Soil Classification and Site Investigation Requirements

What This Requirement Covers

Soil classification and site investigation requirements set out how a site must be assessed before designing and constructing foundations, slabs, retaining walls and other geotechnical elements. The purpose is to identify ground conditions that affect foundation performance - such as clay reactivity, compressibility, fill, collapse potential, high groundwater or landslip risk - so designers can select appropriate footing types, dimensions and mitigation. These requirements protect buildings from damage caused by ground movement, poor bearing capacity or unexpected subsidence, and they underpin safe, durable and code-compliant structural design.

These requirements apply to building designers, structural engineers, certifiers, builders and homeowners commissioning foundations or earthworks. They are referenced across the National Construction Code (NCC) and ABCB Housing Provisions and are implemented by following Australian Standards (in particular AS 2870, AS 1726, and AS 3798) and any applicable state or territory schedules in the NCC. The level of investigation and the soil classification influence whether prescriptive footing provisions in the ABCB Housing Provisions may be used or whether site-specific engineered design (eg. by a geotechnical engineer) is required.

Key Requirements

1. Site classification must be determined in accordance with AS 2870 Residential slabs and footings - Construction where footings and slabs are to be located, and the site must be assigned one of the standard site classes: A, S, M, H, E, P, or Class A-P divisions as defined in AS 2870 (see ABCB Housing Provisions 4.2.2). Refer to ABCB Housing Provisions Standard 2022, Part 4.2.2 and NCC Volume Two, H1D4.
2. Minimum scope of site investigation: investigations must follow AS 1726 Geotechnical site investigations and AS 3798 Guidelines on earthworks for commercial and residential developments. Typical requirements include:
3. Subsurface exploration by test pits and/or boreholes to the depths appropriate to the foundation type and expected zones of moisture change - commonly to at least 1.0 m for shallow footings and deeper where expansive clays, fills or soft soils are suspected; deeper investigation to assess deep-seated movement where indicated (AS 1726 guidance).
4. Collection of representative disturbed and undisturbed samples for laboratory testing where required (moisture content, Atterberg limits, particle size, unit weight, consolidation/compressibility, shear strength; kPa values where bearing capacity is used).
5. Identification and reporting of fill, organic soils, soft or collapsible soils, high water table, rockhead depth and any evidence of landslip or mine subsidence (class P or A-P as applicable).
6. Prescriptive use limitation: The ABCB Housing Provisions (NCC Volume Two H1D4) permit using Housing Provisions prescriptive footing solutions only where the site is classified A, S or M (in accordance with AS 2870) and has a uniform bearing capacity. For H, E, P or A-P (filled or problematic) sites, reference must be made to AS 2870 for design and construction, and engineered design is generally required (NCC Volume Two, H1D4(2)).
7. Foundation design criteria: Where engineered design is required, design must comply with structural standards such as AS 2870, AS 3600 (concrete), relevant parts of AS 4100 (steel) and other referenced standards. Design inputs from geotechnical reports typically include:
8. Characteristic soil bearing pressures (kPa) or allowable bearing capacity values for footing sizing.
9. Design parameters for retaining structures (eg. earth pressure parameters, cohesion and internal friction angle in degrees) in accordance with AS 4678 for retaining walls where relevant.
10. Classification of site reactivity and recommended footing depth, type and reinforcement.
11. Reporting and documentation:
12. A geotechnical site investigation report (prepared in accordance with AS 1726) must provide the site class (AS 2870), subsurface profile, laboratory test results with numeric values (eg. moisture contents %, Atterberg limits, kPa strengths), groundwater level, and recommended foundation types, depths and design parameters.
13. Geotechnical recommendations must include any imposed limits on building dimensions for prescriptive solutions (eg. slab length or width not exceeding 18 m where relevant under H1D4(2)(b)(i-ii)).
14. Relevant NCC and ABCB references:
15. NCC 2022 Volume Two - H1D4 Footings and slabs (including H1D4(1)-(2))
16. ABCB Housing Provisions Standard 2022 - Part 4.2 Footings, slabs and associated elements and Table 4.2.2 site class descriptions
17. NCC 2022 Volume One - Part A6 Building classification (for applicability)
18. Australian Standards: AS 2870, AS 1726, AS 3798, AS 3600, AS 4678, AS 4100 where structural design is required

Residential vs Commercial

1. Residential (Class 1 and 10):
2. Site classification and foundation design for typical detached houses and associated Class 10 structures follow the ABCB Housing Provisions and AS 2870. Where the site is A, S or M with uniform bearing capacity and meets the Housing Provisions limitations (eg. slab not more than 18 m long or wide and no permanent joints), prescriptive footing and slab details in the Housing Provisions may be used (NCC Volume Two, H1D4).
3. For sites classed H, E, P or A-P, or where conditions fall outside Housing Provisions limitations, a geotechnical investigation and engineered design to AS 2870 is required.
4. Commercial (Class 2-9):
5. These are governed by NCC Volume One and have higher demands for site investigation and geotechnical design. AS 1726 and AS 3798 are routinely required; engineered foundation design is expected for most Class 2-9 buildings. Soil parameters must be quantified (kPa bearing values, angles of internal friction, compression parameters) and integrated into structural design to AS 3600, AS 4100, or other relevant structural standards.
6. Commercial projects frequently require deeper investigation (multiple boreholes, geotechnical modelling), continuum stability assessment, retention design (AS 4678), and consideration of service loads, basement excavation, surcharge and adjacent structures.

Exceptions and Exemptions

1. Small, simple structures entirely covered by prescriptive Housing Provisions and located on a clearly A, S or M site with uniform bearing capacity may be constructed without a full geotechnical report provided the Housing Provisions conditions are met (NCC Volume Two, H1D4(1)
2. (b) and ABCB Housing Provisions 4.2.2).
3. Sites classified as H, E, P or A-P, sites with evidence of deep-seated movement, landslip, mine subsidence or corrosive conditions are exempt from the simplified Housing Provisions and must be dealt with by referring to AS 2870 and/or a geotechnical engineer for a Performance Solution.
4. Alternative solutions (Performance Solutions) are available under the NCC if a design demonstrates compliance with Performance Requirements; these commonly rely on engineered geotechnical assessment and structural verification rather than prescriptive compliance clauses.

State and Territory Variations

1. The NCC is national, but each state and territory may include a schedule in NCC Volume One that modifies or augments national provisions for local conditions. Notable points:
2. Some state schedules explicitly require adherence to additional guidance or local standards for footings and earthworks - always check the relevant state schedule in NCC 2022 Volume One (Schedules 4-12).
3. For example, Queensland and New South Wales have particular attention to flood, coastal and landslip risks and may require additional geotechnical assessment for certain coastal, floodplain or mine-affected areas; refer to the relevant state schedule and local authority requirements.
4. Local councils or certifiers may require a geotechnical report for certain classes of work or for any site with visible fill, recent earthworks, imported material, or where historical maps indicate prior disturbance.
5. Termite management, moisture control and local climate effects can also be subject to state-adopted variations; check the state schedule in NCC Volume Two and local planning/council requirements.

Practical Compliance Tips

1. Engage a competent geotechnical engineer early - order a site investigation (AS 1726) before finalising slab or footing design to avoid costly redesign or remedial works.
2. Confirm the site class in writing in the geotechnical report and ensure the site classification method follows AS 2870; do not rely on visual inspection alone for sites with any uncertainty (fill, vegetation, cracking, ponding).
3. For residential projects wanting to use Housing Provisions prescriptive details, verify all limitations: site must be A, S or M, have uniform bearing capacity, and slab dimensions and jointing must comply (eg. slab not more than 18 m long or wide where referenced in H1D4(2)(b)).
4. Provide clear ground level, finished floor level and proposed building footprint information to the geotechnical consultant so recommended investigation depths and locations are appropriate.
5. Keep laboratory test results and numeric design parameters (eg. allowable bearing pressure in kPa, Atterberg limits, groundwater depth) in the design pack for certifiers and future maintenance reference; these are commonly requested by certifiers.
6. Where fill, organic soils or collapsing sands are present (site class P or filled sites A-P), plan for deeper investigation, possible remediation (strip and replace, compaction), piled footings or engineered slab systems rather than shallow footings.
7. Check local council or certifier requirements and state NCC schedules early - some councils require site-specific geotechnical reports for any new dwelling or for works on sloping, waterfront, or historically filled land.

Sources: NCC 2022 Volume Two - H1D4 Footings and slabs; ABCB Housing Provisions Standard 2022 Part 4.2 Site classification and Table 4.2.2; NCC 2022 Volume One A6 Building classification and state schedules; Australian Standards AS 2870, AS 1726, AS 3798, AS 3600, AS 4678, AS 4100.