Tay Bridge disaster
However as the bridge extended out into the river, it became clear in December 1873, that the bedrock really lay much deeper. It was too deep to act as a foundation for the bridge piers.  Bouch had to redesign the bridge. He reduced the number of piers and increased the span of the girders. The pier foundations were no longer taken down to bedrock; instead they were built by sinking brick-lined wrought-iron caissons onto the riverbed,