Elastic analysis of axial load-displacement behavior of single driven piles
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Deep foundations are commonly recommended when large displacements are expected. Typically, though, their design involves only checking and providing for sufficient capacity to carry the applied loads. Load-displacement behavior of piles is considered secondary to the axial capacity; displacements are ordinarily overlooked or not calculated if and when the estimated pile capacity is two to three times the design or expected loading. However, in cases, such as long piles or piles in dense cohesionless soils, displacements can be the critical factor in design or it could be a structural requirement to limit the displacements. In this dissertation, the displacements of axially loaded single piles are investigated by conducting analyses with the aid of an approach based on elasticity. The original solution predicting displacements due to a vertical load within a semi-infinite soil mass has been modified for varying soil conditions and layering, and assumptions of stresses and displacements acting on the soil-pile interface. Aside from the available/known factors of the pile (length, diameter, cross-sectional area, etc.) and the layering of the surrounding soil, Young's modulus and Poisson's ratio of the soil encompassing a pile are the unknowns required as input to obtain predictions based on the elastic method. In this study, attention is directed towards determining Young's modulus because the range and variability of Poisson's ratio is not significant in displacement calculations. Axial pile load testing data were provided by the California Department of Transportation as part of a project to improve its general approach to pile design. All of the tested piles were driven into the ground. Measurements of displacements and loads were made only at the top of the pile. Supplementary in-situ testing involving cone penetration (CPT) and standard penetration (SPT), drilling, and sample collection, were conducted in addition to laboratory testing to enhance the available information. In this research, predicted displacements are compared with those deduced from pile load tests. Two sets of predictions based on elastic method are conducted for comparing displacements. First, various correlations for Young's modulus are employed to determine how accurately each predicts the actual measured displacement. The chosen correlations utilize laboratory triaxial undrained shear strength and standard penetration test blowcount for cohesive and cohesionless soils, respectively. Secondly, the same data are also utilized to obtain back-calculated values of Young's moduli for analyses involving the elastic method. The measured displacements at loads of a third, a half, twothirds, and equal to the failure load were matched iteratively. Results from this research are deemed to have an impact on engineering practice by improving the determination of Young's modulus for displacement analyses involving the elastic method. A unique approach that has potential is the reconciliation of load ratios (percentage of failure load) with displacement calculations to provide a better overview of the range of load ratios for which these newly formulated correlations may be employed. Through this research, it is anticipated that better determination of soil parameters for elastic analysis of axial pile displacements can be made by researchers and engineers alike.