Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
GEOTECHNICAL STRENGTH LIMIT STATE OF LATERALLY LOADED PILES
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| 6.2 GEOTECHNICAL STRENGTH LIMIT STATE OF LATERALLY LOADED PILES Typically, laterally loaded deep foundation elements are deep enough such that they will bend and deflect rather than rotate. If the foundation element is not sufficiently deep, then geotechnical failure of the soil material can occur (geotechnical strength is exceeded) and the foundation element behaves as a rigid element that rotates about a point at or near the bottom of the element this is referred to as short pile behavior or pushover or “fence-posting” and is illustrated in Figure a. If the foundation element is deep enough to avoid pushover failure, then its tip will remain essentially fixed and the element will bend and deflect in a ductile manner under applied lateral load, referred to as long pile behavior and illustrated in Figure b. In this case, the geotechnical strength is not exceeded. This is the general behavior for the vast majority of applications and is controlled by limiting deflections and/or structural resistance of the foundation element. The intent of a Geotechnical Strength Limit State analysis is to verify that the strength limit state of the foundation material exceeds the structural resistance of the pile in flexure, and therefore the foundation will behave in a ductile manner and pushover failure will not occur. For these types of deep foundations, the Geotechnical Strength Limit State can be performed by using a p-y analyses and performing a pushover type of analysis (the p-y method of analysis is described in the subsequent section. Fora pushover analysis, the factored axial and lateral loads are used in computer analyses. Loads are factored according to the Strength Limit States as discussed in Chapter 4. P-y curves developed for the soils or rock at the site are used to model the geotechnical resistance and soil-structure interaction. For piles in groups, p-multipliers, which account for overlapping zones of influence of the piles, must be included (analyses of groups of deep foundations is discussed in detail in Chapter 7). Note that p- multipliers, which are less than 1.0, are used to address overlapping zones of geotechnical resistance and should not be confused with or take the place of resistance factors. Appropriate resistance factors are incorporated into the analyses based on the project design criteria and considerations, as discussed in Chapter 4. |