Salinity, sodic, dispersive and reactive soils: AU residential primer

TL;DR

AS 2870-2011 classifies residential sites A through E by clay shrink/swell severity, and Class P for problem sites requiring site-specific engineering. But salinity, sodic soils, and dispersive soils sit outside the shrink/swell framework entirely and can wreck a build independently. Sandy soil is the trap: stable at the surface, potentially sodic and dispersive at depth, or carrying soluble salts that crystallise inside concrete and brickwork. The NSW Section 10.7 certificate will flag a salinity overlay if the land is mapped; no state instrument reliably catches sodic or dispersive sites. If anything in the soil profile looks unusual, the cheap call is a geotech report under AS 1726-2017 before you pour.

What this article is for

Four overlapping soil conditions affect Australian residential sites beyond acid sulfate soils. Each has different causes, effects, and mapping coverage. Use this national primer to understand what each condition does to your structure, where in Australia it is most common, which state datasets to check, and when a geotech report is required rather than the prescriptive AS 2870 path. Acid sulfate soils are cross-referenced below but covered in detail at Acid sulfate soil in NSW.

The four soil-condition concerns

Condition	What it is	Main effect on residential builds
Salinity	Soluble salts in the soil or groundwater, either naturally present (primary) or mobilised by rising water tables (secondary)	Salt crystallisation inside concrete pores and brick masonry; progressive spalling and corrosion of steel reinforcement
Sodic soils	Soils with high exchangeable sodium percentage (ESP >6%); sodium disrupts clay bonding	Loss of structural bearing capacity when wet; dispersion at depth under footings and slabs
Dispersive soils	Clay soils that shed individual clay particles into suspension when contacted by fresh water; assessed by Emerson Class Number test	Subsurface erosion, tunnel erosion, and subgrade loss; embankments and retention systems particularly vulnerable
Reactive soils	Clay soils that expand when wet and shrink when dry; classified under AS 2870-2011 by characteristic surface movement (ys)	Footing and slab movement, cracking of masonry and finishes; design class determines prescribed footing type

Salinity

Primary salinity means the salt was always in the ground: ancient marine sediments, naturally saline groundwater. Secondary salinity (dryland or irrigation salinity) is human-induced: clearing deep-rooted native vegetation reduces water uptake, groundwater rises, and dissolved salts come with it. Irrigated agriculture has the same effect.

High-risk regions:

NSW: Western slopes, Hunter Valley, Western Sydney Basin, Murray-Darling irrigation districts. Urban salinity affects over 40 towns in the Murray-Darling Basin and the lower Hunter Valley (NSW Department of Environment, verified 2026-05-23).
VIC: Mallee, Wimmera, Murray Basin. Catchment management authorities (Mallee, Goulburn Broken, North Central, Wimmera, North East CMAs) hold salinity mapping by region (Agriculture Victoria VRO, verified 2026-05-23).
WA: The Wheatbelt is Australia’s most extensively salt-affected dryland landscape. DPIRD and CSIRO satellite mapping confirmed 1.75 million hectares of salt-affected land plus 0.67 million hectares newly detected in 2022 (WA DPIRD, verified 2026-05-23).
SA: Around 1.5 million hectares affected by watertable-induced dryland salinity; Upper South East and Eyre Peninsula worst affected; Mallee active seep formation under sandy dune soils (SA EPA SOE 2023, verified 2026-05-23).
QLD and NSW Riverina: Irrigation-induced salinity along Murray-Darling tributary valleys.

Soluble salts travel with moisture through concrete and masonry pores and crystallise at evaporation points, typically at or below the DPC line. Crystallisation pressure can exceed 200 MPa; high-strength concrete has a tensile strength around 10 MPa (Efflock, verified 2026-05-23). Typical damage: spalling brickwork below DPC, eroded mortar joints, reinforcement corrosion once chloride thresholds are breached. Salt attack can appear within five years on affected sites.

Sodic and dispersive soils

Soil is sodic when the exchangeable sodium percentage (ESP) exceeds 6% of total cation exchange capacity. Sodium weakly bonds clay platelets, so sodic clay loses structural cohesion on contact with fresh water. At ESP above 15% the soil is strongly sodic. Sodic soils are widespread: around 63% of SA’s potentially arable land, 59% of Victorian soils, large areas of NSW Murray-Darling Basin clays, and extensive WA duplex soils with dispersive clay beneath sandy topsoils (CSIRO / Agriculture Victoria, verified 2026-05-23).

Dispersive soils shed individual clay particles into suspension in water. The standard test is the Emerson Class Number (AS 1289.3.8.1): Class 1 = strongly dispersive, Class 2 = high risk, Class 3 = moderate risk, Classes 4-8 = low to negligible. Dispersive soils erode through subsurface cracks in a process called tunnel erosion, which can undermine subgrades and drainage structures without visible surface signs until failure.

The construction trap: sandy topsoil over a dispersive clay subsoil looks stable at the surface. The clay layer saturates under wet-season rain or irrigation, and subgrade failure may not show as slab cracking until years two or three. Visual inspection and shallow soil tests miss it. A geotechnical investigation under AS 1726-2017 with Emerson testing and ESP measurement at depth will catch it.

Reactive soils and AS 2870 classification

AS 2870-2011 is the primary standard for residential slab and footing design in Australia. It classifies sites by their characteristic surface movement (ys): the total vertical movement expected between fully dry and fully wet soil conditions.

Class	ys (mm)	Description
A	0	Sand or rock; no reactive movement
S	Up to 20	Slightly reactive clay; minor movement
M	20-40	Moderately reactive clay
H1	40-60	Highly reactive clay
H2	60-75	Very highly reactive clay
E	>75	Extremely reactive; may exceed 100 mm seasonal movement
P	Varies	Problem site; prescriptive AS 2870 tables do not apply

Class P is not a single soil type. A site is classified P when it has characteristics that AS 2870’s prescriptive footing tables cannot safely accommodate: uncontrolled fill, soft ground, highly variable or erodible soils, dispersive subsoil, or conditions combining reactivity with other hazards. A Class P classification does not prevent building; it means the footing design must be done by a geotechnical or structural engineer using a site-specific assessment rather than the standard tables (AS 2870-2011, Section 2).

Salinity and sodicity interact with reactive soil classification. A highly reactive H2 or E site in a saline groundwater zone needs both the footing movement design from AS 2870 and concrete and reinforcement specification suited to chemical exposure (exposure classification from AS 3600 or NCC Volume 1 Part B1). A building certifier focused on site class alone may miss the chemical durability requirement.

Acid sulfate cross-reference

Acid sulfate soils (ASS) are a separate category of soil-condition hazard distinct from salinity, sodicity, and reactivity. Pyrite-bearing sediments oxidise when exposed by excavation or drainage lowering, generating sulfuric acid that corrodes concrete, steel, and copper plumbing. NSW-specific coverage: the mapping classes (1-5), CDC exclusions for Class 1 and 2, and the ASSMP process are covered in Acid sulfate soil in NSW. Queensland and Victorian coastal ASS regimes are also noted there.

State risk mapping resources

State	Salinity dataset	Sodic/dispersive	Reactive/geotech
NSW	EPI Salinity dataset via NSW Planning Portal (SEED); flags LEP/SEPP salinity overlays by lot	No statewide sodic map; site-specific geotech required	No statewide reactive soil map; site class from geotech report
VIC	Agriculture Victoria / Victorian Resources Online (VRO) salinity province maps by CMA region	59%+ of Victorian soils sodic; no parcel-level public map	No statewide map; AS 2870 site class from geotech
WA	DPIRD Soil Landscape Land Quality: Salinity Risk layer (data.wa.gov.au, SLIP Public Services)	Agriculture WA dispersive/sodic soils guidance by soil-landscape unit (agric.wa.gov.au)	No statewide AS 2870 map; geotech report per site
SA	SA Department for Environment and Water: Land Salinity Monitoring; WaterConnect salinity mapping (waterconnect.sa.gov.au)	ESP data by soil landscape from PIRSA; no parcel-level map	No statewide map
QLD	Queensland Murray-Darling and irrigation-area salinity data via QLD Globe spatial portal	DSITI/DAF sodic soil guidance; limited parcel-level coverage	No statewide map

The S10.7 planning certificate in NSW is the closest thing to a statutory salinity check in any Australian state: it discloses the EPI Salinity overlay if it applies to the lot (see Section 10.7 certificates NSW). No other state has an equivalent lot-level statutory instrument check. In all other states, identifying salinity or sodicity risk requires either checking the relevant state soil mapping portal or commissioning a site-specific investigation.

What can go wrong

Skipping geotech because council didn’t ask. The NSW Section 10.7 certificate flags a salinity EPI overlay if the lot is mapped, but many saline and sodic sites are not mapped at fine resolution, and the certificate will not warn about dispersive subsoil. Some certifiers accept a visual site assessment for Class 1a residential. That is a legal minimum, not a safety minimum. Request borings to at least 1.5 m below slab level on any site where surface soil behaviour looks unusual.

Specifying N20 concrete in a saline environment. AS 3600 and the NCC require concrete specified to an exposure classification matching actual sulfate and chloride conditions. A saline groundwater zone may require B1 or B2 exposure classification, higher cement content, and increased reinforcement cover. Getting this wrong shows up as a professional indemnity claim five years later, not a defect at handover.

Treating Class P as a dead end. Class P means the prescriptive AS 2870 footing tables do not apply; it does not mean the site cannot be built. An engineered footing design from a geotechnical engineer resolves most Class P sites without abandoning the project.

References

Standards Australia, AS 2870-2011 Residential slabs and footings. Overview via Oz Geos AS 2870 guide (verified 2026-05-23).
Standards Australia, AS 1726-2017 Geotechnical site investigations. Standards Australia catalogue (verified 2026-05-23).
Standards Australia, AS 1289.3.8.1-2017 Emerson class number. WGLS Emerson Class Test (verified 2026-05-23).
NSW Department of Environment and Heritage, Salinity locations and mapping. https://www.environment.nsw.gov.au/topics/land-and-soil/soil-degradation/salinity/locations-and-mapping (verified 2026-05-23).
NSW Planning Portal, EPI Salinity dataset. https://www.planningportal.nsw.gov.au/opendata/dataset/epi-salinity (verified 2026-05-23).
Agriculture Victoria VRO, Victoria’s Salinity Provinces. https://vro.agriculture.vic.gov.au/dpi/vro/vrosite.nsf/pages/lwm_salinity-provinces (verified 2026-05-23).
WA DPIRD, Soil Landscape Land Quality: Salinity Risk (DPIRD-009). https://catalogue.data.wa.gov.au/dataset/soil-landscape-land-quality-salinity-risk (verified 2026-05-23).
WA Agriculture and Food, Dispersive sodic soils in WA. https://www.agric.wa.gov.au/dispersive-and-sodic-soils/dispersive-sodic-soils-western-australia (verified 2026-05-23).
SA EPA, State of the Environment 2023: Soils. https://soe.epa.sa.gov.au/environmental-themes/land/state-of-our-land/soils (verified 2026-05-23).
Douglas Partners, Sodic Soils and Design/Construction Implications, Central Western QLD. https://www.douglaspartners.com.au/knowledge-sharing/sodic-soils-design-construction-implications-recreational-lake-queensland/ (verified 2026-05-23).
Efflock, Salinity in Australia: a big problem that builders need to consider. https://www.efflock.com.au/blogs/news/12671681-salinity-in-australia-a-big-problem-that-builders-need-to-consider (verified 2026-05-23).

Acid sulfate soil in NSW: how to identify, assess, and manage it before you dig
Australian planning instrument hierarchy: state-by-state comparison: how salinity and other site-constraint overlays fit within LEPs, DCPs, and state codes.
Section 10.7 planning certificates NSW: what they reveal and how to read them