Undrained instability of loose sand under plane-strain conditions and its engineering application

When a loose sand specimen is loaded under an undrained condition, it may become unstable leading to a static iiqucfaction beahviour. Static liquefaction or instability of loose sand under undrained conditions is one of the most frequent causes of flow failures in granular soil slopes. However, the instability behaviour of sand has seldom been studied under plane-strain conditions. Experimental results are presented in this paper to study the undrained instability behaviour of sand under plane-strain conditions. The data shows that the undrained behaviour of sand under plane-strain conditions is similar to that under axisymmetric conditions. Using the state parameter concept, a unified relationship can be established between the normalized slope of instability line and the state parameters for both axisymmetric and plane-strain conditions. This relationship can be used to determine instability conditions for both axisymmetric and plane-strain problems.