Ethan Clarke
The Atterberg limits are a basic measure of the critical water content of a fine-grained soil, determining its shrinkage limit, plastic limit, and liquid limit. The plastic limit (PL) is defined as the gravimetric water content at which a rolled thread of soil breaks apart at a diameter of 3.2 mm. A high plastic limit indicates that a soil remains plastic over a wide range of moisture contents. This is because the plasticity index (PI), which is the difference between the liquid limit and plastic limit, is higher, indicating greater toughness and shear strength.
| Characteristics | Values |
|---|---|
| Plastic Limit (PL) | The gravimetric moisture content where the thread breaks apart at a diameter of 3.2 mm (about 1/8 inch) |
| Plasticity Index (PI) | The range of moisture contents where the silt or clay remains plastic |
| Plastic Limit Test | Performed by rolling a moist clay or silt soil into a 3 mm diameter 'worm' by hand, then remoulding the sample into a ball and repeating |
| Plasticity | The plasticity index, in combination with water content at the liquid limit, indicates how sensitive the soil is to changes in moisture content |
| Plastic Limit State | The water content at which soil changes from a plastic state to a solid state |
| Plastic Limit and Brittleness | Unlike the LL-state transition, the onset of brittleness does not correspond to a fixed value of undrained shear strength |
| Plastic Limit and Clay | The shearing strength of clay at the plastic limit is a measure of its toughness |
| Plastic Limit and Activity | If activity is less than 0.75, the soil is inactive. If activity exceeds 1.4, the soil is termed active |
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What You'll Learn
Plastic limit test
The plastic limit test is a part of the Atterberg limits tests, which are laboratory tests used to evaluate the behaviour of soils under varying moisture content. The Atterberg limits tests were developed in 1911 by Swedish chemist and agricultural scientist Albert Atterberg. They were later refined by Arthur Casagrande, an Austrian geotechnical engineer.
The plastic limit test specifically determines the water content at which the behaviour of soil changes from a plastic to a semi-solid state. This test involves rolling out a thread of the fine portion of soil on a flat, non-porous surface. If the soil is at a moisture content where its behaviour is plastic, this thread will retain its shape down to a very narrow diameter. The sample can then be remoulded and the test repeated. As the moisture content falls due to evaporation, the thread will begin to break apart at larger diameters. The plastic limit is defined as the moisture content where the thread breaks apart at a diameter of 3.2 mm (approximately 1/8 inch). A soil is considered non-plastic if a thread cannot be rolled out to 3.2 mm at any moisture content.
The plastic limit test is an important tool for geotechnical engineers to predict the behaviour of soils that are intended to support structures, pavements, or other loads. The test can be used to evaluate various soils that will have structures built upon them.
The plastic limit of a soil is also related to its activity. The activity of a soil is the ratio of the plasticity index to the clay size fraction. If the activity is less than 0.75, the soil is considered inactive, while if the activity exceeds 1.4, the soil is termed active. Soils with high activity are very chemically reactive and exhibit large volume increases when wetted and large shrinkage when dried.
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Plasticity index
The plasticity index (PI) is a measure of the plasticity of soil. It is the size of the range of water contents where the soil exhibits plastic properties. The PI is the difference between the liquid and plastic limits (PI = LL-PL).
Soils with a high PI tend to be clay, those with a lower PI tend to be silt, and those with a PI of 0 (non-plastic) tend to have little or no silt or clay. A high PI indicates an excess of clay in the soil, which results in greater plasticity. If the soil's plasticity index is small, this means the soil is plastic for a very short range of water content. This type of soil can hold very little water and, with a small increase in water, it reaches its liquid limit and starts flowing.
The plastic limit (PL) is determined by rolling out a thread of the fine portion of soil on a flat, non-porous surface. If the soil is at a moisture content where its behaviour is plastic, this thread will retain its shape down to a very narrow diameter. The plastic limit is defined as the gravimetric moisture content where the thread breaks apart at a diameter of 3.2 mm (approximately 1/8 inch). A soil is considered non-plastic if a thread cannot be rolled out to 3.2 mm at any moisture level.
The liquid limit (LL) is the water content at which the behaviour of clayey soil changes from the plastic state to the liquid state. The transition from plastic to liquid behaviour is gradual over a range of water contents, and the shear strength of the soil is not actually zero at the liquid limit. The liquid limit is determined by mixing a pat of clay in a round-bottomed porcelain bowl of 10-12 cm diameter. A groove is cut through the clay with a spatula, and the bowl is then struck many times against the palm of one's hand.
The plasticity index is used to determine the amount and type of clay present in a soil. It is also used to determine the soil's compressibility. The liquidity index (LI) is used to scale the natural water content of a soil sample to the limit. The consistency index (Ic) indicates a soil's consistency (firmness).
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Atterberg limits
The Atterberg limits are a basic measure of the critical water content of fine-grained soils, such as silt and clay, as they transition from a solid to a liquid. Depending on its water content, soil may appear in one of four states: solid, semi-solid, plastic, and liquid. In each state, the consistency and behaviour of the soil are different, and consequently, so are its engineering properties. Thus, the boundary between each state can be defined based on a change in the soil's behaviour.
The Atterberg limits can be used to distinguish between silt and clay and to distinguish between different types of silts and clays. The water content at which soil changes from one state to another is known as consistency limits, or Atterberg's limit.
The liquid limit (LL) is defined as the water content at which the behaviour of a clayey soil changes from the plastic state to the liquid state. The liquid limit characterizes the critical moisture content of soil transforming from the liquid state to the plastic state. The precise definition of the liquid limit is based on standard test procedures. The plastic limit (PL) is determined by rolling out a thread of the fine portion of a soil on a flat, non-porous surface. If the soil is at a moisture content where its behaviour is plastic, this thread will retain its shape down to a very narrow diameter. The plastic limit characterizes the critical moisture content of soil transforming from the plastic state to the solid state.
The shrinkage limit (SL) is the water content at which further loss of moisture will not result in more volume reduction. The shrinkage limit is much less commonly used than the liquid and plastic limits. The plasticity index (PI) is a measure of the plasticity of soil. The PI is the difference between the liquid and plastic limits. Soils with a high PI tend to be clay, those with a lower PI tend to be silt, and those with a PI of 0 (non-plastic) tend to have little or no silt or clay. The activity of soil is the ratio of the plasticity index to the clay size fraction. If the activity is less than 0.75, the soil is inactive. If the activity exceeds 1.4, the soil is termed active. If the activity lies within the above values, the soil will be moderately active.
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Liquid limit
The liquid limit (LL) is a measure of the water content at which the behaviour of a clayey soil changes from a plastic state to a liquid state. The transition from plastic to liquid behaviour is gradual over a range of water contents, and the shear strength of the soil is not zero at the liquid limit. The liquid limit is one of the Atterberg limits, which are a basic measure of the critical water contents of a fine-grained soil. The other Atterberg limits include the shrinkage limit and the plastic limit.
The Atterberg limits were created by Swedish chemist and agronomist Albert Atterberg in 1911 and later refined by Austrian geotechnical engineer Arthur Casagrande. Atterberg's original liquid limit test involved mixing clay in a round-bottomed porcelain bowl, cutting a groove through the clay with a spatula, and then striking the bowl repeatedly against the palm of one's hand. Casagrande subsequently standardised the apparatus and procedures to make the measurement more repeatable.
Today, the most popular instrument for determining the liquid limit is the liquid-plastic limit combined tester (cone), which is suitable for soils with a particle size of no more than 0.5 mm and an organic content of no more than 5% of the total mass of the sample. Another method for measuring the liquid limit is the fall cone test, also called the cone penetrometer test, which is more prevalent in Europe as it is less dependent on the operator in determining the liquid limit.
The liquid limit is determined by placing a clay sample in a standard cup and making a separation (groove) using a spatula. The cup is then dropped until the separation vanishes, and the water content of the soil is obtained from this sample. The test is performed again with increasing water content, with the number of blows required for the groove to close being recorded. The liquid limit of a clay (LL) is defined as the water content that corresponds to 25 blows.
The liquid limit is an important parameter in geotechnical engineering, as it helps evaluate the behaviour of soils intended to support structures, pavements, or other loads under applied forces and variable moisture conditions. The liquid limit also helps determine the adhesiveness index of the soil, which can be used to evaluate the risk of clogging in shield tunnelling through soil with high clay content.
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Engineering applications
The Atterberg limits are a basic measure of the critical water contents of a fine-grained soil. Depending on its water content, soil appears in one of four states: solid, semi-solid, plastic, and liquid. Consequently, the engineering properties of soil differ in each state.
The plastic limit (PL) is determined by rolling out a thread of the fine portion of soil on a flat, non-porous surface. If the soil is at a moisture content where its behaviour is plastic, this thread will retain its shape down to a very narrow diameter. The plastic limit is defined as the gravimetric moisture content where the thread breaks apart at a diameter of 3.2 mm.
The plasticity index (PI) is the range of moisture contents where the silt or clay remains plastic. It is the numerical difference between the liquid limit and the plastic limit for a particular material. The PI is a measure of the cohesive qualities of the binder resulting from the clay content. It also gives an indication of the amount of swelling and shrinkage that will result in the wetting and drying of the soil.
The Atterberg limits are used to distinguish between silt and clay and to distinguish between different types of silts and clays. The plasticity index, in combination with water content at the liquid limit, indicates how sensitive the soil is to changes in moisture content. This is important in engineering applications as it can determine the suitability of soil for a particular purpose. For example, in shield tunnelling, the liquid limit, plastic limit, and water content of the soil can be used to calculate the adhesiveness index to evaluate the risk of clogging.
Furthermore, the Atterberg limits can be used to determine the amount and type of clay present in a soil. This is important as the liquid limit of steel slag cannot be obtained due to the non-cohesiveness or reduced cohesiveness of the particles.
In summary, the plastic limit and plasticity index are important engineering applications of the Atterberg limits. They help determine the suitability of soil for a particular purpose, such as in shield tunnelling, and provide information about the amount and type of clay present in the soil.
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Frequently asked questions
A high plastic limit refers to the gravimetric moisture content at which a soil sample transforms from a plastic state to a solid state.
The plastic limit is determined by rolling out a thread of the fine portion of soil on a flat, non-porous surface. If the soil is at a moisture content where its behaviour is plastic, this thread will retain its shape down to a very narrow diameter. As the moisture content falls, the thread will begin to break apart at larger diameters.
The liquid limit is the water content at which the behaviour of a clayey soil changes from the plastic state to the liquid state. The plastic limit is the water content at which the soil changes from the plastic state to the solid state. The difference between these two limits is called the plasticity index, which indicates the magnitude of the range of moisture content over which the soil remains plastic.
