A practical guide to representative soil sampling, sample types, handling, identification and chain of custody.
Soil Sampling for Geotechnical Investigations
Soil sampling provides material for classification, strength, compressibility, compaction, permeability, chemical and environmental testing. A laboratory result is only useful when the sample is representative, suitable for the test and traceable to the correct location and depth.
Note: The sampling plan, equipment, preservation and testing must follow the investigation objective, applicable standards, laboratory requirements and site-specific safety controls.
Start with the investigation objective
The sampling plan should define what decisions the data must support. Examples include:
- Soil description and classification
- Foundation strength and settlement parameters
- Earthworks suitability and compaction control
- Pavement subgrade assessment
- Permeability or hydraulic assessment
- Aggressivity and durability testing
- Contamination delineation or waste classification
One sample type is not suitable for every test.
Common sample categories
Disturbed samples
Disturbed samples retain material composition but not the original soil structure. They may be suitable for moisture content, particle-size distribution, Atterberg limits, compaction, CBR and selected chemical tests, subject to the test method.
Bulk samples
Bulk samples provide enough material for tests requiring a larger mass, such as compaction curves, CBR, stabilisation trials or aggregate-related testing. The required mass should be confirmed with the laboratory before fieldwork.
Relatively undisturbed samples
Tube, piston or block samples aim to preserve structure, density and moisture condition for strength or compressibility testing. Sample quality depends on soil type, sampler geometry, drilling disturbance, recovery, extrusion, sealing, transport and storage.
The term undisturbed should not be treated as proof that the specimen is free from disturbance.
Rock core
Rock core requires separate recovery, orientation, fracture, weathering and strength considerations. Core boxes, run markers and photographs should preserve the sequence and depth record.
Sampling methods
Methods may include auger cuttings, split-spoon or drive samples, thin-wall tubes, piston samplers, block samples, test-pit samples and coring. Selection should consider soil type, groundwater, required sample quality, borehole stability and the intended laboratory test.
Drill cuttings can be mixed across depth or altered by the drilling process. Their limitations should be recorded clearly.
Sample identification
Every sample should have a unique identifier linked to:
- Project and site
- Borehole, test pit or sampling location
- Sample number and type
- Top and bottom depths
- Date and time
- Sampler and drilling method
- Recovery and observed condition
- Requested tests
- Sampler or logger initials
Labels should remain legible during transport and storage. The external label, container and field log should agree.
Handling and preservation
Depending on the test program, controls may include:
- Immediate sealing against moisture loss
- Waxing or end caps for tube samples
- Correct orientation and protection from vibration
- Temperature control for environmental samples
- Chemical-compatible containers and preservatives
- Short holding times for volatile analytes
- Secure core boxes and depth markers
- Protection from heat, sunlight, freezing and impact
The receiving laboratory should be consulted before mobilisation when specialised testing is planned.
Chain of custody
A chain-of-custody record should document transfer from the field team to transport and laboratory custody. It should include sample identifiers, containers, requested analyses, preservation, dispatch and receipt dates, and signatures or secure digital acknowledgements.
Geotechnical programs may use a sample submission record even where a formal environmental chain of custody is not required. Traceability remains essential.
Sampling quality controls
- Compare recovered material with the field log and expected geology.
- Record poor recovery, loss, contamination or mixing.
- Prevent cross-contamination between locations and depths.
- Clean equipment using the procedure required for the program.
- Use duplicates, blanks or other QA samples where the environmental plan requires them.
- Do not composite samples unless the objective and governing guidance allow it.
- Retain enough material for repeats when practical.
Geotechnical and environmental sampling are different
Geotechnical sampling focuses on engineering properties and stratigraphy. Environmental sampling may require statistical design, contaminant-specific containers, decontamination, preservation, quality-control samples and regulated chain-of-custody procedures.
If contamination is suspected, the investigation should follow the appropriate environmental framework rather than treating a routine geotechnical sample as sufficient.
Related resources
- Geotechnical Site Investigation
- Borehole Drilling and Soil Logging
- Soil Classification Workflow for AS 1726
- Environmental Site Assessment