Soil Pressure Distribution

A vertical loading applied to the footing is resisted by soil pressure acting on the lower surface of the footing. The distribution of pressure depends on the type of soil at the site. Typical distributions for sand and clay type soils are shown in Fig. 7.5. In practice, we approximate the actual pressure distribution due to a concentric load with an “average uniform” distribution.

Depending on the column loading and the location of the column with respect to the centroid of the footing area, one of the distributions shown in Fig. 7.6a–c is normally assumed in order to establish the dimensions of the footing. A uniform

distribution is the most desirable distribution. Since soil cannot resist tensile stress, one wants to avoid the case illustrated in Fig. 7.6c. We will describe a strategy for selecting the footing dimensions so as to avoid this situation in the following section.

The allowable pressure varies with the type of soil. Soil is a natural material in contrast to steel, which is manufactured with close quality control. Consequently, there is considerable variability in soil properties. We list typical allowable soil pressures for various types of soils in Table 7.1. These values are useful for estimating initial footing dimensions.

Table 7.1 Allowable soil pressures—Reference Terzaghi and Peck