Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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Soldier Rev B
| 11.2.1 Effective Length and Buckling Deep foundation elements are detailed and installed in two configurations. The element can either be completely underground, or a portion of the element may project above the ground surface. Any portion of the foundation element that projects above the ground surface is considered laterally unsupported unless structurally connected to a bracing member. The foundation element type, installation procedure, and ground conditions will all contribute to the distance below the ground surface at which point the foundation element can be considered continuously supported by the soil. This information is used to determine the unsupported length of the foundation element when checking the foundation for stability against buckling. The point of continuous lateral support is not the same as the point of fixity, as the point of fixity occurs below the point of lateral support. From a structural view, foundation elements act as columns and therefore under axial and moment loads, an effective length could be considered for simplified frame analyses. The structural properties of the foundation element and the end conditions are used to approximate an effective length factor, K, as shown in Table 11-1, where the foundation element toe is generally assumed fixed for both translation and rotation (pinned. In the absence of sufficient bracing, (e.g., very soft soils, piles/shafts extended though water, large scour, etc) the foundation element head may experience lateral displacement sideways) and rotation, and therefore cannot be considered as fixed for these conditions, the design value of K should be determined based on the anticipated head restrain condition. For example, if a foundation element extends above the ground surface and is connected to a rigid pier cap, rotation is generally prevented by the pier cap mass and stiffness, but the free-length of the element may result in lateral movement (reduced lateral bracing) in combination with existing loads. In this case, a fixed rotation and free translation condition may exist, as illustrated in Table 11-1. In pile bents, depending on the foundation’s connection to the superstructure, the bent cap could allow rotation and translation perpendicular to the long axis of the bent cap, but free translation with fixed rotation along the long axis of the bent cap. To have a rotationally fixed foundation element top condition, Rollins and Stenlund (2010) observed that rather than defining a rule-of-thumb for minimum foundation element embedment length into the cap, the moment capacity of the cap to foundation element connection should be designed with foundation element embedment and cap reinforcement details such that the moment capacity of the connection exceeds the moment capacity of the foundation element. Buckling is generally of concern when foundation elements extend through water or air, or for liquefaction, where an absence or reduction of confining stress is clearly recognizable. Very soft soils or peat are often considered to provide insufficient lateral support for providing resistance to buckling. 145 To characterize buckling resistance in soft soils, a load test program was performed by the Bethlehem Steel Corporation which suggested that even soft soils provide adequate support (Bethlehem Steel Corporation 1970). One such H-pile in this study extended through 31 feet of water and 29 feet of soft organic silt where the pile sank under its own weight. An applied axial load of 200 tons produced a gross settlement of 0.63 inches but no pile buckling occurred. In addition, Coduto et al. (2016) suggests that, even the softest soils provide enough lateral support to prevent underground buckling in piles subject only to axial loads, especially when a cap is present and provides rotational fixity to the pile top A more conservative approach to this issue would be to determine the critical buckling load using computer software, such as LPILE. For this method, a foundation element-soil model is generated and incremental loads are applied to evaluate the resulting deflection. This method may provide the design engineer with a deflected pile shape to assess buckling fora given factored load in lieu of using prescriptive minimum soil strength values to characterize an unbraced length. The unbraced length, l, or laterally unsupported length is defined by AASHTO (2014) as the distance between two braced points that resist buckling or distortion modes. For embedded foundation elements, the unbraced length is considered for scour and element stickup through air and/or water. For preliminary analysis, when lateral loads are applied, the effective length, K, for flexural resistance calculations is taken as the total unsupported length, plus an embedded depth to fixity If a lateral pile analysis with p-y curves for soil-structure interaction has been performed as discussed in Chapter 6, the depth to fixity concept is unnecessary—most software with lateral analysis also includes additional features to determine a pile’s buckling capacity given the soil model and a pile model with the expected stickup above the ground level. For preliminary calculations, however, depth to fixity below the ground maybe evaluated based on soil type and soil strength parameters as shown in Eq. 11-2 to Eq. 11-4 and discussed in Chapter 6. For sands, Table 11-2 contains the rate of increase in soil modulus, n h , and should be used as applicable in the following depth to fixity estimates. |