Foundation, the base upon which a structure is built or placed, as the foundation of a building, a bridge-pier or an engine. The object is to furnish a base which shall not settle or, if it does settle in the least, that this settling shall be perfectly uniform. The best foundation-base is solid rock, as this is firm and not affected by water, but some gravels and clays are equally good, if they are not saturated with water. The most difficult soils for foundations are soft, compressible earths saturated with water. Where it is possible, it is best to go down until rock, gravel or "hard-pan" is reached. It is often necessary to go down 75 or 100 feet to find such firm material. Where the soil is of a yielding nature, the area of the base must be so enlarged as to decrease the pressure per square foot to safe limits. A common method in soft soils is to drive piles, saw them off at the surface and erect the foundation on a platform on these piles. The whole of the Back-Bay district in Boston is on pile-foundations. Sometimes masonry, concreted timber or iron platform foundations are used. Thus in Chicago, where the soil is boggy and it is impracticable to reach good rock, the foundations of many of the heaviest buildings have been erected on an iron and concrete platform. A cubical pile of crossed iron-rails is made and filled in with concrete, and the building is placed on these. For foundation-bases under water, or under soils saturated with water, coffer-dams and caissons must be used to get the masonry-work down to a firm base. (See Cofferdam and Caisson.) The Romans seem to have been the first to make a serious study of foundations. It is said that the sites of the cathedrals of the middle ages were selected primarily with reference to getting a suitable foundation, and that, even so, many mediaeval structures have disappeared on account of improper foundations. The subject of foundations is one of the most important problems of modern engineering, and it is often found necessary to spend large sums in preliminary borings to determine the character of the soils.
