Maya Ellison
The plastic limit of soil is a fundamental concept in geotechnical engineering, defining the moisture content threshold at which soil transitions to a plastic state. This limit is crucial for classifying soils and understanding their engineering properties, such as compressibility and permeability. To determine the plastic limit, laboratory tests are conducted to identify the moisture content where soil crumbles when rolled into threads of approximately 3.2 mm in diameter. This limit was first established by Albert Atterberg, a Swedish chemist, and has since been refined by several scientists, providing a basis for soil analysis and geotechnical design.
| Characteristics | Values |
|---|---|
| Definition | The plastic limit of soil is the moisture content at which soil begins to behave as a plastic material. |
| Plastic Limit vs. Liquid Limit | The liquid limit is the water content at which the behaviour of clayey soil changes from the plastic state to the liquid state. |
| Plastic Limit vs. Semi-Solid Limit | The boundary of water between the plastic states and semi-solid states of the soil is known as the plastic limit. |
| Plastic Limit vs. Solid Limit | When the water content decreases beyond a limit, the soil starts to behave as a solid. |
| Plastic Limit vs. Liquid Limit and Semi-Solid Limit | The liquid limit and plastic limit are basic soil properties used in geotechnical engineering. |
| Plastic Limit Calculation | The plastic limit is calculated by determining the average water or moisture content of samples. |
| Plastic Limit Test | The test involves rolling out a thread of soil on a flat, non-porous surface and observing if it retains its shape at a narrow diameter. |
| 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 | Measure weights of moisture cans, dry soil samples in an oven, calculate moisture content of each can, and calculate the average moisture content. |
| Plasticity Index | The plasticity index is the difference between the liquid limit and the plastic limit. Soils with a high plasticity index tend to be clay, while those with a low index tend to be silt. |
| Consistency Index | The consistency index indicates the soil's firmness and is calculated using the liquid limit, water content, and plastic limit. |
| Liquidity Index | The liquidity index scales the natural water content of a soil sample to the limit and is calculated using the natural water content, plastic limit, and liquid limit. |
| Flow Index | The flow index is the slope of the flow curve obtained by plotting water content against the number of blows on a logarithmic scale. |
| Bar Linear Shrinkage (LS) Parameter | The LS parameter is a measure of the longitudinal shrinkage of a remoulded soil paste specimen and is used as a control parameter for construction materials. |
| Brittleness | The onset of brittleness does not correspond to a fixed value of undrained shear strength and is not reliably measured by FC-based methods. |
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What You'll Learn
Plastic limit defined as the water content where soil behaves as plastic
The plastic limit of soil is a basic soil property commonly used in geotechnical engineering. It is defined as the water or moisture content percentage of a soil sample at which its behaviour changes from a plastic state to a semi-solid state. In other words, it is the water content at which the soil transitions from being mouldable to being brittle.
The plastic limit is one of the four Atterberg limits, which describe changes in the consistency state of fine-grained soils with varying water content. The other three Atterberg limits are the liquid limit, the shrinkage limit, and the solid limit. These limits were first defined by Swedish chemist and agronomist Albert Atterberg in 1911 and later refined by Austrian geotechnical engineer Arthur Casagrande.
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 its plastic limit, this thread will retain its shape down to a very narrow diameter of 3.2 mm (approximately 1/8 inch). 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 3.2 mm.
To calculate the plastic limit, the moisture content of the soil sample must first be determined. This can be done by drying the soil sample in an oven and then measuring its weight. The plastic limit is then calculated by finding the average water or moisture content of multiple samples.
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Plastic limit test procedures and calculations
The plastic limit test is a laboratory test used internationally to classify soils. The test is performed on material prepared for the liquid limit test, with the starting procedures for both tests being common. The plastic limit of a soil is the water or moisture content as a percentage of its dried weight.
The following apparatus is needed to carry out the test: a laboratory porcelain dish, spatula, soil sample, balance to an accuracy of 0.01g, wash bottle with distilled water, drying oven, glass plate, and similar-weight moisture cans.
- Measure the weights of the moisture cans and record their weights with their respective names on a sheet.
- Take the soil sample that passed the No. 40 sieve pan and slowly add water from the wash bottle while mixing the sample with a spatula.
- Form a ball from the watered soil sample using your palms. The ball should be non-sticky.
- Roll the thread until it achieves 3mm or 1/8 inches in diameter.
- Break the thread into pieces and repeat the above two steps for those broken pieces. Repeat this procedure until the rolling thread crumbles.
- Measure the weight of the crumbled soil collected in the moisture cans.
- Place the moisture cans (without the lids) in an oven for at least sixteen hours to dry them.
- Obtain the dry weights of all soil samples separately by measuring them at a temperature of 115 degrees Celsius.
- Calculate the moisture content of each can.
- Finally, calculate the plastic limit by calculating the average water or moisture content of the samples.
The plasticity index, which is defined as the difference between the liquid limit and the plastic limit, can be calculated using the results from the liquid limit and plastic limit tests. Soils with a high plasticity index tend to be clay, while those with a low plasticity index tend to be silt.
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Plastic limit and liquid limit are Atterberg limits
Plastic limit and liquid limit are two of the three Atterberg limits, which also include the shrinkage limit. These limits were first conceptualized by Swedish chemist and agricultural scientist Albert Atterberg in 1911. Atterberg's limits are a basic measure of the critical water content thresholds of a fine-grained soil beyond which it transitions between the solid, semi-solid, plastic, and liquid states. 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 plastic limit (PL), on the other hand, is the water content at which the soil transitions from a plastic to a semi-solid state.
The liquid limit is determined through three distinct methods. 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 against the palm of one's hand. Casagrande later standardized the apparatus and procedure, making the measurement more repeatable. The fall cone test, which is prevalent in Europe, is less dependent on the operator and provides more reliable results. The third method is the Casagrande test, widely used across North America.
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, 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 gravimetric moisture content where the thread breaks apart at a diameter of 3.2 mm (approximately 1/8 inch).
The plasticity index (PI) of the soil is defined as the difference between the liquid limit and the plastic limit. Soils with a high plasticity index tend to be clay, while those with a low plasticity index tend to be silt. The plasticity index is a measure of the plasticity of the soil and is used to classify soils for engineering and construction purposes. 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 considered inactive, while if it exceeds 1.4, the soil is considered active.
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Plasticity index and its calculation
The plasticity index (PI) is a measure of the plasticity of soil. It is the size of the range of water contents over which the soil exhibits plastic properties. In other words, the PI is the difference between the liquid and plastic limits. The liquid limit (LL) is the water content at which soil changes from a plastic to a liquid state. The plastic limit (PL) is the water content at the change from a plastic to a semi-solid state.
The plasticity index is calculated as follows: 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 an important concept in soil mechanics, particularly in the design of structures. Clays and silts interact with water, causing them to change size and have varying shear strengths. The plasticity index is used to ensure that the soil will have the correct amount of shear strength and will not change in volume too much as it expands and shrinks with different moisture contents.
The plastic limit is determined by rolling out a thread of the fine portion of a soil sample 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 decreases 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.
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Plastic limit and engineering behaviour
The plastic limit of soil is a fundamental concept in civil and geotechnical engineering, providing valuable insights into the behaviour of fine-grained soils, particularly those with plastic properties. This limit is defined as the water or moisture content threshold beyond which soil transitions from a plastic state to a semi-solid or solid state.
At the plastic limit, the soil loses its plasticity and becomes prone to crumbling when rolled into threads or cylinders with a diameter of approximately 3 mm. This behaviour is utilised in laboratory tests to determine the plastic limit of a soil sample. The procedure involves preparing a soil sample, drying it, and then rolling it into threads or cylinders until they begin to crumble. By measuring the water content at this point, engineers can establish the plastic limit.
The plastic limit is closely related to other concepts such as the liquid limit, shrinkage limit, and plasticity index, which collectively describe the consistency state of fine-grained soils. The liquid limit (LL) represents the moisture content at which the soil transitions from a liquid to a plastic state, gaining shear strength and resistance to flow. The shrinkage limit (SL) is the point at which the soil no longer changes volume upon drying, as the loss of moisture is compensated by air entering the pores.
The plasticity index (PI), calculated as the difference between the liquid limit and the plastic limit (PI = LL - PL), quantifies the range of moisture content over which the soil exhibits plastic behaviour. Soils with higher plasticity indices tend to be clay-rich, while those with lower indices tend to be silt-dominant. The plasticity index is a critical parameter in soil classification and engineering design, helping engineers predict and analyse various soil behaviours, including compaction, swelling potential, and shear strength.
In summary, understanding the plastic limit and its associated properties is essential in engineering practice. It enables engineers to characterise and predict the behaviour of fine-grained soils, particularly those with plastic characteristics. By considering the plastic limit and related parameters, engineers can make informed decisions regarding foundation design, earthworks, and other construction activities that are influenced by the mechanical properties of soil.
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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 3.2 mm (1/8 in) in diameter.
The liquid limit (LL) is the water content at which the behaviour of a clayey soil changes from a plastic state to a liquid state. The plastic limit (PL) is the boundary of water content where the soil changes from a plastic state to a semi-solid state.
The plasticity index (PI) is the difference between the liquid limit and the plastic limit: PI = LL - PL. Soils with a high PI tend to be clay, while those with a lower PI tend to be silt.
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.
You will need a laboratory porcelain dish, spatula, soil sample, balance to an accuracy of 0.01g, wash bottle with distilled water, drying oven, glass plate, and similar-weight moisture cans.
