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