buried culvert, so the soil has been disturbed and no amount of packing, tamping etc. can compare with the densities of the undisturbed "virgin" soils on each side of culvert. Rain water/runoff slowly erode the disturbed dirt and whamo. :;:smokin or something like that
that's my uneducated stab at it
looks to me like they put in a huge metal culvert - you can see it crushed on the one end - and the other end you can see the big opening of the section that hasn't collapsed yet
There are sections of "creek" overlain in Fargo using that approach but they always use massive concrete culverts. I don't think metal ones are capable of bearing the weight of parked cars sitting on them all day.
And maybe flowing water "scoured out" soil from under the culvert too, causing it to fold up that much more easily.
either way the consulting engineers who designed that are going to end up taking the blame - rightfully so I think
- - - Updated - - -
you can see the crinkled culvert's outlet at the very beginning