Stress History Approach To Analysis Of Soil Resistance To Pile Driving

Abstract

ABSTRACT A method has been developed to predict soil resistance during continuous pile driving in clay based on interpretation of pile installation experience and related soils data in terms of the soil overconsolidation ratio. Soil resistance, obtained from offshore pile driving records using standard wave equation analysis, can be less than or greater than the predicted static pile capacity in normally consolidated and heavily overconsolidated clays, respectively. Analysis of installation data from large diameter pipe piles in soft to hard clays provides an empirical factor for computing soil resistance from the predicted static pile capacity. Values of the factor for six case histories show a consistent relationship with overconsolidation ratio, and this relationship is represented mathematically. The procedure incorporates overconsolidation ratios determined from undrained shear strength and soil index properties. The method for computing soil resistance during driving for a given soil profile based on the static pile capacity and the empirical adjustment factor is described. INTRODUCTION Drivability of offshore pipe piles is currently predicted by estimating the resistance that a pile-hammer combination can overcome and comparing this with the likely resistance of the foundation soils. The capability of pile-hammer combinations to overcome resistance can be analysed using one-dimensional stress wave theory (Smith, 1962). A logical starting point for assessing the soil resistance is the predicted static axial compressive pile capacity. However, pile installation data indicate that soil resistance during continuous pile driving is less than the predicted static pile capacity in normally consolidated clays (McClelland et aI, 1969; Aurora, 1980; Stockard, 1980), and can exceed the static capacity in heavily overconsolidated clays (McClelland et al, 1969; Fox et al, 1970; Durning and Rennie, 1978). In order to examine the relationship between static pile capacity, the soil resistance mobilised during driving, and the degree of overconsolidation, six installation case histories of large diameter pipe piles in silty and sandy clays were analysed. The results indicate that the overconsolidation ratio of cohesive soils has a significant influence on pile drivability. The paper presents results of this study and a procedure for computing soil resistance during continuous pile driving. STRESS WAVE THEORY Stress wave theory (Smith, 1962) has been used routinely for many years in offshore pile drivability studies. Nonetheless, uncertainty exists regarding values for some of the numerous parameters used in the mathematical solution. The more important of these variables are the hammer impact energy and the parameters for springs and dashpots that model soil resistance. Hammer input energy is not routinely measured in offshore construction. Conflicting published values for quake and damping, representing the resistance springs and dashpots respectively, indicate an imperfect knowledge of appropriate values for soils. Application of a reasonable range of values for quake and damping for a given soil results in variation by a factor of at least two in the predicted resistance that a given pile-hammer combination can overcome. Variation of hammer efficiency increases the range of predicted resistance.