Initial Development Of Ageneral Effective Stress Method For The Prediction Of Axial Capacity For Driven Piles In Clay

Abstract

ABSTRACT A general effective stress method for the prediction of the axial capacity of full displacement piles driven into clay is presented. The method first requires that the initial state of stress in the ground prior to pile driving be defined. The changes in stress associated with the major events in the life of a pile (pile driving, reconsolidation after driving and axial loading) are then estimated from models of pile-soil behavior and added to the initial state of stress to determine the state of stress at failure. Pile capacity is predicted using some of the concepts of critical state soil mechanics described in a companion paper (Kirby and Wroth, 1977). Comparison of predictions with measured capacities at three sites suggest that the effective stress method is promising. The advantages and limitations of the method are described. INTRODUCTION Few would disagree that, in principle, the available shear resistance along the shaft of a pile, Tav is controlled by the effective normal stress on the pile shaft at the time of failure, õ ff, and the effective stress friction angle for soil sliding on the pile material, ?ss' The friction angle, ?ss' can be measured, with difficulty, in the laboratory. However, a satisfactory procedure for prediction of õ ff is not available. A rational procedure for estimating off must begin with definition of the state of stress prior to pile installation and then consider explicitly the changes in stress associated with the major events in the life of a driven pile. These events include pile driving, reconsolidation after pile driving, and pile loading. In other words, prediction of the effective stress at the pile-soil interface, at failure, is a problem of addition: (Calculation available in full paper) Definition of the stress changes for pile driving, reconsolidation, and pile loading is a formidable task. In the past, other researchers have avoided the problem by correlating the available shear resistance at the pile-soil interface (back calculated from load tests) with untrained shear strength of the undisturbed soil and/or vertical effective stress prior to pile driving. The major limitation of such a procedure is Late all the factors influencing pile capacity are lumped into a single, empirically determined, correlation factor. Consequently, there is no basis for extrapolation of correlation factors when soil conditions, loading conditions, pile size, or pile installation methods are different from those included in the load test data from which the correlation factors were derived. An exploratory study has been completed to determine if recent advances in the understanding of soil behavior, and recently developed analytical tools, could be applied to the development of a general effective stress method for the prediction of axial capacity for driven piles in clay. During this study, consideration was given to each of the events in the life of a driven pile. The fundamental concepts of critical state soil mechanics were used to describe soil behavior at large strains.