AS 1726 (Geotechnical site investigations)
AS 1726:2017 sets the methodology for geotech investigations: boreholes, SPT, sample logging, soil description, reporting format. The basis of every soil report.
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AS 1726:2017 is the Australian Standard governing how geotechnical site investigations are conducted. It sets out the methodology for borehole drilling, in-situ testing (SPT, DCP), sample classification, soil description language, and the reporting format. Every soil report a builder reads is (or should be) prepared under AS 1726. The Standard is not the same as AS 2870 (residential slabs and footings), AS 1720 (timber structures), or any of the other geotech-adjacent standards; AS 1726 is specifically about HOW the investigation is done, not what the slab is designed to.
Why builders care
The geotech report drives:
- Site classification (Class A, S, M, H1, H2, E) under AS 2870.
- Foundation system selection (waffle slab vs piered).
- Construction sequencing (excavation depth, water table considerations).
- Cost (a Class E site can cost 2-3x more on the foundation than Class A).
A soil report not prepared to AS 1726 is at risk of being:
- Inadequate for the building it’s classifying.
- Disputed when the engineer or certifier reviews it.
- Invalid for warranty purposes.
Engaging a geotech who follows AS 1726 is the cheapest insurance on a non-trivial residential site.
What AS 1726 covers
| Element | What it specifies |
|---|---|
| Investigation planning | Scope, locations, depth, density of testing |
| Borehole drilling | Hollow-stem auger, rotary, CPT methods |
| In-situ testing | SPT (Standard Penetration Test), CPT (Cone Penetration Test), DCP (Dynamic Cone Penetrometer), pressuremeter |
| Sample retrieval | Disturbed and undisturbed samples |
| Sample classification | Particle size, plasticity, shrink-swell, moisture content |
| Soil description language | Standardised terminology for geotech reports |
| Bore logs | Format for borehole records |
| Reporting | What the geotech report must include |
Typical residential investigation
A standard 3-bedroom residential build typically gets:
| Test | Number per site | Cost (2026 AUD ex-GST) |
|---|---|---|
| Boreholes (3-6 m typical depth) | 1-3 | $400-$800 per hole |
| SPT samples | At every 1.5 m depth | Included in borehole cost |
| DCP (surface) | 2-4 | $50-$100 each |
| Lab classification (Atterberg, particle size) | 2-4 samples | $100-$300 each |
| Report preparation | 1 | $400-$800 |
| Total | $1,500-$3,500 |
For complex sites (sloping, reactive, water table close, contamination), costs scale up.
What’s in a compliant geotech report
A compliant AS 1726 report includes:
- Investigation scope and methods used.
- Bore log for each borehole: depth, soil description, samples, observations.
- Laboratory results: Atterberg limits, particle size, moisture, shrink-swell.
- Site description: surface conditions, vegetation, slope, drainage.
- Soil profile: layered description with depths.
- Water table observations (where present).
- Site classification under AS 2870 (for residential).
- Bearing capacity at relevant depths.
- Foundation recommendations.
- Limitations and assumptions.
A short letter-style “soil classification” without bore log detail does NOT satisfy AS 1726 for any non-trivial site.
Borehole spacing and depth
AS 1726 gives guidance on test density:
| Site complexity | Typical borehole spacing | Typical depth |
|---|---|---|
| Simple uniform site | 1 per 200-400 m² (1 per house typical) | 3-5 m |
| Reactive clay site | 1 per 100-200 m² | 5-8 m |
| Sloping site | 2-3 holes positioned to bracket the slope | 5-10 m (variable) |
| Suspected fill | 1 per 100 m² + targeted in fill zones | Through fill + into competent material |
| Contamination risk | Site-specific; often more boreholes for spatial coverage | Through contamination + below |
The geotech makes the call on scope based on site complexity. A builder accepting the cheapest quote without checking scope is shopping price, not quality.
Common builder issues
- Cheapest geotech quote misses reactive clay below 3 m: shallow investigation passes, build proceeds, slab cracks at year 5. Multi-hundred-thousand-dollar rework.
- No borehole at the actual building location: geotech sampled the front yard but the house sits on the back slope. Investigation invalid.
- Verbal “she’ll be right” without report: no AS 1726 documentation; insurer or warranty body rejects claim.
- Report dated >2 years old: stale for site classification; soil conditions can change.
- Different geotech for report vs supervision: report-only geotech may not visit during construction; defects discovered later have no continuity.
For builders
- Brief the geotech on the proposed foundation system so the investigation reaches the right depth.
- Confirm the geotech investigates AT the building location: footprint pegs first, then boreholes.
- Read the full report, not just the site class. The bore logs and lab results tell you whether the classification is conservative or marginal.
- Use a geotech with construction-phase availability: pier holes need inspection; ad-hoc verification is part of the investigation chain.
- Re-investigate stale reports (over 2 years old) for high-value sites.
References
-
AS 1726:2017 (purchase via SAI Global or Australian Geotechnical Society).
-
Australian Geotechnical Society: https://www.australiangeomechanics.org (verified 2026-05-15).
Related
See also
Last updated: 2026-05-15. Verified: 2026-05-15.