[mabelkitty]

IOW, the contractor skimmed off the top and used cheap materials to fill out the road.

[Alberta's Child]

Uh, no. That's how bridges are designed. They'd buckle under extreme heat if they didn't have expansion joints with bearing plates (where the slab can slide back and forth in tiny increments as it expands and contracts).

The graphic below shows a similar collapse at the Bay Bridge in San Francisco during the major earthquake in the late 1980s. Notice how neither the slabs nor the steel girders actually failed -- the slabs simply "slid" off their supports at one end.