Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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| (a) (b) Figure 6-2: Illustration of short pile behavior and long pile behavior under lateral loads. For a pushover type analysis, the following analysis procedure can be followed (Brown et al. 2010): 1. The deep foundation is modeled as a simple linear elastic beam with the elastic modulus equal to that of the foundation material (concrete or steel. The moment of inertia is based on an equivalent uncracked section in the case of concrete. 2. The soil profile is developed. Separate profiles are used as appropriate depending onsite variability and design conditions (scour, liquefaction, etc. 3. The load at the head of the foundation element is applied in multiples up to and exceeding the factored design load to compute deflections. An unstable condition will result in the computer program being unable to converge or converging at extremely large deflection values. a. The computed deflection should be a reasonable value at and slightly above the factored design loads to ensure that geotechnical strength requirements are satisfied. Some judgment is needed to assess the reasonableness of the computed deflection. For drilled shafts, a value of 10 percent of the diameter is suggested by Brown et alb. Local practice or agency requirements may also define the limiting deflection. For example, as indicated in Chapter 4, the Idaho Transportation Department (2008) defines the Geotechnical Strength Limit State for seat type abutments as a deflection of 2 inches in combination with a resistance factor of 0.9. 4. The geotechnical strength, as evidenced by a reasonable deflection at or exceeding the factored design load, must be adequate to ensure that a ductile lateral foundation response exists i.e., that the foundation element does not fail through pushover. a. The geotechnical strength can be increased, and the deflection decreased, by increasing the length of the foundation element and/or its size (diameter or width) to engage more geotechnical resistance. Once the initial model parameters are established (loads, soil profile, etc, repeated trials of different lengths and shaft diameters can be performed quickly to determine a minimum length and diameter that satisfies the criteria. For most laterally loaded deep foundations, there is a point at which an additional increase in depth does not result in an additional decrease in lateral head deflection this is often referred to as the critical depth or critical penetration concept and is 67 illustrated in Figure 6-3 (Reese 1986). If the foundation element being analyzed has reached the critical depth under the applied loads and has not satisfied the deflection criteria, then the diameter or width of the element must be increased, or additional foundation elements can be provided to reduce the load demand, or a different type of element must be considered. b. Site variability, both in terms of the profile used and the design parameters, should be considered in assessing the geotechnical strength. For relatively variable conditions, adequate or reserve geotechnical resistance maybe desirable when assessing the Geotechnical Strength Limit State. c. Note that in assessing the Geotechnical Strength Limit State, failure is not necessarily geotechnical failure of the soil, rather it is defined by a deflection limit for consistency and applicability of the p-y approach. The analysis must converge at a reasonable deflection limit to represent a stable condition. 5. The factored loads may exceed the strength of the trial foundation element. This is not an issue though as the intent of this Geotechnical Strength Limit State analysis is to verify that the embedment is adequate for the pile to behave as a ductile element rather than to fail through rotation (geotechnical failure of the soil. It is for this reason that the Geotechnical Strength Limit State is modeled with the foundation element as a linearly elastic beam. The Structural Strength Limit State is addressed as a separate section. Download 6.03 Mb. Share with your friends:
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