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Lab Services: Soil testing

Lab Services: Soil testing

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Test Method Title

Standard Test Methods for Liquid Limit, Plastic Limit and Plasticity Index of Soils

Reference Number

ASTM D4318



Test Property

Liquid Limit, Plastic Limit, Plasticity Index

Description of Test

The sample is processed to remove any material retained on a 425-mm (#40) sieve. The liquid limit is determined by performing trials in which a portion of the sample is spread in a brass cup, divided in two by a grooving tool, and then allowed to flow together from the shocks caused by repeatedly dropping the cup in a standard mechanical device. The multi-point liquid limit, Method A, requires three or more trials over a range of water contents to be performed and the data from the trials plotted or calculated to make a relationship from which the liquid limit is determined. The one-point liquid limit, Method B, uses the data from two trials at one water content multiplied by a correction factor to determine the liquid limit. The plastic limit is determined by alternately pressing together and rolling into a 3.2-mm (1/8-in.) diameter thread a small portion of plastic soil until its water content is reduced to a point at which the thread crumbles and can no longer be pressed together and rerolled. The water content of the soil at this point is reported as the plastic limit. The plasticity index is calculated as the difference between the liquid limit and the plastic limit.

Preferred Test Sample Size

Number of Test Specimens

3 Liquid Limit 2 Plastic Limit and 1 moisture content

Test Specimen Size

Liquid Limit:150-200 grams, passing the #40 sieve, for all three test specimens, Plastic Limit: 20 grams, passing the 340 sieve, for both test specimens, Moisture content: at least 50 grams as received material


Atterberg Limits, liquid limit, plasticity index, plastic limit, limits.

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