Soil Atterberg Limits Testing: Determination of Liquid Limit, Plastic Limit, and Plasticity Index

The soil Atterberg limits test is a fundamental geotechnical laboratory procedure used to determine the liquid limit, plastic limit, and plasticity index of fine-grained soils, providing essential index properties for soil classification and engineering design. Accredited third-party laboratories perform this test in accordance with recognized standards such as ASTM D4318, ISO 17892-12, and BS 1377-2 to support foundation design, pavement engineering, and construction quality control. This article presents a comprehensive technical overview of the test principles, sample requirements, testing methods, and practical applications.

Technical Overview

The Atterberg limits test, originally developed by Swedish chemist Albert Atterberg and later standardized for engineering practice, defines the water contents at which fine-grained soils transition between different consistency states. As water content changes, a cohesive soil passes through four states: liquid, plastic, semi-solid, and solid. The boundary water contents separating these states are known as the Atterberg limits, which serve as the most widely used index properties for characterizing the plasticity of clays and silts.

Three primary limits are defined in geotechnical engineering:

• Liquid Limit (LL or wL): the water content at which a soil transitions from a plastic state to a viscous liquid state, corresponding to a nominal undrained shear strength of approximately 1.7 to 2.5 kPa.
• Plastic Limit (PL or wP): the water content at which a soil transitions from a plastic state to a semi-solid, brittle state, identified by crumbling when rolled into a thread of approximately 3 mm diameter.
• Shrinkage Limit (SL): the lowest water content at which the soil remains fully saturated; below this value, further drying produces no additional volume reduction.

The plasticity index (PI), calculated as PI = LL − PL, quantifies the range of water content over which a soil exhibits plastic behavior. Together with the Casagrande plasticity chart, these parameters form the basis of the Unified Soil Classification System (USCS) and the AASHTO soil classification system, and they correlate strongly with engineering properties such as compressibility, permeability, swelling potential, and undrained shear strength.

Testing Samples

Fine-Grained Soils

The test is applicable to cohesive soils and fine-grained fractions of mixed soils, including clays, silts, clayey sands, and silty sands. Only the fraction passing the 425 μm sieve (No. 40 sieve) is tested, since Atterberg limits are meaningful only for materials containing sufficient fines to exhibit plasticity.

Disturbed (Remolded) Samples

Disturbed bulk samples are the most common sample type, as the Atterberg limits are inherently measured on remolded soil. Samples are typically obtained from split-spoon samplers, auger cuttings, stockpiles, borrow pits, or compacted fills. A minimum of approximately 250 to 500 grams of material passing the 425 μm sieve is generally recommended for a complete multi-point test sequence.

Undisturbed Samples

Undisturbed tube or block samples may be submitted when the natural water content and liquidity index must be evaluated alongside the limits. Testing the natural water content of an undisturbed specimen in combination with the Atterberg limits allows calculation of the liquidity index (LI), which indicates the in-situ consistency of the deposit relative to its plastic range.

Sample Preparation

Depending on the specified method, samples are either air-dried prior to testing or processed in the as-received moisture condition, as drying can irreversibly alter the plasticity of certain clays, particularly those containing heat-sensitive minerals or organic matter. The material is mixed with distilled water to a uniform paste, cured for an appropriate equilibration period (commonly up to 24 hours), and then tested. Contamination, segregation, and incomplete hydration of the clay fraction must be avoided during preparation.

Testing Items

A standard Atterberg limits testing program typically reports the following parameters:

• Liquid Limit (LL), expressed as a percentage of dry mass
• Plastic Limit (PL), expressed as a percentage of dry mass
• Plasticity Index (PI = LL − PL)
• Liquidity Index (LI = (w − PL) / PI), when natural water content is determined
• Consistency Index (CI = (LL − w) / PI), where applicable
• Shrinkage Limit (SL), where specified by the project scope
• Natural (as-received) water content, where required
• Classification plotting position on the Casagrande plasticity chart (CL, CH, ML, MH, CL-ML, OL, or OH designation)

Non-plastic (NP) soils that cannot be rolled into a thread at any water content are reported as non-plastic, which is itself an important classification result.

Testing Methods

Casagrande Liquid Limit Device Method (ASTM D4318 Method A/B)

The classical Casagrande method uses a brass cup that drops onto a hard rubber base. A groove is cut through a soil pat placed in the cup using a standard grooving tool, and the cup is dropped repeatedly at a rate of approximately two blows per second. The number of blows required to close the groove over a length of 13 mm is recorded together with the corresponding water content. In the multi-point method (Method A), at least three to five trials spanning blow counts of roughly 15 to 35 are performed, and the liquid limit is read from a flow curve (water content versus log of blow count) at 25 blows. A one-point method (Method B) is also permitted when properly validated.

Fall Cone Penetrometer Method (ISO 17892-12, BS 1377-2)

The cone penetrometer method, preferred in European and many international practice contexts, uses a standardized cone (typically 80 g with a 30° apex angle, or 60 g with a 60° angle depending on the standard) released to penetrate the soil surface for five seconds. The liquid limit is defined as the water content at which the cone penetration equals 20 mm (or 17 mm for the 60 g/60° configuration), determined from a calibration curve of penetration versus water content. This method offers improved repeatability and reduced operator dependence compared with the Casagrande cup.

Thread Rolling Method for Plastic Limit (ASTM D4318)

The plastic limit is determined by rolling a portion of the soil paste into a thread between the fingers or with a rolling device on a ground-glass plate. The soil is repeatedly rolled to a diameter of 3.2 mm (1/8 inch) and reaggregated until the thread crumbles at that diameter. The water content of the crumbled portions is measured, and the plastic limit is reported as the average of acceptable trials. Instrumental and pressure-based plastic limit methods are also available under ISO 17892-12 where specified.

Shrinkage Limit Determination

Where required, the shrinkage limit is determined by measuring the volume change of a soil pat during oven drying, using either the mercury displacement technique or the water displacement method, in accordance with ASTM D4943 or the relevant national standard.

Commonly referenced standards include ASTM D4318 and AASHTO T89/T90 (North American practice), ISO 17892-12 (international geotechnical practice), BS 1377-2 (United Kingdom), and the GB/T 50123 geotechnical testing standard series. Test method selection should align with the project specification, and results from different methods should not be interchanged without recognizing potential systematic differences.

Applications

Atterberg limits test results are used throughout geotechnical and geoenvironmental engineering, including:

• Soil classification under the Unified Soil Classification System (USCS) and AASHTO system, using the Casagrande plasticity chart with the A-line and U-line boundaries.
• Identification and ranking of expansive soils with high swell-shrink potential, supporting foundation mitigation design for light structures.
• Subgrade characterization and pavement design, including resilient modulus correlations and frost susceptibility screening.
• Quality control and acceptance of engineered fill, embankments, compacted clay liners, and dam cores, where plasticity specifications are frequently imposed.
• Assessment of soil consistency and sensitivity through the liquidity index, supporting slope stability and settlement evaluations.
• Empirical correlations with compressibility (compression index), permeability, and remolded undrained shear strength used in preliminary design.
• Screening of fine-grained tailings and dredged sediments for dewatering behavior and disposal classification.
• Supplementary input to liquefaction susceptibility screening of low-plasticity silts.

Because plasticity reflects clay mineralogy and particle surface chemistry, the test also provides insight into material suitability for construction, soil stabilization with lime or cement, and groundwater interaction in containment applications.

FAQ

What is the soil Atterberg limits test used for?

The test determines the liquid limit, plastic limit, and plasticity index of fine-grained soils. These index properties are used to classify soils, estimate their engineering behavior, evaluate expansive soil risk, and verify compliance with construction specifications.

How much sample is required for testing?

Approximately 250 to 500 grams of material passing the 425 μm (No. 40) sieve is typically sufficient for a complete multi-point determination of both the liquid limit and plastic limit. Additional material is needed if shrinkage limit or natural water content is also required.

Which test method should be selected, the Casagrande cup or the cone penetrometer?

The choice depends on the governing project specification and regional practice. ASTM D4318 traditionally uses the Casagrande cup, while ISO 17892-12 and BS 1377-2 emphasize the fall cone penetrometer, which generally provides better repeatability. Both methods yield comparable liquid limits for most inorganic soils when performed correctly.

Can the test be performed on organic or highly calcareous soils?

Yes, but results should be interpreted with caution. Organic matter and some minerals alter the plasticity behavior, and standards provide modified classification zones for organic soils. Drying method selection (air-dried versus as-received) is particularly important for these materials.

What does it mean if a soil is reported as non-plastic?

A non-plastic (NP) result means the soil cannot sustain rolling into a 3 mm thread at any water content. Such soils, typically clean silts, sands, or gravels with minimal fines, are classified accordingly and generally pose low shrink-swell risk.

How long does testing take?

Standard turnaround is typically two to five working days after sample receipt, which includes moisture equilibration, multi-point flow curve determination, plastic limit trials, oven drying, and calculation. Expedited scheduling may be available from accredited third-party laboratories when project timelines demand it.