Anatomy of a Stone Bridge...
Stone arch bridges aren't solid stone. They're mostly dirt. Two parallel stone walls hold compacted dirt between them. Only the arch itself is solid stone, and the very apex is only one layer thick. Only pressure from the other stones in the arch pushing equally in opposite directions holds the keystone in place--and keeps your feet dry!
If
you look carefully at the photo, right, you can see the dirt fill
material between the remaining stone walls. We don't really know for sure
exactly how the bridges were built. The most popular idea is that the
stream was first diverted, then a mound of dirt was constructed where the arch
would be. The stones were placed, and the dirt removed leaving only the
stone arch.
Well, that's the cut-and-dried theory, anyway. Only trouble is that some of these bridges don't appear to have a single keystone. And the stones are of different sizes and number on each side of the arch. We don't really know what makes them stand--or not stand--as was the case with this bridge. The bridge had been routinely inspected a few days before its unexpected collapse, and everything seemed OK . Information is lost from each generation to the next. These bridges were built by long-gone craftsmen, and they took many of their secrets with them.
Walter Sharp, Bridge Builder...