It is convenient to think of a building as consisting of three major parts: the superstructure, which is the above-ground portion of the building; the substructure, which is the habitable below-ground portion; and the foundations, which are the components of the building that transfer its loads into the soil (Figure 2.29).

There are two basic types of foundations: shallow and deep.  Shallow foundations are those that transfer the load to the earth at the base of the column or wall of the substructure. Deep foundations, either piles or caissons, penetrate through upper layers of incompetent soil in order to transfer the load to competent bearing soil or rock deeper within the earth. Shallow foundations are generally less expensive than deep ones and can be used where suitable soil is found at the level of the bottom of the substructure, whether this be several feet or several stories below the ground surface.

Figure 2.29 Superstructure, substructure, and
foundation. The substructure in  this example contains two levels of
basements, and the foundation consists  of bell caissons. (In some buildings, the
substructure and foundation may be partly  or wholly the same.)
The primary factors that affect the choice of a foundation type for a building are:

•  Subsurface soil and groundwater conditions
•  Structural requirements, including foundation loads, building configurations, and depth

Secondary factors that may be important include:

•  Construction methods, including access and working space
•  Environmental factors, including noise, traffic, and disposal of earth and water
•  Building codes and regulations
•  Proximity of adjacent property and potential impacts on that property
•  Time available for construction
•  Construction risks

The foundation engineer is responsible for assessing these factors and, working together with other members of the design and construction team, selecting the most suitable foundation system.

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