Effect of overconsolidation and the direction of initial static shear stress on the liquefaction susceptibility of sand Mohammad Shahsavari Department of Civil Engineering – University of Toronto, Toronto, Ontario, Canada Siva Sivathayalan Department of Civil and Environmental Engineering – Carleton University, Ottawa, Ontario, Canada ABSTRACT The effect of overconsolidation on current liquefaction design methodologies remains uncertain since stress level ( ܭ ఙ ) and static shear ( ܭ ఈ ) correction factors used in such analysis have generally been determined based on tests on normally consolidated sands. The effect of overconsolidation ratio (OCR) on the cyclic response of Fraser River sand and its influence on correction factor ܭ ఈ are studied in this research. Cyclic triaxial tests were performed on overconsolidated samples that were prepared by water pluviation. Specimens were tested at different OCR levels at a confining pressure of 100 kPa, but with different initial levels of static shear. Also, tests were also performed on samples with initial relative densities of 19%, 40%, and 65%. It is observed that while overconsolidated sands are stronger, a higher OCR will adversely affect the ܭ ఈ correction factor. Also, a negative initial static shear stress ሺ ௦௧ ൌ ߪ ௩௖ ᇱ െ ߪ ௛௖ ᇱ ሻ would have a dramatic effect on the cyclic resistance of soil and will lead to flow failure of a soil which would fail due to cyclic mobility if the same amount of initial static shear stress is being applied in the positive direction. RÉSUMÉ L'effet de sobre-consolidación sur les méthodes actuelles de conception de liquéfaction reste incertain puisque les facteurs niveau de stress ( ܭ ఙ ) et de cisaillement statique ( ܭ ఈ ) correction utilisés dans cette analyse ont généralement été déterminée sur la base des tests sur les sables normalement consolidés . L' effet du razón de sobre-consolidación ( OCR ) sur la réponse cyclique de sable du fleuve Fraser et de son influence sur le facteur de correction ܭ ఈ sont étudiés dans cette recherche . Essais triaxiaux cycliques ont été effectuées sur des échantillons surconsolidés qui ont été préparés par pluviation de l'eau . Les échantillons ont été testés à différents niveaux de razón de sobre-consolidación à une pression de confinement de 100 kPa , mais avec différents niveaux initiaux de cisaillement statique . En outre , des essais ont également été effectués sur des échantillons de densités relatives initiales de 19 %, 40% et 65% . On observe que, bien que les sables surconsolidés sont plus forts , un OCR supérieur affectera négativement le facteur de correction ܭ ఈ . En outre, un stress négatif initial statique de cisaillement ( ௦௧ ൌ ߪ ௩௖ ᇱ െ ߪ ௛௖ ᇱ ) aurait un effet dramatique sur la résistance cyclique du sol et mènera à couler l'échec d'un sol qui échouer en raison de la mobilité cyclique si la même quantité de première contrainte de cisaillement statique est appliquée dans le sens positif. (French Abstract will be revised for the final version) 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