The faulty bolt and epoxy assemblies that led to more headlines about Boston’s Big Dig/Central Artery Tunnel Project was just the kind of engineering who-done-it engineers find irresistible. Was the wrong epoxy used? Was the design faulty? Or was poor installation the reason for the failure?
“Tension anchors such as this are required to have a factor of safety (by code) of at least 4. Considering that, it appears that the assembly had been holding for a while and gave way,” posts a reader on Eng-tips.com. “That points more so toward an installation anomaly, likely exacerbated by vibration or other repetitive loading, or potentially a material failure (excessive creep under load, embrittlement, corrosion, etc.), or as usual, some combination of those.”
The investigation, which found faulty assemblies, was sparked by the July 10 death of a 39-year-old woman after 12 tons of cement ceiling panels fell on the car her husband was driving inside the I-90 connector tunnel more than two weeks ago.
As a result, several portions of Boston’s $14.6 billion Big Dig/Central Artery Tunnel Project have been shut down. Ongoing investigations by federal and state officials are uncovering additional problems almost on a daily basis.
On July 12, according to a Boston Globe report, inspectors found at least 60 faulty bolt fixtures in the ceiling of the tunnel. On that same day, Attorney General Tom Reilly says tests conducted in 1999 showed that the ceiling bolts had a tendency to come loose.
On Wednesday, July 26, the Herald reported they obtained documents revealing that Big Dig officials noted six years ago that epoxy ceiling supports were failing in the same tunnel section where the deadly accident occurred, but allowed the continued use of that “superglue” system to support 3-ton concrete slabs.
On Thursday, July 27, Mass Turnpike Authority Chairman Matt Amorello stepped down from his position, overseeing the project, just before a hearing with Gov. Mitt Romney – a strong proponent of Amorello’s resignation.
While doubts about the infrastructure’s safety are only coming to light now – a December 1998 Inspector General’s Report reviewing the project’s use of anchor bolts documents numerous problems with the bolts and glue used to secure the ceiling in the Ted Williams Tunnel which opened to traffic in 1995. The I-90 connector connects the Mass Pike with the Ted Williams Tunnel.
The Inspector General’s Report discloses several findings which would indicate a problem could occur:poor design specifications, paying contractors to test improperly installed anchor bolts and lack of consultation with tunnel designers before allow contractors to drill through steel reinforcements in the tunnel roof.
It also documents numerous problems with the bolts and glue used, including bolts that were too short and trouble with the epoxy used to glue the bolts into the concrete.
In the past two weeks, forums and blogs have been popping up all over the Internet where many techies are eager to speculate on what happened, why it happened and what the next steps should be under the cloak of anonymity.
Engineers, for example, have been posting their two cents on Eng-tips.com since the news broke about the deadly accident.
“Now why hire a geotechnical engineer to study structural failure,” posted one reader after reading a newspaper report about a geotechnical engineer hired to conduct an investigation on behalf of the Turnpike Authority.
Others are offering their thoughts on how these assemblies failed, what materials should have been used, and how the problem can be corrected.
One blogger, whose profile notes that he is an ICC Reinforced Concrete Special Inspector and an ICC Pre-stressed Concrete Special Inspector, among other specialties, says he has nearly 20 years of experience performing both placement and post-placement inspections of rebar, post-tensioning systems, concrete, masonry, etc. He says if structural engineers who specify epoxy for dowels and the like believe that the work is being done correctly then they live in a world unfamiliar to him. “The key instructions to correctly epoxy in a dowel are either unknown to or widely ignored by contractors. You would be astonished at the number of so-called epoxied dowels that I can pull out with one bare hand,” he posted.
Another posting says all engineers with substantial field experience who have commented on the site agree that epoxy anchors are fraught with danger and that their use in the situation at hand was a poor choice.
A consulting engineer with 30 years of experience in structures, materials, materials testing, failure analysis of buildings, pavements, and products agrees.
Another posting explains that very few epoxies work in overhead applications.
“Most are too viscous. Most anchors require careful cleaning to develop proper capacities. Proper mixing and distribution of the epoxy in the hole is also critical. Cored holes need to be scored. In many applications the full capacity of the anchor is not needed, so sloppy installation works. Here installation was obviously critical, as well as proper product selection and preoff testing. I would not have used epoxy overhead unless I had to. On thing that bothers me are the turn bukles (sic). The rods provided a highly redundant support path for the panels. The loads in the hanger anchors could vary considerably from what may have been assumed for design based on how the rods were designed. This may have a significant contribution to the failure.”
“Was there a structural necessity to have big thick concrete panels on the inside of the tunnel,” posts one reader. “My guess would be that it was not necessary.
He adds that he thinks the problem is due to the lack of oversight in these types of projects to ensure appropriate standards are being met.
State inspectors began strength tests on a sampling of ceiling bolts in the Ted Williams Tunnel this week. But while there’s still uncertainty about exactly what happened, one thing is for certain: Engineers will continue to debate the issue.
For more discussion on the issue of materials failure in Boston’s Big Dig project, check out our Materials Forum.
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