Soil Dynamics and Earthquake Engineering 134 (2020) 106162 Available online 16 April 2020 0267-7261/© 2020 Elsevier Ltd. All rights reserved. Three-dimensional constitutive model for cyclic behavior of soil-structure interfaces Miad Saberi a, ** , Charles-Darwin Annan b, * , Jean-Marie Konrad b a GHD, Montr� eal, Canada b Department of Civil and Water Engineering, Universit� e Laval, Qu� ebec City, Qu� ebec, Canada A R T I C L E INFO Keywords: Three-dimensional loading Soil-structure interaction Constitutive model Soil-structure interface Monotonic and cyclic loading ABSTRACT Many soil-structure interaction systems experience a three-dimensional loading condition (i.e. shear coupling) at their interfaces. In this study, an elasto-plastic constitutive formulation for interfaces in soil-structure interaction problems is proposed considering the effects of 3D shear coupling loading conditions. The proposed model is capable of simulating granular soil-structure interfaces for both monotonic and cyclic loading over a wide range of normal stress and normal stiffness using a single set of eleven calibration parameters. The model is capable of simulating a number of complex interface behaviour, including hardening and softening, compaction, dilation and phase transformation, stress path dependency, accumulative contraction and stabilization, stress degradation and particle breakage under monotonic and cyclic loading. The constitutive model performance is examined using available experimental data for gravelly and sandy soil-structure interfaces subjected to monotonic and cyclic loads involving shear coupling. 1. Introduction The performance of many soil-structure interaction (SSI) systems is highly dependent on the load–displacement relationships and volu- metric behaviour of their interfaces. The soil-structure interface problem can be two-dimensional (2D) or three-dimensional (3D) depending on the loading condition to which it is subjected. There have been many experimental studies from interface shear tests on granular soil-structure interfaces under 2D loading conditions to understand the effects of normal stress, soil type and relative density [1,2], surface roughness [1–5], interface anisotropy [6,7], stress paths [8,9], temperature [9], axial and torsional shearing [4], surface texture [10], cyclic loading condition [9,11–14]. However, very limited data exists on interface behaviour under 3D loading state [15–18]. There are also various 2D constitutive models proposed to numerically predict the response of granular soil-structure interfaces under monotonic loading in the framework of elastoplastic Mohr-coulomb models [19,20], bidimen- sional continuum [21], disturbed state concept [3], bounding surface theory [22,23], and hypoplasticity [24]. Moreover, different 2D inter- face models have been developed for simulating granular soil-structure interfaces under cyclic loading using the theories of DSC [25], generalized plasticity [26,27], and two-surface plasticity [28–34]. These are useful for the understanding of interfaces in many common SSI systems such as axially loaded piles. However, many other SSI problems are subjected to 3D loading conditions involving shear coupling. Earthquake excitations in different directions, wind waves, lateral loads and combined compression/uplift loads and seismic waves can result in 3D loading conditions in SSI systems. Fig. 1 illustrates schematically the 3D condition of loading in the interface of an axially-laterally loaded pile foundation. The interface zone experiences a normal stress σ n due to confnement pressures of surrounding soil, τ axial resulting from axial force (F), and τ lat due to lateral force (P). An effcient design and analysis of this type of SSI systems would require a combined effort in both experimental and numerical studies to provide an understanding of the complex 3D behaviour under monotonic and cyclic loading conditions. Based on laboratory observations on sand-structure interfaces [15] and gravel-structure interfaces [16–18] under 3D loading conditions, the interface zone experiences nonlinear behaviour such as strain hardening/softening, compaction and dilation, cyclic stress degrada- tion, cyclic accumulative contraction, stress path dependency and par- ticle breakage, similar to observations under a 2D loading condition. However, under a 3D loading condition, the interface zone experiences * Corresponding author. ** Corresponding author. E-mail addresses: miad.sabery@gmail.com (M. Saberi), Charles-Darwin.Annan@gci.ulaval.ca (C.-D. Annan). Contents lists available at ScienceDirect Soil Dynamics and Earthquake Engineering journal homepage: http://www.elsevier.com/locate/soildyn https://doi.org/10.1016/j.soildyn.2020.106162 Received 5 January 2020; Received in revised form 24 February 2020; Accepted 26 March 2020