Geotechnical Investigation for Site Classification

A site classification under AS 2870-2011 is only as reliable as the geotechnical investigation behind it. The classification itself (A, S, M, H1, H2, E or P) is an output; the borehole or test pit program, the soil sampling, and the laboratory testing are what actually generate the data used to derive it. In Australia, this investigation is the mandatory first step before a structural engineer can select standard footing details or a council can accept a Development Application founded on AS 2870.

This page focuses on the investigation program itself: how it is scoped, what fieldwork and laboratory testing it involves, and how the results are converted into a characteristic surface movement and a site class. For the classification framework and class descriptions, see Site/Lot Classification (AS 2870) and AS 2870 Site Classes.

Why the investigation drives the classification

AS 2870 does not permit a site class to be assigned from desktop information, a neighbouring lot's report, or soil maps alone. The National Construction Code Housing Provisions adopt AS 2870 as the Deemed-to-Satisfy pathway for residential footing and slab design, and that pathway is conditional on a site-specific classification supported by an investigation carried out in accordance with AS 1726:2017 Geotechnical Site Investigations. Councils and certifiers generally will not accept a classification report where the subsurface data, sampling, and testing cannot be traced back to the specific lot.

Scoping the investigation

The scope should reflect the size, layout and complexity of the proposed development rather than a fixed template. Matters to confirm before fieldwork include:

• Building footprint, number of storeys and any basement or cut/fill levels
• Site area: single residential lots are typically covered by two investigation points, while larger footprints, multi-dwelling sites or unusual geometries warrant additional coverage
• Known or suspected fill, made ground, or previous site disturbance
• Slope, drainage, and proximity to trees or waterways
• Regional soil suction and climate characteristics, which affect the design depth of moisture change
• Any prior geotechnical, contamination or subsidence history for the lot or surrounding area

Field investigation

Investigation points

A standard residential lot is commonly investigated using two boreholes or hand-augered holes, positioned to represent the footprint of the proposed dwelling while allowing for siting flexibility. Larger, sloping, or non-uniform sites need additional points so that soil variability across the footprint is properly represented. A single pair of holes at one corner of a large or irregular block will not adequately characterise the site.

Depth of investigation

Holes need to extend through the full depth of soil that can undergo moisture-related volume change, commonly to around 1.5–3 m on typical suburban lots, but deeper where investigation of fill, groundwater, or a thick reactive clay profile is warranted. In low-rainfall, high-suction inland and arid climate zones, the design depth of suction change can extend to 3 m or beyond, which is what triggers the deep-seated classification discussed below. Depth should never be fixed in advance without reference to the observed profile. Drilling should continue (or be extended) where reactive material persists at the base of the planned hole.

In-situ testing and logging

Dynamic Cone Penetrometer (DCP) testing is typically carried out in each hole to profile relative density and strength with depth, informing both footing behaviour and the practicality of excavation. Soil and rock encountered is logged and described in accordance with AS 1726, including geological origin, consistency or density, moisture condition, and any evidence of fill, fissuring, root activity or perched groundwater. See Dynamic Cone Penetrometer (DCP) Testing and Borehole Drilling and Soil Logging.

Sampling

Disturbed and, where required, undisturbed samples are recovered at depths chosen to represent the significant soil layers, particularly the clay horizons that control reactivity. Sample handling, sealing, and transport need to preserve natural moisture content, since moisture condition at the time of sampling directly affects several of the laboratory results used in classification. See Soil Sampling for Geotechnical Investigations.

Laboratory testing

Samples are tested at a NATA-accredited laboratory under the AS 1289 series. The core test suite for an AS 2870 classification typically includes:

See also Shrink/Swell Testing for Reactive Soils and Atterberg Limits Indices: PI, LI and CI.

From test results to a site class

AS 2870 assigns a class based on the characteristic surface movement (ys), the predicted vertical ground movement due to soil suction (moisture) change. Deriving ys from the investigation results involves:

1. Instability index (Ipt): derived from the shrink-swell index (Iss) results for each contributing clay layer, adjusted by a lateral restraint factor.
2. Depth and magnitude of suction change: the design change in soil suction and the depth over which it occurs (Hs), taken from the relevant climatic zone data or site-specific measurement rather than assumed.
3. Layer contribution: each soil layer's contribution to movement is calculated over the affected depth and summed, commonly using the method set out in AS 2870 (often referred to as the Mitchell method).
4. Rounding and classification: the resulting ys value is rounded up to the nearest 5 mm and compared against the AS 2870 class boundaries to assign A, S, M, H1, H2 or E.

The calculation depends entirely on investigation quality: an inadequate number of holes, insufficient depth, or poor sample handling will understate or overstate ys and can lead to an incorrect class being assigned.

Deep-seated (-D) classification

Where the design depth of suction change (Hs) reaches 3.0 m or more (typically in drier, inland climate zones rather than temperate coastal areas), AS 2870 requires the classes M, H1, H2 and E to carry a "-D" suffix (M-D, H1-D, H2-D, E-D), signalling that movement is occurring from deep, slow-reacting moisture changes rather than shallow seasonal wetting and drying. Investigations in these zones need to extend deep enough, and use appropriate suction-change data, to identify whether a deep-seated classification applies. A shallow 1.5 m investigation program is not adequate in these areas.

Problem sites (Class P)

Regardless of the calculated ys, a site is classified P where the investigation identifies conditions AS 2870 treats as a problem site, including:

• Uncontrolled or poorly documented fill
• Soft, collapsing, or highly compressible soils
• Abnormal moisture conditions from leaking services, poor drainage, or established trees
• Sloping sites where instability, erosion, or mass movement is credible
• Mine subsidence or karst/collapse-prone ground
• Highly saline or dispersive soils affecting foundation performance

A P classification cannot be resolved by a standard AS 2870 footing selection. It requires a site-specific footing or slab design from a structural engineer, informed directly by the geotechnical findings.

Reporting

A site classification report founded on a proper investigation should include, at minimum:

• Investigation methodology, number, location and depth of boreholes or test pits, referenced to AS 1726
• Factual borehole/test pit logs and DCP results
• Laboratory test certificates from a NATA-accredited laboratory
• The basis of the ys calculation, including assumed climatic and suction-change parameters
• The assigned classification, including any -D suffix or P designation and the reasoning
• Site observations relevant to footing design: slope, drainage, fill, and vegetation
• Limitations of the investigation and any recommendation for further assessment

The report should be prepared or reviewed by a suitably qualified and experienced geotechnical practitioner before it is issued for use by a structural engineer or lodged with a council or certifier.

Related Resources

• Site/Lot Classification (AS 2870)
• AS 2870 Site Classes (A Complete Reference)
• Geotechnical Site Investigation
• Shrink/Swell Testing for Reactive Soils
• Borehole Drilling and Soil Logging
• Dynamic Cone Penetrometer (DCP) Testing
• Foundation Design for Slabs and Footings

Authoritative References

• AS 2870:2011 Residential Slabs and Footings, Standards Australia
• AS 1726:2017 Geotechnical Site Investigations, Standards Australia
• NCC Housing Provisions, Part 4.2 – Footings, Slabs and Associated Elements