Foundations and other structures may be subjected to uplift forces under special circumstances. For those foundations, during the design process it is desirable to apply sufficient factor of safety against failure by uplift. During the last thirty or so years, several theories have been developed to estimate the ultimate uplift capacity of foundations embedded in sand and clay soils, and some of those theories are detailed in this chapter. The chapter is divided into two major parts: foundations in granular soil and foundations in saturated clay soil (Φ = 0).
Figure 8.1 shows a shallow foundation of width B. The depth of embedment is Df. The ultimate uplift capacity of the foundation Qu can be expressed as
If the foundation is subjected to an uplift load of Qu, the failure surface in the soil for relatively small Df /B values will be of the type shown in Fig. 8.1.
The intersection of the failure surface at the ground level will make an angle α with the horizontal. However, the magnitude of " will vary with the relative density of compaction in the case of sand and with the consistency in the case of clay soils.
When the failure surface in soil extends up to the ground surface at ultimate load, it is defined as a shallow foundation under uplift. For larger values of Df /B, failure takes place around the foundation and the failure surface does not extend to the ground surface. These are called deep foundations under uplift. The embedment ratio, Df /B, at which a foundation changes from shallow to deep condition is referred to as the critical embedment ratio, (Df /B)cr. In sand the magnitude of (Df /B)cr can vary from 3 to about 11 and, in saturated clay, it can vary from 3 to about 7.
FIGURE 8.1 Shallow foundation subjected to uplift