Predicting the Onset of Static Liquefaction of Loose Sand with Fines

Loose sandy soil subject to undrained shearing manifests deviatoric strain softening, and such a behavior has been referred to as collapse, static liquefaction, or instability. This paper uses the term instability and characterizes its triggering by the corresponding effective stress ratio, referred to as the instability ratio, ${\eta }_{IS}$. To capture the influence of fines on ${\eta }_{IS}$, the state parameter, $\psi$, as originally proposed by Been and Jefferies, was generalized to an equivalent granular state parameter, ${\psi }^{*}$. This is achieved simply by replacing the void ratio, $e$, with the equivalent granular void ratio, $e^{*}$. The conversion from $e$ to $e^{*}$ was achieved by a predictive approach, and backanalysis is not required. It was hypothesized that, provided the fines content is less than the threshold value, ${\eta }_{IS}$ and ${\psi }^{*}$, at the start of undrained shearing, can be described by a single relationship irrespective of the fines content. Two published databases and a series of undrained triaxial tests results are used to evaluate this hypothesis. The importance of this relationship is significant, because it can be used to predict ${\eta }_{IS}$ for sand with different fines content.