The Effect of Fine Percent and Its Plasticity on State Boundary Surface Characteristics for Clayey Sands

After observing some phenomena like liquefaction and flow failures which generally occur in loose and saturated sands, extensive investigation have been conducted in sands and effective parameters on their critical state behavior. Preliminary conducted researches have mainly been focused on clean sand or its mixtures with non-plastic fines and the critical state of clayey sands have not been much paid attention. May be it was thought that plastic properties in clay, prohibit occurring flow behaviors and liquefaction. However, carried out researches after some earthquakes like Northridge 1994, Kokaeli 1999, Chi Chi 1999 and Niigata 2004 indicated that notable settlements have been observed where soils included considerable amount of clay, resulting notable damages. These studies demonstrated that more detailed investigations should be undertaken to determine the critical state behavior of clayey sands. Previous researches have shown that a surface called state boundary surface can be defined for loose sands in the three dimensional space that determine the shape of stress path after collapse and strain softening. Unsteady states that sand show at this condition, may lead to occur phenomenon such as liquefaction and flow behaviors during the undrained loading. This research is aimed at evaluating existence of boundary state surface for 161 Firoozkooh sand and its mixtures with different plasticity clays. In fact, the main objective is to assess the effect of fine percent and its plasticity on characteristics of boundary state surface in the three dimensional space through employing proper parameters and mathematic equations. The obtained results indicated that state boundary surface characteristics depend mainly on the fine percent and its plasticity. Since state boundary surface characteristics mainly determine the behavior characteristics of soil sample in the critical state, the effect of changes in the shape and characteristics of this surface on critical state soil behavior has been investigated in the next step. In this way, valuable results have been obtained about the effect of fine percent and its plasticity on the behavior characteristics and liquefaction potential of clayey sands.