Omar Khalid
The Atterberg limits, named after Swedish chemist Albert Atterberg, define the boundaries between four states of soil consistency: solid, semi-solid, plastic, and liquid. The liquid limit (LL) and plastic limit (PL) are determined by the water content at which a change in soil consistency occurs. The liquid limit is the boundary between the liquid and plastic states, while the plastic limit is the boundary between the plastic and semi-solid states. These limits are important in geotechnical engineering as they help predict the behaviour of soil used in foundations, embankments, and pavements. The plasticity index (PI), calculated as the difference between the liquid and plastic limits, is an indicator of soil properties, including its clay content and potential to resist liquefaction.
| Characteristics | Values |
|---|---|
| Liquid Limit (LL) | Water content at which soil changes from a plastic to a liquid state |
| Plastic Limit (PL) | Water content at the change from a plastic to a semi-solid state |
| Plasticity Index (PI) | The range of water content over which a soil behaves plastically |
| Shrinkage Limit (SL) | Water content where further moisture loss will not result in more volume reduction |
| Liquidity Index (LI) | Used to scale the natural water content of a soil sample to the limit |
| Consistency Index (CI) | Indicates a soil's consistency (firmness) |
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What You'll Learn
Liquid limit test methods
The liquid limit of a soil sample is the water content at which the soil changes from a plastic to a liquid state. The transition from plastic to liquid behaviour occurs gradually over a range of water contents, and the shear strength of the soil is not zero at the liquid limit. The liquid limit is used in the classification of soils and provides an idea about the plasticity of the soil.
There are several methods to test the liquid limit of a soil sample. One method involves placing a portion of the soil paste in a cup of mechanical liquid limit device and spreading it with a spatula. The soil is then trimmed to a depth of 1 cm at the point of maximum thickness, and the excess soil is returned to the dish. The soil in the cup is then struck many times against the palm of one hand, or by turning a crank at the rate of two revolutions per second, until the two halves of the soil cake come in contact with each other for a length of about 12 mm. The number of blows required to cause the groove to close is recorded. The test is then repeated at least three more times for blows between 15 and 35. A 'flow curve' is then plotted on a semilogarithmic graph, and the moisture content corresponding to 25 drops is reported as the liquid limit of the soil.
Another method, known as the Casagrande test, involves mixing a pat of clay in a round-bottomed porcelain bowl of 10-12 cm diameter. A groove is then cut through the pat of clay with a spatula, and the bowl is struck many times against the palm of one hand. This method has been standardised by incorporating a crank-rotated cam mechanism to standardise the dropping action.
The fall cone test is another method used to determine the liquid limit of a soil sample. This test is more prevalent in Europe and elsewhere as it is less dependent on the operator and hence provides more reliable results.
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Plastic limit test methods
The plastic limit of a soil sample is the moisture content at which it transitions from a plastic to a semi-solid state. This is an important test for geotechnical engineers to evaluate the behaviour of soil that is intended to support structures, pavements, or other loads.
The plastic limit test involves rolling a thread of the fine portion of the soil sample on a flat, non-porous surface. The soil sample is rolled into a thread until it reaches a point where it crumbles. The test is repeated at least three times, and the average of the results is taken. 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 moisture content at which the thread breaks apart at a diameter of 3.2 mm (approximately 1/8 inch). The plastic limit test procedure is defined in ASTM Standard D 4318.
In the case of sandy soils, it is important to determine the plastic limit first. If the plastic limit cannot be determined, the plasticity index should be reported as non-plastic. The plasticity index is a measure of the range of water contents at which the soil exhibits plastic properties. It is calculated by subtracting the plastic limit from the liquid limit. Soils with a high plasticity index tend to have a higher clay content.
The liquid limit and plastic limit tests were first defined by Swedish chemist and agricultural scientist Albert Atterberg in 1911. Atterberg's original liquid limit test involved mixing clay in a round-bottomed porcelain bowl and striking the bowl against the palm of the hand. Arthur Casagrande, an Austrian geotechnical engineer, later refined and standardised the tests, incorporating a crank-rotated cam mechanism to standardise the dropping action. The Casagrande test is widely used across North America, while the fall cone test is more prevalent in Europe and elsewhere as it is less dependent on the operator.
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Plasticity index
The plasticity index (PI) is a measure of the plasticity of a soil sample. It is the difference between the liquid and plastic limits of a soil sample, and it is calculated using the formula: PI = LL-PL, where LL is the liquid limit and PL is the plastic limit. The liquid limit is the water content at which the behaviour of a clayey soil changes from a plastic to a liquid state, and the plastic limit is the water content at which the soil changes from a semi-solid to a plastic state. Atterberg originally defined the liquid limit by 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 later standardised the apparatus and procedures to make the measurement more repeatable.
The plasticity index is the size of the range of water contents where the soil exhibits plastic properties. Soils with a high PI tend to be clay, while those with a lower PI tend to be silt. Soils with a PI of 0 (non-plastic) tend to have little to no silt or clay. The plasticity index is used to determine the activity number of a soil sample, which indicates whether the soil is inactive, moderately active, or active. If the activity is less than 0.75, the soil is inactive; if the activity exceeds 1.4, the soil is active; and if the activity falls in between these values, the soil is moderately active.
The plasticity index is also used to calculate the liquidity index (LI) and consistency index (CI) of a soil sample. The liquidity index is a measure of the natural water content of a soil sample relative to its limit, and it is calculated using the formula: LI = (W-PL)/(LL-PL), where W is the natural water content. The consistency index is a measure of the firmness of a soil sample, and it is calculated using the formula: CI = (LL-W)/(LL-PL), where W is the existing water content. The soil at the liquid limit has a consistency index of 0, the soil at the plastic limit has a consistency index of 1, and if W > LL, then the consistency index is negative.
The plasticity index is used extensively in geotechnical engineering to evaluate the behaviour of soils. It is used to correlate with engineering behaviour such as compressibility, hydraulic conductivity (permeability), shrink-swell, and shear strength. The plasticity index is also used to evaluate the weathering characteristics of clay-shale materials. When clay-shale materials are subjected to repeated wetting and drying cycles, their liquid limits tend to increase, and the amount of increase is considered a measure of the shale's susceptibility to weathering.
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Soil mechanics applications
One of the primary applications of the liquid and plastic limits is in soil classification. The liquid limit (LL) and plasticity index (PI) are used to distinguish between different types of clays and silts. Soils with a high PI tend to be clay, while those with a lower PI tend to be silt, and non-plastic soils have a PI of 0. The plasticity index is calculated as the difference between the liquid and plastic limits (PI = LL-PL).
The liquidity index (LI) is another important concept in soil mechanics. It scales the natural water content of a soil sample to its limit. The LI is calculated using the formula: LI = (W-PL)/(LL-PL), where W represents the natural water content. Soils with a high LI are closer to the liquid state, while those with a LI of 0 or lower are harder and more brittle.
The consistency index (CI) or relative consistency is also a critical parameter in soil mechanics. It indicates the firmness or consistency of a soil sample. The CI is calculated as CI = (LL-W)/(LL-PL), where W is the existing water content. The consistency index plays a pivotal role in understanding the behaviour of saturated fine-grained soils in field conditions. A CI of 0 indicates the liquid limit, while a CI of 1 represents the plastic limit. If the consistency index is negative, it means that the natural water content of the soil is higher than its liquid limit, causing it to behave like a liquid.
Additionally, the Atterberg limits are used to predict the behaviour of soil infills, embankments, and pavements. They help assess shear strength, estimate permeability, and identify potentially expansive soils. These limits are also valuable in the preliminary stages of designing structures, ensuring that the soil has the required shear strength and minimal volume change during expansion and shrinkage due to varying moisture contents.
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Limitations
The Atterberg limits (liquid limit, plastic limit, and shrinkage limit) are useful for indicating the properties of soils, but they do have some limitations. One limitation is that the Atterberg limits are determined using standardised lab tests, which may not perfectly reflect the behaviour of soil in the field. The Casagrande test, for example, is widely used in North America, but it is more operator-dependent than the fall cone test used in Europe, which can lead to less reliable results.
Another limitation of the Atterberg limits is that they apply specifically to fine-grained clay and silt soils. Soils with larger particle sizes, such as sand, are not evaluated using these limits. Additionally, the Atterberg limits do not account for all the factors that can affect soil behaviour. For example, the presence of other materials in the soil, such as rocks or organic matter, can impact its properties and behaviour but are not considered in the calculation of the liquid and plastic limits.
The Atterberg limits also assume a gradual transition between the four states of consistency (solid, semi-solid, plastic, and liquid), but in reality, this transition can be more complex and non-linear. The limits are defined based on changes in the behaviour of the soil, but these changes can vary depending on factors such as temperature, pressure, and the specific composition of the soil.
Furthermore, the liquid limit and plastic limit tests focus on the water content of the soil, but other liquids or chemicals in the soil can also impact its behaviour. For example, the presence of salts or other solutes can affect the soil's consistency and strength. The Atterberg limits also do not account for dynamic changes in soil behaviour over time, such as the effects of weathering or the growth of roots and vegetation.
Finally, while the Atterberg limits provide a useful indication of the properties of soils, they are just one tool in soil mechanics. Other tests and analyses are often required to fully characterise a soil's behaviour and determine its suitability for a particular application, such as foundation design or pavement construction.
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Frequently asked questions
The liquid limit (LL) is the water content at which the behaviour of clayey soil changes from a plastic state to a liquid state. It is the boundary between the liquid and plastic states.
The plastic limit (PL) is the water content at which soil changes from a plastic state to a semi-solid state. It is the boundary between the plastic and semi-solid states.
Liquid and plastic limits are used to classify soils and predict their behaviour under different moisture conditions. The limits are particularly useful for determining the properties of soils that will have structures built on them.
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