Plasticity chart is a graph between plasticity index (IP) and liquid limit (WL) in percentage which is used for classification of fine-grained soils as per the Indian Standard Soil Classification System(ISSCS). If more than 50% percent of soil passes through 75micron sieve, then it is classified as fine-grained soil.
Equation of A-line as represented in the given chart relates plasticity index and liquid limit as
IP = 0.73 (WL – 20).
The equation of A-line gives value of plasticity index ranges from 0 to 58.4 corresponding to values of liquid limit from 0 to 100%.
The plasticity characteristic of fine grained soil based on different liquid limit range is shown below.
From the plasticity chart,
CL: Clay with low plasticity
ML: Silt with low plasticity
CH: Clay with high plasticity
MH: Silt with high plasticity
OH: organic soil with high plasticity
CI: Clay with intermediate plasticity
MI: Silt with intermediate plasticity
OI: organic soil with intermediate plasticity
The soil fall above A-line in plasticity chart are clayey soils and that fall below A-line are silt and organic soils. If plasiticity index ranges between 4–7%, soil is represented in dual symbol. That is CL-ML.