Why did the slabs give way?

Friday18 and her husband used to work for a materials testing company - hired to test construction material on certain very large public-works projects in the Boston, many involving the same company responsible for the part of the Big Dig tunnel with the collapsing concrete slabs. And she explains why this makes her completely unsurprised by the collapse.

Bruce, who has written before about Big Dig construction deficiencies, today is left wondering what happened?

... Now, you've got to ask yourself, which is more likely? That this was a fluke occurrence, and that all the other suspended panels in that section of tunnel are just fine? Or, that this failure was just the proverbial tip of the iceberg, and the whole ceiling could likely give way anytime now? If it's proven to be the former, good luck convincing all the people who used to drive through this tunnel of that fact. ...

He wonders if maybe somebody cut corners and never properly bolted the panels in place. Channel 4 is reporting the panels might only have been held in place by glue.