Alexei Volkov
The plasticity index is a measure of the range of moisture content over which a given soil will behave as a plastic material. It is the numerical difference between the liquid limit and the plastic limit for a particular material. A low plasticity index indicates that the soil contains fewer clay particles and, consequently, captures less water to become plastic. This article will explore the implications of a low plasticity index and how it affects soil behaviour.
| Characteristics | Values |
|---|---|
| Plasticity Index range | < 8.0 |
| Plasticity Index value | < 20-24 |
| Water content | Very short range |
| Water retention capacity | Low |
| Clay particles | Less |
| Coarse-grained soils | Cannot achieve a plastic state |
| Organic soils | Low Plasticity Index |
| Particle size | Larger |
| Sand proportion | Higher |
| Plastic material presence | Influences plasticity properties |
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What You'll Learn
Low PI value indicates less clay content
The plasticity index (PI) is a measure of the cohesive qualities of the binder resulting from the clay content in the soil. It indicates the magnitude of the range of moisture content over which the soil remains plastic. In other words, it is the size of the range of water contents where the soil exhibits plastic properties.
The PI is calculated as the numerical difference between the liquid limit (LL) and the plastic limit (PL) for a particular material. 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 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 PI of soil depends on the amount of clay present in the soil. A low PI value indicates that the soil contains fewer clay particles and, consequently, captures less water to make the soil plastic. Coarse-grained soils, for example, cannot achieve a plastic state of consistency because they do not contain clay minerals. Their liquid and plastic limits are said to coincide because, at higher water content, the soil starts flowing without even reaching the plastic limit.
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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Soil plasticity classification
The Atterberg limits consist of the shrinkage limit, plastic limit, and liquid limit. The shrinkage limit (SL) is the water content at which further moisture loss will not lead to additional volume reduction. This limit is less commonly used compared to the liquid and plastic limits.
The plastic limit (PL) is determined by rolling out a thread of fine soil on a flat, non-porous surface. If the soil is plastic, this thread will retain its shape down to a narrow diameter. The plastic limit is defined as the moisture content at which the thread breaks apart at a diameter of 3.2 mm. Soils are considered non-plastic if a thread cannot be rolled out to this diameter at any moisture level.
The liquid limit (LL) is the water content at which clayey soil transitions from a plastic to a liquid state. However, this transition is gradual, and the soil's shear strength is not zero at the liquid limit. The liquid limit is determined through standardised test procedures.
The plasticity index (PI) of soil is the range of moisture content over which the soil exhibits plastic behaviour. It is calculated as the difference between the liquid limit and the plastic limit. A low PI value indicates that the soil contains fewer clay particles and, consequently, holds less water to achieve a plastic state. In contrast, a high PI value suggests the soil can retain more water while remaining in a plastic state due to its higher clay content. Coarse-grained soils cannot attain a plastic state as they lack the necessary clay minerals.
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Plastic limit testing
The plasticity index (PI) of a soil sample is a measure of the range of moisture content over which the soil will exhibit plastic behaviour. It is the difference between the liquid limit (LL) and the plastic limit (PL). A low PI value indicates that the soil contains fewer clay particles and, therefore, holds fewer water molecules. Consequently, the soil will require less water content to become plastic.
The plastic limit test can be used to determine whether a soil is plastic or non-plastic. If a thread cannot be rolled out to 3.2 mm at any moisture content, the soil is considered non-plastic. Conversely, if the thread retains its shape at 3.2 mm, the soil is deemed plastic.
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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 Atterberg limits are: the shrinkage limit, plastic limit, and liquid limit. The shrinkage limit (SL) is the water content where 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 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 of 3.2 mm. The plastic limit is defined as the gravimetric moisture content where the thread breaks apart at a diameter of 3.2 mm. The liquid limit (LL) is conceptually defined as the water content at which the behaviour of a 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, and the shear strength of the soil is not actually zero at the liquid limit.
The plasticity index (PI) is a measure of the plasticity of soil. The plasticity index 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 plasticity index is the range of moisture content where a given soil will behave as a plastic material. A low plasticity index means that the soil contains fewer clay particles and, consequently, captures less water to make the soil plastic.
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Plasticity index and load
The plasticity index (PI) is a measure of the range of moisture content over which a given soil will behave as a plastic material. It is the numerical difference between the liquid limit (LL) and the plastic limit (PL) for a particular material. The liquid limit is the water content at which the behaviour of a clayey soil changes from a plastic to a liquid state. The plastic limit is determined by rolling out a thread of the fine portion of soil on a flat, non-porous surface and seeing if it retains its shape as the moisture content decreases due to evaporation.
Soil with a low plasticity index contains fewer clay particles and, therefore, captures less water to become plastic. The load simply increases the number of contacting asperities in each state of deformation. A plasticity index of < 0.5 will correspond to almost fully elastic behaviour, whatever the load. Conversely, a plasticity index of > 8.0 will correspond to almost fully plastic behaviour, whatever the load.
The plasticity index can be used to classify soils. For example, clay (C) is designated for soil combinations above the "A-line" for soils with PI > 7. Soil below the A-line and PI > 4, and above the A-line with below PI < 4 are considered silt, designated "M". Soils with LL > 50 are considered high plasticity, while those with LL < 50 are considered low plasticity.
The plasticity index of a soil can be altered by adding certain materials. For example, the plasticity index of stabilized soil decreases with the addition of CFA (Class C), with a greater rate of reduction for high plasticity clay due to its higher moisture content.
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Frequently asked questions
The Plasticity Index (PI) is the difference between the liquid limit and the plastic limit for a particular material. It indicates the range of moisture content over which the soil behaves like a plastic material.
A low plasticity index means that the soil contains fewer clay particles and, therefore, less water, making the soil less plastic.
The Plasticity Index is used to classify fine-grained soils. Soils with a PI greater than 7 are considered clay, while those with a PI between 4 and 7 are considered silt. Soils with a PI less than 4 are non-plastic.
A low PI indicates that the soil will have lower cohesive qualities and will be more susceptible to ravelling during dry weather due to the deficiency of clay binder.
