Omar Khalid
The plastic limit of soil is a measure of its moisture content, specifically the point at which a fine-grained soil can no longer be remoulded without cracking. This limit is important in geotechnical engineering as it helps to determine the soil's classification and engineering properties such as compressibility, permeability, and strength. The plastic limit is one of the Atterberg limits, which describe changes in the consistency state of fine-grained soils with varying water content. These limits were created by Swedish chemist Albert Atterberg in 1911 and later refined by Austrian geotechnical engineer Arthur Casagrande.
| Characteristics | Values |
|---|---|
| Definition | The plastic limit (PL) is the moisture content at which a fine-grained soil can no longer be remolded without cracking. |
| Determination | The plastic limit is determined by rolling out a thread of the fine portion of soil on a flat, non-porous surface. |
| Test Procedure | ASTM Standard D 4318. If the soil is plastic, the thread will retain its shape down to a narrow diameter. As moisture content falls, the thread will break apart at larger diameters. |
| Plastic Limit Defined | Gravimetric moisture content where the thread breaks at a diameter of 3.2 mm (approximately 1/8 inch). |
| Non-Plastic Soil | If a thread cannot be rolled out to 3.2 mm at any moisture content, the soil is considered non-plastic. |
| Plasticity Index (PI) | A measure of the plasticity of soil; the range of water contents where the soil exhibits plastic properties. |
| Relationship with PI | The plasticity index is the difference between the liquid and plastic limits (PI = LL - PL). |
| PI and Soil Type | Soils with a high PI tend to be clay, lower PI soils tend to be silt, and non-plastic soils have little to no silt or clay. |
| Atterberg Limits | Used to distinguish between silt and clay and between different types of silts and clays. |
| Atterberg Limit Creator | Albert Atterberg, a Swedish chemist and agronomist, in 1911. |
| Atterberg Limit Refinement | Arthur Casagrande, an Austrian geotechnical engineer, refined the limits in collaboration with Karl Terzaghi (both pioneers of soil mechanics). |
| Applications | Used for soil classification and to estimate geotechnical design parameters such as compaction, swelling potential, and shear strength. |
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What You'll Learn
- Plastic Limit (PL) is the moisture content at which fine-grained soil can no longer be remoulded without cracking
- The PL is determined by rolling out a thread of soil on a flat, non-porous surface
- Atterberg's liquid limit test involves mixing clay in a bowl and striking it against the palm of a hand
- The plasticity index (PI) is a measure of the plasticity of soil and is the range of water contents where the soil exhibits plastic properties
- The PI is the difference between the liquid and plastic limits (LL and PL)
Plastic Limit (PL) is the moisture content at which fine-grained soil can no longer be remoulded without cracking
The plastic limit (PL) is a measure of the moisture content at which fine-grained soil can no longer be remoulded without cracking. It is one of the Atterberg limits, which describe changes in the consistency state of fine-grained soils with varying water content. These limits were first 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 standardised the apparatus and procedures to make the measurement more repeatable.
The plastic limit is determined by rolling out a thread of fine soil on a flat, non-porous surface. If the soil is at a moisture content where it behaves plastically, this thread will retain its shape down to a very narrow diameter. The sample can then be remoulded and the test repeated. As moisture content falls due to evaporation, the thread will begin to break apart at larger diameters. 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 a plastic to a liquid state. The plasticity index (PI) is a measure of the plasticity of soil and 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.
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The PL is determined by rolling out a thread of soil on a flat, non-porous surface
The plastic limit (PL) is the moisture content at which a fine-grained soil can no longer be remoulded without cracking. The plastic limit is determined by rolling out a thread of the fine portion of soil on a flat, non-porous surface. This procedure is defined in ASTM Standard D 4318.
If the soil is at a moisture content where its behaviour is plastic, the 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 gravimetric moisture content where the thread breaks apart at a diameter of 3.2 mm (about 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 liquid limit (LL) is the water content at which the behaviour of clayey soil changes from the plastic state to the liquid state. However, the transition from plastic to liquid behaviour is gradual over a range of water contents. The precise definition of the liquid limit is based on standard test procedures.
Atterberg's original liquid limit test involved mixing a pat of clay in a round-bottomed porcelain bowl of 10-12 cm diameter. A groove was cut through the pat of clay with a spatula, and the bowl was then struck many times against the palm of one hand. Casagrande later standardised the apparatus and procedures to make the measurement more repeatable.
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.
The standard thread rolling method for determining the plastic limit of soil has been criticised for requiring considerable judgement from the operator that carries out the test. The thread bending test is an alternative method that has been shown to produce similar results to the thread rolling test, but with minimal operator interference.
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Atterberg's liquid limit test involves mixing clay in a bowl and striking it against the palm of a hand
The plastic limit of soil is the moisture content at which a fine-grained soil can no longer be remoulded without cracking. It is one of the two Atterberg limits, along with the liquid limit, which are used to identify and classify soils. These limits were first defined by Swedish chemist and agricultural scientist Albert Atterberg in 1911. Atterberg's liquid limit test involves mixing clay in a bowl and striking it against the palm of a hand.
The procedure for Atterberg's liquid limit test is as follows: a pat of clay is mixed in a round-bottomed porcelain bowl with a diameter of 10-12 cm. A groove is then cut through the clay using a spatula. The bowl is then struck against the palm of one's hand repeatedly. Arthur Casagrande later standardised this procedure by incorporating a crank-rotated cam mechanism to standardise the dropping action. This mechanism lifts the bowl and then drops it from a height of 10mm.
Casagrande also defined the procedure for the plastic limit test, which involves rolling an ellipsoidal-sized soil mass by hand on a non-porous surface. The plastic limit is the water content at which a thread of soil, with a diameter of 3mm, just crumbles.
The liquid limit is the water content at which the soil changes from a plastic to a liquid state. The plastic limit is the water content at which the soil changes from a plastic to a semi-solid state. These two limits are used to determine the consistency and behaviour of clayey soils, which can vary depending on their moisture content. The Atterberg limits are used internationally for soil identification, classification, and strength correlations.
The plasticity index is a measure of the plasticity of soil and is the difference between the liquid and plastic limits. Soils with a high plasticity index tend to be clay, while those with a lower index tend to be silt. The Atterberg limit tests are used to establish the moisture contents at which fine-grained clay and silt soils transition between the four states of solid, semi-solid, plastic, and liquid.
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The plasticity index (PI) is a measure of the plasticity of soil and is the range of water contents where the soil exhibits plastic properties
The plasticity of a soil refers to its ability to change shape without cracking or breaking apart. The plasticity index (PI) is a measure of the plasticity of soil and is defined as the range of water contents within which the soil exhibits plastic properties. In other words, it is the difference between the liquid limit (LL) and the plastic limit (PL) of the soil, i.e., PI = LL - PL.
Soil can exist in one of four states: solid, semi-solid, plastic, and liquid. The transition between these states depends on the water content in the soil, which changes its consistency and behaviour. Atterberg's limits define the boundaries between these states, with the liquid limit being the point at which the soil transitions from a plastic to a liquid state, and the plastic limit being the point at which it transitions from a semi-solid to a plastic state.
The plastic limit of a soil is determined by rolling out a thread of the fine portion of the soil on a flat, non-porous surface. If the soil is at a moisture content where it behaves plastically, the thread will retain its shape down to a very narrow diameter. As the moisture content decreases due to evaporation, the thread will begin to break apart at larger diameters. The plastic limit is defined as the moisture content at which the thread breaks apart at a diameter of 3.2 mm (approximately 1/8 inch).
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 to no silt or clay. The PI is used to classify soils and is related to other engineering properties such as compressibility, permeability, and strength.
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The PI is the difference between the liquid and plastic limits (LL and PL)
The plastic limit of a soil sample is determined through a laboratory test and refers to the moisture content percentage at which the soil passes from a plastic to a semi-solid state. At this limit, the soil can be molded but does not flow—a critical parameter in geotechnical engineering for understanding soil behavior and designing foundations and structures. The plastic limit is one of the key indicators used to classify soils, along with the liquid limit, and these two values are used to calculate another important soil property, the plasticity index (PI).
The plasticity index is a simple measure of the plasticity of a soil, which is its ability to deform without breaking or cracking. Soils with a high PI are generally considered more plastic and include clay and silty clay soils. Soils with a low PI are less plastic and tend to be sandy or gravelly. The PI is calculated as the difference between the liquid limit and the plastic limit (LL and PL), providing an indication of the range of water content over which the soil exhibits plastic behavior.
To put it simply, the liquid limit tells us the moisture content at which a soil specimen becomes plastic, while the plastic limit tells us the moisture content at which it becomes semi-solid and loses its plasticity. The difference between these two limits, as indicated by the PI, gives us an idea of the width of this plastic range. A higher PI indicates a wider range, suggesting that the soil can maintain its plasticity over a broader range of moisture contents. Conversely, a lower PI indicates a narrower plastic range.
It's important to note that the plastic limit and resulting plasticity index are not applicable to all types of soils. For example, purely sandy soils do not have a plastic range and therefore do not have a defined plastic limit or plasticity index. This is because sandy soils do not exhibit plastic behavior; they do not maintain a given shape when molded. As a result, the plastic limit and plasticity index are most commonly used for clayey soils and soils with a significant fine-grained fraction.
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Frequently asked questions
The plastic limit (PL) is the moisture content at which a fine-grained soil can no longer be remoulded without cracking.
The plastic limit is determined by rolling out a thread of fine soil on a flat, non-porous surface. If the soil is at a moisture content where it behaves plastically, the thread will retain its shape down to a narrow diameter. The sample can then be remoulded and the test repeated.
The liquid limit (LL) is the water content at which the behaviour of 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.
Atterberg's original liquid limit test involved mixing clay in a round-bottomed porcelain bowl. A groove was then cut through the clay, and the bowl was struck against the palm of one's hand. This procedure was later standardised by Casagrande, who incorporated a crank-rotated cam mechanism to standardise the dropping action.
The plasticity index (PI) is a measure of the plasticity of soil. It 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 tend to have little to no silt or clay.
