They describe how fine-grained soils behave as moisture content changes, and in doing so they underpin soil classification, design assumptions and risk management across construction and infrastructure projects. When interpreted correctly, they help engineers understand how the ground will perform. When they are wrong, the consequences can be expensive - or worse.

Understanding Atterberg Limits

Atterberg Limits define the boundaries between different states of consistency in fine-grained soils:

• Liquid Limit (LL) marks the transition from plastic to liquid behaviour
• Plastic Limit (PL) marks the transition from semi-solid to plastic behaviour
• Plasticity Index (PI), the difference between LL and PL, expresses the range of moisture contents over which a soil remains plastic

Together, they provide a practical framework for understanding how soils respond to changes in water content — something that sits at the heart of most geotechnical risk.

Soil classification: the starting point for good design

Atterberg Limits are fundamental to soil classification in accordance with recognised British standards: BS 5930:2015+A1:2020, BS EN ISO 14688-1:2018 and BS EN ISO 14688-2:2018.

They allow engineers to distinguish between clays, silts and intermediate materials, and to identify whether a soil is cohesive, moisture-sensitive or prone to volume change.

This classification is not academic. It directly informs ground models, design assumptions and construction strategies. If the classification is wrong, everything built on it is vulnerable.

Predicting engineering behaviour

Atterberg Limits also correlate strongly with key engineering properties that affect performance on site:

• Strength: Soils with higher LL and PI values often exhibit lower shear strength at elevated moisture contents
• Compressibility: High-plasticity clays typically compress more under load, increasing settlement risk
• Permeability: Highly plastic soils tend to have lower permeability, influencing drainage and pore pressure behaviour
• Workability: Low-plasticity soils are generally easier to excavate, place and compact during earthworks

These relationships help engineers design foundations, embankments, pavements and retaining structures with appropriate safety margins - and realistic expectations of ground behaviour.

Shrink–swell risk and ground movement

One of the most critical uses of Atterberg Limits is in assessing shrink–swell potential.

Soils with high LL and PI values are more likely to expand when wet and shrink when dry. This behaviour can cause significant ground movement, particularly beneath structures that are sensitive to differential settlement such as housing, roads, pipelines and ground-bearing slabs.

Atterberg Limits are therefore routinely used in heave risk assessments and to determine whether soil treatment or stabilisation, for example using lime or cement, is required.

Earthworks and material suitability

For highways, rail and general earthworks, specifications often impose limits on LL and PI to control performance and durability. Atterberg Limits testing helps determine whether a material is suitable for reuse as fill, requires treatment, or should be excluded altogether.

In this context, accurate testing is essential for cost control, programme certainty and long-term performance.

Why UKAS accreditation matters

UKAS (United Kingdom Accreditation Service) is the national body responsible for assessing the competence of laboratories. When a laboratory is UKAS‑accredited for Atterberg Limits testing the laboratories are independently verified for compliance against BS EN ISO/IEC 17025:2017 “General requirements for the competence of testing and calibration laboratories”, this would cover areas such as:

• Equipment calibration including traceability to national standards
• Staff competence
• Testing procedures verified to be in line with current national testing standards, BS 1377-2:2022 and BS EN ISO 17892-12 2018+A2:2022
• Regular internal audits of testing and the supporting system
• Independent audits of testing and the supporting system by experts
• Comparative testing with other accredited laboratories
• Clients and regulators gain confidence that the data is reliable and defensible.

The risk of getting it wrong

Inaccurate Atterberg Limits results can lead to:

• Misclassification of soils
• Incorrect foundation or pavement design
• Unanticipated shrink–swell movement
• Earthworks failure or costly remediation

UKAS accreditation significantly reduces these risks by ensuring that testing is carried out to a consistently high and auditable standard.

A dependable foundation for geotechnical decisions

Atterberg Limits remain one of the most powerful tools for understanding how fine-grained soils behave under changing moisture conditions. They inform classification, predict performance and support safe, economical design.

When these tests are undertaken within a UKAS-accredited laboratory, the results carry the assurance of competence, consistency and compliance with recognised standards - providing a dependable foundation for sound geotechnical engineering.