Figure 6-8: Deflection pattern of laterally loaded long pile/shaft and associated strain wedge (from

77 The SWM can directly accommodate the effect of stratigraphic variations on soil properties and behavior at a specific elevation, and thereby address a theoretical limitation of the p-y approach which uses independent non-liner springs to model the soil resistance (Brown et al. 2010). Variable subsurface conditions and soil properties can be addressed through the inclusion of multiple soil material layers. The shape of the wedge and strain patterns within each separate material wedge are modified to accommodate a theoretical composite wedge as shown in Figure 6-9.
Figure 6-9: Geometry of a compound passive wedge (Ashour et al. 1988).
Compared to the p-y methodology, the SWM has advantages for analysis of laterally loaded deep foundations in the following areas Soil response curves are developed not only as a function of soil properties and pile/shaft width but also to account for pile/shaft-head fixity, pile/shaft cross-section shape, bending stiffness, and soil property distribution with depth. This advantage is significant for the analysis of large diameter shafts because not all of these aspects can be accounted for using the conventional p-y method. For pile/shaft group analyses
– This method allows evaluating group response by overlapping the effects of passive wedges on the relative stiffness of the soil-pile system (p-multipliers, as discussed in Chapter 7 for the p-y method, are not required.
– The presence of a pile/shaft cap can be accounted for by also evaluating the development of a passive wedge over the depth of the pile/shaft cap (Refer to Chapter 7 for discussion regarding the use pile cap resistance.
– Individual members in a group are analyzed based on their location in the group, longitudinal and transverse spacing, and the soil type that surrounds them. These aspects are not currently addressed in the current p-y practice. The analysis of individual piles/shafts and/or groups in liquefied soil maybe more realistic as the development of excess pore pressure can be accounted for.

78 The effect of lateral soil spreading on individual piles/shafts and/or groups can be assessed with a rational method. The effect of the vertical side shear on the lateral response of large diameter shafts can be explicitly considered. The SWM has inherent limitations due to simplifications of the problem and inherent limitations of predicting in-situ stresses and stress-strain behavior of geomaterials; such limitations are common to all complex D modeling of nonlinear soil-structure interaction problems. The SWM is not as widely used or accepted in practice for transportation structures, and therefore there is not as much experience or correlation of results with testing databases for this method compared to the more widely used p-y method. It is therefore recommended that for long piles, the SWM method be used to supplement a p-y analyses to provide additional understanding and evaluation of the problem conditions, rather than replace the p-y analysis. The use of SWM in additional to p-y analyses maybe useful for large or complex problems. Also, full scale load testing can be used to provide field verification testing of any designs that are based on SWM. The SWM maybe more applicable than the Broms method for shorter, large diameter foundations elements that tend to rotate rather than bend (AASHTO 2014).

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