the, installation,piers, by, caisson, method
Large piers (such as for bridges) are often installed by caisson method. The caisson is a hollow boxlike structure that is sunk to the bearing stratum by excavating from the interior, and becomes a permanent part of the completed pier. The lower edges of the caisson walls are provided with cutting shoes to make the penetration easier. As material is excavated and the caisson sinks, the side walls are built up to increase weight and to facilitate sinking. As the excavation deepens, pressure outside the caisson increases and surrounding material may fall into the caisson and that may. Cause considerable settling around the working area and also can tilt the caisson. This difficulty may be avoided by maintaining pressure within the caisson with water through which the material is removed by open dredging. Large caissons are subdivided into several vertical cells, and tilting can be controlled by dredging to a greater depth in some cells than in others. When hand labor is necessary to remove obstacles below the cutting edge or to prepare the rock surface for concreting, a chamber for the workers may be provided in the lower part of the caisson. Air is introduced into the chamber at a pressure a little greater than the pressure required to balance the outside pressure and the chamber must be dry. Air locks must be provided for passage of men and materials. Men cannot work effectively under pressures exceeding65 lb. per square inch, and compressed-air caissons are limited to depths of about 120 ft.
