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

277 From AS 5100 for bridge design, foundations must be designed for the ultimate geotechnical strength state and for serviceability state, similar to AASHTO requirements. For the ultimate geotechnical strength state, the maximum load factor combination should be used, geotechnical material strengths are unfactored, but the overall geotechnical resistance is factored. The design equation to be satisfied for design of foundations is Where
S* = Design action loads.
R
ug
= Ultimate geotechnical strength.
φ
g
= Geotechnical strength reduction factor. The design action loads, S, are to developed based on the combination of factored loads that produces the most adverse effect on the foundation in accordance with Australian Standard AS 5100.2. The value of g varies from 0.4 to 0.9 for design of piles depending on the amount of site investigation, the complexity of the calculations, the degree of construction testing, causes of failure, cyclic loading, and use of general or site-specific correlations. Without load testing, the values are in the range of 0.4 to 0.65; with load testing, values are in the range of 0.5 to 0.9 depending on the type of testing and whether the testing is carried to failure or not. The focus of the discussion on piles is based on axial load tests there is no mention of lateral load tests or any differences in the design procedures for lateral pile design. There is no indication or guidance regarding specific procedures for lateral piles analysis or allowable lateral deflections. The standard indicates that for cases or conditions not specifically addressed for the geotechnical strength reduction factor, to use the published factors and conditions within the standard as a guide. For serviceability design, the standard states that foundations are to be designed by controlling or limiting settlement, horizontal displacement, and cracking. Deflections and horizontal displacements should be limited to ensure that the foundations and structure remain serviceable and that allowable displacements should be established and should consider the tolerance of the structure to deformation. The serviceability state does not use a geotechnical strength reduction factor. Load factors for serviceability state are 1.0 to provide an accurate estimation of movements. AS 2159 presents minimum requirements for design of piles for civil engineering and building structures on land. It states that AS 5100 series (summarized above) should be considered for design of foundations for bridges. For the ultimate geotechnical strength, the geotechnical strength is factored similar to the AS 5100 Standard. However, there are additional steps in developing the geotechnical strength reduction factor, g. These include Individual Risk Ratings (IRR) that serve to create a quantitative measure of the relative risk associated with the design, such as the level of investigation, the basis for the geotechnical strengths, the amount of construction testing, the level of construction control, etc. These IRR factors are then used in developing the value of g, with lower g values corresponding to higher relative risk. Basic geotechnical strength reduction factors range from 0.40 to 0.76, but can be increased if construction testing is used with higher values allowed for static testing compared to dynamic testing.
*
g ug
R
S
φ
>

278 Lateral load tests are included in the discussion regarding pile testing, but only briefly. The majority of the discussion on testing pertains to axial testing of piles. The text indicates that for proof tests, including lateral load tests, the acceptance criteria has to be defined before the test, whereas for ultimate geotechnical strength tests, the testis conducted to geotechnical failure and no acceptance criteria is provided before the test. For design of a pile subjected to lateral loads, the ultimate geotechnical design strength is determined as the lesser of Short pile failure, which is the ultimate lateral resistance of the soil surrounding the pile fully mobilized along the entire pile length, or Long pile failure, in which the structural strength of the pile is fully mobilized before the ultimate soil resistance along the entire length of the pile. For pile groups, the ultimate geotechnical strength, in the absence of an alternative method, is taken as the lesser of The sum of the ultimate strength of individual piles, or The ultimate geotechnical strength of a block containing the piles and the soil between them. Fora piled foundation, in addition to the design of an individual pile, the geotechnical strength of the pile group must be analyzed for failure under the loading on the group. Serviceability design is consistent with the AS 5100 standard. It indicates that the deflections for the serviceability state design should be limited to values appropriate for the intended design of the pile, but the methods for analyzing deflections and the actual allowable deflection values are not provided. No reduction factors are applied for serviceability Limit States. The Standard indicates that for piles subjected to lateral ground movements, the bending moments, shear forces, and axial actions must be determined using an appropriate soil-structure interaction. Otherwise, there is no guidance with regard to the type of analyses to be performed, the use of p-y curves, etc.

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