Jacked piles in London clay: interaction and group behaviour under working conditions

Abstract

This Paper describes a field investigation of the behaviour of a row of two piles and a row of three piles installed at close spacings in London Clay. It follows directly the paper by the same authors (1979) presenting observations of the shear stresses and shear strains around a single pile and the measurement techniques described in that paper have been employed and extended. The three pile row was tested under conditions of equal pile loading and equal pile displacement and also with the piles linked by a rigid concrete cap on the clay surface. In theoretical studies of pile group behaviour the principle of superposition is frequently applied to soil stresses and displacements in order to determine the settlement of each pile in the group. The measurements reported in this Paper demonstrate the validity of a superposition approach both to the estimation of the pile settlements and to the determination of the load carried by each pile. The results of theoretical studies have generally been presented in the form of interaction factors relating the additional settlement caused by a loaded pile nearby to the settlement of each pile under its own load. Interaction factors measured in the field investigation are compared with published theoretical values and also with observations of the variation of vertical displacement of the soil with radial distance from the pile axis. Design curves relating the settlement of piled foundations to the settlement of a single pile are presented. The variation of load transfer along each pile shaft with the complexity of the foundation is discussed. Cet article décrit des recherches effectuées in situ sur le comportement d'une rangée de deux pieux et une rangée de trois pieux implantés à intervalles rapprochés dans de l'argile londonienne. Il fait directement suite à un autre article des mêmes auteurs (1979) dans lequel étaient exposées des observations faites sur les contraintes et déformations de cisaillement autour d'un seul pieu; les techniques de mesures décrites dans cet article ont été utilisées et développées. La rangée à trois pieux a été soumise à des essais dans les conditions de charge et déplacement des pieux identiques, et également avec une semelle en béton rigide reliant les pieux à la surface de l'argile. Dans les études théoriques du comportement de groupes de pieux, le principe de superposition est souvent appliqué aux tensions et déplacements du sol en vue de déterminer le tassement de chacun des pieux du groupe. Les mesures que rapporte cet article démontrent la validité du principe de superposition tant pour l'évaluation du tassement des pieux que pour la détermination de la charge portée par chaque pieu. Les résultats des études théoriques ont généralement été présentés sous la forme de coefficients d'interaction mettant en rapport le tassement additionnel causé par un pieu chargé proche avec le tassement de chaque pieu sous sa propre charge. Les coefficients d'interaction mesurés lors de recherches in situ sont comparés aux valeurs théoriques publiées ainsi qu'à des observations de la variation du déplacement vertical du sol à distance radiale de l'axe du pieu. L'article présente des courbes de calcul mettant en relation le tassement de fondations sur pieux et le tassement d'un seul pieu. La variation du transfert de charge le long de chaque fût de pieu en fonction de la complexité de la fondation est également examinée.

Piled raft foundations on stiff clays—a contribution to design philosophy

“…The measurements made by Cooke et al (1980) suggest that the interaction between piles is negligible at spacings greater than about ten pile diameters. The corner and centre piles of a 7' group at k = 2.5 would not therefore influence each other perceptibly since they are IO.6 pile diameters apart.…”

“…Much larger instrumented piles were tested under field conditions by Koizumi & Ito (1967) and by Cooke et al (1980). Koizumi and Ito loaded a 3' group of closed end pipe piles 300 mm dia.…”

“…The settlement interaction curve derived from these measurements was very similar to that obtained theoretically by Banerjee & Davies (1977) for groups in cohesive soils increasing in stiffness with depth. Simple calculations using the interaction curve prepared by Cooke et al (1980) for a 3' group loaded through a rigid cap clear of the clay surface suggest that at k = 3 each corner pile would carry twice and each centre side pile 1.5 times the centre pile loading. This is consistent with the results of the tests described earlier.…”

“…The increase in stiffness resulting from the addition of a small rigid pile cap to a row of three instrumented steel piles has been measured by Cooke, Price & Tarr (1980). The cap overhung the piles in each direction to half the pile spacing so that it corresponded to the raft element which would be associated with the three piles if they were forming part of a larger group at the same spacing.…”

FEM modelling of a large piled raft for settlement control in weak rock

“…Therefore, piles would transmit the entire structural load to the soft granite (SG) layer; bypassing the potentially troublesome residual soil and weathered granite layers. Validity of this assumption is supported by Butterfield and Banerjee [4] and Cooke et al [28], who have shown that the contact between raft and soil does not significantly affect the settlement of the pile group even if the load taken by the raft-soil system is as high as 50% of the total applied load.…”