PNEIIMATIO FOUNDATIONS FOR BUILDINGS. The pneu matic process was devised for laying foundations of bridges under considerable depths of water or water-bearing soil, and for a number of years was used exclusively for that purpose and in tunneling; but since 1894 the pneumatic process has been used extensively in laying foundations for buildings in New York City, and has there been carried to a great degree of refinement. In addition to the advantages of the pneumatic process for foundations in general (¢ 908), the two conditions which primarily led to the introduction of the pneumatic process for building-foundations were: (1) the tall buildings required so great a supporting power that it was necessary to carry the foundation to bed-rock, which is from 60 to 80 feet below the street surface, and (2) the necessity of using a process of excavat ing through the water-bearing soil that would not disturb the soil under adjacent buildings. For many of the large buildings at the lower end of Manhattan Island, a pneumatic pile or caisson was sunk for each column.
In some cases, the so-called caisson was virtually a pneumatic pile (I 863-66) made of wood, stone masonry, or steel plates; but in most cases the foundation consists of a pneumatic caisson proper made of wood or steel plates surmounted by brick masonry or con crete. The steel caisson is usually preferred because the thinner sides give greater space in the working chamber, and also because the caisson can be brought to the building ready for sinking. The
method of operation is the same as in bridge foundations except in three particulars, viz.: 1. Since the caissons are comparatively small, have vertical sides, and are sunk all the way through earth, the weight of the masonry on the caisson was insufficient to overcome the friction and the upward pressure of the compressed air, and hence extra weight is usually required to sink the caissons. 2. To prevent the soil from escaping from under the shallower foundations of adjacent buildings, it is necessary to make the excavation without reducing the air pressure. 3. The sinking must be continuous, as otherwise the soil will settle around the caisson and make it impos. Bible to start again without releasing the air pressure.* For an account of recent improvements in the details of pneumatic foundations for buildings, see Transactions of the American Society of Civil Engineers, Vol. LxI (1908), pages 211-37. The more important of these improvements are: (1) the elimination of the roof of the caisson, (2) the doing away with the coffer-dam; (3) the elimination of the shaft-lining, (4) the substitution of cylindrical for rectangular caissons, and (5) surrounding the foundation area with a row of pneumatic caissons which together act as a coffer-dam.