Improved Evaluation of Equivalent Top-Down Load-Displacement Curve from a Bottom-Up Pile Load Test

Abstract

A modified procedure is presented in this study to evaluate the equivalent top-down load-displacement curve in a bottom-up pile load test considering elastic shortening. On the basis of the results of a parametric study on a bored pile in normally consolidated cohesive soils under undrained conditions, varying shear strength distribution and pile slenderness ratio, it was concluded that the pile shortening caused by the skin-friction component of the load in a top-down test can be related to the measured elastic shortening in a bottom-up test. A λ-factor is used to define this relationship, that is, the ratio of the top-down to bottom-up pile shortening. The factor λ=1.0 is used for the case of a pile in soil with uniform shear strength profile, λ=2.0 for linear profiles, 1.0<λ<2.0 for nonlinear profiles varying above linear, and λ>2.0 for nonlinear profiles varying below linear. In addition, the method suggests taking the corresponding readings of the skin-friction load component from the upward displacement curve of the top of the pile, which is a closer approximation to rigid pile displacement than the bottom when corrections for elastic pile shortening are to be applied. Assuming a fully mobilized skin-friction, a logarithmic relation for the factor λ to the normalized area under the shear strength profile was generally formulated and is limited to the assumptions on which they were derived. The suggested procedure in this study has produced the equivalent top-down load-displacement curves that are in close agreement with the measured top-down curve, as validated in the case studies.