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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

3The lateral resistance of bridge foundations is often a crit- ical component in the design of highway bridges. Lateral loads can be produced by earthquakes, wind, wave action, ship impact, and traffic. In practice, bridge foundation design often is governed by load demands of the bridge column. The size and number of piles as well as the pile group layout are designed to resist service level moments, shears, and axial loads and the moment demands induced by the column plastic hinge mech- anism. The foundation designer must verify that the lateral capacity of the foundation exceeds the lateral demand transmit- ted by the column. When the lateral capacity of the foundation is inadequate, the designer develops a strategy for increasing the lateral resistance of the foundation. These strategies typically include thickening the pile cap to increase passive resistance or increasing the number or diameter of piles. When existing bridge foundations are found to have inadequate lateral resist- ance, additional piles, drilled shafts or micro-piles are added to increase the lateral resistance. Furthermore, an expanded pile cap or connecting beam often is required to structurally con- nect the new piles to the existing pile group. Although these structural approaches provide the required lateral resistance, they may also be relatively expensive and time consuming An alternative approach is to use soil improvement tech- niques to increase the strength and stiffness of the surrounding soil and thereby increase the lateral resistance of the pile group. Ground improvement methods have the potential to increase (1) the passive resistance of the pile cap and (2) the lateral resistance of the underlying piles. The improved zone could potentially be relatively shallow because the lateral resistance of piles is typically transferred within 5 to 10 pile diameters. Although soil improvement techniques have the potential for being cost-effective and reducing construction time, relatively few tests have been performed to guide engineers in evaluating the actual effectiveness of this approach. In addition, numeri- cal models to evaluate this approach have not been validated. As a result, no general procedures are available for designing pile foundations in soils that have been improved in zones sur- rounding the piles. For these reasons, soil improvement meth- ods for increasing lateral pile group resistance have rarely been implemented in practice. Two different improvement schemes might be employed depending on whether the pile foundation is a new foundation or an existing foundation. Soil improvement for a new pile foundation is relatively straightforward since it can take place prior to installing the piles using a variety of techniques. Lat- eral pile stiffness is typically affected by the soil stiffness within the zone of significant soil-pile interaction, which in most poor sites is approximately 4 to 5 pile diameters from the ground surface. For this case, soil improvement could be performed on the entire block of soil within the pile cap footprint, extending laterally about 3 to 4 pile diameters from the perimeter pile and vertically about 4 to 5 pile diameters as illustrated in Figure 1-1. In this case, all of the piles would be located in improved soil and increased resistance could be substantial. For the case of existing pile foundations, the soil improve- ment frequently would be limited to the perimeter of the pile group because of practical access to the interior piles as shown in Figure 1-2. In this case, increased lateral resistance might be concentrated in piles at the edge of the group, and relatively little increase could occur for the interior piles. Alternatively, the soil under the foundation could be improved for a new foundation or even for an existing foundation with a technique such as jet grouting. Improving the soil under the foundation would have the potential for producing greater increases in lateral resistance than just improving around the perimeter because the improvement would reach interior piles. In addi- tion, the process of creating a cemented “soilcrete” zone around pile foundations could potentially produce a zone that would behave like a reinforced “superpile” with increased structural stiffness. C H A P T E R 1 Introduction

4D 4-5D 3-4D Improved Soil (a) Soil Improvement of Entire Block Prior to New Foundation (b) Construct New Foundations Figure 1-1. Soil improvement around new foundation. Figure 1-2. Soil improvement around existing footing. Improved Soil 4-5D 3-4D (a) Existing Foundation (b) Soil Improvement at Perimeter

Next: Chapter 2 - Available Ground Improvement Case Histories and Approaches »
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 697: Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils examines guidance for strengthening of soils to resist lateral forces on bridge pile foundations.

The report presents computational methods for assessing soil-strengthening options using finite-element analysis of single piles and pile groups and a simplified approach employing commercially available software.

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