Types of Foundation Structure

The design of foundations embodies three essential operations, namely, calculating the loads to be transmitted by the foundation structure to the soils or rocks supporting it, determining the engineering performance of these soils and rocks, and then designing a suitable foundation structure.

Footings distribute the load to the ground over an area sufficient to suit the pressures to the properties of the soil or rock. Their size therefore is governed by the strength of the foundation materials. If the footing supports a single column, it is known as a spread or pad footing, whereas a footing, beneath a wall is referred to as a strip or continuous footing.

The amount and rate of settlement of a footing due to a given load per unit area of its base is a function of the dimensions of the base, and of the compressibility and permeability of the foundation materials located between the base and a depth that is at least one and a half times the width of the base. If footings are to be constructed on cohesive soil, it is necessary to determine whether or not the soil is likely to swell or shrink according to any seasonal variations. Fortunately, significant variations below a depth of about 2 m are rather rare.

Footings usually provide the most economical type of foundation structure, but the allowable bearing capacity must be available to provide an adequate factor of safety against shear failure in...