Skip to main content
Question
Ricardo Kuhn
Construction Foreman
Asked a question 2 years ago

# What is the equation of A-line in the plasticity chart?

What is the equation of A-line in the plasticity chart?

### Where am I?

In TheConstructor you can ask and answer questions and share your experience with others!

Wilma Caldwell
Architectural technologist

Fine grained soils are classified based on the plasticity chart. The equation of A line is ;

Ip=0.73(liquid limit -20)

Ip=plasticity index

Hi,

According to ISSCS, plasticity chart, depending on the values of liquid limit (WL) and plasticity index (IP). The ‘A’ line is illustrated as IP is 0.73 (WL – 20).  fine soils are split up into clays(C), silts (M), and organic soils (O). High  plasticity:  WL> 50% Low plasticity:  WL< 35% Intermediate plasticity:  35% < WL< 50% Refer attached images for more information.

Hi,  Depending upon the point in the chart, we can categorize fine soil into

• clay
• silt

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.

According to ISSCS, plasticity chart, depending on the values of liquid limit (WL) and plasticity index (IP).

Terry Collins
Sustainability consultant

Hi,

According to ISSCS, plasticity chart, depending on the values of liquid limit (WL) and plasticity index (IP). The ‘A’ line is illustrated as IP is 0.73 (WL – 20).  fine soils are split up into clays(C), silts (M), and organic soils (O). High  plasticity:  WL> 50% Low plasticity:  WL< 35% Intermediate plasticity:  35% < WL< 50% Refer attached images for more information.

## Plasticity chart:

The plasticity chart is mainly based on the values of a liquid limit as well as a plastic limit.

The A-line in this chart is expressed as;

Ip= 0.73(Wl- 20)

Depending upon the point in the chart, we can categorize fine soil into

• clay
• silt
• organic soil

Thank you

Henry Newman
Water resource engineer

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.