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<!DOCTYPE article
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<article article-type="research-article" dtd-version="1.0" specific-use="sps-1.6" xml:lang="en" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">
	<front>
		<journal-meta>
			<journal-id journal-id-type="publisher-id">dyna</journal-id>
			<journal-title-group>
				<journal-title>DYNA</journal-title>
				<abbrev-journal-title abbrev-type="publisher">Dyna rev.fac.nac.minas</abbrev-journal-title>
			</journal-title-group>
			<issn pub-type="ppub">0012-7353</issn>
			<publisher>
				<publisher-name>Universidad Nacional de Colombia</publisher-name>
			</publisher>
		</journal-meta>
		<article-meta>
			<article-id pub-id-type="doi">10.15446/dyna.v85n204.59676</article-id>
			<article-categories>
				<subj-group subj-group-type="heading">
					<subject>Artículos</subject>
				</subj-group>
			</article-categories>
			<title-group>
				<article-title>Evaluation of coupled porewater pressure and stress-strain constitutive model in granular soils</article-title>
				<trans-title-group xml:lang="es">
					<trans-title>Evaluación de modelos constitutivos esfuerzo-deformación acoplados con presión de poros en suelos granulares</trans-title>
				</trans-title-group>
			</title-group>
			<contrib-group>
				<contrib contrib-type="author">
					<name>
						<surname>Moreno-Torres</surname>
						<given-names>Oscar</given-names>
					</name>
					<xref ref-type="aff" rid="aff1"><sup>a</sup></xref>
				</contrib>
				<contrib contrib-type="author">
					<name>
						<surname>Chang-Nieto</surname>
						<given-names>Gustavo</given-names>
					</name>
					<xref ref-type="aff" rid="aff2"><sup>b</sup></xref>
				</contrib>
				<contrib contrib-type="author">
					<name>
						<surname>Salas-Montoya</surname>
						<given-names>Andrés</given-names>
					</name>
					<xref ref-type="aff" rid="aff3"><sup>c</sup></xref>
				</contrib>
			</contrib-group>
			<aff id="aff1">
				<label>a</label>
				<institution content-type="original"> Civil Engineering Department, Universidad del Magdalena, Santa Marta, Colombia and Civil Engineering Department, Universidad de la Guajira, Riohacha, Colombia. oshemoreno@yahoo.com</institution>
				<institution content-type="normalized">Universidad del Magdalena</institution>
				<institution content-type="orgdiv1">Civil Engineering Department</institution>
				<institution content-type="orgname">Universidad del Magdalena</institution>
				<addr-line>
					<named-content content-type="city">Santa Marta</named-content>
				</addr-line>
				<country country="CO">Colombia</country>
			</aff>
			<aff id="aff2">
				<label>b</label>
				<institution content-type="original"> Civil Engineering Department, Universidad del Magdalena, Santa Marta, Colombia. gchang@engineer.com</institution>
				<institution content-type="normalized">Universidad del Magdalena</institution>
				<institution content-type="orgdiv1">Civil Engineering Department</institution>
				<institution content-type="orgname">Universidad del Magdalena</institution>
				<addr-line>
					<named-content content-type="city">Santa Marta</named-content>
				</addr-line>
				<country country="CO">Colombia</country>
				<email>gchang@engineer.com</email>
			</aff>
			<aff id="aff3">
				<label>c</label>
				<institution content-type="original"> Civil Engineering Department, Universidad Nacional de Colombia, Manizales, Colombia, asalasmo@unal.edu.co</institution>
				<institution content-type="normalized">Universidad Nacional de Colombia</institution>
				<institution content-type="orgdiv1">Civil Engineering Department</institution>
				<institution content-type="orgname">Universidad Nacional de Colombia</institution>
				<addr-line>
					<named-content content-type="city">Manizales</named-content>
				</addr-line>
				<country country="CO">Colombia</country>
				<email>asalasmo@unal.edu.co</email>
			</aff>
			<pub-date pub-type="epub-ppub">
				<season>Jan-Mar</season>
				<year>2018</year>
			</pub-date>
			<volume>85</volume>
			<issue>204</issue>
			<fpage>248</fpage>
			<lpage>256</lpage>
			<history>
				<date date-type="received">
					<day>19</day>
					<month>08</month>
					<year>2016</year>
				</date>
				<date date-type="rev-recd">
					<day>29</day>
					<month>03</month>
					<year>2017</year>
				</date>
				<date date-type="accepted">
					<day>25</day>
					<month>11</month>
					<year>2017</year>
				</date>
			</history>
			<permissions>
				<license license-type="open-access" xlink:href="http://creativecommons.org/licenses/by-nc-nd/4.0/" xml:lang="en">
					<license-p>This is an open-access article distributed under the terms of the Creative Commons Attribution License</license-p>
				</license>
			</permissions>
			<abstract>
				<title>Abstract</title>
				<p>The evaluation of performance of three coupled pressure (PWP) generation models and stress-strain constitutive models are applied to granular soils. Those constitutive models are used to recommend them for subsequent application in seismic site-response analysis in effective stresses. The performance of the three-coupled models were evaluated using a database of 25 selected high quality cyclic simple shear tests. The conducted analysis suggested that the simple Coupled GMP and stress-strain constitutive model reasonably capture PWP behavior observed in the laboratory tests, they are analyzed better than using advanced constitutive models, and all of them can be used to perform effective stress-based one-dimensional site-response analysis.</p>
			</abstract>
			<trans-abstract xml:lang="es">
				<title>Resumen</title>
				<p>En el presente artículo se evaluó el desempeño de tres modelos constitutivos esfuerzo-deformación acoplados con presión de poros a suelos granulares con el objetivo de recomendar su posterior aplicación en el análisis sísmico de respuesta de sitio. El desempeño de los tres modelos acoplados se evaluó utilizando una base de datos de 25 ensayos de corte simple cíclico de alta calidad. Los análisis realizados sugieren que el modelo acoplado de esfuerzo-deformación y GMP captura razonablemente el comportamiento de presión de poros observado en los ensayos de laboratorio de una mejor manera que los modelos constitutivos más avanzados y todos ellos se pueden utilizar para realizar análisis unidimensional de respuesta de sitio considerando esfuerzos efectivos<bold>.</bold></p>
			</trans-abstract>
			<kwd-group xml:lang="en">
				<title>Keywords:</title>
				<kwd>constitutive models</kwd>
				<kwd>granular materials</kwd>
				<kwd>laboratory tests</kwd>
				<kwd>seismic site-response analysis</kwd>
			</kwd-group>
			<kwd-group xml:lang="es">
				<title>Palabras clave:</title>
				<kwd>análisis sísmico de respuesta de sitio</kwd>
				<kwd>ensayos de laboratorio</kwd>
				<kwd>materiales granulares</kwd>
				<kwd>modelos constitutivos</kwd>
			</kwd-group>
			<counts>
				<fig-count count="8"/>
				<table-count count="1"/>
				<equation-count count="14"/>
				<ref-count count="40"/>
				<page-count count="9"/>
			</counts>
		</article-meta>
	</front>
	<body>
		<sec sec-type="intro">
			<title>1. Introduction</title>
			<p>Seismic site-response analysis is routinely used to quantify the influence of surficial soil layers on ground motion propagation. Typically, one-dimensional (1D) frequency-domain (i.e., equivalent linear) or time-domain (i.e., nonlinear) analyses are performed in a total stress framework. However, downhole array and surface acceleration records available at sites that experienced level-ground cyclic liquefaction-for example, 1964 Niigata (Kawagishi-cho apartment buildings; Ishihara and Koga, 1981); 1987 Superstition Hills (Wildlife site [<xref ref-type="bibr" rid="B1">1</xref>])-illustrate that porewater pressure (PWP) generation and concurrent strain-softening can significantly alter the acceleration time histories and response spectra [<xref ref-type="bibr" rid="B2">2</xref>]. When these sites are analyzed using a total-stress, equivalent-linear constitutive formulation, poor agreement generally is observed between predicted and field measured response [<xref ref-type="bibr" rid="B3">3</xref>-<xref ref-type="bibr" rid="B6">6</xref>].</p>
			<p>In order to improve the predicted seismic site-response, researchers have proposed PWP generation models (e.g., Vucetic [<xref ref-type="bibr" rid="B4">4</xref>], Matasovic [<xref ref-type="bibr" rid="B5">5</xref>], Seed et al. [<xref ref-type="bibr" rid="B7">7</xref>], Martin et al. [<xref ref-type="bibr" rid="B8">8</xref>], Dobry et al. [<xref ref-type="bibr" rid="B9">9</xref>,<xref ref-type="bibr" rid="B10">10</xref>], Green et al. [<xref ref-type="bibr" rid="B11">11</xref>], Green [<xref ref-type="bibr" rid="B12">12</xref>], Polito et al. [<xref ref-type="bibr" rid="B13">13</xref>], Ivsic [<xref ref-type="bibr" rid="B14">14</xref>]) and effective stress-based soil constitutive models (e.g., Wang et al. [<xref ref-type="bibr" rid="B15">15</xref>], Elgamal et al. [<xref ref-type="bibr" rid="B16">16</xref>], Park and Byrne [<xref ref-type="bibr" rid="B17">17</xref>], Jefferies and Been [<xref ref-type="bibr" rid="B18">18</xref>], Boulanger and Ziotopoulou [<xref ref-type="bibr" rid="B19">19</xref>]) that can be used for seismic site-response analysis. For example, Booker et al. [20] first proposed an effective stress-based model (named GADFLEA) to study the generation and dissipation of PWP.</p>
			<p>In this context, the objective of this study is to evaluate the performance of three coupled PWP and stress-strain constitutive models, used in granular soils, and further recommend them for subsequent application in site-response analysis in effective stresses. For this evaluation, the authors collected 12 bins cyclic simple shear tests (25 individual tests) [<xref ref-type="bibr" rid="B21">21</xref>] available in the referred literature. To facilitate comparisons, the laboratory tests were binned based on similar soil response. The PWP coupled stress-strain constitutive models were then evaluated using the laboratory test data, and statistics (i.e., residuals) were used to identify the most effective model.</p>
			<p>The three PWP coupled stress-strain constitutive models evaluated corresponded to the:</p>
			<p>
				<list list-type="bullet">
					<list-item>
						<p>Green et al. [<xref ref-type="bibr" rid="B11">11</xref>]/Green [<xref ref-type="bibr" rid="B12">12</xref>]/Polito et al. [<xref ref-type="bibr" rid="B13">13</xref>] model, termed the GMP model coupled with hyperbolic stress-strain model, </p>
					</list-item>
					<list-item>
						<p>Elgamal model [<xref ref-type="bibr" rid="B16">16</xref>], and</p>
					</list-item>
					<list-item>
						<p>PM4SAND model [<xref ref-type="bibr" rid="B19">19</xref>]. </p>
					</list-item>
				</list>
			</p>
		</sec>
		<sec sec-type="methods">
			<title>2. Methodology</title>
			<p>In this paper, the authors collected 12 bins cyclic simple shear tests (25 individual tests) [<xref ref-type="bibr" rid="B21">21</xref>] available in the referred literature. Those tests were reproduced using three PWP coupled stress-strain constitutive models and the framework of the models used in here is presented next. The results of the numerical simulation were compared with the data obtained in the Laboratory tests. The comparisons with the laboratory tests were evaluated using statistics (i.e., residuals) to identify the most effective model.</p>
			<sec>
				<title>2.1. Coupled porewater pressure (PWP) and stress-strain constitutive models</title>
				<p>In response to undrained cyclic loading, saturated soils generate excess PWP, which can be separated into transient and residual components. The transient excess PWP results from changes in the mean normal stress during cyclic loading [<xref ref-type="bibr" rid="B22">22</xref>,<xref ref-type="bibr" rid="B23">23</xref>] and thus do not affect the soil effective stress. The residual excess PWP develops from the tendency for volume decrease during undrained cyclic loading, and therefore result in a decrease in effective stress and stiffness. This residual excess PWP can be defined as the PWP in excess of hydrostatic conditions when the cyclic stress is zero [<xref ref-type="bibr" rid="B9">9</xref>,<xref ref-type="bibr" rid="B11">11</xref>]. Most PWP generation models were developed to represent residual excess PWP and they can be incorporated in 1D site-response analysis studies to account for changes in effective stress and stiffness related to PWP generation. A brief description of coupled PWP and stress-strain constitutive models is subsequently presented.</p>
				<sec>
					<title>2.1.1. GMP model coupled with hyperbolic stress-strain model</title>
					<p>Building on the work by Hardin and Drnevich [<xref ref-type="bibr" rid="B24">24</xref>], Matasovic [<xref ref-type="bibr" rid="B9">9</xref>] proposed two degradation indices that introduce excess PWP-induced material softening into a simplified hyperbolic constitutive model. The modulus degradation index (δ<sub>G</sub>) and stress degradation index (δ<sub>τ</sub>) reduce the shear stress mobilized during the loading-unloading process as a result of PWP increase [<xref ref-type="bibr" rid="B9">9</xref>], and are defined as: </p>
					<p>
						<disp-formula id="e1">
							<graphic xlink:href="0012-7353-dyna-85-204-00248-e1.png"/>
						</disp-formula>
					</p>
					<p>where r<sub>u</sub> = excess PWP/σ'<sub>vo</sub> or Δu/σ'<sub>vo</sub>; and ϑ = dimensionless exponent generally equal to 3.5 [<xref ref-type="bibr" rid="B9">9</xref>] based on matching the stress-strain hysteresis loops over a wide range of r<sub>u</sub>-values for Santa Monica Beach sand, Wildlife Site sands A and B, Heber Road point bar (PB) and channel fill (CF) sands. The advantage of the degradation indices is that they can use r<sub>u</sub> values defined by any PWP generation model. </p>
					<p>The modified hyperbolic model presented by Phillips and Hashash [<xref ref-type="bibr" rid="B25">25</xref>] was further modified to incorporate the degradation indices previously defined. Moreno-Torres et al. [<xref ref-type="bibr" rid="B26">26</xref>] proposed the following equations to compute shear stresses (τ) during loading and unloading-reloading, respectively, corresponding to a given strain.</p>
					<p>Loading:</p>
					<p>
						<disp-formula id="e2">
							<graphic xlink:href="0012-7353-dyna-85-204-00248-e2.jpg"/>
						</disp-formula>
					</p>
					<p>Unloading - Reloading:</p>
					<p>
						<disp-formula id="e3">
							<graphic xlink:href="0012-7353-dyna-85-204-00248-e3.jpg"/>
						</disp-formula>
					</p>
					<p>where γ<sub>c</sub> = given shear strain; γ<sub>rev</sub> = reversal shear strain; τ<sub>rev</sub> = reversal shear stress; γ<sub>m</sub> = maximum shear strain; γ<sub>r</sub> = reference shear strain; t = dimensionless exponent; β<sup>'</sup> = dimensionless factor; δ<sub>G</sub> = modulus degradation index; δ<sub>τ</sub> = stress degradation index; F(γ<sub>m</sub>) = reduction factor; and G<sub>0</sub> = initial shear modulus.</p>
					<p>Based on the energy-based models proposed by Green et al. [<xref ref-type="bibr" rid="B11">11</xref>], Green [<xref ref-type="bibr" rid="B12">12</xref>], Polito et al. [<xref ref-type="bibr" rid="B13">13</xref>], Davis and Berrill [<xref ref-type="bibr" rid="B27">27</xref>], [<xref ref-type="bibr" rid="B28">28</xref>] and Berrill and Davis [<xref ref-type="bibr" rid="B29">29</xref>], proposed the following empirical expression, termed the GMP model:</p>
					<p>
						<disp-formula id="e4">
							<graphic xlink:href="0012-7353-dyna-85-204-00248-e4.png"/>
						</disp-formula>
					</p>
					<p>where W<sub>s</sub> = energy dissipated per unit volume of soil divided by the initial mean consolidation stress (i.e., normalized unit energy), and PEC = a calibration parameter termed the pseudoenergy capacity. The GMP model was developed using results from stress-controlled, undrained cyTxC tests performed on nonplastic silt-sand mixtures that ranged from clean sands to pure silts. For each test, W<sub>s</sub> can be computed as:</p>
					<p>
						<disp-formula id="e5">
							<graphic xlink:href="0012-7353-dyna-85-204-00248-e5.jpg"/>
						</disp-formula>
					</p>
					<p>where n = number of load increments imposed during cyclic loading on the stress-strain curve until r<sub>u</sub> = 1 [<xref ref-type="bibr" rid="B11">11</xref>]; (<sub>i</sub> and (<sub>i+1</sub> = applied shear stress at load increment i and i+1, respectively; and (<sub>i</sub> and (<sub>i+1</sub> = shear strain at load increment i and i+1, respectively. Simply stated, <xref ref-type="disp-formula" rid="e4">Eq. (4)</xref> employs the trapezoidal rule to compute the area bounded by the hysteretic stress-strain loop.</p>
					<p>For the GMP model, It is used the model parameter correlations reported by Polito et al. [<xref ref-type="bibr" rid="B13">13</xref>], which are defined as:</p>
					<p>
						<disp-formula id="e6">
							<graphic xlink:href="0012-7353-dyna-85-204-00248-e6.jpg"/>
						</disp-formula>
					</p>
					<p>where PEC = pseudoenergy capacity; D<sub>r</sub> = Relative Density; and FC = fines content.</p>
				</sec>
				<sec>
					<title>2.1.2. Elgamal model</title>
					<p>The numerical framework of this model uses two-phase (fluid and solid) fully-coupled FE (Finite Element) formulation [<xref ref-type="bibr" rid="B30">30</xref>,<xref ref-type="bibr" rid="B31">31</xref>] based on Biot’s theory [<xref ref-type="bibr" rid="B32">32</xref>]. The saturated soil system is analyzed such as two-phase material. The <italic>u-p</italic> formulation (displacement of the soil skeleton, <italic>u</italic>, and pore pressure <italic>p</italic> are the primary unknowns) is a simplified numerical framework of this theory [33]. This model was implemented for the simulation of two-dimensional (2D) and 3D response scenarios [<xref ref-type="bibr" rid="B30">30</xref>,<xref ref-type="bibr" rid="B31">31</xref>,<xref ref-type="bibr" rid="B34">34</xref>,<xref ref-type="bibr" rid="B35">35</xref>]. </p>
					<p>The <italic>u-p</italic> formulation is defined by an equation of motion for the solid-fluid mixture, and by an equation of mass conservation for the fluid phase that incorporates equation of motion for the fluid phase and Darcy's law [<xref ref-type="bibr" rid="B33">33</xref>]. The two governing equations are presented in the following finite element matrix form [<xref ref-type="bibr" rid="B33">33</xref>]:</p>
					<p>
						<disp-formula id="e7">
							<graphic xlink:href="0012-7353-dyna-85-204-00248-e7.jpg"/>
						</disp-formula>
					</p>
					<p>where [M] is the total mass matrix, {Ü} is the second derivative of displacement vector, [B]<sup>T</sup> the strain-displacement transposed matrix, {σ<sup>'</sup>} the effective stress tensor, Ω is the body volume, dΩ is derivative of the body volume, Q is the discrete gradient operator coupling the solid and fluid phases, [Q]<sup>T</sup> is the discrete transposed gradient operator coupling the solid and fluid phases, {p} is the pore pressure vector, [S] is the compressibility matrix, {0} is a cero vector, and [H] is the permeability matrix. The vectors {f <sup>s</sup>} and {f <sup>p</sup>} represent the effects of body forces and prescribed boundary conditions for the solid-fluid mixture and the fluid phase, respectively. Those Equations are integrated in the time domain using a single-step predictor multi-corrector scheme of the Newmark type [<xref ref-type="bibr" rid="B30">30</xref>,<xref ref-type="bibr" rid="B33">33</xref>].</p>
					<p>For cyclic and seismic loading scenarios, multi-surface plasticity pressure independent (Von-Mises) and pressure dependent models are able to reproduce the cyclic shear stress-strain response characteristics of the soil conditions. A soil constitutive model pressure-dependent for frictional soils [<xref ref-type="bibr" rid="B30">30</xref>,<xref ref-type="bibr" rid="B31">31</xref>,<xref ref-type="bibr" rid="B34">34</xref>] was developed based on the Prevost [<xref ref-type="bibr" rid="B36">36</xref>] original framework. The focus of the formulation was placed on simulating the liquefaction-induced shear strain accumulation mechanism in clean medium-dense sands [<xref ref-type="bibr" rid="B31">31</xref>,<xref ref-type="bibr" rid="B37">37</xref>]. Special attention was given to the deviatoric-volumetric strain coupling (dilatancy) under cyclic loading, because this causes increased shear stiffness and strength at large cyclic shear strain excursions (i.e., cyclic mobility). The main modeling parameters [<xref ref-type="bibr" rid="B38">38</xref>] include standard dynamic soil properties such as low-strain shear modulus and friction angle, as well as parameters to control the dilatancy effects (phase transformation angle, contraction, and dilation), and the level of liquefaction-induced yield strain (γ<sub>y</sub>).</p>
				</sec>
				<sec>
					<title>2.1.3. PM4SAND model</title>
					<p>The sand plasticity model presented follows the basic framework of the stress-ratio controlled, critical state compatible, bounding-surface plasticity model for sand presented by Dafalias and Manzari [<xref ref-type="bibr" rid="B39">39</xref>]. The Dafalias and Manzari [<xref ref-type="bibr" rid="B39">39</xref>] model extended the previous work by Manzari and Dafalias [<xref ref-type="bibr" rid="B40">40</xref>] by adding a fabric-dilatancy related tensor quantity to account for the effect of fabric changes during loading. The fabric-dilatancy related tensor was used to macroscopically model the effect that microscopically observed changes in sand fabric during plastic dilation have on the contractive response upon reversal of loading direction.</p>
					<p><bold>
 <italic>Basic stress and strain terms</italic>
</bold> </p>
					<p>The basic stress and strain terms for the model are described. The basic model is supported on effective stresses and the conventional prime symbol is skipped from the stress terms for convenience. All stresses are effective for the model considered. The stresses are represented by the tensor σ, the principal effective stresses σ<sub>1</sub>, σ<sub>2</sub>, and σ<sub>3</sub>, the mean effective stress p, the deviatoric stress tensor (s), and the deviatoric stress ratio tensor (r). The implementation was simplified by forming various equations and relationships in terms of the in-plane stresses only. This procedure limits the implementation to plane-strain applications and is not correct for general cases. This has the advantage of simplifying the implementation and improving computational speed by reducing the number of operations. Expanding the implementation to include the general case should not affect the general features of the model. Consequently, the relationships between the various stress terms can be summarized next: </p>
					<p>
						<disp-formula id="e8">
							<graphic xlink:href="0012-7353-dyna-85-204-00248-e8.jpg"/>
						</disp-formula>
					</p>
					<p>
						<disp-formula id="e9">
							<graphic xlink:href="0012-7353-dyna-85-204-00248-e9.png"/>
						</disp-formula>
					</p>
					<p>
						<disp-formula id="e10">
							<graphic xlink:href="0012-7353-dyna-85-204-00248-e10.jpg"/>
						</disp-formula>
					</p>
					<p>
						<disp-formula id="e11">
							<graphic xlink:href="0012-7353-dyna-85-204-00248-e11.png"/>
						</disp-formula>
					</p>
					<p>where σ<sub>xx</sub> is the stress in the plane x, σ<sub>yy</sub> is the stress in the plane y, σ<sub>xy</sub> is the shear stress in the plane xy and I is the unitary matrix.</p>
					<p>The strain model is represented by a tensor (ε) (strains) that can be separated into the volumetric strain (ε<sub>v</sub>) and the deviatoric strain tensor (e). The volumetric strain is represented as:</p>
					<p>
						<disp-formula id="e12">
							<graphic xlink:href="0012-7353-dyna-85-204-00248-e12.png"/>
						</disp-formula>
					</p>
					<p>where ε<sub>xx</sub> is the strain in the plane x, ε<sub>yy</sub> is the strains in the plane y and the deviatoric strain tensor (e) is represented by: </p>
					<p>
						<disp-formula id="e13">
							<graphic xlink:href="0012-7353-dyna-85-204-00248-e13.jpg"/>
						</disp-formula>
					</p>
					<p>In incremental form, the deviatoric (de) and volumetric strain (dε<sub>v</sub>) terms are decomposed into an elastic and a plastic part as they are presented next. </p>
					<p>
						<disp-formula id="e14">
							<graphic xlink:href="0012-7353-dyna-85-204-00248-e14.png"/>
						</disp-formula>
					</p>
					<p>where de<sup>el</sup> is the elastic deviatoric strain increment tensor, de<sup>pl</sup> is the plastic deviatoric strain increment tensor, dε<sub>v</sub>
 <sup>el</sup> is the elastic volumetric strain increment and dε<sub>v</sub>
 <sup>pl</sup> is the plastic volumetric strain increment.</p>
				</sec>
			</sec>
		</sec>
		<sec sec-type="results">
			<title>3. Results</title>
			<p><bold>3.1. Prediction of PWP using coupled PWP and stress-strain constitutive models</bold></p>
			<p>The authors used 12 cySS bins for the prediction exercise (<xref ref-type="table" rid="t1">Table 1</xref>). Model parameters for each of the models were determined using recommendation and formulation for that purpose. Figs. 1 - 3 compare the PWP time histories computed using the GMP model coupled with hyperbolic stress-strain, Elgamal and PM4SAND models to the measured responses of cySS tests in bins S3B, S6B, and S8B, respectively. In general, the GMP model coupled with hyperbolic stress-strain reasonably predict PWP generation in loose to medium-dense specimens (<xref ref-type="fig" rid="f1">Fig. 1 (a) </xref> and <xref ref-type="fig" rid="f1">(b)</xref>, respectively). As expected, the model more poorly predict PWP generation in dense specimens (<xref ref-type="fig" rid="f1">Fig. 1 (c) </xref>). The Elgamal and PM4SAND models fairly predict PWP generation in loose to dense specimens (<xref ref-type="fig" rid="f2">Figs. 2</xref> through <xref ref-type="fig" rid="f3">3</xref>, respectively) for different CSR magnitudes (i.e., different FSliq).</p>
			<p>
				<table-wrap id="t1">
					<graphic xlink:href="0012-7353-dyna-85-204-00248-gt1.jpg"/>
					<table-wrap-foot>
						<fn id="TFN1">
							<p><bold>Table 1.</bold> Summary of cySS tests used in prediction phase of the models.</p>
						</fn>
						<fn id="TFN2">
							<p><bold>Source:</bold> The authors.</p>
						</fn>
					</table-wrap-foot>
				</table-wrap>
			</p>
			<p>
				<fig id="f1">
					<label>Figure 1</label>
					<caption>
						<title>Comparison of calculated GMP model coupled with hyperbolic stress-strain constitutive model and measured PWP ratios for cySS tests: (a) loose specimens; (b) medium-dense specimens; and (c) dense specimens. (Note the differences in the time scales for the various plots.)</title>
					</caption>
					<graphic xlink:href="0012-7353-dyna-85-204-00248-gf1.jpg"/>
					<attrib><bold>Source:</bold> The authors.</attrib>
				</fig>
			</p>
			<p>
				<fig id="f2">
					<label>Figure 2</label>
					<caption>
						<title>Comparison of calculated Elgamal model and measured PWP ratios for cySS tests: (a) loose specimens; (b) medium-dense specimens; and (c) dense specimens. (Note the differences in the time scales for the various plots.)</title>
					</caption>
					<graphic xlink:href="0012-7353-dyna-85-204-00248-gf2.jpg"/>
					<attrib><bold>Source:</bold> The authors.</attrib>
				</fig>
			</p>
			<p>
				<fig id="f3">
					<label>Figure 3</label>
					<caption>
						<title>Comparison of calculated PM4SAND model and measured PWP ratios for cySS tests: (a) loose specimens; (b) medium-dense specimens; and (c) dense specimens. (Note the differences in the time scales for the various plots.)</title>
					</caption>
					<graphic xlink:href="0012-7353-dyna-85-204-00248-gf3.jpg"/>
					<attrib><bold>Source:</bold> The authors.</attrib>
				</fig>
			</p>
			<p>
				<xref ref-type="fig" rid="f4">Fig. 4</xref> presents the residuals for measured and predicted r<sub>u</sub> values (at each cycle) for the 12 cySS bins (25 individual cySS tests) in the prediction dataset. These data illustrate that the coupled GMP with stress-strain constitutive model yields small residuals (±0.25), but contain some bias. The Elgamal and PM4SAND models yield medium residuals (±0.5) with high bias concentrated in the underprediction side for Elgamal model and PM4SAND presents bias in the under and overprediction sides. However, all models qualitatively capture initial liquefaction.</p>
			<p>
				<fig id="f4">
					<label>Figure 4</label>
					<caption>
						<title>Residuals computed from r<sub>u</sub> time histories for tests considered using: (a) GMP model coupled with hyperbolic stress-strain constitutive model; (b) Elgamal model and (c) PM4SAND model.</title>
					</caption>
					<graphic xlink:href="0012-7353-dyna-85-204-00248-gf4.jpg"/>
					<attrib><bold>Source:</bold> The authors.</attrib>
				</fig>
			</p>
			<p><bold>3.2. Evaluation of predicted stress-strain behavior using coupled PWP and stress-strain constitutive models</bold></p>
			<p>The authors used the same 12 cySS bins considered for the PWP prediction exercise to evaluate the stress-strain behavior of the models. Model residuals were calculated using computed and measured shear stresses when the measured shear strain, γ<sub>c</sub> = 0%.</p>
			<p>
				<xref ref-type="fig" rid="f5">Figs. 5</xref> through <xref ref-type="fig" rid="f7">7</xref> compare the stress-strain and excess r<sub>u</sub>-strain behavior computed using the GMP model coupled with hyperbolic stress-strain, Elgamal and PM4SAND models to the measured responses of cySS tests in bins S3B, S6B, and S8B, respectively. In general, the three models reasonably predict stress-strain and r<sub>u</sub>-strain behavior in loose to dense specimens for strains less that γ<sub>c</sub> = 5%. As expected, the coupled GMP model with stress-strain constitutive model more poorly predict stress-strain and r<sub>u</sub>-strain behavior when dilation becomes more pronounced (γ<sub>c</sub> &gt; 5% and r<sub>u</sub> &gt; 0.65) and significant modulus degradation occurs. Instead, Elgamal and PM4SAND can capture the initial part of dilation spikes and the degradation of the modulus in a good manner when γ<sub>c</sub> &gt; 5% and r<sub>u</sub> &gt; 0.65, but the both codes are not able to support more than a couple of cycles after r<sub>u</sub> &gt; 0.98 and the codes stop the calculation.</p>
			<p>
				<fig id="f5">
					<label>Figure 5</label>
					<caption>
						<title>Comparison of calculated GMP coupled with hyperbolic stress-strain, Elgamal and PM4SAND models and measured PWP ratios for cySS test loose specimens.</title>
					</caption>
					<graphic xlink:href="0012-7353-dyna-85-204-00248-gf5.jpg"/>
					<attrib><bold>Source:</bold> The authors.</attrib>
				</fig>
			</p>
			<p>
				<fig id="f6">
					<label>Figure 6</label>
					<caption>
						<title>Comparison of calculated GMP coupled with hyperbolic stress-strain, Elgamal and PM4SAND models and measured PWP ratios for cySS test medium-dense specimens.</title>
					</caption>
					<graphic xlink:href="0012-7353-dyna-85-204-00248-gf6.jpg"/>
					<attrib><bold>Source:</bold> The authors.</attrib>
				</fig>
			</p>
			<p>
				<fig id="f7">
					<label>Figure 7</label>
					<caption>
						<title>Comparison of calculated GMP coupled with hyperbolic stress-strain, Elgamal and PM4SAND models and measured PWP ratios for cySS test dense specimens.</title>
					</caption>
					<graphic xlink:href="0012-7353-dyna-85-204-00248-gf7.jpg"/>
					<attrib><bold>Source:</bold> The authors.</attrib>
				</fig>
			</p>
			<p>
				<xref ref-type="fig" rid="f5">Figs. 5</xref> through <xref ref-type="fig" rid="f7">7</xref> compare the stress-strain and excess r<sub>u</sub>-strain behavior computed using the GMP model coupled with hyperbolic stress-strain, Elgamal and PM4SAND models to the measured responses of cySS tests in bins S3B, S6B, and S8B, respectively. In general, the three models reasonably predict stress-strain and r<sub>u</sub>-strain behavior in loose to dense specimens for strains less that γ<sub>c</sub> = 5%. As expected, the coupled GMP model with stress-strain constitutive model more poorly predict stress-strain and r<sub>u</sub>-strain behavior when dilation becomes more pronounced (γ<sub>c</sub> &gt; 5% and r<sub>u</sub> &gt; 0.65) and significant modulus degradation occurs. Instead, Elgamal and PM4SAND can capture the initial part of dilation spikes and the degradation of the modulus in a good manner when γ<sub>c</sub> &gt; 5% and r<sub>u</sub> &gt; 0.65, but the both codes are not able to support more than a couple of cycles after r<sub>u</sub> &gt; 0.98 and the codes stop the calculation.</p>
			<p>
				<xref ref-type="fig" rid="f8">Fig. 8</xref> presents the residuals determined for measured and predicted τ/σ'<sub>vo</sub> values (CSRs) at each cycle for the 12 cySS bins (25 individual cySS tests) in the prediction dataset. These data illustrate that the coupled GMP with stress-strain constitutive model and PM4SAND yield relatively low residuals (&lt; ±0.25) for r<sub>u</sub> &lt; 0.65 although the models generally overestimate the values of r<sub>u</sub>. For high r<sub>u</sub>-values (r<sub>u</sub> &gt; 0.65), the residual increases (±0.35), although the bias decreases for both models. Instead, Elgamal model yields very low residuals (&lt; ±0.05) through all r<sub>u</sub>-values, and no bias is presented with means that this model can reproduce the strain-stress behavior in a good shape.</p>
			<p>
				<fig id="f8">
					<label>Figure 8</label>
					<caption>
						<title>Residuals computed from measured and computed CSR values (τ/σ'<sub>vo</sub>) for tests considered using: (a) GMP model coupled with hyperbolic stress-strain constitutive model; (b) Elgamal model and (c) PM4SAND model.</title>
					</caption>
					<graphic xlink:href="0012-7353-dyna-85-204-00248-gf8.jpg"/>
					<attrib><bold>Source:</bold> The authors.</attrib>
				</fig>
			</p>
		</sec>
		<sec sec-type="conclusions">
			<title>4. Conclusion</title>
			<p>In this paper, three coupled PWP and stress-strain constitutive models (termed GMP, Elgamal and PM4SAND) are described. Those constitutive model uses different approach to incorporate porewater pressures (PWP) predicted by PWP generation models available in the literature. The evaluation process consisted of a prediction phase, the coupled GMP and stress-strain constitutive model exhibited relatively low residuals compared with Elgamal and PM4SAND model (more advanced constitutive models), but the bias in the coupled GMP and stress-strain constitutive model is more reduced than the other models considered here.</p>
			<p>Given the ability of the coupled GMP and stress-strain constitutive model to reasonably predict PWP generation in the prediction phase over the widest range of r<sub>u</sub> and over all values of D<sub>r</sub>, it was expected a reasonable stress-strain prediction. The modified hyperbolic constitutive model coupled with the GMP model qualitatively captures the stress-strain behavior for r<sub>u</sub> ≤ 0.65, but the inability of the hyperbolic constitutive model to reproduce dilation at r<sub>u</sub> values exceeding approximately 0.65 reduces the accuracy of the coupled GMP and stress-strain model compared with the Elgamal and PM4SAND models can reproduce the dilation process in better way. The advanced models predict PWP fairly reasonably until initial liquefaction with bias more pronounced. Even, the found limitations that were exposed, all the models can be used in site response analysis with good results in reproduction of the soil response in an earthquake event when liquefaction is an issue.</p>
		</sec>
		<sec>
			<title>5. List of Symbols</title>
			<p>
				<inline-graphic xlink:href="0012-7353-dyna-85-204-00248-i024.jpg"/>
			</p>
		</sec>
	</body>
	<back>
		<ack>
			<title>Acknowledgments</title>
			<p>The authors want to thank Dr. Jiaer Wu for sharing his cySS tests with the authors. The authors also thank the Universidad del Magdalena, Universidad de la Guajira and Universidad Nacional de Colombia Sede Manizales for providing the conditions required by authors to further explore and analyze the soils database to be able to complete this study.</p>
		</ack>
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