ABSTRACT
In literature, the concept of state parameter, which is a difference between the current void ratio and the steady-state void ratio at the current stress level, is presented to be a useful state index for the comparison of shear behaviour under drained conditions. This is due to the nature of state parameter to simultaneously take into account the effects from void ratio and pressure as both control the volume change potential and hence, the shear behaviour. However, the analysis on undrained shear behaviour of some laboratory-standard sands shows that relationships between the state parameter and stress ratio at initial peak strength (which mobilizes upon the onset of strength reduction or softening leading to liquefaction) are influenced by the level of consolidation pressure. For some samples, at the same state parameter, the increase in consolidation pressure showed the increase in stress ratio mobilized at the instability state which means that the effect of densification (cause of dilative potential) overpowers the effect of pressure increase (cause of contractive potential). Whereas, other samples showed the decrease in stress ratio mobilized at the instability state with the increase in consolidation pressure against the same state parameter. This represents bias in the behavioural comparisons just on the basis of initial states which is also influenced by the curved geometry of the steady state line on e-log(p’) plane. These equivocal effects on the basis of state parameter also hinder the applicability of such correlations in judging the influence of material properties on the instability state.
