Atterberg's Limit Test

Atterberg’s limits test is a soil classification test used to determine the plasticity characteristics of a soil sample. The test is based on the concept that the consistency of soil changes as the water content changes. The test determines four limits:

Liquid limit: The water content at which soil changes from a plastic state to a liquid state.

Plastic limit: The water content at which soil changes from a plastic state to a semi-solid state.

Shrinkage limit: The water content at which soil changes from a plastic state to a solid state without further shrinkage.

Plasticity index: The difference between the liquid and plastic limits, used to classify soil into different groups such as clay, silt, or sand.

The test is performed by using a device called a liquid limit apparatus, which measures the water content of a soil sample while it is subjected to shearing. The test results are used to determine the soil’s suitability for various engineering purposes such as road construction, foundations, and retaining walls.

The Atterberg Limits Test as per the Indian Standard Test (IS 2720) involves the following steps:

Sample preparation: A soil sample is prepared by removing any foreign materials and thoroughly mixing the soil. The sample is then air-dried and sieved through a 2 mm sieve to remove any coarse particles.

Liquid limit determination: A standard grooving tool is used to make a groove in a soil patty. The groove is then closed with a crank-operated cam, and the number of drops of the cam required to close the groove is counted. The test is repeated several times and the average number of drops is used to determine the liquid limit of the soil.

Plastic limit determination: A small soil sample is rolled into a thread that is 3.15 mm in diameter. The water content of the soil is then determined and the plastic limit is calculated.

Shrinkage limit determination: The soil sample is placed in a dish and the water content is determined. The dish is then placed in an oven at 110 ± 5°C for 24 hours, and the water content is determined again. The shrinkage limit is calculated as the difference between the two water content values.

Plasticity index determination: The plasticity index is calculated as the difference between the liquid limit and the plastic limit.

The test results are used to classify the soil into different groups based on the plasticity index and liquid limit values. The Indian Standard IS 2720 provides tables and charts for soil classification based on these values.