Problems with the “Big Dig” tunnel deepen, will last months at a minimum

Not bad for a $2.5 billion dollar project that has ballooned to over $15 billion.

I agree with you, Kevin. The problem is, there is not enough oversight in these projects to ensure that the appropriate standards are being met. We supplied some equipment for a project on a military base with 11″ epoxy type anchors. The equipment failed, be we were not called out to look at the failure mode until 4 years after the fact. When we inspected, it was at first very clear that the bolts had been installed improperly due to the presence of epoxy and concrete dust on the outside of the bolts. Further inspection showed that the bolts had been cut/shortened. We began to pull all of the bolts and found one that was only 4.5″ long. Fortunately, this was not a life or death situation.

My guess is that there was a widespread belief that the anchors were overdesigned for the installation, there was a belief that there was no way the panels would come down, and that there were probably union or worksite regulations regarding the frequency of inspection.

The big question to ask regards the necessity of the concrete paneling. Wouldn’t foam or sheet metal have worked just as well? Was there a structural necessity to have big thick concrete panels on the inside of a tunnel? My guess would be that it was not necessary.

I just don’t understand why they are suspending THREE TON CONCRETE PANELS from the ceiling over the motorists’ heads. Is that just an aesthetic thing, or do they serve some structural purpose?

Being from California, I often wonder about failures that will show up in the next big quake. Every time there is a major quake in others parts of the world that result in a lot of deaths, they talk about our high safety standards. But I can’t help but think about how many shortcuts and payoffs were done by greedy developers to bypass those standards.

In a system like this aren’t bolts and concrete sections suppose to be the load bearing components with the epoxy there to stabilize the bolt in the material? Or is there some epoxy that is suppose to acceptable for load bearing applications?

Maybe the cost and the failure both have the same problem. It used to be the engineer who called the shots on a job and he had his reputation on the line, success or failure either made or broke you, so you didn’t allow for failure. Now there is over site, with over site, and then there is over site. All of these overseers get paid and no one needs to worry that their career will be distroyed by a mistake that someone else should have caught. How about every level that didn’t catch the error or errors give back the money they made on the job that had anything to do with it happening, and don’t forget the lawyers who drew up the contracts with a few million disclaimers, nor the ones who will make out like bandits on the fallout from it. While were at it maybe we should include every tax payer who wants the moon but thinks someone else should pay for it. Our press who seem to love to tell us about every wrong after the fact, but never seem to really discouver anything before it happens. Or the elected offical that loves to note the funding of projects that are not well thought out in advance. The idiot that thinks his party would do a better job should be at the top of my list. Make it simple one guy is in charge and it is real its his neck if it is wrong, period. #1 it would never get built, #2 nobody would be paying for it.

The schematic of the installation shows the bolt bonded to concrete in a 3/4 inch diameter x 5 inch length hole. If the expected load on each bolt is 4000 pounds the stress on the bond would equal about 400 pounds per square inch in shear. Epoxy materials should be capable of more than 10 times that stress. That factor might have led to overconfidence in the design.
For a bonded joint the strength is determined significantly by the surface adhesion, in this case to the concrete. Surface preparation is critical in any bonded joint. One would expect that adhesion to be highly variable in a hole in concrete. Concrete dust or even normal concrete surfaces may drastically affect the actual strength of the bond. A ‘fast-setting” epoxy may also harden before there is significant wetting and possible penetration of the epoxy into the pores of the cement reducing its bond strength compared to a slower reacting epoxy formulation.
As mentioned in one of the other postings, once one bolt installation fails or even has moderate displacement , the loads on the others increase and an “unzipping” failure affecting a panel installation could result.
Process factors, like voids in the epoxy, the potential use of a “fast-set” epoxy formulation, dislocation of some “filler plugs” reduce the expected strength even more. In this case, the reduction was too much.
Such a scenario points to a simple engineering error that probably could have been avoided if the design had been reviewed by someone knowledgeable about adhesive joints.There is no need to blame politicians, inspectors, workmanship, suppliers, contractors or the phases of the moon.
The Big Dig was celebrated as a prime example and success of the engineering discipline of “Systems Engineering”. It may be that overall, but it takes the knowledgable specialist to ferret out the subtle errors in assumptions that brings the roof down.