Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
Figure 7-6: Pile group interaction in the SWM model (from Ashour et al. 2006, originally from
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| Figure 7-6: Pile group interaction in the SWM model (from Ashour et al. 2006, originally from Brown et al. 1988). The SWM approach can be used to analyze pile groups. As discussed in Chapter 6, the SWM approach models the response of the soil as a wedge of soil resistance. Ina group analysis, the wedges of different piles in the group overlap. The overlap of the SWM wedges decreases with depth, even in uniform soil, because the wedges vary with depth. This is illustrated in in Figure 7-6. The overlap of the wedges must betaken into account in the SWM group analysis (Ashour et al. 2006). It is therefore possible to perform lateral analysis of pile groups using the SWM method. However, as discussed in Chapters 5 and 6, the p-y method for analysis is preferred and there are several commercially available programs for group analysis. The use of the SWM approach should be limited to cases where shallow pile behavior controls, or the method should be used as a supplemental analysis to p-y methods for complex or detailed design conditions, as discussed in Chapter 6. 106 7.3 LATERAL RESISTANCE CONTRIBUTION OF THE CAP The cap fora group of deep foundation elements can provide a significant lateral resistance. Several studies have indicated that the cap can provide 40 to 50 percent or more of the overall contribution to the lateral resistance of a group of deep foundation elements (Beatty 1970;, Kim and Singh 1974; Rollins et al. 2000; Zafir and Vanderpool 1998; and, Mokwa and Duncan 2001). The lateral resistance of the cap is generated by passive pressures on the cap front, and shear resistances on the sides of the cap. The base resistance under a cap is often disregarded because separation between the cap base and soil is likely due to soil settlement, particularly in soft ground conditions. Similarly, if scour or future excavations are likely to remove the soil in front of the cap, then the passive resistance and side resistance should not be relied upon. Soil shrinkage, frost action, or erosion may reduce the contact between the cap and the adjacent soil, which may reduce the side shear resistance or may increase the required deflection to mobilize the passive resistance. Use of passive pressure and side shear also requires verification of backfill placement and compaction around the cap to ensure that the soil is in full contact with the cap and properly compacted for the assumed strengths. For these reasons, the cap is often not relied upon for contribution to lateral load resistance of deep foundations. However, AASHTO does allow the effects of the lateral resistance provided by an embedded cap to be considered in the evaluation of horizontal resistance and displacements, but does not provide detailed guidance regarding this approach (2014). Based on the findings from full-scale tests performed by Mokwa and Duncan (2001) and centrifuge tests conducted by McVay et al. (1995), the lateral resistance provided by a pile cap depends primarily on two factors 1. Passive resistance in front of the cap, which is a function of the stiffness and strength of soil in front of the cap. The passive resistance that can be developed in front of a pile cap is directly related to the backfill strength and stiffness. The lateral resistance increases as the stiffness and strength of soil around the cap increases 2. Depth of cap embedment, as increasing cap thickness results in smaller lateral deflections at the same load (Mokwa and Duncan 2001). Additionally, increasing depth of cap embedment results in larger lateral resistance at the same deflection (McVay et al. 1995). The inclusion of lateral resistance of the pile cap can be a complex problem to solve. The passive resistance in front of the cap is dependent on the amount of lateral deformation that occurs under the lateral load. For the Service Limit State, it is possible that passive resistance will not be fully mobilized. Considering that large ground displacements are typically needed to develop full passive soil resistance, the soil resistance used in design should be based on the results of a foundation displacement analysis and appropriate correlations between passive resistance and displacement. The approach to analyzing the contribution of the pile cap involves consideration of loads and deformation, and an iterative process between p-y analysis of piles and assessment of deformations and reactions of the surrounding soils. This is a complex problem that is unlikely to be solved efficiently or accurately without the use of robust computational tools. However, software programs are available that can consider the contribution of the pile cap in addition to the pile-soil-pile interaction of the pile group (p- y analysis accounting for p-multipliers). |