Study of creep effects in single piles using a five-parameter linear viscoelastic soil model

Anumita Mishra, Nihar Ranjan Patra

In the proceedings of: GeoOttawa 2017: 70th Canadian Geotechnical Conference; 12th joint with IAH-CNC

Session: Field Cases III

ABSTRACT: The present study utilizes a five-parameter model (a Hookean spring element in parallel with two Maxwell elements in series) to study the creep effects in single piles. Using a five-parameter soil model would give the advantage of accurately fitting the material parameters that would best characterize the actual soil behavior. Laplace transforms are utilized to simplify the viscoelastic problem into an equivalent elastic problem. The results obtained from the proposed approach are compared with the available experimental results and are found to be within an error of ±10%. The effects of slenderness ratio, pile elastic modulus and effect of enlarged base on the time-dependent settlement of piles have been studied. This model also discusses the distribution of stresses around the periphery of pile and the rearrangement of stresses on real-time basis as the creep settlement in the soil progresses. Keywords: creep, viscoelastic, settlement, pile

RÉSUMÉ: La présente étude utilise un modèle de sol viscoélastique ayant cinq paramètres (un élément à ressort connecté en parallèle avec deux éléments de Maxwell en série) pour étudier les effets de fluage dans des piles simples. L'avantage d'utiliser un modèle viscoélastique avec cinq éléments est qu'il détermine avec précision les paramètres qui caractérisent le mieux le comportement réel du sol. La transformée de Laplace est utilisée pour résoudre le problème viscoélastique en la transformant en un problème élastique équivalent. Les résultats obtenus à partir de l'approche proposée sont comparés aux résultats expérimentaux disponibles et l'erreur est inférieure à ± 10%. Les effets du rapport L / D de la pile, du module élastique de la pile et de la base agrandie sur le règlement à long terme de la pile ont été étudiés. Ce modèle traite également de la répartition de la charge autour du périmètre de la pile et du réarrangement de la charge sur une période de temps. 1. INTRODUCTION Experimental investigations and several case studies have revealed that soils experience time-dependent settlements long after the construction of structure is complete. On a microscopic level, soils undergo particle rearrangement, deformation and destruction over time as a response to sustained loading. However, since only phenomenological behavior is important from engineering point of view, soils are treated as viscoelastic materials. The viscoelastic behavior of soil is modeled using a combination of mechanical components like springs and dashpots. The arrangement of the mechanical components should be such that it would be capable to simulate both the instantaneous and steady state creep of soil. The time-dependent settlement response of structures has been modeled using viscoelastic models using standard solid models by Booker and Poulos (1976), Bartolomey et al. (1991), Guo (2000) etc. Other models like the Nishihara model and Burger four-element model have also been used for some special cases. However, the application of five-element models in prediction of time-dependent settlement of piles has been limited in the past because of the complexities involved in handling larger number of parameters. Though the mathematical formulation may be slightly complex, the five-element model gives the advantage of more accurately fitting the viscoelastic parameters according to the actual time-dependent strain behavior of soil. Edil and Mochtar (1988) studied the long-term displacement due to creep between the interface of pile and soil through a series of model tests on single piles. Wei (2000) analyzed the creep behavior of single piles by developing closed-form solutions for viscoelastic soil. Danno and Kimura (2009) numerically analyzed the time-dependent settlement behavior of pile groups using coupled FEM analysis. Wu et al. (2012) utilized virtual-soil pile model to study the time-dependent creep behavior of single piles. Feng et al. (2014) predicted the post construction settlement of a railway bridge due to creep effects by using Mesri-creep model. Li et al. (2015) determined the time-dependent behavior of soft clay by using elastic-viscoplastic three-dimensional Nishihara model. This paper presents a five-parameter linear viscoelastic soil model to study the time-dependent creep of single piles. The effects of pile slenderness ratio, enlargement of pile base and pile elastic modulus on the time-dependent creep settlement behavior have been studied. The mobilization of shaft resistance over the pile length and its gradual rearrangement with time has also been discussed. The non-uniform displacement of pile elements coupled with the effect of pile slenderness ratio has been studied.

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