AS 2870-2011 "Residential Slabs and Footings" is the Australian Standard that governs the classification of residential building sites and the design of slab and footing systems.
The Seven Site Classes
Class A — Sand or Rock Sites
Expected surface movement: 0 mm
Sites where the foundation can be placed on sand, rock, or other stable materials with negligible ground movement.
- Typical soils: Sand, sandstone, granite, limestone
- Foundation: Slab on ground, light edge beam
- Design: Minimal reinforcement required
- Testing: Basic site inspection sufficient
Class S — Slightly Reactive Sites
Expected surface movement: 0–20 mm
Sites with low-reactivity clay or silt where minor ground movement is expected.
- Typical soils: Sandy clay, silty clay (low plasticity)
- Foundation: Waffle raft, stiffened raft with shallow beams
- Design: Standard design procedures apply
Class M — Moderately Reactive Sites
Expected surface movement: 20–40 mm
The most common classification in many Australian regions. Moderate ground movement expected from moisture changes in the clay subgrade.
- Typical soils: Medium plasticity clay
- Foundation: Stiffened raft with 450–600 mm beams
- Design: Increased reinforcement for soil movement
Class H1 — Highly Reactive Sites
Expected surface movement: 40–60 mm
High expected ground movement requiring increased foundation stiffness.
- Typical soils: High plasticity clay, basaltic clay
- Foundation: Stiffened raft with 600–750 mm beams
- Design: Significant reinforcement, articulation joints
- Prevalent in: Western Sydney (Bringelly Shale), parts of QLD
Class H2 — Highly Reactive Sites
Expected surface movement: 60–75 mm
Very high ground movement. Requires heavy foundation design.
- Typical soils: Very high plasticity clay
- Foundation: Stiffened raft with 750–900 mm beams
- Design: High reinforcement content, deep beams
- Prevalent in: Parts of QLD and NSW with deep reactive clay profiles
Class E — Extremely Reactive Sites
Expected surface movement: > 75 mm
Extreme ground movement risk. Requires deeply founded structures.
- Typical soils: Extremely reactive clay (montmorillonite)
- Foundation: Pier and beam system, deep piers
- Design: Piers founded below the active zone (typically 2–4 m depth)
- Prevalent in: Adelaide plains, parts of WA
Class P — Problem Sites
Expected surface movement: Site-specific
Sites with conditions that cannot be classified in the A–E system:
| Condition | Description |
|---|---|
| Uncontrolled fill | Fill not placed under engineering supervision |
| Soft / collapsing soils | Soils with high compressibility or collapse potential |
| Abnormal moisture conditions | Sites with perched water tables, artesian conditions |
| Mine subsidence | Areas with underground mine workings |
| Landfill / contaminated ground | Sites with waste material or chemical contamination |
| Sloping sites | Slopes requiring additional stability assessment |
| Tree effects | Sites where tree root moisture extraction causes severe differential movement |
| Settling fill | Fill material subject to long-term settlement |
P class sites require a site-specific engineered foundation design.
What Determines the Site Class?
The site class is determined by the characteristic surface movement (Yₛ):
$$ Y_s = \frac{(I_{ss}) \times H_s}{100} $$Where:
- Iss = Shrink/swell index (from AS 1289.7.1.1 laboratory testing)
- Hs = Design depth of seasonal moisture change (AS 2870 specifies values for different climatic zones)
| Site Class | Yₛ Value |
|---|---|
| A | 0 mm |
| S | 0 < Yₛ ≤ 20 mm |
| M | 20 < Yₛ ≤ 40 mm |
| H1 | 40 < Yₛ ≤ 60 mm |
| H2 | 60 < Yₛ ≤ 75 mm |
| E | Yₛ > 75 mm |
Climate Zones
AS 2870 defines three climatic zones that influence the depth of seasonal moisture change:
| Zone | Description | Examples |
|---|---|---|
| Zone 1 | Coastal strip — consistently moist | Sydney coastal, Gold Coast |
| Zone 2 | Inland — moderate seasonal variation | Western Sydney, Brisbane western suburbs |
| Zone 3 | Inland — large seasonal variation | Canberra, Dubbo, Toowoomba |
Additional Classifications
Wind Classification (AS 4055)
The site's wind class is also determined during site classification:
| Wind Class | Description | Typical Regions |
|---|---|---|
| N1–N2 | Low wind | Inland sheltered areas |
| N3 | Medium wind | Most suburban areas |
| N4 | High wind | Coastal areas, open terrain |
| N5–N6 | Very high wind | Coastal, exposed ridges |
| C1–C2 | Cyclonic | North QLD, NT |
| C3 | Very cyclonic | Far north QLD, coastal |
Fill Classification
Where fill is encountered, it must be assessed:
| Fill Type | Description | Foundation Approach |
|---|---|---|
| Controlled fill | Placed and compacted under AS 3798 | Can be built on with standard design |
| Uncontrolled fill | No compaction records | P class — deep piers or ground improvement |
| Domestic fill | From previous site regrading | Investigate depth and compaction |
How to Read a Site Classification Report
A compliant AS 2870 site classification report should clearly state:
- Site class (A, S, M, H1, H2, E, or P)
- Characteristic surface movement (Yₛ) in mm
- Iss value from shrink/swell testing
- Climate zone (1, 2, or 3)
- Wind classification (N1–N6, C1–C3)
- Fill assessment (if applicable)
- Borehole logs — soil profile to 3 m (or more)
- Laboratory test results
- Foundation recommendations — slab type, beam depth, reinforcement
- Conditions and limitations — specific site issues that affect design
Frequently Asked Questions
Can a site class change over time?
The site classification reflects conditions at the time of testing. However, changes in site conditions (new fill, altered drainage, tree removal/planting, changed water table) can affect ground behaviour. A new classification may be required if significant changes occur.
What if I disagree with the site class?
Site classification is a geotechnical engineering assessment. If you believe the class is incorrect, you may request additional testing (e.g., deeper boreholes, additional shrink/swell tests) from your geotechnical provider.
Can a P class site be upgraded?
Not in the same way that other classes are ranked. P is a classification for problem conditions — the solution is specific engineering design, not reclassification. However, if the problem condition is removed (e.g., uncontrolled fill is excavated and replaced with controlled fill), the site may be reclassified.
Is a site classification valid for any soil type?
Yes, AS 2870 covers all soil types. The classification system handles sand and rock sites (Class A) through to extremely reactive clays (Class E) and problem sites (Class P).