Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
Figure 6-3: Illustration of the critical depth concept for deep foundations (from Reese 1986)
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| Figure 6-3: Illustration of the critical depth concept for deep foundations (from Reese 1986). Local practice may vary from the procedure described above or local agencies may require a different approach. Designers should ensure that their analyses procedures comply with applicable design standards including local agency guidelines or requirements, especially as the state of the practice may continue to evolve based on research or local experience. 68 For larger diameter, relatively short foundation elements such as short drilled shafts or relatively short and stiff piles, the foundation element may rotate rather than bend, i.e., develop a “fence-posting” or pushover failure. For such cases, the p-y analysis will not converge at a reasonable depth and error messages may result this is a result of the pile not being long enough to behave as along pile, and indicates that the pile is more likely to fail through rotation rather than bending. For these types of foundations, the Geotechnical Strength Limit State can be analyzed using theoretical analyses such as Strain Wedge Model method (SWM) to verify that the foundation materials have adequate strength to support the factored loads. For multiple deep foundations with potentially overlapping zones of influence, the effect of the overlap must be accounted for in the analysis p-multipliers for p-y analyses are specific to that method and are not applicable to Broms method or SWM. The Broms method can be used as well for short pile behavior analysis of the Strength Limit State, but the Broms method is typically only used for simple structures (such assign post or light pole foundations) or for preliminary analyses. For non-gravity cantilevered walls, analyses should be performed as described in Chapter 9. 6.3 P-Y METHOD The analysis of a laterally loaded deep foundation element is a complex problem that involves nonlinear reactions of the foundation element and nonlinear reactions of the surrounding geomaterial. The most common nonlinear analysis method used in practice is the p-y method. This method captures the essential mechanisms of the problem, has a wide industry acceptance, a history of use in the transportation industry, and can be performed with readily available commercial software. The p-y method is the recommended method for design of all major deep foundation projects. The p-y method can accommodate variable subsurface layers, axial loads, lateral deflections as inputs, distributed loads, sloping ground conditions, fixed or free head conditions, nonlinear bending characteristics (such as cracked sections in drilled shafts, and nonlinear soil response. The outputs of the p-y method include distributions versus depth of lateral displacement, shear forces, bending moments, soil resistance, and soil and pile moduli. These distributions can be tabular or graphical. The output information also allows for an analysis of the foundation’s structural resistance. An illustration of conceptual p-y analysis graphical results is provided in Figure 6-4. The p-y method was originally developed under an FHWA research grant and was developed into the DOS-based computer program COM. COM has been discontinued, but has been succeeded by other commercial software programs for p-y analyses. Any commercially available software program that properly performs the p-y method should be adequate for analysis. |