Where the soil is too yielding or the necessary area is not available, foundations are sometimes carried to the bedrock. This is done by the use of caissons which are sunk through the soft material to the rock.

These caissons consist of a steel chamber, having the bottom edge extending below an air-tight floor, far enough to form a working chamber into which compressed air is forced, which keeps out the water and soft material, and enables the workmen to excavate and allow the caisson to sink to the bedrock. The excavated material is hoisted up through air-locked shafts, and the masonry of the foundation is built within the caisson, and helps to sink it down. When bedrock is reached, the working chamber is filled solid with concrete and a solid foundation is secured.

This is called the plenum or compressed air process. Caissons are also sunk by the vacuum process, which consists in exhausting the air from the interior of the caisson, and allowing it to sink by the aid of the pressure of the atmosphere. By this process, the water is caused to flow under the lower edge of the caisson, loosening the soil and assisting the sinking of the caisson.

Pile foundations are also used as already described, sometimes in connection with a timber grillage, but generally with a foundation of concrete or stone. (Fig. 206.)