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It collapsed in the center from the photograph but all the locals were worried about was "ground stability". The stuff on the sides looked really loose. I'd suspect a "blowout" on the sides which imparted a shear force on the concrete instead of a compressive one.

In compression, non-pozzolan or other enhanced concrete is usually good to 4,000 PSI. In tension or shear, it drops down to just about nothing. That's what prestressed concrete is all about. You run a cable or bar through it and purposely put compression on it. Any shearing motion will tend to "stretch" the steel tensile member which causes it to put even more compression on the concrete, which in turn prevents shear from affecting it.

In this case, if the dirt on the sides wasn't consolidated, it would have allowed the concrete tunnel to push the sides out and flex in the middle under load...and collapse.
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