Influence of pullout loads on the lateral response of pile foundation

Mahmoud N. Hussien, Mourad Karray, Tetsuo Tobita, Susumu Iai

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Earth Walls and Foundations

ABSTRACT: of pullout loads on the lateral response of pile foundation Mahmoud N. Hussien & Mourad Karray Department of Civil Engineering, Sherbrooke University (QC), Canada Tetsuo Tobita & Susumu Iai Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan ABSTRACT The influence of both compression and pullout loads on the lateral response of vertical piles installed in sandy soil is studied through two-dimensional finite elements. The interaction between the pile and the surrounding soil in three-dimensional type was idealized in the two-dimensional analysis using soil-pile interaction springs with hysteretic non-linear load displacement relationships. Soil models were idealized by multiple shear mechanism of hyperbolic type. Joint elements with separation-contact mechanism were used to idealize the separation effect at the soil-pile interface. Unlike the compression vertical loads that tend to increase the lateral capacity of a vertical pile installed sandy soil, numerical results show that pullout loads generally decrease the ultimate lateral capacity of piles. The increase or decrease in the confining pressures in the sand deposit at shallow depths is the major driving factor to contribute the increase or decrease in the lateral resistance of piles, depending on the direction of the applied vertical load. RÉSUMÉ L'influence de deux types de chargement, compression et arrachement, sur la réponse latérale de pieux verticaux installés dans un sol sablonneux est étudiée par éléments finis en deux dimensions. L'interaction entre le pieu et le sol environnant dans les trois dimensions a été idéalisé dans l'analyse en deux dimensions en utilisant des ressorts d'interaction sol-pieu avec des relations charge-déplacement hystérétique non-linéaire. Les sols ont été idéalisés par un mécanisme de cisaillement multiple de type hyperbolique. Les éléments communs avec mécanisme contact-séparation ont été utilisés pour idéaliser l'effet de séparation à l'interface sol-pieu. Contrairement aux charges verticales en compression qui tendent à augmenter la capacité latérale d'une pile verticale installé un sol sablonneux, les résultats numériques montrent que les charges d'arrachement diminuent généralement la capacité latérale ultime des pieux. L'augmentation ou la diminution de la pression de confinement dans le dépôt de sable à faible profondeur, est le facteur majeur qui contribue à l'augmentation ou la diminution de la résistance latérale de pieux, en fonction du sens de la charge verticale appliquée. 1 INTRODUCTION Pile foundations have been used for many years as common foundation solutions for offshore structures. Offshore structures are in general subjected to a combination of lateral loads and overturning moments due to wind loads, wave pressures, earth pressures, and ship impacts. The overturning moments are transferred to the structure foundations in the form of compression axial forces on some piles and pullout forces on others. Although it is obvious that the presence of these axial forces may affect the lateral carrying capacities of piles, all traditional methods of pile analysis consider that axial and lateral loads to act independently, and the interaction between these loads was not significant (Hussien et al. 2014). In the conventional methods of pile analysis, soil is modelled by spring elements attached to the pile at different depths. These springs generally have nonlinear load-displacement characterizations called (t-z) and (p-y) curves for axial and lateral loading, respectively. These two types of springs are generally uncoupled and therefore soil reactions along the corresponding degrees of freedom are also uncoupled. In other word, the influence of load acting in one direction on the characteristics of the spring in the other direction is neglected. However, recent studies based on finite element analyses of single piles (i.e. Karthigeyan and Ramakrishna (2006) and Karthigeyan et al. (2007), and Hussien et al. (2012), (2014)) suggested a significant increase in the lateral resistance of piles in the presence of vertical loads. In fact, the scopes of these recent studies were limited to the behavior of single piles subjected to compressive vertical loads. Yet, little work has been devoted to the behavior of piles subjected to the combined action of pullout axial and lateral loads. In this study, the influence of compression and pullout loads on the lateral capacities of vertical piles installed in sandy soil is studied through two-dimensional (2D) finite elements (FE) analyses. The interaction between the pile and the surrounding soil in three-dimensional (3D) type was idealized in the 2D analysis using soil-pile interaction springs with hysteretic non-linear load displacement relationships. Soil models were idealized by multiple shear mechanism of hyperbolic type, FLIP (Iai et al. 1992). Joint elements with separation-contact mechanism are used to idealize the separation effect at the soil-pile interface (Hussien et al. 2010). Initial results from this study are presented and the primary findings were summarized as conclusions. 2 FINITE ELEMENT MODELING AND MATERIAL PROPRTIES 2.1 Finite elements The 2D FE program FLIP was employed to analyze the behavior of piles under pure lateral loads and a

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Mahmoud N. Hussien; Mourad Karray; Tetsuo Tobita; Susumu Iai (2014) Influence of pullout loads on the lateral response of pile foundation in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper316,author = Mahmoud N. Hussien; Mourad Karray; Tetsuo Tobita; Susumu Iai,title = Influence of pullout loads on the lateral response of pile foundation ,year = 2014}