Sofia Martinez
The plastic limit of soil is the moisture content at which it begins to behave as a plastic material. This limit is determined by rolling out a thread of soil on a flat, non-porous surface. If the soil is at its plastic limit, the thread will retain its shape down to a diameter of 3.2mm or 1/8 inches. To calculate the plastic limit, the weight of the soil sample is measured before and after it is heated to 115 degrees Celsius. The plastic limit is used in conjunction with the liquid limit to determine the plasticity index of the soil, which is a measure of the plasticity of the soil.
| Characteristics | Values |
|---|---|
| Plastic limit definition | Boundary of soil between the plastic and solid state |
| Plastic limit of soil | Moisture content as a percentage of its dried weight |
| Plasticity index (PI) | Measure of the plasticity of soil |
| Liquidity index (LI) | Scales the natural water content of a soil sample to the limit |
| Consistency index (Ic) | Indicates a soil's consistency (firmness) |
| Liquid limit (LL) | Water content at which the behaviour of clayey soil changes from plastic to liquid |
| Plastic limit determination | Rolling out a thread of soil on a flat, non-porous surface |
| Plastic limit test apparatus | Laboratory porcelain dish, spatula, soil sample, balance, wash bottle with distilled water, drying oven, glass plate, moisture cans |
| Plastic limit test procedure | Mix soil sample with water, form a ball, roll into a uniform thread, break and repeat, measure weight, dry in oven, obtain dry weight |
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What You'll Learn
- The plastic limit of soil is the moisture content at which soil behaves as a plastic material
- The plasticity index is the difference between liquid and plastic limits
- The liquidity index is used to scale the natural water content of a soil sample
- The consistency index indicates a soil's consistency or firmness
- The liquid limit is the point at which the behaviour of clayey soil changes from plastic to liquid
The plastic limit of soil is the moisture content at which soil behaves as a plastic material
The plastic limit of soil is the moisture content at which it begins to behave as a plastic material. The boundary between the different states of soil – solid, semi-solid, plastic, and liquid – can be defined by a change in the soil's behaviour, and this is known as the Atterberg limit. The plastic limit is, therefore, the point at which soil transitions from a semi-solid to a plastic state.
The plastic limit of soil is determined by rolling out a thread of soil on a flat, non-porous surface. The thread should be rolled until it reaches a diameter of 3.2mm or 1/8 inches. If the soil is at its plastic limit, the thread will retain its shape at this narrow diameter. As the moisture content decreases due to evaporation, the thread will begin to break apart at larger diameters. The plastic limit is then defined as the moisture content at which the thread breaks apart at a diameter of 3.2mm.
The plastic limit test is a laboratory test used internationally to classify different types of soils. The test is often carried out at the same time as the liquid limit test, as the starting procedures for both are common. The plastic limit of soil is expressed as a percentage of its dried weight.
The plasticity index (PI) is a measure of the plasticity of soil and is calculated by subtracting the plastic limit from the liquid limit (PI = LL-PL). The liquidity index (LI) is used to scale the natural water content of a soil sample to the limit and is calculated as LI = (W-PL)/(LL-PL), where W is the natural water content. The consistency index (Ic) indicates a soil's consistency or firmness and is calculated as CI = (LL-W)/(LL-PL), where W is the existing water content.
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The plasticity index is the difference between liquid and plastic limits
The plasticity of soil is a measure of its ability to act in a plastic manner. 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).
Soil may appear in one of four states depending on its water content: solid, semi-solid, plastic, and liquid. The boundary between each state can be defined based on a change in the soil's behaviour. The water content at which soil changes from one state to another is known as consistency limits or Atterberg's limit. These limits were created by Albert Atterberg, a Swedish chemist and agronomist, in 1911. 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 plastic limit (PL) is the moisture content at which soil begins to behave as a plastic material. At this water content, the soil will crumble when rolled into threads of 3.2mm (1/8in) in diameter.
The PI is used to classify soils. 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 PI of soil depends on the amount of clay present in the soil and gives a good indication of the soil's compressibility. The PI is also used to determine the amount of clay present in a soil sample.
The liquidity index (LI) is used to scale the natural water content of a soil sample to the limit. It can be calculated as a ratio of the difference between natural water content, plastic limit, and liquid limit: LI=(W-PL)/(LL-PL), where W is the natural water content. The consistency index (Ic) indicates a soil's consistency (firmness). It is calculated as CI = (LL-W)/(LL-PL), where W is the existing water content. The soil at the liquid limit will have a consistency index of 0, the soil at the plastic limit will have a consistency index of 1, and if W > LL, Ic is negative.
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The liquidity index is used to scale the natural water content of a soil sample
The liquidity index (LI) is used to scale the natural water content of a soil sample. It is a ratio of the difference between the natural water content of a soil sample and its plastic limit to its plasticity index. The plasticity index (PI) is the difference between the liquid and plastic limits. The plastic limit of a soil is the moisture content at which the soil begins to behave as a plastic material.
The plastic limit test is one of the laboratory tests used internationally to differentiate or classify soils into groups. The test is defined by ASTM Standard D 4318. 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 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 this diameter at any moisture level.
The liquidity index of soil begins from zero at the plastic limit and increases as the water content increases. When the water content reaches the liquid limit, the liquidity index becomes 1. If the water content is increased further, the value of the liquidity index becomes greater than 1, indicating that the soil is in a liquid state. At water content lower than the plastic limit, the soil is relatively harder and brittle in nature, and the liquidity index will be negative.
The liquidity index is also related to the consistency index (Ic), which indicates a soil's consistency or firmness. The consistency index of the soil at the plastic limit is 1, and at the liquid limit, it is 0. The sum of the liquidity index and consistency index is always equal to 1.
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The consistency index indicates a soil's consistency or firmness
The consistency index (Ic) indicates a soil's consistency or firmness. It is calculated as CI = (LL-W)/(LL-PL), where W is the existing water content. The consistency index is also called relative consistency. It is the ratio of the difference between the liquid limit and the natural water content of the soil to its plasticity index.
The liquidity index (LI) is used to scale the natural water content of a soil sample to the limit. It can be calculated as a ratio of the difference between natural water content, plastic limit, and liquid limit: LI = (W-PL)/(LL-PL), where W is the natural water content. The sum of the Liquidity index and Consistency index is equal to 1.
Soils with a high plasticity index (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 PI is the difference between the liquid and plastic limits (PI = LL-PL).
The plastic limit of a soil is the moisture content at which soil begins to behave as a plastic material. At this water content (plastic limit), the soil will crumble when rolled into threads of 3.2mm (1/8in) in diameter. 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).
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 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.
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The liquid limit is the point at which the behaviour of clayey soil changes from plastic to liquid
The liquid limit, along with the plastic limit, is used to calculate the plasticity index (PI), which 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 calculated by subtracting the plastic limit from the liquid limit (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 liquid limit is determined by testing the consistency and behaviour of the clayey soil. Swedish scientist Albert Atterberg was the first person to define the limits of soil consistency for the classification of fine-grained soils and later, they were refined by Arthur Casagrande. Atterberg's original liquid limit test involved mixing clay in a round-bottomed porcelain bowl of 10-12 cm diameter. A groove was cut through the clay with a spatula, and the bowl was struck many times against the palm of one hand. Casagrande subsequently standardised the apparatus and procedures to make the measurement more repeatable.
The liquid limit can be determined by plotting the moisture content of the soil, in percentage, and the corresponding number of blows on a semi-logarithmic graph. The moisture content corresponding to N 25, determined from the curve, is the liquid limit of the soil. The liquid limit can also be calculated using the flow curve equation: W = - If Log N + C. The liquidity index (LI) is another way to scale the natural water content of a soil sample to the liquid limit.
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Frequently asked questions
The plastic limit of soil is the moisture content at which soil begins to behave as a plastic material. At this water content, the soil will crumble when rolled into threads of 3.2mm or 1/8 inches in diameter.
The plastic limit is one of the parameters used to classify soils into groups. It is also used to distinguish between different types of clay and silts.
To calculate the plastic limit, you need to perform a laboratory test known as the plastic limit test. Here are the general steps:
- Take a soil sample and add water gradually while mixing.
- Form a non-sticky ball from the watered soil sample using your palms.
- Roll the soil ball on a glass plate using your palms or fingers to form a uniform thread. Apply pressure by exerting 80-90 strokes per minute.
- Continue rolling the thread until it reaches a diameter of 3mm or 1/8 inches.
- Break the thread into pieces and repeat the rolling process for those pieces.
- Collect the crumbled soil in moisture cans and dry them in an oven for at least 16 hours.
- Obtain the dry weights of the soil samples by measuring them at a temperature of 115 degrees Celsius.
