Geotechnical Engineering Journal of the SEAGS & AGSSEA ISSN 0046-5828
Vol. 46 No.2 June 2015
Operational Soil Stiffness from Back-Analysis of Pile Load Tests within Elastic Continuum Framework
Fawad S. Niazi and Paul W. Mayne
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 (Gmax) is derived from the measured shear wave velocity (Vs) 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/Gmax) vs. logarithm of pseudo-strain: γp = wt/d, where wt = 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.
KEYWORDS: Pile foundations, Displacements, Load tests, Shear modulus, Shear wave velocity, Soil stiffness, Plasticity index
