2727
Contributing factors on soil setup and the effects on pile design parameters
Facteurs contribuant au durcissement du sol et leur effet sur les paramètres de conception
des pieux
Fakharian K., Attar I.H., Sarrafzadeh A., Haddad H.
Department of Civil and Environmental Engineering, Amirkabir University of Technology, Tehran, IRAN,
ABSTRACT: Most of the pile setup studies in literature have focused on the variations of total capacityand/or tip and skin friction
resistances. But it is of practical importance to propose realistic parameters for design applications such as pile shaft resistance β-
parameter. Pile dynamic (PDA) and staticload test results are available for 5% of total of 6000 drivenpre stressed spun piles with OD
of 450 mm at a petrochemical utility plant in southwest Iran.The range of β-parameter for each of the three layers along which the
piles are embedded are determined for design applications in the regionwith back-calculationfromdynamic tests results at restrike. An
axisymmetric finite element numerical model isused to simulate the cavity expansion developedduring pile driving and corresponding
generation of excess pore water pressure (EPWP). The setup effects are evaluated allowing sufficient time for dissipation of EPWP
and simultaneous increase in radial effective stresses along the pile shaft. The difference between shaft capacity obtained from the
back-calculated β of the field data and that from the numerical model for EPWP dissipation analysisonly, is assumed to be attributed
to aging effects. Increase in the interface shear strength between pile-soil with time isconsidered to account for the aging component
of the soil setup. The results show that the dissipation of EPWP has been the dominant factor in soil setup corresponding to increase
in the frictional resistance of the pile shaft of the study area in which 75% has been resulted from radial cosolidation (EPWP
dissipation) and 25% from aging.
RÉSUMÉ : La plupart des études configuration de pieuxdans la littératureontmisl'accentsurles variations de la capacitétotale et / ou la
pointe et les résistances de frottementlatéraldans le temps. Maisilestd'une importance pratique de proposer des paramètresréalistes
pour les applications de conception telles le paramètre β décrivant la résistance de fût. Des essaisdynamiques (PDA) ainsique des
résultatsd'essais de charge statiquesontdisponibles pour 5% des 6000 pieux de bétonpréfabriquésprécontraint d'un diamètreextérieur
de 450 mm dansuneusinepétrochimique du sud-ouest de Iran. La plage des valeurs du paramètre β pour chacune des trois couches le
long desquelles les pieuxsontenfoncésestdéterminée pour les applications de conception dans la région à partir de rétro-calculsd'après
les résultatsd'essaisdynamiqueslors du rebattage. Un modèlenumérique par éléments finis axisymétrique estutilisé pour
simulerl'expansion de la cavitédéveloppée au cours du battage de pieux et de la production correspondante de la
pressiond'eauinterstitielle en excès (EPWP). Les effets de mise en place sontévaluées en permettant un temps suffisant pour la
dissipation de l'augmentation de la pressiond'eau et l'accroissementsimultané des contraintes effectives radiales le long du fût du pieu.
La différence entre la capacité de fûtobtenued'après les valeurs de β rétro-calculées à partir des données de terrain et cellesobtenues à
partir du modèlenumériqued'analyse de dissipation de la pressiond'eauinterstitielle, estsupposéêtreattribués à des effets de
vieillissement. L'augmentation de la résistance au cisaillementd'interface entre le pieu et le sol en fonction du temps
supposétenircompte de la composante du vieillissement du durcissement du sol. Les résultatsmontrentque la dissipation de la
pressioninterstitielle a été le facteur dominant dans le durcissement du sol correspondant à l'augmentation de la résistance de
defûtdans la zone d'étudeoù 75% de l'augmentationestattribuable à la consolidation radiale (dissipation de la
pressiond'eauinterstitielle) et 25% au vieillissement.
KEYWORDS: pile driving,numerical modelling,effective stress analysis, soil setup, aging, PDA test, cavity expansion
1 INTRODUCTION
Driven piles are frequently used in industrial projects in
southern lowlands of Iran near Persian Gulf.One of the
important issues in driven piles is variation of bearing capacity
with time after the initial drive. This important issue is well-
understood in literature and it is pointed out that depending on
the soil type, either “soil setup” or “soil relaxation” may occur
with time. Soil setup results in eventual increase in the pile
capacity, whereas in the soil relaxation condition, the bearing
capacity decreases with time.
Different reasons are stated for these phenomena and the
types of soil in which either of the setup or relaxation may occur
(Svinkin, 1996; Seidel and Kolinowski, 2000; Rausche et al.,
2004; Bullock et al., 2005 and 2005b).In majority of reported
cases however, the setup has occurred and relaxation has
seldom been reported (Komurka et al., 2003; Axelsson, 2000).
The bearing capacity variations are observed to be rapid with
time initially, the rate of which substantially decreases with time
elapse.
The stated reasons for setup can be summarized as: (1)
generation of excessive pore water pressure during pile driving
and subsequent dissipation with time, (2) aging. Most of the
studies available in literature, however, have focused on pile
capacity variation with time and it is stated that (for example by
Svinkin, 2000) the most portion of setup is related to dissipation
of effective pore water pressure. Little attention has been paid
on distinguishing between the contribution of dissipation of
EPWP and aging.
The main objective of this study is a numerical approach to
study the setup effects on a single pile embedded in layered
strata on the basis of back-calculated parameters from an
industrial unit in southwest Iran. An axisymmetric nonlinear
finite element scheme is adopted to simulate the cavity
expansion as a result of pile driving and its subsequent
dissipation of EPWP and aging effects. A case in fajr II utility
