Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities

12.1
PRELIMINARY DESIGN
The Broms method is widely used and accepted by the majority of the entities investigated. Some accept the method for final design, while others limit it to preliminary design or for structures with relatively low lateral loads. As such it is recommended that for the purposes of preliminary design or for special design cases (simple structures or structures with low lateral loads, Broms method should be included in GEC 9. However, it is not recommended that Broms method be recommended for structures regardless of size where lateral load controls either the structural design section or embedment depth. Additionally, it is recognized that establishment of a preliminary point of fixity for structural engineers to perform an equivalent cantilever beam analysis is important in the early stages of a project for the purposes of estimating anticipated element sizes and spans. The method proposed by Davisson (1965), which is recommended in AASHTO LRFD, is considered an acceptable approach which has been used successfully in practice for many years.

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12.2
FINAL DESIGN
The procedure for final design is anticipated to be modeled after that in the FHWA manual for drilled shafts (GEC 010), with modifications to account for long and/or slender elements. The basic outline presented for discussion includes these general steps
1. Determine whether the deep foundation elements) classify as short or long and determine the point of fixity. a. Guidance for procedures for determining short versus long, as well as point of fixity, will be provided in the GEC; however, at the time of this Literature Review Report, the available methods are currently under review and a final approach has not yet been determined.
2. If the element classifies as short, then analyze a. Geotechnical Strength Limit State—pushover analysis for uncracked section with factored loads and factored resistance b. Structural Strength Limit State—check cracked structural section for factored loads and nominal resistance c. Service Limit State—check cracked structural section deflections are within acceptable limits for service loads and nominal resistance
3. If the element classifies as long, then analyze a. Structural Strength Limit State—check cracked structural section for factored loads and nominal resistance b. Service Limit State i. Check cracked structural section deflections are within acceptable limits for service loads and nominal resistance ii. Determine embedment required (with criteria according to critical depth or the second point of zero deflection) for service loads and nominal resistance Guidance within GEC 009 is expected to be supplied for Use of P-multipliers related to group effects General procedure for the development of liquefaction loads Selection of p-y curves and soil properties for liquefied strata Head fixity
– Note that current literature review suggests that fixed head condition results in more accurate deflection estimates Use of Strain Wedge Model (SWM) method
– This literature review has found that the p-y method is more widely used and accepted than the
SWM, and there appears to be a larger body of testing and case histories as well as region- or state-specific parameters applicable to the p-y method. Therefore, the p-y method will be presented as the preferred method for analysis. However, the PWM method will be considered to be a valid alternative method and can be used in cases where the designer is comfortable with the method and software, where there is adequate precedence or site/region specific data, and/or where an additional assessment is desired, such as cases where lateral capacity is critical, where subsurface conditions are unusual, and/or where the results of the p-y method appear to be

289 unusual or questionable. However, the SWM method should not be used in lieu of the p-y method where there is wide acceptance in local practice and/or site or region specific p-y parameters based on DOT guidelines or testing. Responsibilities of structural and geotechnical engineers Based on general practice from State DOTs it is anticipated that it will be recommended that geotechnical engineers supply soil properties or p-y curves to structural engineers who perform the lateral analysis. Additionally, it will be suggested that a final review be conducted of the results by the geotechnical engineer.