Evaluation of liquefaction potential for a dam using combination of traditional and performance based seismic design methods

Hassan Ali, Ali Nasseri-Moghaddam

In the proceedings of: GeoSt. John's 2019: 72nd Canadian Geotechnical Conference

Session: Geotechnical Characterization

ABSTRACT: Liquefaction potential assessment of an earth dam is a multidisciplinary problem which involves correct evaluation of subsurface conditions of fill material and dam foundation, seismicity of the area, and evaluation method. Single scenario method for liquefaction assessment are useful however, the assessment techniques using performance based approach are a promising tool in correctly identifying the risk of dam due to liquefaction. A case study is presented where a combination of deterministic, simple probabilistic and performance based methods are successfully used to identify the liquefaction potential of a dam. Preliminary geotechnical investigation showed presence of loose sand layers at the foundation of the dam. Considering the high hazard potential classification for the dam and for a return period of 1 in 10,000 years, the liquefaction analysis showed that loose sandy layer may liquefy in case of extreme seismic event. The initial analysis was carried out using a single scenario approach. Thus, a second phase of geotechnical investigation was completed with additional deep boreholes and CPT testing to identify the thickness of loose sandy layers. A detailed liquefaction assessment was completed using a combination of deterministic, probabilistic and performance based approaches. The results of the second phase showed that existing loose sandy soil layers are not wide spread and have limited thickness. For dams, simple deterministic method is not enough and probabilistic and performance based approach for liquefaction analysis show promising results.

RÉSUMÉ: L'évaluation du potentiel de liquéfaction d'un barrage en terre est un problème multidisciplinaire qui implique une évaluation correcte des conditions souterraines du matériau de remblai et de la fondation du barrage, de la sismicité de la zone et de la méthode d'évaluation. La méthode à scénario unique pour l™évaluation de la liquéfaction est utile, mais les techniques d™évaluation utilisant une approche basée sur les performances sont un outil prometteur pour identifier correctement le risque de barrage dû à la liquéfaction. Une étude de cas est présentée dans laquelle une combinaison de méthodes déterministes, probabilistes simples et basées sur la performance est utilisée avec succès pour identifier le potentiel de liquéfaction d'un barrage. Une étude géotechnique préliminaire a montré la présence de couches de sable en vrac à la fondation du barrage. Compte tenu de la classification du potentiel de risque élevé pour le barrage et pour une période de retour de 1 sur 10 000 ans, l'analyse de liquéfaction a montré qu'une couche de sable lâche peut se liquéfier en cas d'événement sismique extrême. L'analyse a été réalisée en utilisant une approche à un seul scénario. Ainsi, une deuxième phase d™investigation géotechnique a été complétée avec des forages profonds supplémentaires et des tests CPT pour identifier l™épaisseur des couches de sable lâches. Une évaluation détaillée de la liquéfaction a été réalisée à l'aide d'une combinaison d'approches déterministes, probabilistes et basées sur la performance. Les résultats de l™étude de la deuxième phase ont montré que les couches de sol sableux lâches à très lâches existantes ne sont pas étendues et ont une épaisseur limitée. Pour les barrages, la méthode déterministe simple ne suffit pas et une approche probabiliste et basée sur les performances pour l'analyse de la liquéfaction doit être complétée. INTRODUCTION Assessment of the liquefaction potential of subsurface soils needs a detailed geotechnical investigation, laboratory and in-situ testing and calculations, which are usually beyond the scope of work of conventional geotechnical investigations. Specifically, in medium to low risk seismic areas with limited history of extreme seismic events detail subsurface investigations for this purpose often does not have financial justification. However, in heavily urbanized areas even if the probability of a major seismic event is low the risk of such event can be high with significant adverse consequences. Therefore, proper techniques for evaluation of liquefaction potential is necessary for identifying possible remediation measures in case of ground shaking due to seismic events. This paper presents a case study where a combination of deterministic, simple probabilistic and performance based methods are successfully used to identify the liquefaction potential of a dam. Preliminary geotechnical investigation showed presence of loose sand layers at the foundation of the dam. Considering the high hazard potential classification for the dam and for a return period of 1 in 10,000 years, the liquefaction analysis showed that loose sandy layer may liquefy in case of extreme seismic event. The initial analysis was carried out using a single scenario approach. Thus, a second phase of geotechnical investigation was completed with additional deep boreholes and CPT testing to identify the thickness of loose sandy layers. A detailed liquefaction assessment was completed using a combination of deterministic, probabilistic and performance based approaches. The

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Cite this article:
Hassan Ali; Ali Nasseri-Moghaddam (2019) Evaluation of liquefaction potential for a dam using combination of traditional and performance based seismic design methods in GEO2019. Ottawa, Ontario: Canadian Geotechnical Society.

@article{Geo2019Paper148,author = Hassan Ali; Ali Nasseri-Moghaddam,title = Evaluation of liquefaction potential for a dam using combination of traditional and performance based seismic design methods,year = 2019}