In 1911, Swedish chemist and agricultural scientist Albert Atterberg discovered a way to distinguish between different types of finely-grained soil using the Atterberg Limits Test.
He found that plasticity is a unique property of cohesive soils such as clay and silt.
As a result, Atterberg suggested classifying these soils with a particle size of 2µm (0.002mm) or less as clays to differentiate them from silt.
In the early 1930s, engineers Karl Terzhagi and Arthur Casagrande recognized the value of characterizing soil plasticity for use in geotechnical engineering applications.
Casagrande helped to refine and standardize the tests.
Today, his methods determine the liquid limit, plastic limit, and shrinkage limit of soils.
In this blog, we’ll primarily focus on the Atterberg Limits Test, how these test methods work, and the significance of the limit values and calculated indexes.
Let’s get started.
1. What is the Atterberg Limits Test?
Before it can support structures, pavements, or other heavy loads, soil must be evaluated by geotechnical engineers to predict its behavior under applied forces and variable conditions.
Soil mechanics test in geotechnical laboratories measure all the following factors:
Particle size distribution
The potential for expansion or shrinkage of cohesive soils
Atterberg Limits Tests establish the moisture contents at which fine-grained clay and silt soils transition between various states (soil, semi-solid, plastic, and liquid), which is an important step in determining the soil’s strength, consistency and behavior.
2. Why are Atterberg Limits Tests important?
When moisture contents increase, clay and silt soils go through four distinct states of consistency.
Each of these stages shows significant differences in strength, consistency, and behavior.
Atterberg Limit Tests accurately define the boundaries between these states using moisture contents at the points where the physical changes occur.
The test values and derived indexes have direct applications in the foundation design of structures and in predicting the behavior of soil infills, embankments, and pavements.
These values assess the following:
Potentially expansive soils
3. What is liquid limit, plastic limit, and shrinkage limit?
Part of Atterberg Limits Test are the individual tests for liquid, plastic, and shrinkage.
The limits of all of these are obtained by direct measurement using standard test methods.
Here are the Atterberg Limits definitions of liquid limit, plastic limit, and shrinkage limit:
Liquid Limit (LL): The boundary between the liquid and plastic states.
Plastic Limit (PL): The boundary between the plastic and semi-solid states.
Shrinkage Limit (SL): The boundary between the semi-solid and solid states.
Here are the Casagrande definitions of liquid limit, plastic limit, and shrinkage limit.
The limits have been more specifically defined by Casagrande.
Liquid Limit (LL): The water content at which soil changes from a plastic to a liquid state when the soil specimen is just fluid enough for a groove to close when jarred in a specified manner.
Plastic Limit (PL): The water content at the change from a plastic to a semi-solid state.
This test involves repeatedly rolling a soil sample into a thread until it reaches a point where it crumbles.
Shrinkage Limit (SL): The water content where the further loss of moisture does not cause a decrease in specimen volume.
It’s the lowest water content at which soil can still be saturated.
4. What is the toughness index?
The shearing strength of clay at its plastic limit is a measure of its toughness.
It provides an indication of the shear strength of the soil.
5. What is the plasticity of soil?
The plasticity of soil is defined as the property of cohesive soil that allows it to undergo changes of shape without rupture or a change in volume.
Non-plastic soils also exist.
These are soils that do not have a plastic limit, or do not go through a plastic phase, such as fairly clean sand, rock dust, etc.
Typically, non-plastic soils make excellent road materials when properly confined under wearing course (i.e., rock dust).
That said, using non-plastic soil as a base can sometimes create problems in construction.
In this next section, we’ll discuss the plasticity index, which is the range of water content over which a soil behaves plastically.
It indicates the degree of plasticity of the soil and is technically defined as the difference between the liquid limit and plastic limit.
6. How do you calculate Atterberg soil indexes?
The Atterberg soil indexes compare the test values mathematically to express different plasticity and consistency characteristics.
Plasticity Index (PI) Calculation: PI = PL – LL
The plastic index is the plastic limit subtracted from the liquid limit.
It indicates the size of the range between the two boundaries.
Soils with a high PI have a higher clay content.
If the PI value is higher than mid-to-low 20s, then the soil may be expansive under wet conditions or shrink under dry conditions.
Liquidity Index (LI) Formula: LI = (PL – Natural Water Content) / PI
The liquidity index is determined by subtracting the plastic limit from the natural water content from the sample.
Then, you divide it by the plasticity index.
Soils with a LI of 1 or more will be close to the liquid state.
A liquid index of 0 or lower indicates soils that are harder and more brittle.
The liquid index permits the prediction of soil properties at different moistures.
Consistency Index (CI) of Soil Formula: CI = (LL – Natural Moisture Content) / PI
The consistency index (or relative consistency) is the liquid limit of the soil minus the natural moisture content divided by the plasticity index.
It is related to the liquidity index, and it is an indicator of the relative shear strength.
As the consistency index increases, the firmness (or shear strength) of the soil also increases.
7. How do you calculate the activity number?
The activity number of a soil sample is the ratio of the plasticity index to the clay fraction (or the percentage of the soil that is clay).
If soils have an activity number of 1.25, then they are considered active and will have an increased volume change in response to moisture conditions.
These soils will expand in wet conditions and shrink in dry conditions.
8. What are the Atterberg Limits Test procedure?
For the Atterberg Limits Test, soil samples are passed through a No. 40 (425µm) test sieve.
These soils should be prepared for each testing using wet or dry methods described in the standards.
The moisture in the test specimens will be adjusted by adding water and being mixed with a spatula.
The soil must be allowed to condition for at least 16 hours.
Here are the procedures for each individual test:
Liquid limit: It’s measured by spreading a portion of the soil sample in the brass cup of a liquid limit machine and dividing it using a grooving tool.
The liquid limit is the moisture content when the groove closes for ½ inch after 25 drops of the cup.
The test methods used are ASTM D4318 and AASHTO T 89.
Plastic limit: It’s determined by repeatedly remolding a small ball of moist plastic soil and manually rolling it out into a 1/8-inch thread.
A plastic limit roller device can also be used to perform this test.
The plastic limit is the moisture content at which the thread crumbles before being completely rolled out.
Standard test methods are ASTM D4318 and AASHTO T 90.
Shrinkage limit: It’s performed by molding a soil pat of moist test material into a special shrinkage dish.
The dish and soil are oven-dried and weighed.
The volume of the specimen is determined by water displacement.
The shrinkage limit is performed the least often and it’s described in ASTM D4943.
9. What equipment do you need for the Atterberg Limits Test?
Here is the equipment you’ll need to perform the Atterberg Limits Test broken down into categories.
Sample Preparation and Processing
- Evaporating Dishes – used to mix specimens to the desired moisture content
- Spatula – used to mix, form, and smoothen soil specimen
- Wash Bottle – used to dispense mixing water
- Aluminum Containers – used for soil moisture samples
- Mortar and Pestle – used for particle size reduction
- Washing Pan – used for convenient clean-up of bowls and spatulas
- Liquid limit and Plastic Limit Test Accessory Set – includes items needed to run most Atterberg Limits Tests
- Soil Grinder – optional for efficient particle size reduction
- Digital Lab Scale or Balance – must have a 0.01g readability
- Drying Laboratory Oven – used for moisture content tests
Liquid Limit Test
- Liquid Limit Machine – either motor-driven or manually operated
- AASHTO or ASTM (Casagrande) Grooving Tool
Plastic Limit Test
- Glass Plate – used to roll out threads
- Plastic Limit Apparatus – used as an optional plastic limit roller apparatus for fast and consistent rolling of samples
Shrinkage Limit Test
- Shrinkage Limit Test Apparatus – includes items needed for shrinkage limit test
- Shrinkage Dish – used to coat specimens, supplied in 5lbs (2.3kg) quantities
- Microcrystalline Wax – used to coat shrinkage dish
- Petroleum Jelly – used for suspending weight of the specimen
- Glass Plate – used for calibration of the dish
- Wax Melting Pot – used to prepare wax for sample immersion
10. When would you use the Atterberg Limits Test in daily life?
As you research the Atterberg Limits Test, it seems complex, and it may leave you wondering, “When would I use this in daily life?”
These soil tests are most important to use when you’re trying to build with or on plastic soils.
The Atterberg Limits represent the moisture contents as which a specific soil’s behavior changes from soil to plastic (Plastic Limit) and from plastic to liquid (Liquid Limit).
This is essential information to have to ensure your structural design is sound.
The Atterberg Limits Test is a classification test used to determine the moisture content at which fine-grained clay and silt soils transition between different phases.
Performing these tests allow you to evaluate the shrink and swell potential of surface soil.
You’ll want to perform these tests in the early stages of structural design to ensure that the soil you’re working with performs as expected during construction.
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