Instability Behaviour for Sandy Soils

The concept of collapse line, CL, or instability line, IL have been reported in literature for analysing instability behaviour of sands under undrained shearing. The similarity, but different implications of these two concepts sometimes led to some confusion. Thus this paper uses a different stress parameter, the instability stress ratio, in studying the relationship between onset of instability and state of sand with fines. However, the behaviour of sand with fines for a given void ratio, e, is strongly influenced by fines content. Equivalent granular void ratio, e*, when used in lieu of e may capture the effects of fines content. This paper critically examines three different approaches of getting e*. The concept of state parameter as first proposed by Been and Jefferies is then generalized to an equivalent granular state parameter in order to capture the effects of fines. A good correlation between instability stress ratio and equivalent granular state parameter is obtained irrespective of fines contents.