What Does Atterberg Limits Test Mean?
The Atterberg limits test is a classification test used to determine the moisture content at which fine-grained clay and silt soils transition between the different phases. The test for Atterberg limits is performed on the fraction of soil that will pass through a No. 40 or 425µm or 0.425mm sieve as per ASTM D 4318-00.
The test aids in the classification of soil and its plasticity characteristics and evaluates the shrink/swell potential of near-surface soil. It can be used to distinguish between silt and clay in its different types and determines the shrinkage limit (SL), plastic limit (PL), and liquid limit (LL) of the soil sample.
The Atterberg limits test is named after the Swedish chemist Albert Atterberg who was the first to develop a classification system to determine the different states and limits of soil consistency. Karl Terzhagi and Arthur Casagrande later refined and standardized the tests which are now widely used to determine the LL, PL, and SL of soils.
Clay soil changes consistency and behavior depending on its moisture content. The boundary at which the change in behavior takes place is defined based on the behavior of the soil sample. Depending on the moisture content, the soil may be in any of the four states:
The liquid limit (LL) and plastic limit (PL) are commonly used and are dependent on soil parameters such as particle size and specific surface area of the particles able to attract water molecules.
Trenchlesspedia Explains Atterberg Limits Test
The Atterberg limits test is critical in the early stages of structural design to ensure that the soil performs as expected. The test is performed only on soil fraction that passes through sieve No. 40 as per ASTM D 4318.
The apparatus for sample preparation and processing and determination of LL, PL, and SL consists of:
- Evaporating dishes – to mix specimen to desired moisture content.
- Spatula – to mix, form and smooth the soil specimen.
- Aluminum containers – for soil moisture samples.
- Mortar and pestle – to reduce particle size.
- Digital scale with 0.01g readability.
- Drying oven – for moisture content test.
- Liquid limit test accessory set including liquid limit machine and Casagrande grooving tool.
- Plastic limit test apparatus including plastic limit roller and glass plate.
- Shrinkage limit test apparatus including shrinkage dish, microcrystalline wax, petroleum jelly, fine thread, glass plate, and wax melting pot.
The soil samples for each test consist of soil that is able to pass through a No. 40 sieve and is prepared using standard methods. Moisture is adjusted by adding water and thoroughly mixing it. The sample is allowed to condition for at least 16 hours.
- Liquid Limit (LL) Test – A portion of the soil sample is spread in the brass cup of the liquid limit machine. It is then divided at the center using the Casagrande grooving tool. The liquid limit is reached when the groove closes a distance of 0.5inches along the bottom of the groove after 25 blows. The moisture content is noted. The test is conducted at varying moisture content for the same soil with the number of blows varying between 15 and 35.
- Plastic Limit (PL) Test – A small ball of moist plastic soil is repeatedly remolded and rolled out into a 1/8th inch (3.18 mm) thread. The moisture content at which the thread crumbles before it is completely rolled out is the plastic limit.
- Shrinkage Limit (SL) Test – A soil pat from the moist soil sample is molded into a special shrinkage dish. The dish along with the soil pat is oven-dried and weighed and the volume of the specimen is determined. The test is described in ASTM D4943.