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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

Analysis of model sheet pile walls with plastic hinges / P. J. Bourne-Webb in Géotechnique, Vol. 61 N° 6 (Juin 2011) 

Public ISBD [article]  in Géotechnique > Vol. 61 N° 6 (Juin 2011) . - pp. 487–499 Titre : Analysis of model sheet pile walls with plastic hinges Type de document : texte imprimé Auteurs : P. J. Bourne-Webb, Auteur ; D. M. Potts, Auteur ; D. Konig, Auteur Année de publication : 2011 Article en page(s) : pp. 487–499 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Limit  state  design/analysis  Numerical  modelling  Centrifuge  modelling  Standards  Sheet  piles  and  cofferdams Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : As part of a wider project investigating the implications of Eurocode 3, Design of steel structures – Part 5: Piling, centrifuge testing of model sheet pile walls and numerical back-analysis were undertaken. The aims of the study were to examine the effect of plastic hinging on embedded retaining wall response, and to verify calculation methods for use in wider generic calculations. Physical modelling of an anchored wall embedded in dry sand was undertaken. In some of the tests a hinge zone was introduced into the wall in order to reproduce a kinematic mechanism similar to that associated with plastic hinge formation. Finite-element calculations were undertaken using Lade's double-hardening cap model to represent the behaviour of the sand. The analyses generally yielded good accord with the test results in many aspects of the wall behaviour. In terms of the aims of the testing, the study of the effect of plastic hinging was not realistically captured, because the hinge zone was present from the outset of the test, allowing greater than expected earth pressure redistribution, and because the moment–curvature characteristic of the notched wall did not reproduce the buckling effects seen in the plastic bending response of steel sheet piles. On the other hand, the satisfactory agreement between the test observations and the numerical predictions gave confidence in the use of the calculation model for further generic calculations using realistic steel sheet pile moment–plastic curvature characteristics. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.9.p.061 [article] Analysis of model sheet pile walls with plastic hinges [texte imprimé] / P. J. Bourne-Webb, Auteur ; D. M. Potts, Auteur ; D. Konig, Auteur . - 2011 . - pp. 487–499. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 61 N° 6 (Juin 2011) . - pp. 487–499 Mots-clés : Limit  state  design/analysis  Numerical  modelling  Centrifuge  modelling  Standards  Sheet  piles  and  cofferdams Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : As part of a wider project investigating the implications of Eurocode 3, Design of steel structures – Part 5: Piling, centrifuge testing of model sheet pile walls and numerical back-analysis were undertaken. The aims of the study were to examine the effect of plastic hinging on embedded retaining wall response, and to verify calculation methods for use in wider generic calculations. Physical modelling of an anchored wall embedded in dry sand was undertaken. In some of the tests a hinge zone was introduced into the wall in order to reproduce a kinematic mechanism similar to that associated with plastic hinge formation. Finite-element calculations were undertaken using Lade's double-hardening cap model to represent the behaviour of the sand. The analyses generally yielded good accord with the test results in many aspects of the wall behaviour. In terms of the aims of the testing, the study of the effect of plastic hinging was not realistically captured, because the hinge zone was present from the outset of the test, allowing greater than expected earth pressure redistribution, and because the moment–curvature characteristic of the notched wall did not reproduce the buckling effects seen in the plastic bending response of steel sheet piles. On the other hand, the satisfactory agreement between the test observations and the numerical predictions gave confidence in the use of the calculation model for further generic calculations using realistic steel sheet pile moment–plastic curvature characteristics. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.9.p.061

Comparison of finite element and limiting equilibrium analyses for an embedded cantilever retaining wall / A. B. Fourie in Géotechnique, Vol. 39 N°2 (Juin 1989) 

Public ISBD [article]  in Géotechnique > Vol. 39 N°2 (Juin 1989) . - pp. 175 –188 Titre : Comparison of finite element and limiting equilibrium analyses for an embedded cantilever retaining wall Type de document : texte imprimé Auteurs : A. B. Fourie, Auteur ; D. M. Potts, Auteur Année de publication : 2007 Article en page(s) : pp. 175 –188 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Retaining  walls  Finite  elements  Soil-structure  interaction  Earth  pressure  Diaphragm  walls Résumé : The design of embedded cantilever retaining walls is often based on approximate limit equilibrium calculations. In this Paper the results of a limit equilibrium approach are compared with the results of a finite element study. Predictions of both embedment depth and maximum bending moments are compared and the finite element study is used to show the influence of such parameters as initial soil stress conditions and construction procedure. For the finite element study an elasto-plastic constitutive law is used to model the soil behaviour. The soil is assumed to behave in a fully drained manner with zero pore fluid pressures everywhere. Effective stress parameters are used to define the soil strength. The results of the comparison indicate that both the limit equilibrium method used in the Paper and the finite element approach give similar predictions of the embedment depth required to ensure stability. The maximum bending moment was calculated using two different limit equilibrium approaches. Generally it was found that predictions based on these approaches were greater than the corresponding values from the finite element analyses. In particular, for embedded cantilever walls formed by excavation in low K 0 soils or by bachtIlling one of the limit equilibrium approaches provided an overestimate of almost 50% of the maximum bending moment value predicted by the finite element method at a factor of safety of 2. The results of the study indicate that some reduction in bending moment as calculated by the limit equilibrium method may therefore be warranted. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.1989.39.2.175 [article] Comparison of finite element and limiting equilibrium analyses for an embedded cantilever retaining wall [texte imprimé] / A. B. Fourie, Auteur ; D. M. Potts, Auteur . - 2007 . - pp. 175 –188. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 39 N°2 (Juin 1989) . - pp. 175 –188 Mots-clés : Retaining  walls  Finite  elements  Soil-structure  interaction  Earth  pressure  Diaphragm  walls Résumé : The design of embedded cantilever retaining walls is often based on approximate limit equilibrium calculations. In this Paper the results of a limit equilibrium approach are compared with the results of a finite element study. Predictions of both embedment depth and maximum bending moments are compared and the finite element study is used to show the influence of such parameters as initial soil stress conditions and construction procedure. For the finite element study an elasto-plastic constitutive law is used to model the soil behaviour. The soil is assumed to behave in a fully drained manner with zero pore fluid pressures everywhere. Effective stress parameters are used to define the soil strength. The results of the comparison indicate that both the limit equilibrium method used in the Paper and the finite element approach give similar predictions of the embedment depth required to ensure stability. The maximum bending moment was calculated using two different limit equilibrium approaches. Generally it was found that predictions based on these approaches were greater than the corresponding values from the finite element analyses. In particular, for embedded cantilever walls formed by excavation in low K 0 soils or by bachtIlling one of the limit equilibrium approaches provided an overestimate of almost 50% of the maximum bending moment value predicted by the finite element method at a factor of safety of 2. The results of the study indicate that some reduction in bending moment as calculated by the limit equilibrium method may therefore be warranted. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.1989.39.2.175

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

A numerical simulation of underwater slope failures generated by salt diapirism combined with active sedimentation / N. Kovacevic in Géotechnique, Vol. 62 N° 9 (Septembre 2012) 

Public ISBD [article]  in Géotechnique > Vol. 62 N° 9 (Septembre 2012) . - pp. 777 –786 Titre : A numerical simulation of underwater slope failures generated by salt diapirism combined with active sedimentation Type de document : texte imprimé Auteurs : N. Kovacevic, Auteur ; R. J. Jardine, Auteur ; D. M. Potts, Auteur Année de publication : 2012 Article en page(s) : pp. 777 –786 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Failure  Numerical  modelling  Slopes  Landslides  Offshore  engineering Résumé : This paper describes advanced finite-element simulations of a geologically active and periodically unstable 700 m high underwater escarpment located in the Gulf of Mexico that has been created by uplifting salt diapirs combined with sediment deposition. The site's geological history, soil permeability and clay shear strength characteristics govern current slope stability, and determine the timing of any future failure. The geological and geotechnical conditions are reported before presenting fully coupled, large-displacement numerical simulations employing critical state formulations of the slope's evolution over the last 600 000 years. The latter form the basis for strain-softening analyses of the pore pressure equalisation and progressive failure processes that control current slope stability. The analyses also address the ground movement patterns that precede instability, giving useful information for geohazard risk assessment and early warning monitoring system design. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.12.OG.004 [article] A numerical simulation of underwater slope failures generated by salt diapirism combined with active sedimentation [texte imprimé] / N. Kovacevic, Auteur ; R. J. Jardine, Auteur ; D. M. Potts, Auteur . - 2012 . - pp. 777 –786. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. 62 N° 9 (Septembre 2012) . - pp. 777 –786 Mots-clés : Failure  Numerical  modelling  Slopes  Landslides  Offshore  engineering Résumé : This paper describes advanced finite-element simulations of a geologically active and periodically unstable 700 m high underwater escarpment located in the Gulf of Mexico that has been created by uplifting salt diapirs combined with sediment deposition. The site's geological history, soil permeability and clay shear strength characteristics govern current slope stability, and determine the timing of any future failure. The geological and geotechnical conditions are reported before presenting fully coupled, large-displacement numerical simulations employing critical state formulations of the slope's evolution over the last 600 000 years. The latter form the basis for strain-softening analyses of the pore pressure equalisation and progressive failure processes that control current slope stability. The analyses also address the ground movement patterns that precede instability, giving useful information for geohazard risk assessment and early warning monitoring system design. ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.12.OG.004

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) 

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