applying certain tre~tments to the soil. Further, some structures, such as steel storage tanks, can be supported directly on a specially prepared pad of soil without the benefit of intervening structural members. Thus the word foundation refers to the soil under the structure as well as any intervening load carrying member, i.e., foundation refers to the material whose behavior the civil engineer has analyzed in order to provide satisfactory and economical support for the structure. Indeed, the word foundation is used to describe the material that supports any type of engineering structure such as building, dam, highway embankment, or airfield runway. 111 modern usage, the term shallow foundation is used to describe anarrangement where structural loads are carried by the soil directly under the structure, and deep foundation is used for the case where piles, caissons, or piers are used to carry the loads to firm soil at some depth.
In the design of any foundation system. the central problem is to prevent settlements large enough to damage the structure or impair its functions. Just how much settlement is permissible depends on the size, type, and use of structure, type of foundation, source in the subsoil of the settlement, and location of the structure. In most cases, the critical settlement is not the total settlement but rather the differential settlement, which is the relative movement of two parts of the structure.
In most metropolitan areas of the United States and Western Europe, owners of buildings usually are un• willing to accept settlements greater than a few inches, ' since unattractive cracks are likely to occur if the settle• ments are larger. For example, experience has shown that settlements in excess of approximately 5 in. will cause the brick and masonry walls of buildings on the

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