Post liquefaction response of initially overconsolidated Fraser River sand

Mohammad Shahsavari, Siva Sivathayalan

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

Session: Soil Mechanics

ABSTRACT: efaction response of initially overconsolidated Fraser River sand Mohammad Shahsavari Department of Civil Engineering Œ University of Toronto, Toronto, Ontario, Canada Siva Sivathayalan Department of Civil and Environmental Engineering Œ Carleton University, Ottawa, Ontario, Canada ABSTRACT An experimental study aimed at improving the understanding of post-liquefaction behaviour of sands is presented in this paper. Overconsolidated samples of Fraser River sand at three levels of initial relative density (19%, 40%, & 65%) were first subjected to cyclic triaxial loading leading to liquefaction (either by flow failure or cyclic mobility mechanisms), and then sheared undrained under triaxial extension and compression loading. Tests were conducted on samples consolidated to a mean effective confining pressure of 100 kPa, but with different consolidation stress history to yield overconsolidation ratios (OCR) of 1.00, 1.25, and 2.00. The effect of overconsolidation on both the ‚post-liquefaction shear strength' and the strain range over which the sand shears at essentially zero stiffness is evaluated from these tests. It is shown that the ‚zero stiffness strain' range is only marginally dependent on OCR, but it is highly dependent on the past maximum shear strain. RÉSUMÉ Une étude expérimentale visant à améliorer la compréhension du comportement post- liquéfaction des sables est présenté ici. Échantillons surconsolidées de sable du fleuve Fraser à trois niveaux de densité relative initiale (19%, 40% & 65%) ont d'abord été soumis à un chargement triaxial cyclique conduisant à la liquéfaction (soit par une insuffisance de écoulement ou pour mécanismes de mobilité cycliques), puis cisaillées non drainé sous extension triaxiale ou sous charge de compression. Les essais ont été effectués sur des échantillons consolidés à une pression de confinement effectif moyen de 100 kPa, mais avec des antécédents de stress de consolidation pour produire des taux de surconsolidation (OCR) de 1,00, 1,25, et 2,00. L'effet de préconsolidation à la fois, la "résistance au cisaillement après liquéfaction" et l'intervalle de déformation sur laquelle la cisaille de sable à essentiellement zéro rigidite est évaluée à partir de ces essais. Il est montré que la gamme "zéro rigidité souche" est peu dépendant de l'OCR, mais il est très dépendante de la déformation en cisaillement maximal passé. 1 INTRODUCTION There have been several instances of large ground deformation (including flow failure) reported due to earthquake loading during the past few decades. Such failures can take place either during or following the cessation of earthquake shaking, and often occur on account of soil liquefaction in granular soils. The most notable of these deformations include the Mochikoshi tailings dam failure (Ishihara et al. 1978), Lower San Fernando dam failure (Seed et al., 1985), and the Lake Merced bank failure (Ross, 1968). The catalogue of case histories involving liquefaction flow failures presented in Olson and Stark (2001), and the list of earthquake induced failures in California by Youd and Hoose (1978) clearly indicate that such failures have been widespread. Depending on the regional setting, such failures may pose a serious threat to life and property. Better understanding of the nature and stress-strain behaviour of liquefied soils is essential to safeguard against, or minimize the impact of, such failures. Finn et al. (1986) and Seed (1987) represent the initial attempts to deal with the design problems in liquefied soils. A residual shear strength that was uniquely related to the density (or SPT ) and reduced initial shear modulus in a hyperbolic stress-strain curve (Byrne et al. 1992) were employed to predict the behaviour of the liquefied soil. The updated form of the original Seed (1987) residual strength - correlations (Seed and Harder, 1990) has been widely used in engineering practice for several years. But, the use of normalised strength ratios, originally proposed by Stark and Mesri (1992) is becoming the commonly adopted tool in liquefaction design practice in recent years. While the design solutions were based on field observations, only a systematic laboratory study could provide insights into the characteristics of the liquefied soils. Such understanding is essential to provide confidence to the design methods. Vaid and co-workers at the University of British Columbia led the initial efforts to better understand the mechanical behaviour of liquefied soils (Kuerbis, 1989; Vaid and Thomas, 1995; Vaid and Sivathayalan, 1997). Figure 1 shows the stress-strain response of a tailings material during the cyclic loading leading to liquefaction and following liquefaction. The last cycle represents post-liquefaction deformation (Kuerbis, 1989). Vaid and Thomas (1995) were the first to formally identify three distinct phases of deformation in liquefied soils as shown in Figure 2. The stiffness of the liquefied sand in region 1

RÉSUMÉ: liquefaction response of initially overconsolidated Fraser River sand Mohammad Shahsavari

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Mohammad Shahsavari; Siva Sivathayalan (2014) Post liquefaction response of initially overconsolidated Fraser River sand in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper294,author = Mohammad Shahsavari; Siva Sivathayalan,title = Post liquefaction response of initially overconsolidated Fraser River sand,year = 2014}