A companion paper by Esrig et al {1977} describes the initial development of a general effective stress method for the prediction of axial capacity for driven piles in clay. A dominant physical characteristic of driven piles in clay is a zone of severely remolded soil around the pile. The available shear resistance of the soil in the remolded zone controls axial capacity for driven piles in clay. This paper summarizes how critical state soil mechanics was used during an initial study to describe the engineering properties and stress conditions of the soil in the remolded zone.
It is assumed that the reader is not familiar with the concepts of critical state soil mechanics. Those concepts relevant to the pile problem are briefly explained and procedures for estimating the necessary soil parameters by direct testing, or indirectly from correlations with index tests, are presented.
The changes in stress during pile driving and reconsolidation after pile driving are illustrated by stress paths.
A companion paper by Esrig et al {1977} describes the initial development of a general effective stress method for the prediction of axial capacity for driven piles in clay. As noted in that paper, several distinct zones can be identified in the vicinity of a driven pile in clay. These zones are illustrated in Fig. 1. A zone of severe remolding due to pile driving encircles the pile as indicated by Zone A. The soil in this zone has experienced large displacements and has been "turned upside-down" by the pile driving operation. For displacement piles, the zone of severe remolding extends to at least 1.4 pile radii from the center of the pile based on volume considerations.
Outside the zone of severe remolding, the soil has been stressed by the increase in volume caused by pile installation. In this zone {Zone B}, the soil accommodates the pile installation by relatively small radial displacements. The soil in Zone B may become overstressed but it is not "turned upside-down" by the pile driving process. Soil displacements due to pile driving are influenced by the ground surface to some limited depth; below this depth, the soil displacements are in the radial direction only. Near the bottom of the pile the displacements are influenced by the pile tip.
An essential step in the development of a general effective stress method for prediction of axial capacity, of piles driven into clay, is the prediction of the effective stress in Zone A immediately after driving. This requires the use of a model for soil behavior which adequately describes the behavior of soils at large strains. The concepts of "critical state soil mechanics" {Schofield and Wroth, 1968} were developed to permit prediction of the behavior of soils at all strains but the cornerstone of this model is the state of stress at large strains.
