Post-liquefaction Strength of Cohesionless Soils

Abouzar Sadrekarimi

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Laboratory and Field Testing

ABSTRACT: uefaction Strength of Cohesionless Soils Abouzar Sadrekarimi, Ph.D., P. Eng. Western University, London, Ontario, Canada ABSTRACT In this paper, a relationship is developed for estimating undrained post-liquefaction shear strength of sands based on the critical state framework. Using this relationship, the effects of effective stress variation and density on undrained post-liquefaction strength are studied for different combinations of critical state line parameters corresponding to several reference sands. The parametric study indicates that depending on sand void ratio, undrained post-liquefaction strength may increase, remain the same, or decrease as sand shearing-compressibility (critical state line slope) increases. The underlying mechanisms of field failures in dense sands and reverse behaviour of compressible sands are explained through this relationship. It is suggested that the critical state parameter is insufficient for describing the behavior of liquefiable sands and sand shearing-compressibility should be taken into account for estimating undrained shear strength corresponding to the changes in density and effective confining stress to be incorporated in stability analysis. RÉSUMÉ Dans cet article, une relation est développée pour l'estimation de résistance au cisaillement après liquéfaction des sables basé sur le framework d'état critique. En utilisant cette relation, les effets de variation de contrainte effective et de la densité sur la résistance après liquéfaction non drainée sont étudiés pour différentes combinaisons de paramètres de ligne de l'état critique correspondant à plusieurs sables de référence. L'étude paramétrique indique que selon le rapport de vide de sable, la force de post-liquéfaction non égouttée peut augmenter, rester le même, ou diminuer comme la compressibilité de tonte de sable (la pente de ligne publique critique) les augmentations. Les mécanismes sous-tendants d'échecs de terrain dans les sables denses et le comportement contraire de sables compressibles sont expliqués par ce rapport. Il est suggéré que le paramètre public critique est insuffisant pour décrire la conduite de sables liquéfiables et la compressibilité de tonte de sable devrait être tenue compte pour estimer que la force de tondage non égouttée conforme aux changements dans la densité et la tension de confinant efficace est incorporée dans l'analyse de stabilité. 1 INTRODUCTION Flow liquefaction occurs when shear resistance of soil decreases by monotonic (e.g. erosion at the toe of a slope, oversteeping of a slope, dredging, rapid sediment accumulation, drawdown conditions, reservoir filling), cyclic (e.g., earthquake, tidal waves), or dynamic (e.g. blast, vibration) loading at constant volume (Castro, 1969; Terzaghi, et al., 1996). This becomes a severe catastrophic event if the shear resistance of the liquefied soil drops below the existing static shear load (e.g. driving shear stress in a slope). The soil mass will then undergo large shear displacements and flow until the driving shear stress becomes lower than the reduced post-liquefaction shear strength. Soil undrained shear strength mobilized following flow failures and liquefaction is an important parameter in undrained stability analysis for evaluating the occurrence of flow deformation during earthquakes. Correct estimation of undrained post-liquefaction shear strength is particularly necessary for the design of large soil structures such as mine tailings impoundments, earth dams, and building foundations in order to protect them against liquefaction failures. Undrained post-liquefaction shear strength of soil mobilized following liquefaction flow failures is either determined through empirical correlations with in-situ penetration tests based on earlier cases of liquefaction failures (Stark and Mesri, 1992; Olson and Stark, 2002; Idriss and Boulanger, 2008) or laboratory shear tests on representative field samples obtained by ground freezing, or high-quality tube sampling techniques and correcting the undrained strength for sample disturbance effects (Poulos, et al., 1985). While each approach has inherent advantages and limitations, both methods are limited to specific soils involved in the earlier liquefaction flow failures or samples used in laboratory element tests. Particularly empirical methods based on in-situ penetration tests are subject to significant uncertainties and simplifying assumptions. Therefore, our knowledge of soil behaviour is merely within the range of soils that have been tested in the laboratory or encountered in past field failures. This has hampered our current understanding of the underlying mechanisms of field failures in medium dense sands, or the increasing undrained post-liquefaction strength ratio with increasing consolidation stress and decreasing contractiveness of compressible sands (-situ density, stress level, and compressibility) significantly affect the in-situ strength of sands. The separate and combined roles of these factors on sand behaviour need to be properly characterized in order to design safeguards against their adverse effects and failures. This study presents a analytical framework for estimating the undrained post-liquefaction shear strength associated with liquefaction failure of sloping grounds using critical state soil mechanics and critical state parameters obtained from laboratory testing of soils. Parametric analyses are carried out to investigate the roles of soil friction angle, stress, and compressibility on sand undrained strength. Understanding the effects of these factors on sand strength would be helpful for selecting the appropriate strength for many practical problems.

RÉSUMÉ: -liquefaction Strength of Cohesionless Soils Abouzar Sadrekarimi, Ph.D., P. Eng. Western University, London, Ontario, Canada ABSTRACT In this paper, a relationship is developed for estimating undrained post-liquefaction shear strength of sands based on the critical state framework. Using this relationship, the effects of effective stress variation and density on undrained post-liquefaction strength are studied for different combinations of critical state line parameters corresponding to several reference sands. The parametric study indicates that depending on sand void ratio, undrained post-liquefaction strength may increase, remain the same, or decrease as sand shearing-compressibility (critical state line slope) increases. The underlying mechanisms of field failures in dense sands and reverse behaviour of compressible sands are explained through this relationship. It is suggested that the critical state parameter is insufficient for describing the behavior of liquefiable sands and sand shearing-compressibility should be taken into account for estimating undrained shear strength corresponding to the changes in density and effective confining stress to be incorporated in stability analysis. RÉSUMÉ Dans cet article, une relation est développée pour l'estimation de résistance au cisaillement après liquéfaction des sables basé sur le framework d'état critique. En utilisant cette relation, les effets de variation de contrainte effective et de la densité sur la résistance après liquéfaction non drainée sont étudiés pour différentes combinaisons de paramètres de ligne de l'état critique correspondant à plusieurs sables de référence. L'étude paramétrique indique que selon le rapport de vide de sable, la force de post-liquéfaction non égouttée peut augmenter, rester le même, ou diminuer comme la compressibilité de tonte de sable (la pente de ligne publique critique) les augmentations. Les mécanismes sous-tendants d'échecs de terrain dans les sables denses et le comportement contraire de sables compressibles sont expliqués par ce rapport. Il est suggéré que le paramètre public critique est insuffisant pour décrire la conduite de sables liquéfiables et la compressibilité de tonte de sable devrait être tenue compte pour estimer que la force de tondage non égouttée conforme aux changements dans la densité et la tension de confinant efficace est incorporée dans l'analyse de stabilité. 1 INTRODUCTION Flow liquefaction occurs when shear resistance of soil decreases by monotonic (e.g. erosion at the toe of a slope, oversteeping of a slope, dredging, rapid sediment accumulation, drawdown conditions, reservoir filling), cyclic (e.g., earthquake, tidal waves), or dynamic (e.g. blast, vibration) loading at constant volume (Castro, 1969; Terzaghi, et al., 1996). This becomes a severe catastrophic event if the shear resistance of the liquefied soil drops below the existing static shear load (e.g. driving shear stress in a slope). The soil mass will then undergo large shear displacements and flow until the driving shear stress becomes lower than the reduced post-liquefaction shear strength. Soil undrained shear strength mobilized following flow failures and liquefaction is an important parameter in undrained stability analysis for evaluating the occurrence of flow deformation during earthquakes. Correct estimation of undrained post-liquefaction shear strength is particularly necessary for the design of large soil structures such as mine tailings impoundments, earth dams, and building foundations in order to protect them against liquefaction failures. Undrained post-liquefaction shear strength of soil mobilized following liquefaction flow failures is either determined through empirical correlations with in-situ penetration tests based on earlier cases of liquefaction failures (Stark and Mesri, 1992; Olson and Stark, 2002; Idriss and Boulanger, 2008) or laboratory shear tests on representative field samples obtained by ground freezing, or high-quality tube sampling techniques and correcting the undrained strength for sample disturbance effects (Poulos, et al., 1985). While each approach has inherent advantages and limitations, both methods are limited to specific soils involved in the earlier liquefaction flow failures or samples used in laboratory element tests. Particularly empirical methods based on in-situ penetration tests are subject to significant uncertainties and simplifying assumptions. Therefore, our knowledge of soil behaviour is merely within the range of soils that have been tested in the laboratory or encountered in past field failures. This has hampered our current understanding of the underlying mechanisms of field failures in medium dense sands, or the increasing undrained post-liquefaction strength ratio with increasing consolidation stress and decreasing contractiveness of compressible sands (

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Abouzar Sadrekarimi (2014) Post-liquefaction Strength of Cohesionless Soils in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper298,author = Abouzar Sadrekarimi,title = Post-liquefaction Strength of Cohesionless Soils,year = 2014}