Atterberg Limits are used worldwide in the construction industry to classify fine-grained clay and silt soil. They provide an essential guide to understanding how much soil is likely to settle or consolidate under different loads and in certain conditions, depending on its water content.
Varying amounts of water in soil will influence the structure of soil particles in a soil mass by causing present clay and silt minerals to expand or shrink. This process affects the material's natural behaviour and engineering properties, leading to variations in size and shear strength.
When building on or with soil and similar materials, it's important to classify the soil before use to ensure it is suitable and safe for its intended use. Find out more below.
Introduction to Atterberg Limits
Swedish chemist and agronomist, Albert Atterberg, established the Atterberg Limits in 1911. They were later refined and standardised by geotechnical engineer Arthur Casagrande and collaborator Karl Terzaghi. The Atterberg Limits are now used internationally to identify, classify, and correlate soil strengths.
Depending on the moisture content, soil may be classified as either solid, semi-solid, plastic, or liquid. At low moisture content, soil behaves like a solid, while at high moisture content, it flows like a liquid. The Atterberg Limits test is conducted to determine the moisture content at which soils transition between these states.
There are three Atterberg Limits: The Liquid Limit (LL), the Plastic Limit (PL), and the Shrinkage Limit (SL). These can be used to classify engineering behaviours such as shrink / swell potential, shear strength, compressibility, and permeability. For example, if the moisture content is near the liquid limit, more settlement is likely. The opposite is true if the moisture content is near or below the plastic limit.
The 3 Key Atterberg Limit Parameters
The Atterberg Limits are crucial for identifying potential changes in soil behaviour with changes in water content. The three Atterberg Limits consist of the following key values:
- The Liquid Limit - this indicates the moisture content at which the soil no longer flows like a liquid
- The Plastic Limit – this indicates the moisture content at which the soil can no longer be remoulded without cracking
- The Shrinkage Limit – this indicates the moisture content at which the soil no longer changes volume upon drying due to the entry of air compensating for the loss of moisture
Atterberg Limits are especially useful in that they enable soil behaviour to be inferred. For instance, soil tests with similar Liquid Limits and Plasticity Index (PI) will generally have similar water content/strength relationships. This means that if such a relationship is identified for one soil, it can be inferred for a soil with similar Atterberg Limits.
Laboratory Testing Procedure & Interpretation
In the UK, Atterberg Limits testing is carried out in accordance with BS 1377‑2:1990, which provides the standard methods for determining the Liquid Limit, Plastic Limit, and Shrinkage Limit.
Each Atterberg Limit test consists of a sample prepared in its natural state or prepared using material passing through a 425µm test sieve. The samples are then prepared for each test using wet or dry methods described in industry standards.
To test for a Liquid Limit, a portion of the soil sample is spread into a brass cup and divided at the centre using a grooving tool. A machine is then used to lift the brass cup to a specified height before dropping it onto a hard, rubber base. Once the groove closes 0.5 inches after 25 drops, the current moisture content is defined as the Liquid Limit.
To test for a Plastic Limit, a small ball of moist, plastic soil is repeatedly remoulded and manually rolled out into a thread. The moisture content at which the thread crumbles before being completely rolled out is defined as the Plastic Limit.
To test for a Shrinkage Limit, a soil pat from the moist soil sample is moulded into a shrinkage dish. The dish and soil pat are oven-dried and weighed, and then the specimen's volume is determined by water displacement.
In the laboratory, these soil samples are also tested for their Plasticity Index (PI), Liquidity Index (LI), Consistency Index (CI), and Activity Number.
- PI = LL – PL
To calculate the Plasticity Index, the Plastic Limit is subtracted from the Liquid Limit, resulting in the range between the two boundaries. Soils with a high PI have a higher clay content. If the PI is higher than the mid-20s, the soil will expand in wet conditions and shrink in dry conditions.
- LI = (Natural Water Content - PL) ÷ PI
To calculate the Liquidity Index, the Plastic Limit is subtracted from the soil's natural water content, then divided by the Plasticity Index. Soils with an LL of 1 or higher will be closer to a liquid state, while soils with an LI of 0 or lower will be harder and more brittle.
- CI = (LL - Natural Water Content) ÷ PI
To calculate the Consistency Index, the Liquid Limit is subtracted from the soil's natural water content, then divided by the Plasticity Index. This figure is an indicator of relative shear strength – as CI increases, the soil's firmness or strength increases.
The Importance of Classifying Soil Before Use
From an engineering standpoint, in the early stages of structural design, material properties such as shear strength, forecast settlement, permeability, and compressibility are the most important for ensuring that the soil performs as expected.
Understanding the potential attributes of your soil under different conditions is crucial to ensuring the integrity of your construction projects. This could include pavements, embankments, soil infills, and more.
Soils intended to support loads must be evaluated by geotechnical professionals, such as Simtec, to predict their behaviour under applied loads and varying moisture conditions. If not tested, structures will be unreliable and hazardous to the surrounding people, buildings, and wildlife.
Atterberg Limits Testing by Simtec
Simtec's UKAS-accredited laboratories offer a comprehensive range of soil testing services for projects of all sizes across the UK.
Our team of experts can test your soils and earthworks for:
- Moisture content
- Plasticity index
- Frost heave
- Sulphate content
- Permeability
- Shear strength
- And much more
We also provide on-site WAC testing to help you identify potential hazards, ensure compliance, and reduce liability risk.
For more information about our soil testing services, visit our Soil Testing page.