A review workflow for soil description and classification aligned with AS 1726, without reproducing controlled standard tables.
Soil Classification Workflow for AS 1726
Soil classification combines particle-size information, plasticity testing and field observations to produce a consistent engineering description. In Australia, AS 1726 provides requirements for geotechnical site investigations and a standardised system for describing and classifying soils and rocks.
Note: This page explains the soil classification process and does not reproduce the controlled classification tables, boundaries or abbreviations set out in AS 1726. A current licensed copy of the standard, applied by a qualified geotechnical professional, should be used to produce a formal classification.
Classification and description are separate records
A soil description records observed properties such as colour, moisture condition, consistency or density, structure, inclusions and geological context. A classification applies a defined ruleset to laboratory and field data. Both the original description and the derived classification should be preserved as separate records.
Inputs
| Input group | Example fields |
|---|---|
| Sample identity | Project, borehole or test pit, sample number and depth |
| Particle size | Percent gravel, sand and fines, grading record and test method |
| Plasticity | Liquid limit, plastic limit and plasticity index |
| Material indicators | Organic content, cementation, dispersive behaviour or unusual constituents |
| Field description | Moisture, consistency or density, structure, colour and inclusions |
| Method control | Standard edition, company procedure and reviewer |
The plasticity index is calculated as:
$$ PI=LL-PL $$A formal soil class should never be inferred when required grading or plasticity inputs are missing.
Decision workflow
- Validate the sample and test-method identifiers.
- Confirm that percentages and grading data are internally consistent.
- Determine whether the coarse or fine fraction controls the selected ruleset.
- Apply the approved grading and plasticity boundaries from the licensed method.
- Add secondary descriptors and dual symbols only when the approved rules permit them.
- Compare the calculated classification with the logged field description.
- Require review where the laboratory result and field description conflict.
Validation controls
- Liquid limit and plastic limit must use the same percentage convention.
- A negative plasticity index should be rejected or referred for correction.
- Percent fractions should reconcile within the approved tolerance.
- Extrapolation of missing particle-size data should not occur automatically.
- Organic, mixed, fill or unusual materials should trigger a manual review pathway.
- Every classification should store the method version and the input test results.
Worked workflow example
Assume a sample has a complete grading record, liquid limit of 42 percent and plastic limit of 24 percent:
$$ PI=42-24=18\% $$PI can be calculated directly and checked against the grading record on the approved plasticity framework, but the final AS 1726 classification must follow the complete licensed rules, including any boundary and descriptive requirements. PI alone must not determine the classification.
Required output record
- Final classification and full soil description
- Source field log and laboratory result identifiers
- Ruleset and standard edition
- Any warning, override or manual decision
- Reviewer, review date and approval status
Related resources
- Soil Classification in Geotechnical Engineering
- Particle Size Distribution and Grading Coefficients
- Atterberg Limits Indices