Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
TYPES OF DEEP FOUNDATIONS ADDRESSED IN THIS MANUAL
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| 2.2 TYPES OF DEEP FOUNDATIONS ADDRESSED IN THIS MANUAL Lateral load applications typically involve the application of significant lateral forces and/or moments on a deep foundation, resulting insignificant internal bending moments and shear forces. Adequate design for such bending moments and shear forces requires deep foundations with adequate structural section and bending moment resistance. For this reason, most lateral load applications for deep foundations involve the use of piles or drilled shafts. However, micropiles and CFA piles, especially with permanent steel casings (for micropiles) and internal reinforcing, can also be used to resist lateral loads, especially if the lateral loads are not significant. Micropiles can also be used in battered configurations for resistance of lateral loads as discussed in Section 2.1.2. These foundation systems are described below however, the majority of this document focuses on piles and drilled shafts as these are the most common types of deep foundations. Deep foundations systems not addressed in this manual include those with limited tension or moment capacity, such as vibro-compacted concrete columns and unreinforced auger-cast piles and those not typically used in transportation facility applications, such as helical piles, and timber piles. In summary, deep foundation types addressed in this manual include Driven Piles Drilled Shafts • Micropiles Continuous Flight Auger (CFA) Piles 26 2.3 SELECTION OF DEEP FOUNDATION TYPES FOR RESISTANCE OF LATERAL LOADS The selection of the optimum foundation type fora project involves considering first whether shallow foundations are better alternatives than deep foundations. In general, shallow foundations are typically more economical, less complex, and easier to construct than deep foundations. However, deep foundations are preferred or required when a competent stratum is not present within an economical depth, when the footing dimensions required for stability cannot be accommodated, when settlement is excessive, when significant uplift or lateral loads are present, and when the bearing support maybe subject to scour or liquefaction. The evaluation of foundation types must be based on vertical loads and settlement, and lateral resistance and displacement. Vertical loads, axial foundation capacity, and settlement are beyond the scope of this document. The discussion herein focuses on considerations for selection of deep foundation type when lateral loads area significant or controlling aspect of the design. With regard to lateral resistance, shallow foundations generally only offer limited capacity in the form of passive resistance on the side of the footing and/or shear resistance along the base of the footing. Deep foundations maybe more advantageous when lateral loads are large because of the ability to develop high lateral resistance over the depth of deep foundation elements. Deep foundations are generally more economical when overturning loads are dominant (e.g., foundations for noise barrier walls and signposts. Deep foundations can also be used for slope stabilization applications to intercept and develop sufficient lateral resistance below a failure plane. The factors to be considered when selecting deep foundations to resist lateral loads include subsurface conditions and geomaterial properties design loads potential for scour or liquefaction structural properties of the deep foundation elements • constructability • cost-effectiveness structural redundancy acceptable magnitude of lateral displacement In addition, availability of local experience and construction practices maybe a factor when considering one type of deep foundation over another. The factors above are discussed in the following subsections. Download 6.03 Mb. Share with your friends:
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