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[quote user="CShaveRR"] Was the Twin Cities' Stone Arch Bridge's problem a design flaw, or flood-related scouring that caused a pier to drop?[/quote]I mentioned this in another bridge thread, but it fits better here.  In the big flood of 1965, one of the piers (nearer to the east end) of Jim Hill's stone arch bridge sunk about two feet.  As I recall, it happened when a passenger train was passing over it.  The bridge just accommodated the sinking pier like it was made of rubber.  Telephoto shots of the double tracks made it look like a perfect roller coaster dip.  They just filled the sag in with new grout, raised the tracks to proper surface, and did not raise the sunken pier.  Today, if you sight along the sides of the bridge, you can plainly see where all the courses of masonry in the upper portion take a dip.  I believe they also lined the underside of the arches with new concrete on each side of that sunken pier.  I always thought the incident seemed kind of strange.  If the water soured away the bearing material supporting a pier, the material would have to be clay or gravel.  I would have guessed that those piers are standing directly on limestone bedrock rather than soil.  It is also surprising that they could just continue operating the bridge with the sunken pier by merely re-surfacing the deck portion to proper grade.  Maybe they did also do work to reinforce the footing bearing as well.  It also seem a bit amazing that a masonry bridge could move that much under the weight of a train and not fall down.  I suppose the force was so even that it created an appropriate distribution of joint cracking to equally spread out the joint separation.
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