As an intern, I worked for a company that did geotechnical analysis for one section of the Big Dig. I wasn’t privy to it, but the two hours I ever billed to the project were for picking up x-rays of soil cores at MIT. Since then, I have never heard of x-rays being done on cores for any other project, so they took information gathering seriously.
There’s just no way that the plans would have been stamped by a Professional Engineer if the foundation couldn’t be expected to sustain the Big Dig. The liability costs are enormous, and I knew at least one of the engineers that would have stamped them. This was not a guy to do it lightly.
I’ll poke around and see if I can find information about the foundations and clarify things a little.
For some parts of the dig, at least, the problem isn’t keeping it from sinking, it’s keeping it from floating. The tunnels are hollow and watertight (in theory); put them in soil below the water table and they’ll act like the hull of a ship. In some places, like the approach to the Ted Williams tunnel, the concrete under the road is ten feet thick so there will be enough weight to offset the buoyancy.
And from what I understand, Romney has been trying to fire the head of the Turnpike Authority for a long time. Maybe he (the Governor) knew the guy was incompetent and this incident brought it to light, but the story could also play that he’s taking advantage of the tragedy to oust a political rival.
The word is that the concrete ceiling panels (3 tons each) were held up by threaed bolts epoxied into the poured concrete tunnel roof. Supposedly, the bolts were tooo short to support the loads. Will lawyers be able to “prove” negigence? not liely-the Governer can’t even get access to the MTA’s files
I’ll be interested to see if the conclusion is faulty installation (there have already been problems with substandard concrete being supplied) or faulty design. Engineering design always has multiple factors of safety and redundancy built in and most structural engineers take their responsibility for public safety very, very seriously. I’m much more inclined to believe a contractor cut corners somewhere and somehow the inspectors didn’t catch it.
Here’s a diagram of the method used to secure the panel:
If that has already shown up in one of the links then I apologize for the repetition.
tremorviolet I too would take the side of the engineers on this one. There has already been talk of sub-standard materials having been used elsewhere in the “Big Dig”.
From mhendo’s link:
Incidentally, I’m wondering about the credentials of Matthew Amorello - the “Turnpike Authority guy” as he’s been called here.
During Governor Ed King’s administration (late 1970’s), Mass Port Authority appointments requiring Masters Engineering Degrees were being filled with people who had no more than a high school education. Since that resulted in no one’s death, I guess the Massachusetts citizens (including me) accepted this as the typical corruption, patronage, etc that is so rampant here in the Bay State.
Perhaps Tuesday’s fatality will cause some serious scrutiny of our elected officials and state employees.
I don’t know. Without being an engineer, this ceiling system doesn’t, on the face of it, seem like a terrific idea. You’re relying entirely on the bond between the epoxy and the concrete keeping its integrity over time. There’s no mechanical strength to the joint at all. Plus, why are the ceiling panels concrete? Each one apparently weighs several tons. Why not a lighter material?
Oh, and as to the whole Big Dig, I also thought it was a huge boondoggle. That said, it has improved traffic flows in the city in certain important ways. There’s now a connection between Interstates 90 and 93, access to the airport has improved, and some of the connections around Leverett Circle are not as nightmarish as they once were. And certainly tearing down the elevated highway means you can reknit halves of the city that were torn apart by the Central Artery. (The unfortunate corollary is that where you used to get a beautiful view of the city as you passed through on the elevated highway, now you get a view of dreary tunnel walls.)
So yeah, in a general sense, I would say the Big Dig has improved Boston. But was it worth it at the price? That’s the $20 billion question.
One of the news broadcasts I watched last night said that the panels were so thick because they were part of the air plenum system for the tunnel and had to be strong enough to handle the force of the air flow. At least that’s how I recall that part of the report.
The epoxy/bolt systems are not unusual in engineering design and have a good, extensive track record (provided they’re installed properly). When you need to bolt something to a concrete or rock face, you don’t have many other economical options.
This page has some information about anchoring systems. Look at the lowest rated bar in the charts at the bottom of the page (top row, third chart). It has a stated working load (with a factor of safety of four which means they established what it actually could hold and then divided by four) of 4.5 kips. A kip is 1000 pounds or half a ton. The panels weighed only 3 tons or 6 kips which sounds heavy but are actually pretty light. They must be only around half an inch thick.
Anyway, you can see from the illustration wolf_meister linked, each panel has multiple anchors holding it up. The number of anchors was probably driven more by the geometry of the panel supports thanthe minimum number of anchors needed. Things like anchors are pretty cheap and most engineers would throw extra in “just to be safe”. In the illustration, I see six anchors for one end and I would think the other end would have the same configuration (I would actually bet there’s more in the middle of the panel since the panel is so thin). So assume twelve anchors holding up a half of two different panels.
12 anchors x 4.5k = 54 kips or 27 tons >> 3 tons
So, on the surface, it’s really overly designed. I really think it’s going to be a case of improper installation or bad materials.
(Actually it’s the same exact graphic but the detail is much better. The first one I uploaded to Photobucket which limits a file’s size. The new link goes to a free geocities website.)
As for me, I take public transportation which is … well it’s better than walking.
I don’t have a car because (as you may know), the insurance rates in this state are ridiculous. Yeah, this might be an indication for me to leave the state.
One can go through Chelsea to get to Logan (cue Elvis Costello’s "(I Don’t Want To Go To) Chelsea) but it takes forever. Otherwise you can go via the Callahan/Sumner tunnels. You can drive on the surface roads in Government Center, but most likely you would come through the tunnel from I-93 North or South, or Storrow Drive. The concrete ceilings in the I-93 tunnel are apparently different from the ones that were in the I-90 tunnel, but there’s been falling debris and flooding in the I-93 tunnel as well.
They dismantled the “other Green Monster” - the elevated Fitzgerald Expressway - starting before the Democratic Convention in 2004 (which looked totally weird) and it’s now completely gone. But I’m glad. If you were sitting in traffic on the Fitzgerald you could feel the roadway shake - quite a bit - more than I’m comfortable experiencing. Good riddance to bad rubbish, I say. But I don’t recall it ever falling and killing anyone.
Maybe so, but shouldn’t the engineer at least allow for the possibility that the installation will be substandard? This type of installation is hardly foolproof. Moreover, there’s no guarantee that the particular square inch of concrete into which you’re boring the hole for the bolt has all the integrity of the concrete sample in the lab. In fact, there’s a record of failure with this type of installation going back to the late '90s, as today’s paper reveals.
This ceiling was like a sword of Damocles, and even if the installers were to blame, I don’t think the engineering is all that defensible. Off the top of my head, I could come up with a half dozen safer solutions. Sure, they’re probably more expensive, but in the context of the whole job, the cost would probably be negligible.
tremorviolet’s back-of-the-envelope calculations give a factor of safety of 36 (nine was applied to the system by the engineer, four by the manufacturer). That’s a pretty good accounting for the chance of substandard installation, don’t you think? By way of comparison, factors of safety for, let’s say, a dam, will typically range between 1.4 and 3.5 depending on the analysis. My brother, a mechanical engineer, tells me that the overall factor of safety for an airplane is about 1.01 (but that’s word of mouth, so don’t quote me on that, please)
That said, a photo of the anchors was sent around my office, and the general ‘gut feeling’ was that any of us would have put in bigger anchors. If we had looked at the specs for the anchors, though, that might have changed.
Epoxy anchors are pretty standard stuff in bridge rehabilitation. When installed correctly, they’re quite reliable. I’ve never heard of one failing in my state. Exactly what is the function of the ceiling? Is it just aesthetics or something to do with air flow?
