When building foundations or making geotechnical recommendations, one of the key factors is how the soil may behave under various conditions. The water content in cohesive soils is a good indication of their behavior. Soils with little moisture behave more like solids, but as the water content increases, so does its plasticity. Excessive water will turn them into a liquid state. The Atterberg limits test helps soil engineers determine the moisture content at which silt soils transition from one moisture state to another and vice versa.
The results of these tests can tell how soil will behave under load. These tests also help classify silts and clays per their plasticity.
This article provides a comprehensive overview of the Atterberg limits test, including what it is, how to calculate it, the equipment you need, and much more.
Understanding Atterberg Limits Test
The Atterberg Limits tests help determine the behavior and engineering properties of fine-grained soils at different moisture content levels.
Depending on its moisture content, the soil may be in four possible states (solid, semi-solid, plastic, and liquid).
With the help of the Atterberg limits tests, you can tell the moisture level or limits at which the soil changes from one state to the other.
The limits are classified as liquid, plastic, and shrinkage limits of soils.
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Liquid limit: Moisture content required to turn the soil from a liquid state into a plastic state.
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Plastic limit: Moisture content needed to turn the soil from a plastic state into a semi-solid state.
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Shrinkage limit: The point at which a soil sample retains its volume despite losing moisture.
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Brief History of Atterberg Limits
Albert Atterberg, a chemist and agricultural scientist, classified fine-grained soils by defining the limits of soil consistency. However, he was the first person to do this in 1911.
He discovered that clay and silt, also known as cohesive soils, had a unique property of plasticity. He thus suggested that any soil with a particle size of 2µm (0.002mm) or less be classified as clay.
In 1920, Karl Terzhagi and Arthur Casagrande built on Atterbergs’s findings and suggestions. The latter, in particular, refined the test for geotechnical engineering applications.
We still use his approach, which consists of the liquid limit (LL), plastic limit (PL), and shrinkage limit (SL) to this day.
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How is the Atterberg Soil Limit Calculated?
The Atterberg soil limit is calculated as follows:
Moisture Content
You must determine the moisture content (MC) of soil samples to calculate their Atterberg Soil limit. The formula for MC is:
MC = 100% ∗ [(Wwt − Wdt)/ (Wdt − Wt)]
Where:
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Wwt = the weight of the container and wet soil
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Wdt = the weight of the container and dry soil
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Wt = the weight of the empty container
Liquid Limit Test
You can determine the liquid limit test from the “flow curve” in the Casagrande cup method. This method requires no calculation per se. However, if you’re using the One Point method, the formula is:
LL = MC ∗(N/25)0.121
Where:
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MC: Moisture content
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N: The number of blows or taps for the soil groove to close at the moisture content
Plastic Limit Test
The plastic limit is the average water content of soil moisture samples given by:
(MC1 + MC2 +MC3)/3
Plasticity Index (PI), Liquidity Index (LI), Consistency Index (CI)
You can use the values from the liquid limit, plastic limit, and moisture content to determine the plasticity index (PI), liquidity index (LI), and consistency index of soil.
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PI: PL − LL; The PI is the plastic limit subtracted from the liquid limit. Soils with high clay content have high PI values.
