A caisson is a vertical chamber that is sunk deep below the surface to permit construction work to be performed. Often the work is done below the water level of rivers and harbors. In many caissons, men and machines prepare the space for foundation for buildings, bridge abutment, piers and tunnels.
Caissons can be made of wood, steel or concrete. Open caissons are open at the top and bottom. Box caissons have a bottom but no top. Pneumatic caissons are airtight, and have a roofed chamber at the bottom where men work. Compressed air keeps the water out of the working chamber.
Workers called sandhogs descend into the caisson and dig out the soft mud and muck. This is raised through the shaft in buckets through airlocks. As the men work, the caisson sinks deeper by its own weight until it reaches a solid foundation. Wooden timbers or steel beams are often driven into the ground until they reach bedrock. The timbers or beams form a circle which is dug out and filled with concrete. These are used as permanent foundation for the structure that is being built.
Air compressors work constantly to support the needed air pressure in a pneumatic caisson. As the caisson sinks deeper, the air pressure must be increased. Working under such pressures is dangerous. Construction companies follow strict precautions to keep their workers from developing the bends.
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