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Accounting for partial material factors in numerical analysis / D. M. Potts in Géotechnique, Vol. 62 N° 12 (Décembre 2012) 

Public ISBD [article]  in Géotechnique > Vol. 62 N° 12 (Décembre 2012) . - pp. 1053 –1065 Titre : Accounting for partial material factors in numerical analysis Type de document : texte imprimé Auteurs : D. M. Potts, Auteur ; L. Zdravkovic, Auteur Année de publication : 2012 Article en page(s) : pp. 1053 –1065 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Limit  state  design/analysis  Bearing  capacity  Numerical  modelling  Constitutive  relations Résumé : The concept of a safety factor in the design of geotechnical structures has traditionally been developed within the framework of classical soil mechanics, where the analysis methods for its calculation involve simple limit equilibrium or limit analysis approaches. Therefore the inclusion of a safety factor within an advanced analysis method, such as finite elements or finite differences, is a more complex issue. In particular, the problem arises with design codes, such as Eurocode 7, in which partial factors on soil strength (or partial material factors) must be accounted for. Eurocode 7 implies that a numerical analysis should be performed accounting for a characteristic strength, which is reduced by partial factors. There are two ways in which such partial factors can be included in numerical analysis: one in which the strength is reduced at the beginning of the analysis, and the other in which this is done during the analysis. Eurocode 7 gives no guidance as to which one of these two approaches is more appropriate to apply. More importantly, there is no guidance on the appropriate numerical procedure that should be implemented in any software in order to perform the required strength reduction during the analysis in the latter approach. Therefore different software programs account for this in different ways, and mostly only for simple constitutive models. This paper presents, first, a consistent methodology for accounting for partial material factors in finite-element analysis, which can be applied to any constitutive model. It then demonstrates the implications of the two ways the partial material factors can be introduced in any analysis, using the example of a bearing capacity problem and employing constitutive models of increasing complexity. The paper shows that the two approaches for accounting for partial material factors may lead to different results, and that it is therefore necessary to develop a rational set of guidelines for their inclusion in advanced numerical analysis. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.11.P.057 [article] Accounting for partial material factors in numerical analysis [texte imprimé] / D. M. Potts, Auteur ; L. Zdravkovic, Auteur . - 2012 . - pp. 1053 –1065. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 62 N° 12 (Décembre 2012) . - pp. 1053 –1065 Mots-clés : Limit  state  design/analysis  Bearing  capacity  Numerical  modelling  Constitutive  relations Résumé : The concept of a safety factor in the design of geotechnical structures has traditionally been developed within the framework of classical soil mechanics, where the analysis methods for its calculation involve simple limit equilibrium or limit analysis approaches. Therefore the inclusion of a safety factor within an advanced analysis method, such as finite elements or finite differences, is a more complex issue. In particular, the problem arises with design codes, such as Eurocode 7, in which partial factors on soil strength (or partial material factors) must be accounted for. Eurocode 7 implies that a numerical analysis should be performed accounting for a characteristic strength, which is reduced by partial factors. There are two ways in which such partial factors can be included in numerical analysis: one in which the strength is reduced at the beginning of the analysis, and the other in which this is done during the analysis. Eurocode 7 gives no guidance as to which one of these two approaches is more appropriate to apply. More importantly, there is no guidance on the appropriate numerical procedure that should be implemented in any software in order to perform the required strength reduction during the analysis in the latter approach. Therefore different software programs account for this in different ways, and mostly only for simple constitutive models. This paper presents, first, a consistent methodology for accounting for partial material factors in finite-element analysis, which can be applied to any constitutive model. It then demonstrates the implications of the two ways the partial material factors can be introduced in any analysis, using the example of a bearing capacity problem and employing constitutive models of increasing complexity. The paper shows that the two approaches for accounting for partial material factors may lead to different results, and that it is therefore necessary to develop a rational set of guidelines for their inclusion in advanced numerical analysis. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.11.P.057

Evolution of microstructure in compacted London Clay during wetting and loading / R. Monroy in Géotechnique, Vol. 60 N° 2 (Fevrier 2010) 

Public ISBD [article]  in Géotechnique > Vol. 60 N° 2 (Fevrier 2010) . - pp. 105–119 Titre : Evolution of microstructure in compacted London Clay during wetting and loading Type de document : texte imprimé Auteurs : R. Monroy, Auteur ; L. Zdravkovic, Auteur ; A. Ridley, Auteur Année de publication : 2011 Article en page(s) : pp. 105–119 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Compaction  Partial  saturation  Laboratory  tests  Microscopy Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : The influence of fabric on mechanical behaviour is explicitly considered in some of the current constitutive models for unsaturated soils. These are based on assumptions regarding the interaction between different levels of structure, which still require experimental validation. A study has been carried out to investigate the evolution of fabric in a compacted natural clay during wetting and loading, and the results are presented in this paper. Samples of London Clay were statically compacted to the same initial conditions, dry of optimum moisture content in a Proctor plot, and subsequently taken along complex stress paths involving wetting, loading, or a combination of both. Mercury intrusion porosimetry (MIP) and environmental scanning electron microscopy (ESEM) microstructure investigations were carried out to observe and quantify the change in fabric associated with each path. The soil fabric was observed to change from an aggregate to a matrix type structure along all wetting paths. This transition was found to take place only when the suction was reduced to a value close to zero kPa. Results also showed that fabric changes during yielding were stress path-dependent. It was not possible to find a correspondence between the volume of free porosity and the volume of intra-aggregate pores, as suggested by some authors. Finally, the volume of pore water was found to agree closely with the volume of intra-aggregate pores, providing support to the assumption that in an unsaturated aggregate microstructure the clay aggregations are saturated. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.8.p.125 [article] Evolution of microstructure in compacted London Clay during wetting and loading [texte imprimé] / R. Monroy, Auteur ; L. Zdravkovic, Auteur ; A. Ridley, Auteur . - 2011 . - pp. 105–119. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 60 N° 2 (Fevrier 2010) . - pp. 105–119 Mots-clés : Compaction  Partial  saturation  Laboratory  tests  Microscopy Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : The influence of fabric on mechanical behaviour is explicitly considered in some of the current constitutive models for unsaturated soils. These are based on assumptions regarding the interaction between different levels of structure, which still require experimental validation. A study has been carried out to investigate the evolution of fabric in a compacted natural clay during wetting and loading, and the results are presented in this paper. Samples of London Clay were statically compacted to the same initial conditions, dry of optimum moisture content in a Proctor plot, and subsequently taken along complex stress paths involving wetting, loading, or a combination of both. Mercury intrusion porosimetry (MIP) and environmental scanning electron microscopy (ESEM) microstructure investigations were carried out to observe and quantify the change in fabric associated with each path. The soil fabric was observed to change from an aggregate to a matrix type structure along all wetting paths. This transition was found to take place only when the suction was reduced to a value close to zero kPa. Results also showed that fabric changes during yielding were stress path-dependent. It was not possible to find a correspondence between the volume of free porosity and the volume of intra-aggregate pores, as suggested by some authors. Finally, the volume of pore water was found to agree closely with the volume of intra-aggregate pores, providing support to the assumption that in an unsaturated aggregate microstructure the clay aggregations are saturated. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.8.p.125

Implications of the definition of the Φ function in elastic-viscoplastic models / T. M. Bodas Freitas in Géotechnique, Vol. 62 N° 7 (Juillet 2012) 

Public ISBD [article]  in Géotechnique > Vol. 62 N° 7 (Juillet 2012) . - pp. 643 –648 Titre : Implications of the definition of the Φ function in elastic-viscoplastic models Type de document : texte imprimé Auteurs : T. M. Bodas Freitas, Auteur ; D. M. Potts, Auteur ; L. Zdravkovic, Auteur Année de publication : 2012 Article en page(s) : pp. 643 –648 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Time  dependence  Creep  Constitutive  relations Résumé : A discrepancy has been identified in existing overstress theory based elastic-viscoplastic models regarding the definition of the Φ function, which determines the magnitude of the viscoplastic strain increment. Based on existing experimental data, some authors define the function Φ assuming that any given loading surface is a locus of constant volumetric viscoplastic strain rate. Others, instead, assume that a loading surface is a locus of constant Φ. This paper examines the implications of using either of the above approaches. It is shown that the two approaches yield very similar predictions for stress states far from failure (for stress levels up to approximately 0·6), however, critical-state conditions are reproduced only when the current loading surface is assumed to be a locus of constant Φ. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.10.P.053 [article] Implications of the definition of the Φ function in elastic-viscoplastic models [texte imprimé] / T. M. Bodas Freitas, Auteur ; D. M. Potts, Auteur ; L. Zdravkovic, Auteur . - 2012 . - pp. 643 –648. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 62 N° 7 (Juillet 2012) . - pp. 643 –648 Mots-clés : Time  dependence  Creep  Constitutive  relations Résumé : A discrepancy has been identified in existing overstress theory based elastic-viscoplastic models regarding the definition of the Φ function, which determines the magnitude of the viscoplastic strain increment. Based on existing experimental data, some authors define the function Φ assuming that any given loading surface is a locus of constant volumetric viscoplastic strain rate. Others, instead, assume that a loading surface is a locus of constant Φ. This paper examines the implications of using either of the above approaches. It is shown that the two approaches yield very similar predictions for stress states far from failure (for stress levels up to approximately 0·6), however, critical-state conditions are reproduced only when the current loading surface is assumed to be a locus of constant Φ. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.10.P.053

On the relative merits of simple and advanced constitutive models in dynamic analysis of tunnels / S. Kontoe in Géotechnique, Vol. 61 N° 10 (Octobre 2010) 

Public ISBD [article]  in Géotechnique > Vol. 61 N° 10 (Octobre 2010) . - pp. 815-829 Titre : On the relative merits of simple and advanced constitutive models in dynamic analysis of tunnels Type de document : texte imprimé Auteurs : S. Kontoe, Auteur ; L. Zdravkovic, Auteur ; D. M. Potts, Auteur Année de publication : 2011 Article en page(s) : pp. 815-829 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Earthquakes  Numerical  modelling  Constitutive  relations  Tunnels Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : This paper compares simple constitutive models that are widely used in engineering practice with more sophisticated methods in the context of a case study. In particular, four constitutive modelling approaches have been considered: a simple elasto-plastic constitutive model (modified Cam-clay), with and without Rayleigh damping; the same model coupled with a cyclic non-linear model that can simulate pre-yield hysteresis; and finally an advanced kinematic hardening model, which is an improved version of the Al-Tabbaa & Wood two-surface model. These four approaches are used to analyse the seismic response of a section of the Bolu tunnels during the 1999 Duzce earthquake. To shed light on the performance of the constitutive models, simple site response finite-element analyses were first undertaken for the studied site, paying particular attention to the calibration of the Rayleigh damping parameters. The results of these analyses, in terms of maximum shear strain, were then used as input to an analytical elastic method (extended Hoeg method) for calculating the thrust and bending moment acting in the tunnel lining. Finally the results of dynamic time domain plane-strain analyses, employing the four adopted constitutive modelling approaches, are compared against field observations and results obtained by the extended Hoeg method, to investigate the ability of the models, of ranging complexity, to mimic soil response under seismic excitation. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.9.p.141 [article] On the relative merits of simple and advanced constitutive models in dynamic analysis of tunnels [texte imprimé] / S. Kontoe, Auteur ; L. Zdravkovic, Auteur ; D. M. Potts, Auteur . - 2011 . - pp. 815-829. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 61 N° 10 (Octobre 2010) . - pp. 815-829 Mots-clés : Earthquakes  Numerical  modelling  Constitutive  relations  Tunnels Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : This paper compares simple constitutive models that are widely used in engineering practice with more sophisticated methods in the context of a case study. In particular, four constitutive modelling approaches have been considered: a simple elasto-plastic constitutive model (modified Cam-clay), with and without Rayleigh damping; the same model coupled with a cyclic non-linear model that can simulate pre-yield hysteresis; and finally an advanced kinematic hardening model, which is an improved version of the Al-Tabbaa & Wood two-surface model. These four approaches are used to analyse the seismic response of a section of the Bolu tunnels during the 1999 Duzce earthquake. To shed light on the performance of the constitutive models, simple site response finite-element analyses were first undertaken for the studied site, paying particular attention to the calibration of the Rayleigh damping parameters. The results of these analyses, in terms of maximum shear strain, were then used as input to an analytical elastic method (extended Hoeg method) for calculating the thrust and bending moment acting in the tunnel lining. Finally the results of dynamic time domain plane-strain analyses, employing the four adopted constitutive modelling approaches, are compared against field observations and results obtained by the extended Hoeg method, to investigate the ability of the models, of ranging complexity, to mimic soil response under seismic excitation. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.9.p.141