Effects of overconsolidation and the direction of principal stresses on liquefaction susceptibility of Fraser River sand

Mohammad Shahsavari, Siva Sivathayalan

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

Session: Soil Mechanics

ABSTRACT: overconsolidation and the direction of principal stresses on liquefaction susceptibility of 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 The effect of overconsolidation ratio (OCR) on the cyclic response of Fraser River sand and its influence on correction factor are studied in this research. Cyclic triaxial tests were performed on overconsolidated samples that were prepared by water pluviation at three relative density states (19%, 40%, and 65%). Specimens were tested at a confining pressure of 100 kPa, but with different levels of initial static shear and OCR. It is observed that while overconsolidated sands have higher cyclic resistance, increasing OCR will adversely affect the correction factor. Also, a 'negative' initial static shear stress () could dramatically reduce the cyclic resistance and lead to flow failure of a soil which would fail due to cyclic mobility if the same amount of initial static shear stress is applied in the 'positive' () direction. RÉSUMÉ L'effet du taux de surconsolidation (OCR) sur la réponse cyclique de sable du fleuve Fraser et de son influence sur le facteur de correction sont étudiés dans cette recherche. Essais triaxiaux cycliques ont été effectuées sur des échantillons surconsolidés qui ont été préparés par pluviation de l'eau à trois états de densité relative (19%, 40 % et 65 %). Les échantillons ont été testés à une pression de confinement de 100 kPa, mais avec différents niveaux de cisaillement statique initiale et OCR. On observe que, bien que les sables surconsolidés ont une résistance cyclique supérieure, augmentant OCR affecter le facteur de correction. En outre, une contrainte "négative" initiale de cisaillement statique () pourrait considérablement réduire la résistance cyclique et mener à couler l'échec d'un sol qui échouer en raison de la mobilité cyclique si la même quantité de contrainte initiale de cisaillement statique est appliqué dans le " positif " () direction. 1 INTRODUCTION Saturated granular materials can lose much of their shear strength and undergo excessive deformation due to "liquefaction" when sheared undrained. The main cause of liquefaction is the development of excess pore water pressure under either dynamic loads (such as earthquakes) or monotonic loads (such as the stresses caused by building a dam). Depending on the initial conditions of the soil in the field, this excess pore pressure development can cause limited or unlimited deformations. Liquefaction phenomenon has been observed in many historical earthquakes; one of the first reported incidents was during the 1891 Mino-Owari earthquake in Japan and the most recent one in the 2011 Christchurch earthquake in New Zealand. However, systematic research on liquefaction began after the failures in the 1964 earthquakes in Alaska, USA and Niigata, Japan. Since then, most of our understanding of liquefaction and the effect of various factors controlling this phenomenon have been derived from controlled laboratory experiments. These experiments have made it possible to study the effect of each factor individually and obtain a better insight into this phenomenon. The early characterization of liquefaction susceptibility was based on the empirical analysis of case histories (Seed and Idriss, 1971; Seed et al., 1985). Seed et al. (1985) provided a relationship between corrected SPT blow counts (considered a proxy for the relative density of the soils) and liquefaction potential for given cyclic stress. This methodology to derive the liquefaction potential of a specific soil was based on the Seed and Idriss (1971) "simplified procedure" and has been widely used since that time. In determining the liquefaction susceptibility of a specific site, usually site specific measurements are not made. Instead, correction factors derived from the literature are used to consider the effect of various state variables. The state variables include: void ratio (relative density), effective confining stress level, soil fabric, static shear stress, strain/stress history, and stress path. Among these variables, the effect of relative density and confining stress level are well understood, and commonly considered (Vaid and Sivathayalan, 2000). On the other hand, a comprehensive study of the effects of soil fabric and strain/stress history is not available in the literature. In current design practice, the effect of prior stress history (e.g. overconsolidation) is ignored due to the belief that a normally consolidated soil generally has a more contractive response than overconsolidated soil while other variables are not changing. This practice often leads to a conservative design. On the other hand, only limited

RÉSUMÉ: ts of overconsolidation and the direction of principal stresses on liquefaction susceptibility of

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Mohammad Shahsavari; Siva Sivathayalan (2014) Effects of overconsolidation and the direction of principal stresses on liquefaction susceptibility of Fraser River sand in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper293,author = Mohammad Shahsavari; Siva Sivathayalan,title = Effects of overconsolidation and the direction of principal stresses on liquefaction susceptibility of Fraser River sand,year = 2014}