Soil classification gives engineers a consistent language for grouping and describing soil. It combines observations with test results such as particle size distribution and Atterberg limits.
Classification is useful for communication and preliminary interpretation. It does not, by itself, provide design values for bearing capacity, settlement, permeability or shear strength. Those properties require project-specific investigation, testing and engineering assessment.
Classification and description are different
These terms are related but not interchangeable:
- Classification assigns a soil to a group using defined laboratory criteria.
- Description records what was observed in a sample or exposure, including composition, colour, moisture condition, consistency or density, structure and other relevant features.
- Interpretation considers what the observations and test results mean for the proposed works.
A good geotechnical log may include a field description and a provisional classification. Laboratory results can later confirm or refine the classification.
Information used to classify soil
Two laboratory data sets are commonly important:
| Information | What it shows |
|---|---|
| Particle size distribution | The proportions of gravel, sand and fine material |
| Liquid limit and plastic limit | The plasticity characteristics of the fine fraction |
| Organic or unusual material | Whether separate description or testing is needed |
| Field observations | Features that may not be preserved or represented in a small laboratory sample |
Sampling quality matters. A disturbed, segregated or unrepresentative sample can produce a classification that does not describe the broader soil unit.
Broad soil groups
Soils are commonly considered in broad groups before a more detailed symbol or description is assigned:
- Gravel and sand are coarse-grained materials.
- Silt and clay form the fine fraction and are distinguished using behaviour and plasticity, not particle size alone in routine engineering classification.
- Organic soils and peat need specific recognition because their behaviour can differ significantly from mineral soils.
The boundary rules, naming conventions and dual symbols depend on the classification system being used. Refer to the current applicable standard rather than relying on a simplified online chart for formal reporting.
Unified Soil Classification System group symbols
The Unified Soil Classification System, commonly called USCS, uses letter combinations to communicate a broad soil group.
| Symbol | General meaning |
|---|---|
| GW or GP | Gravel, with grading descriptor |
| GM or GC | Gravel with silty or clayey fines |
| SW or SP | Sand, with grading descriptor |
| SM or SC | Sand with silty or clayey fines |
| ML or MH | Silt of lower or higher liquid limit range |
| CL or CH | Clay of lower or higher liquid limit range |
| OL or OH | Organic fine-grained soil |
| Pt | Peat or highly organic soil |
Dual symbols can apply near classification boundaries or where the fines content falls within a transition range. The symbol should be supported by the specified test data and classification rules.
Plasticity and the plasticity chart
Fine-grained soils are commonly assessed using the liquid limit, plastic limit and plasticity index:
$$ PI = LL - PL $$The position of the result on a plasticity chart helps distinguish clay-like and silt-like behaviour within the relevant system. The A-line is commonly expressed as:
$$ PI = 0.73(LL - 20) $$A chart position is only one part of classification. Organic content, sample condition and the applicable standard also need consideration.
Read the Atterberg Limits guide for the meaning of liquid limit, plastic limit and plasticity index.
Soil description under AS 1726
AS 1726:2017, Geotechnical site investigations, provides a standardised Australian system for describing and classifying soils and rocks. A field or borehole description commonly records relevant attributes such as:
- principal and secondary soil components
- colour and moisture condition
- consistency for cohesive soils or density condition for granular soils
- structure, inclusions and other distinctive features
- interpreted origin or geological unit where supportable
The order, terminology and abbreviations should follow the current project requirements and the applicable edition of AS 1726. The standard itself should be consulted for formal descriptions.
Field identification and laboratory confirmation
Visual and manual observations can support a provisional field description. Engineers may consider texture, dilatancy, dry strength, toughness, plasticity and the appearance of the sample.
Field identification has limitations. It can be affected by moisture, weathering, cementation, mixed fill and the experience of the logger. Laboratory testing is appropriate when classification affects design, material acceptance or compliance.
Worked classification concepts
Coarse-grained example
A predominantly sandy sample has a small fines content. Sieve analysis provides the grading curve, while plasticity testing may be needed to characterise the fines. The final group symbol depends on the grading and fines criteria in the selected system.
Fine-grained example
A fine-grained sample has a liquid limit of 62 and plastic limit of 28:
$$ PI = 62 - 28 = 34 $$The plasticity chart position can then be assessed. This calculation illustrates the process, but the formal group symbol must follow the complete rules of the applicable standard.
Why classification matters
Classification can help teams:
- maintain consistent borehole and test pit records
- select suitable laboratory tests
- identify material variability
- communicate preliminary construction considerations
- organise subsurface units for engineering interpretation
It should not be used to assign generic design parameters without supporting evidence. Two soils with the same group symbol can have different density, structure, stress history, mineralogy and engineering performance.
Related resources
- Soil Properties
- Soil Classification Workflow for AS 1726
- Atterberg Limits
- Dispersive Soil Testing (Pinhole Dispersion and Emerson Class)
- Soil Compressibility
- Shear Strength of Soil
- California Bearing Ratio Testing
Authoritative reference
This guide is educational. Project documentation, current standards and advice from a suitably qualified geotechnical professional take precedence.