How to Determine the Atterberg Limits

Based on moisture content, a soil can exist in four possible states:

  1. Solid
  2. Plastic
  3. Liquid

The Atterberg limits, created by swedish chemist Albert Atterberg, are simply the moisture content, usually reported in percent, at which the soil changes state. They are defined as:

  • Plastic Limit (solid – plastic)
  • Liquid Limit (plastic – liquid)

Some textbooks often refer to a third Atterberg limit, the “shrinkage limit” which defines the boundary between “solid” and a further “semi-solid” state, but in my experience this is rarely tested and reported in industry.

Plastic Limit

The threshold between the solid and plastic states is the plastic limit. When it has reached its plastic limit it is moldable and easy to change its shape. Plastic soil is like play-doh.

There is an ASTM standard for testing the plastic limit (D4318), but you can do a quick test in the field. Grab a handful of the soil and roll it into a cylinder with both hands. If you can keep rolling it down to about a quarter-inch diameter without breaking, it’s plastic. If it breaks before then, it’s solid. In the ASTM test, the process is standardized and the breaking point is 1/8″ but because of field conditions a larger threshold diameter should keep you on the straight and narrow.

I have done this test many times when fill arrived at my job sites. If the soil is plastic, the moisture content is probably high and compaction will be difficult.

The plastic limit is also used to determine the Plasticity Index.

Liquid Limit

The liquid limit is the point at which the soil flows. The ASTM test method for liquid limit involves placing a soil sample into a bowl, cutting a groove into the soil, and counting the number of blows on the bottom of the bowl to make the groove close.

The liquid limit is used in the classification of soils.

ASTM Test Method

The test method for atterberg limits is ASTM D4318.

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