Operational Soil Stiffness from Back-Analysis of Pile Load Tests within Elastic Continuum Framework

Abstract

New sets of shear stiffness reduction curves are developed from the back-analyses of 299 static axial pile load tests from 61 sites towards the implementation of a non-linear load-displacement (Q-w) response method for pile foundations. The initial shear modulus (G_max) is derived from the measured shear wave velocity (V_s) profiles at the pile sites, usually obtained from seismic cone penetration tests (SCPT). A Randolph-type closed-form elastic continuum solution for axial compression loading is used, modified for additional applications involving axial tension loading cases. The back-analysis of shear moduli at various load levels results in derivations of new shear modulus reduction curves, specifically normalized shear stiffness (G/G_max) vs. logarithm of pseudo-strain: γ_p = w_t/d, where w_t = pile top displacement and d = pile diameter. These curves incorporate the effects of pile type and installation method, and also show the influence of soil plasticity. A complete step-by-step methodology is presented for use and application of these new stiffness reduction curves within the extended system of closed-form elastic continuum solutions. In a companion paper, these solutions are further extended towards their application to a stacked pile model, representing certain practical field situations of non-homogeneous and non-Gibson type soil profiles, and accounting for the concept of progressive failure with depth.