Volume 22, Issue 2 (June 1999)
Determination of the Plasticity Index from Flow Index
The flow index (If), which is the slope of the water content versus log10 (number of blows) plot in the percussion cup method or the slope of the water content versus log10 (depth of penetration) plot of the cone penetration method of determining the liquid limit of soils is a measure of soil plasticity. Hence, the plasticity index of a soil that represents the soil plasticity can be correlated with the flow index.
Analysis of test data pertaining to soils of widely varying plasticity characteristics and geological origin indicates that there is an excellent correlation between the plasticity index and the flow index, particularly in the cone penetration method. In this paper, a simple equation has been suggested to obtain the plasticity index of a soil from the flow index, thereby eliminating the dependence of the plasticity index determination on the time-consuming and unrefined plastic limit test.