The most common reason bridges collapse in the US is scouring around the foundation. This happens when water moves quickly around a bridge’s piers and abutments and scours sediment. That is, it carries sediment away from these foundation elements and leaves holes behind which then weaken the bridge’s structural integrity.
Since most bridges are built over water, most bridges will have problems with scouring over the course of their useful life. This is a problem because the flood repair cost for bridges runs around $50 million annually for bridges that are on the federal aid system. Also, bridge failures are up to five times more costly than bridge repairs. So, anything that can lead to bridge failure – including scouring – is a big problem.
Buildings along coastlines are unique in that they face things buildings inland don’t, namely waves, the rising and lowering of tides, storm surges, and floods that include debris. All this can cause extensive erosion and the scouring of soil around foundation piers and slabs – which weakens foundations.
Remember, the role of a foundation is to take the load on the building and transfer it into the ground. Therefore, foundations need to be sufficiently deep and they need to be strong. If they’re not, the building will fail. The foundations of coastal buildings need to be embedded deep enough into the ground so that the erosion and scouring of sandy soils which will undoubtedly happen over the building’s lifetime, don’t affect the foundation’s integrity and lead to a collapse.