A Unified Constitutive Model for Simulating Stress Path Dependency of Sandy and Gravelly Soil-

Structure Interfaces

Avant-Propos

Auteurs et affiliation:

 Miad Saberi: Étudiant au doctorat, Université Laval, Faculté des sciences et de génie, Département de génie civil et de génie des eaux, Québec, Canada.

 Charles-Darwin Annan: Professeur, Université Laval, Faculté des sciences et de génie, Département de génie civil et de génie des eaux, Québec, Canada.

 Jean-Marie Konrad: Professeur, Université Laval, Faculté des sciences et de génie, Département de génie civil et de génie des eaux, Québec, Canada.

État : soumis à la revue

Date de soumission: 3 octobre 2017

Revue : International Journal of Non-Linear Mechancis

Titre français : Un modèle constitutif unifié pour la simulation de la dépendance au stress-

chemin des interfaces de structure de sol sablonneux et graveleux

Résumé: Un modèle de plasticité constitutive est proposé pour simuler le comportement

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modèle de plasticité à deux surfaces développées antérieurement pour les interfaces entre les sols graveleux et les matériaux structuraux [1], qui imite l'écrouissage, la dégradation des contraintes et de transformation de phases. Le modèle proposé dans cette étude inclut le comportement d’amenuisement susceptible de se réaliser dans les interfaces sol-structure sableuses denses sous chargement monotone et cyclique, et fournit une formulation unifiée pour simuler le comportement des interfaces sol-structure graveleuses et sableuses. Le modèle tient compte du comportement de dépendance au chemin de contrainte des interfaces, et il exige un ensemble unique de neuf paramètres d'étalonnage, qui peuvent facilement être obtenus à partir des tests de cisaillement d'interface standard. Les performances du modèle d'interface sont évaluées pour les conditions de charge normale constante, de rigidité normale constante et de contrainte constante en comparant ses prédictions aux données expérimentales.

Mots-clés: interface sol-structure sableuse et graveleuse, formulation unifiée, plasticité à

deux surfaces, mécanique des sols à l'état critique

Abstract: A plasticity constitutive model is proposed to simulate the monotonic and cyclic

behavior of granular soil-structure interfaces. The model is built on a two-surface plasticity model previously developed for interfaces between gravelly soils and structural materials (Saberi et al. 2016), which simulates strain hardening, stress degradation and phase transformation behavior. The proposed model in this study incorporates the softening behavior likely to occur in dense sandy soil-structure interfaces under monotonic and cyclic loading, and it provides a unified formulation for simulating the behavior of both sandy and gravelly soil-structure interfaces. The model accounts for the stress path dependency behavior of interfaces, and it requires a single set of nine calibration parameters, which can readily be obtained from standard interface shear tests. The interface model’s performance is evaluated for Constant Normal Load, Constant Normal Stiffness, and Constant Volume stress path conditions by comparing its predictions with experimental data.

Keywords : sandy and gravelly soil-structure interface, unified formulation, two-surface

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Introduction

In geo-structures such as buried pipelines, retaining walls, underground tunnels, embankment dams, and shallow and deep foundations, the transition zone between the soils and the structure, known as interface, may present a critical load transfer mechanism. This mechanism involves the interaction between the soils and the structure, and could play an important role in evaluating the performance of many soil-structure interaction systems. The design and performance of these geo-structures largely depend on mobilized shear strength, load-displacement behavior and cyclic evolution of stress degradation at the interface between the soil and the structure. Based on laboratory observations (e.g. Desai et al. 1985; Zeghal & Edil 2002; Hu & Pu 2004; Mortara et al. 2007; Di Donna et al. 2015; Evgin & Fakharian 1996; Uesugi et al. 1990; Kishida & Uesugi 1987; Desai et al. 2005), the mechanical behavior of these interfaces is largely dependent on loading conditions (i.e. stress path and magnitude of normal stress), soil properties (i.e. relative density and grain mineralogy) and structural surface conditions.

Generally, interface zones in different soil-structure interaction problems may exhibit different boundary conditions, which impose different stress paths. It has been experimentally observed that the stress path largely affects the load transfer mechanism, stress-displacement relation and volumetric behavior of the interface (Johnston et al. 1987; Tabucanon et al. 1995; Fakharian 1996; Evgin & Fakharian 1996; Fakharian & Evgin 1997; DeJong et al. 2003; Zhang & Zhang 2006b; Mortara et al. 2007; DeJong & Westgate 2009). For example, in a typical shallow foundation problem, the average normal stress induced on the soil-foundation interface zone remains constant. This condition is known as constant normal load stress path. The normal stress is, however, not constant in some other soil- structure interaction systems. For example, interfaces between a retaining wall and the backfill soil or pile shaft and its surrounding soils experience varying normal stress under both monotonic and cyclic loading. In effect, understanding the load transfer mechanisms of the interface in different stress paths is required for the analysis and design of soil-structure interaction systems.

In addition to using experimental studies to understand the mechanical behaviour of granular soil-structure interface zones, numerical methods using material constitutive modeling have

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proved to be effective complementary technique. In past studies, the special-purpose elements proposed by Goodman et al. (1968) as zero-thickness elements were used to solve different soil-structure contact problems (e.g. Clough & Duncan 1971; Hu & Pu 2003; Bayraktar et al. 2011). However, their inability to accurately simulate the volumetric behavior of the interface zone has been a major drawback. To overcome this limitation, thin- layer elements were developed by Zienkiewicz et al. (1970) and Desai et al. (1984) to simulate interface zones. The evolution of this element type has led to the development of advanced constitutive models for soil-structure interfaces (e.g. Shahrour & Rezaie 1997; Liu et al. 2006; Liu & Ling 2008; Lashkari 2013; Lashkari & Kadivar 2016; Liu et al. 2014; D’Aguiar et al. 2011; Stutz et al. 2016; Zhang et al. 2015; Desai et al. 2005). However, only a few of these constitutive models have the capability to predict both monotonic and cyclic responses, and stress path dependency behavior of interfaces (Zhang & Zhang 2008; Liu & Ling 2008; Liu et al. 2014), but they do require a significant number of calibration parameters. Moreover, to the best of the authors knowledge, only the model proposed by Liu et al. (Liu et al. 2014) with fifteen calibration parameters has been introduced for both sandy and gravelly soil-structure interfaces. In the field of granular soil-structure contact problems, there is the need for a unified and efficient constitutive formulation for sandy and gravelly interfaces, capable of simulating different normal stresses and soil densities (i.e. loose and dense) under different stress paths. In addition, it would be useful to predict both monotonic and cyclic responses without the need to recalibrate the model parameters.

In this paper, the constitutive model developed by Saberi et al. (2016) for gravelly soil- structure interfaces is extended by incorporating softening behavior to simulate monotonic and cyclic behavior of both sandy and gravelly soil-structure interfaces. Softening in dense sandy soil-structure interfaces results in the reduction of shear strength after peak responses under monotonic and cyclic loading conditions (Evgin & Fakharian 1996; Hu & Pu 2004; DeJong & Westgate 2009). The proposed model is based on a unified constitutive formulation to simulate the complex behaviour of granular soil (gravely and sand)-structure interfaces with different soil densities under monotonic and cyclic loading, and it requires a set of only nine calibration parameters. The model is compatible with the frameworks of two- surface plasticity, critical state soil mechanics (CSSM) and state dependency. The following sections discuss the essential features of the behavior of granular soil-structure interfaces and

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present the formulation of the proposed constitutive equations. The model calibration parameters are subsequently presented, followed by the validation of the proposed model.