Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
Frost/Desiccation Depth, Loss of Contact, Etc
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- 6.7.10 Other Design Considerations
| 6.7.9 Frost/Desiccation Depth, Loss of Contact, Etc. Another consideration for analysis is disturbance to surficial soils that may impact the resistance to lateral loads of near surface soils. In many project settings, there is potential for natural disturbance of the near surface soils due to frost heave, desiccation or shrinkage, surficial erosion due to runoff, or other potential mechanisms that may result in loss of contact between the foundation element and the adjacent soil. These mechanisms may vary with time or year or precipitation patterns, or other outside influence. In such cases, it maybe appropriate to neglect or reduce the soil resistance within the anticipated depths of which disturbance or loss of contact may occur. Construction disturbance, such as trenching for installation of utilities, maybe another reason to neglect a nominal depth of resistance to lateral loads. 6.7.10 Other Design Considerations 6.7.10.1 Variations in Subsurface Conditions Most project sites include some degree of variability with regard to subsurface conditions. The use of the p-y method allows parametric studies to be performed relatively easily. It is recommended that parametric software runs be performed when there are potential variations in subsurface layer thicknesses and depths, subsurface material strengths and stiffness, and significant variations in the water table depth. Selection of the p-y curve for modeling specific materials can also be varied to assess potential impacts of the specific p-y curve on the results, especially for conditions that the available p-y curves may not fit well with the actual subsurface conditions. Brown et al. (2010) present an example of drilled shaft through soft clay socketed into rock that demonstrates that some of the variations in subsurface stratigraphy may produce counterintuitive results. If the site is highly variable, consideration should be given to subdividing the project site into smaller sites with more consistent conditions for analysis. If the results of parametric studies indicate the potential for significant impacts on the overall design, then consideration should be given to performing additional exploration and testing to better define the subsurface conditions and/or construction verification testing to verify the performance of the deep foundation design. 96 6.7.10.2 Anchors and Bracing against Deep Foundations For deep foundation elements that are restrained by anchor devices, such as a tieback or strut, the tieback or strut can be simulated by using a very stiff p-y curve at the location of the support. The p-y curve should be consistent with the stiffness of the restraining element. This curve can be input in the same manner that other p-y curves can be specified for soil layers. 6.7.10.3 Increasing Lateral Resistance around Deep Foundations In some problem cases the near surface soils are particularly weak or soft and may not provide adequate resistance to lateral loading of deep foundations. Atypical approach for such a condition would be to increase the size of the deep foundations (larger diameters or width) and/or add more elements, resulting in a larger pile/shaft group. Another approach would be to use ground improvement to increase the resistance of the soils to lateral loads. Ground improvement methods for improving lateral soil resistance were evaluated and published by Rollins and Brown (2011) in the NCHRP report 698, Design Guidelines for Increasing the Lateral Resistance of Highway Bridge Pile Foundations by Improving Weak Soils Rollins and Brown (2011) indicate that significant increases in lateral resistance of soft clays and loose sands can be achieved through ground improvement and replacement techniques. Examples of treatment areas are shown in Figure 6-17. Ground improvement methods evaluated in the study included jet grouting, soil mixing, placement of flowable fill or compacted fill, and rammed aggregate piers. Cost comparisons were performed to evaluate potential cost savings of implementing ground improvement versus the additional of more piles and enlargement of the cap. Refer to the NCHRP study for additional details. |