other fastening system concerns at the time included “holes being drilled too deep, concerns that cold temperatures could weaken epoxy and that the use of screens was contrary to the epoxy manufacturer’s recommendations,” the Herald reports.

Insufficient testing could also have played a role in the collapse. Investigators are examining test records to see if the contractor tested every bolt as required, the Globe reported.
For now, construction crews have begun to reinforce the ceiling with new heavy-load-tested embedded anchors that passed a 14,000 pound pull test.
Despite the incident, several fastener industry veterans interviewed by FastenerNews.com gave a vote of confidence to chemical anchors in general.
“These are very reliable, highly-engineered systems used in critical application,” Vertullo noted. “When used properly, they work well.” �2006 FastenerNews.com