Caisson | Research & Encyclopedia Articles

Caisson

In the construction of bridge piers and deep-water docks, it is necessary to provide the workers a water-free environment to excavate the foundations. In shallow water and on dry land, coffer dams and open-air caissons are sufficient. For deep water, an enclosed caisson is necessary.

The caisson is a water tight compartment that is open at the bottom. (The earliest caissons were shaped like cylinders, open at both ends.) Workers descend to the caisson through a shaft, or man-lock. As they excavate the bottom, the material is transported to the surface through the excavation shaft, or lock. As the excavating continues, the massive caisson sinks deeper into the soft bed until the bedrock, or any other desired depth, is reached. The downward edges of the caisson walls are wedge-shaped to aid in its downward penetration of the bottom.

Pressurized caissons were used by Isambard K. Brunel in the 1838 construction of the London docks. Compressed air was used to keep water and soil out of the work space. Without it, pressure at depths of just forty feet were great enough to push the water and soil into the bottom opening of the caisson. But it also put the same pressure on the workers; pressures of about 52 lb/in² (360 kilopascals), or about 3.7 times atmospheric pressure, is about all the human body can stand. This corresponds to a caisson depth of 120 ft (37 m).

In 1839, a French researcher observed that miners were emerging from mine shafts with medical complaints that included incoherence, nosebleeds, muscle cramps, and itching. The symptoms sometimes were not experienced until long after the miners returned home, and several even died. These same symptoms were also experienced by caisson workers. Hence, the condition became known as caisson disease, and later by deep sea divers as "the bends." The symptoms are the result of too rapid decompression as a person ascends to the surface from a great depth or pressure, due to the expansion of bubbles of air trapped in joints, muscles, or blood.

One of the most renowned cases of caisson disease is that of Washington Roebling (1837-1926). In 1869, Roebling replaced his father John Roebling as chief engineer of the Brooklyn Bridge building project after the elder Roebling's death. Shortly thereafter, Washington fell victim to caisson disease due to repeated descents to the bases of the bridge piers. Seriously weakened, he was forced to oversee the bridge's completion from his home near the Brooklyn end of the bridge. He never fully recovered. Caisson disease today is averted by slower ascents and by use of decompression chambers. Modern caissons are incorporated directly into the foundation of the structure, sometimes as openings in the massive concrete foundations which are allowed to remain after the completion of the project.