Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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Soldier Rev B
| 7.2.1 Group Efficiency Deep foundation elements in closely spaced groups behave differently from isolated deep foundation elements because the combined interaction of a foundation element with the soil, other piles, and with the cap are more complex than that of an isolated foundation element. In general, deflections of a closely spaced group of deep foundation elements are greater than the deflections of an individual deep foundation for the same foundation size, subsurface conditions, and load (on the individual foundation element. Likewise, maximum bending moments along a deep foundation element within a group will tend to be larger than those in an isolated deep foundation element. This results in an apparent reduction in resistance or reduction inefficiency of a group of deep foundation elements acting together compared to the sum of the same number of foundation elements if being analyzed individually in isolation. This reduction inefficiency is characterized by the term group efficiency, G e , which quantifies the interaction effects in a group. G e is defined as (Prakash and Sharma 1990): Equation 7-1) Where Q g = Lateral load resistance of the group. n = Number of deep foundation elements in the group. Q s = Lateral load resistance of a single deep foundation element of the same design section and length as the elements in the group. In Equation 7-1, it is assumed that all the deep foundation elements in the group have the same characteristics. In general, the lateral load resistance of the group is less than or equal to the sum of the lateral load resistances of the individual foundation elements, or G e ≤ 1.0. Pile group efficiency depends on various factors, including foundation element characteristics, group size, group arrangement, head fixity, subsurface conditions, and displacement of the foundation elements. |