TABLE OF CONTENTS Spread foundations constructed for electric transmission towers are subjected to uplifting force. The uplift capacity of such foundations can be estimated by using the same relationship described in this chapter. During the construction of such foundations, the embedment ratio H/h is usually kept at 3 or less. The native soil is first excavated for foundation construction. Once the foundation construction is finished, the excavation is backfilled and compacted. The degree of compaction of the backfill material plays an important role in the actual net ultimate uplift capacity of the foundation. Kulhawy, Trautmann and Nicolaides (1987) conducted several laboratory model tests to observe the effect of the degree of compaction of the backfill compared to the native soil. According to their observations, in most cases, at ultimate load failure in soil take place by side shear as shown in Figure 2-40. However, wedge or combined shear failure occurs for foundations with H/h < about 2 in medium to dense native soil where the backfill is at least 85% as dense as the native soil (Figure 2-41). Figure 2-42 shows the effect of backfill compaction on the breakout factor F q when the native soil is loose. Similarly, Figure 2-43 is for the case where the native soil is dense. Based on the observations of Kulhawy et al. (1987). this study shows that the compaction of the backfill has a great influence on the breakout factor of the foundation, and the net ultimate uplift...
