are 3 ton slabs of concrete necessary for big dig tunnels?

Question

I was wondering, after the recent tragedy in one of the tunnels that's part of the Big Dig in Boston killed someone driving through it, why is it necessary to have these slabs of concrete (like what's their purpose in the tunnel)?
Thanks

well in the pictures they were just hanging off of the real ceiling.

Answer 1

The panels that actually fell were not part of the structural system of the tunnel. There is concrete that forms the tunnel itself - roof, walls, and floor - to hold out the dirt. However, the panels that fell were hung by steel anchors from the "real" roof of the tunnel. They were there to form a kind of duct that would allow fresh air to be circulated into the length of the tunnel - imagine a big hole with a running car and what would happen to the air.

I'm sure that they chose concrete panels for their durability. Using a lighter weight metal panel would have required the panels to be replaced more frequently as tunnel users damaged them.

So far it looks like the failure was not in the panel itself, but rather in the epoxy anchors that connected it to the "real" roof. The panels were likely precast - formed in a concrete shop with fairly tight quality control. The likely culprit was an installer who was unfamiliar with the type of anchor, or was too lazy to train their personnel correctly.

Answer 2

Of course concrete is necessary in construction of the tunnel. However concrete, though very strong in compression is much weaker in tension. In the ceiling, concrete will not only be under the pressure of the load above the tunnel, it will be under the stress of its own weight. This is why concrete is reinforced with steel. A matrix of steel “re-bar” is part of the structure that when designed and constructed correctly will be an extremely strong member under compression and tension. There will be beams or cross members adding to the support. My guess is that, in an effort to save money, they designed to the hairy edge with respect to the density of re-bar in the concrete. If this is this case, than I feel sorry for the Engineer of Record and the General Contractor. I doubt this is the case. I would be willing to bet that re-bar was deliberately left out during construction in an effort to either keep the profit margin or line the pockets of the construction contractor. As the re-bar is covered by concrete no one would wiser.......

Answer 3

Soil weighs approximately 125 pounds per cubic foot. so if you have 15 feet of soil above that point you have 2,000 pounds per square foot of weight the concrete is there to hold it up. The soil doesnt just hold itself up.


To reply to the answer below me. On a project of that size, cost, and city attention they would have one maybe two inspectors watching everything all day everyday. Therefore the contractor leaving the rebar out of the concrete is pretty much not going to happen without getting noticed.

Answer 4

As to the inspector issue, i disagree with previous answer.
In the structures that have lasted a long time with little maintenance, quite a few inspectors were used to oversee the hen house.
Today, too few inspectors are used to cover all areas.
And, these panels were probably cast in a specialty concrete with little or no oversight.

Answer 5

They're the ceiling. It's gonna be a necessary part of the tunnel.

Sorry...didn't mean to be a smart-aleck. Check Larrybob's answer below...the panels are required to help hold it all in and together.