Field Stiffness Reduction Evaluations from Back-Analysis of Pile Load Tests

Abstract

Starting with the fundamental small-strain shear modulus (Gmax), the mechanical nonlinear stress-strain-strength behavior of soil manifests itself in the form of stiffness reduction under loading. Back analyses of 105 pile load tests combined with a knowledge of soil parameters at the site and pile geometries provide a framework for evaluation of operational stiffnesses at different levels of loading. This paper explains the framework for obtaining the operational stiffnesses from pile load tests and presents stiffness reduction curves (G/Gmax) in terms of logarithm of pseudo-strain (w/d), where w = pile displacement and d = pile diameter. The result affords an approach to evaluating the nonlinear load-displacement response of piles without resort to evaluating the pile capacity. The framework utilizes the Randolph elastic pile model to generate stiffness reduction curves from top-down axial compression load tests, tension tests, and O-cell configurations.