The piers for the St. Louis bridge had to be built upon
bedrock, which lay over 100 feet below the river surface. To reach
bedrock, Eads sank a caisson for each pier. Each caisson was made of heavy
steel, sixty feet in diameter and nine feet high, and open at the bottom
for digging up the sand. As the sand was removed, the caisson sank, and
the masonry pier was built up upon it as it went down. As the caisson went
lower, air pressure had to be increased to keep out the water, and serious
problems with "the bends" were encountered below sixty feet.
The view on the left, from the British journal Engineering, shows the caisson for the East Pier in cross section, with the
pier rising upon it.