Effective Stress and Axial Pile Capacity: Lessons from Empire
Authors: Bogard Dewaine 
Published: October 2008

Abstract
The results of a number of field experiments with instrumented piles and probes in highly plastic normally consolidated clays have led to the development of accurate and reliable axial pile design methods. Alternate methods have been developed using total and effective stress approaches. In this paper, the methods used for developing the ultimate resistance relationships are demonstrated, using the results of eight in situ model pile experiments at a site near Empire, La. as examples.
The choice of the particular design method to be used is generally determined by site-specific variations in the shear strength profile. For classic normally consolidated clays, the methods give identical results, with either the availability or the reliability of the geotechnical data determining the choice of the design method.

Introduction
Comprehensive studies of the behavior of friction piles under static and cyclic loading have been performed for Conoco and other industry sponsors. Based on the results, methods were developed for the design of friction piles in normally consolidated clays (Bogard and Matlock, 1990a; Bogard, Matlock, and Chan, 2000). The design methods include consideration of the setup time, the load-deformation behavior, and the effects of cyclic loading on the axial shear transfer capacity.
The design methods were based on the results of field tests on a fully instrumented 30-inch-diameter pile and on 29 in situ experiments with instrumented model piles. The results of the experiments led to the decision to base the ultimate axial capacity calculations on the undrained shear strength. The decision was made primarily from the standpoint of technical accuracy. However, the total stress approach results in greater economy in the required site investigations and provides more flexibility in its application.
The close agreement among all the experiments was documented in Bogard, Matlock, and Chan (2000). In this paper, the results of eight experiments at the Empire test site will be explored, and the methods used to formulate the design equations for ultimate capacity will be demonstrated.

Experimental Results from Empire, La.
Fourteen pile segment model tests were conducted at the site of two previous axial pile research programs (Bogard and Matlock, 1990b). Eight of the fourteen experiments were performed between depths of 115 and 165 ft, corresponding to Pile 1, as described by Cox and Kraft (1979), and encompassing the test depth reported by Azzouz and Lutz (1986). The remaining six experiments, performed between depths of 210 and 250 ft, disclosed a high degree of variability, suggesting that this depth interval consisted of mixed and interlayered sand, silt, and clay.

Consolidation and Setup Time.
Pore pressure dissipation data from the eight experiments are shown in normalized form in Fig 1. The data were normalized with respect to t50 (the time for dissipation of half the excess pore pressure) in the rawest possible form, without embedding arbitrary permeability or consolidation coefficients in the relationships.