Standards Australia · Published 2011

AS 2870:2011

Residential Slabs and Footings

AS 2870:2011 specifies the design and construction requirements for residential slabs and footings, covering Class 1 (residential) and Class 10 (non-habitable, e.g. garage, shed) structures. The standard prescribes site classification (Class A, S, M, H1, H2, E, P) based on reactive-soil response (clay shrink-swell behaviour), and provides corresponding deemed-to-satisfy slab and footing details — typically waffle pod slabs, raft slabs, and strip footings sized and reinforced according to the site classification and footprint geometry. AS 2870 is the deemed-to-satisfy reference for residential slabs and footings under NCC Volume Two and is the controlling design standard for the vast majority of new residential foundation construction in Australia. The standard's loading basis is harmonised with AS 1170 series; capacity is harmonised with AS 3600 (concrete) and AS 1726 (geotechnical investigation); reactive-soil treatment uses AS 1289 series soil-test methods. The 2011 edition incorporates significant changes from AS 2870-1996 in site classification methodology, slab detail provisions for Class M and H sites, and explicit treatment of edge-thickening and pier-supported configurations. Amendments have been issued covering specific reactive-soil regions and slab-on-pile applications.

Purchase from Standards Australia
TRSC Engineering Interpretation

AS 2870 is rarely directly applied in TRSC's commercial-asset practice, but it is decision-controlling in three categories of engagement: heritage residential foundation assessment (heritage cottages and Queenslanders showing differential settlement on reactive-soil sites), Class 1 to Class 9 reclassification (residential being converted to office or retail, which exceeds the standard's deemed-to-satisfy envelope), and post-disaster residential foundation assessment (flood, subsidence, root-induced settlement). Three application points matter when AS 2870 is relevant. First, the standard's site classification is decision-controlling for any continuing-life or change-of-use foundation Form 15. Pre-1996 residential construction in Brisbane and Ipswich frequently sits on Class M (moderate reactive) or H1/H2 (highly reactive) sites where the as-installed slab does not meet the AS 2870:2011 deemed-to-satisfy requirements. The shrink-swell movement of reactive clay produces the characteristic distortion pattern seen in heritage residential — diagonal wall cracking, door-frame distortion, and ceiling-cornice separation — and the assessment must determine whether the observed distortion is consistent with reactive-soil response within the standard's serviceability envelope or evidence of additional structural distress (typically root-induced or stormwater-induced settlement). TRSC's heritage residential foundation assessments combine AS 1726 design-level geotechnical investigation with automated-total-station settlement monitoring over a wet-dry cycle to derive the characteristic site classification under current site conditions. Second, AS 2870 deemed-to-satisfy details do not cover Class P (problem) sites, which are typical for filled urban sites in Brisbane (Spring Hill, Fortitude Valley, West End), and require AS 2870 engineering pathway (Section 4) with site-specific footing design — typically pile-supported slabs or screw-pile foundations. For existing pre-AS-2870-engineering-pathway construction on Class P sites, the as-installed foundation rarely meets current Class P requirements, and continuing-life Form 15 certification requires either pile-underpinning retrofit or documented engineering basis for accepting the reduced foundation performance under restricted use. Third, AS 2870 site classification is dynamic — the classification can change over the life of the asset due to changes in landscape (vegetation removal, stormwater redirection, neighbouring construction), and the assessment must address current-site conditions rather than original-site conditions. Heritage residential adaptive-reuse projects in Brisbane CBD frequently encounter site-classification changes from the original construction era to current — and the assessment must address the current Class M or H designation, even if original construction met the at-the-time Class A or S deemed-to-satisfy. TRSC's heritage residential adaptive-reuse foundation Form 15 always references the current AS 2870 site classification as the basis for assessment, with explicit documentation of any site-classification change since original construction.

Form 15 RPEQ Certification Implications

Form 15 RPEQ certifications for heritage residential foundation adequacy and Class 1 to Class 9 change-of-use reclassification reference AS 2870:2011 where the structure is within the standard's deemed-to-satisfy envelope, and reference AS 3600 + AS 1726 engineering assessment where it is not (the typical case for change-of-use). The Form 15 file documents the current AS 2870 site classification, the engineering basis for any deviation from deemed-to-satisfy details, and the AS 1726 geotechnical input that supports the foundation assessment. For heritage residential undergoing differential settlement assessment, the Form 15 retains the automated-total-station monitoring data and the site-classification derivation. Where retrofit pile-underpinning is required for continuing-life certification, the Form 15 is issued only after physical verification of the retrofit and AS 3600 capacity confirmation of the supplementary foundation system.

Frequently Asked Questions

Engineering questions about AS 2870:2011

What is reactive-soil site classification under AS 2870?
AS 2870 site classification (Class A, S, M, H1, H2, E, P) categorises sites by their reactive-soil response — the shrink-swell movement of clay soils as moisture content varies through wet-dry cycles. Class A and S are stable (sand, silt, low-plasticity clay); Class M is moderately reactive (typical Brisbane suburban clays); Class H1 and H2 are highly reactive (deeper Brisbane reactive zones, parts of Ipswich and the Lockyer Valley); Class E is extremely reactive (rare); Class P is problem sites that require engineering pathway. The classification determines the deemed-to-satisfy slab and footing details — Class M sites require thicker slabs with edge stiffening; Class H sites require waffle-pod slabs with deeper edge beams; Class P sites require site-specific engineering. For existing-asset assessment, the current site classification matters more than the original.
Does AS 2870 cover heritage cottage foundations?
Heritage cottage foundations (typical pre-1940 timber-framed Queenslander on stumps, brick piers, or strip footings) pre-date AS 2870 and are not within the standard's deemed-to-satisfy envelope. Continuing-life and adaptive-reuse Form 15 certification of heritage cottage foundations requires AS 3600 + AS 1726 engineering assessment with current-site classification. The most commonly observed issue is reactive-soil-induced differential settlement on Class M or H sites where the original strip footing or pier foundation has insufficient depth to escape the active reactive zone — TRSC's typical retrofit specification involves screw-pile underpinning at strategic locations to transfer load below the active zone, with AS 3600 capacity confirmation of the supplementary foundation.
What does change-of-use mean for residential foundations?
Class 1 (residential) to Class 5 to 9 (commercial) change-of-use takes the structure outside the AS 2870 deemed-to-satisfy envelope, because AS 2870 is calibrated to residential imposed action (typically 1.5 kPa) and residential building geometry. Commercial occupancy requires higher imposed action under AS 1170.1 (3.0 kPa office, 4.0 kPa retail) and the existing residential-grade foundation rarely has adequate margin. The assessment requires AS 3600 + AS 1726 engineering pathway, and the Form 15 documents whether the existing foundation is adequate (often the case for compact-floor-plan office conversion of a robust Queenslander) or whether retrofit underpinning is required (typically the case for retail conversion or high-occupancy assembly use).
Relevant Services
Structural Investigation
Heritage Structural Engineering
Certification
Relevant Materials
Hilti Hit Re 500 V4
Sources & Further Reading