Reactive soil — clay-rich ground that swells when wet and shrinks when dry — affects an estimated 60% of metropolitan Melbourne lots and most of the basalt-plain western suburbs (Werribee, Point Cook, Tarneit, Wyndham Vale, Caroline Springs). When seasonal moisture cycles cause the soil to shift, slabs crack, walls bow, and windows stop closing. Underpinning to fix advanced subsidence costs $40,000 to $200,000+. Most buyers don’t check the soil class until they’re rewriting the kitchen tiles five years in.
This guide covers the AS2870 soil classification system, where reactive soil dominates in Victoria, what to look for in inspections, and the rectification options when subsidence has already started.
The AS2870 soil classification
Australian Standard 2870 (Residential slabs and footings) classifies reactive soil sites by characteristic surface movement (the amount the soil moves between dry and wet seasons):
| Soil class | Characteristic surface movement | Common in | Slab cost premium |
|---|---|---|---|
| A — Stable | < 0mm | Sand, rock | Baseline |
| S — Slightly reactive | 0–20mm | Sandy clay | +5–10% |
| M — Moderately reactive | 20–40mm | Most metro Melbourne | +10–20% |
| H1 — Highly reactive | 40–60mm | Western basalt plain | +15–25% |
| H2 — Highly reactive | 60–75mm | Werribee, Point Cook, Tarneit | +20–35% |
| E — Extremely reactive | > 75mm | Specific Werribee/Bacchus Marsh pockets | +30–50% |
For a reactive soil site, the slab and footing design must be engineered to absorb seasonal movement. Compliant new builds use deeper edge beams, additional reinforcement, and articulation joints. Older homes built before AS2870 became standard (pre-1995) may not have these features.
Where reactive soil is worst in Victoria
- Western basalt plain — Werribee, Point Cook, Tarneit, Caroline Springs, Wyndham Vale, Truganina. Class H1–H2.
- Bendigo and Ballarat — basalt-derived clay soils. Class M–H1.
- Mornington Peninsula — much of Mt Eliza, Frankston South, Mt Martha. Class M–H1.
- Eastern suburbs ridges — pockets of class M–H1 across Boroondara, Whitehorse, Manningham.
- Geelong — basalt clay soils across Newtown, Belmont, Highton. Class M–H2.
Detecting subsidence at inspection
Visual signs of subsidence to look for during inspection:
- Diagonal cracking in walls, especially around windows and doors
- Stair-step cracks in brick walls following mortar joints
- Doors and windows that stick or won’t close
- Visible gap between brick courses and door/window frames
- Tiles cracked or popping in kitchens and bathrooms
- Floor unevenness — drop marbles to test slope
- Driveway cracks following the same lines as the house
- Garden bed retaining walls leaning away from the house
Rectification options and costs
| Method | Best for | Cost range |
|---|---|---|
| Resin injection | Slab heave, minor settlement | $15k–$50k |
| Underpinning (concrete piers) | Major settlement, edge subsidence | $40k–$120k |
| Underpinning (screw piles) | Deep stable base needed | $60k–$200k |
| Drainage improvements | Moisture-driven movement | $3k–$15k |
| Tree removal (within 1.5x height of slab) | Tree-induced drying | $1.5k–$6k |
| Full slab replacement | Catastrophic failure | $150k–$400k+ |
Section 32 and contract checks
- Council building permit history. Look for any prior structural rectification works.
- Engineer’s report.If a soil report or structural engineer’s report exists, the vendor must provide it under their disclosure obligations.
- Vendor disclosure. Direct question: has the property had any subsidence, slab heave, or structural rectification works in the last 10 years?
- Independent structural inspection. $800–$1,500 for a structural engineer. Strongly recommended for any property on Class H1 or above.
- Soil report. If not in the Section 32, a soil investigation costs $1,500–$3,500 and provides definitive classification.
Insurance considerations
Most home insurance policies cover damage from sudden subsidence events but exclude gradual settlement caused by reactive soil movement. Read the PDS carefully before assuming you’re covered. Some insurers exclude:
- Damage from tree root drying
- Damage from groundwater changes
- Damage that develops gradually (over months/years)
- Damage to driveways, paths, retaining walls outside the dwelling
Ready to check your contract? Upload your Section 32 or Contract of Sale at precontractreview.com for a pre-contract check — typically in just a few minutes.