Uncontrolled fill
Uncontrolled fill is fill placed without AS 3798 supervision. Under AS 2870 it forces the site to Class P and a fully engineered slab. Common on brownfield sites.
Ask Chalkline about this →Uncontrolled fill is soil or material placed on a site without AS 3798 Level 1 supervision, including any pre-existing fill from earlier developments, demolition, or council works that has no compaction or material-conformance documentation. Under AS 2870, a site with uncontrolled fill cannot use the standard deemed-to-comply slab and footing tables; the site classification drops to Class P and a fully engineered slab and footing design is required.
The contrast with controlled fill:
| Property | Controlled fill | Uncontrolled fill |
|---|---|---|
| Supervision | AS 3798 Level 1 (GITA on site continuously) | None, or supervised below Level 1 |
| Compaction testing | Per AS 3798 schedule | Absent or incomplete |
| Material conformance | Documented | Unknown |
| Compliance report | GITA-issued | Not available |
| Site classification effect | Retains the underlying soil class | Forces Class P under AS 2870 |
| Foundation design path | DTS slab + footing tables | Engineered design |
Where uncontrolled fill comes from:
- Historical site fill from earlier subdivisions, demolitions, or council improvements. The site looks level but has 1 to 3 m of fill of unknown composition.
- Owner-placed fill between purchase and build, often by the previous owner trying to level the lot before sale.
- Spoil from neighbouring civil works dumped or pushed onto the lot in earlier years.
- Demolition fill where a previous house was knocked down and slab/brick rubble buried rather than removed.
- Marine-derived fill in coastal infill subdivisions, often saline and aggressive to footings.
Geotech identification. The soil report flags uncontrolled fill when the geo-engineer can identify:
- Layer interfaces with different colour, texture, or moisture from natural soil.
- Inclusions (rubble, plastic, organic material, bricks, glass).
- Variable compaction in adjacent test pits.
- No prior AS 3798 compliance report for the lot’s previous earthworks.
The cost impact. Class P classification adds typically $15,000 to $40,000+ to the foundation works of a single-storey dwelling, depending on:
- Depth of fill (the deeper, the more pier or pile work to reach competent strata).
- Variability (uniform fill is simpler than layered or pocketed fill).
- Engineer’s solution (piers to natural ground, raft slab over piers, or strip-and-replace and re-classify).
Three resolution paths a builder commonly faces on an uncontrolled-fill site:
- Engineer to Class P: accept the premium, design footings to bypass the fill or distribute over it. Most common path.
- Strip and replace: dig out the uncontrolled fill, replace with controlled fill under AS 3798 supervision, re-classify. Economical only on smaller fill depths (under ~1.5 m).
- Buyer beware: walk away from the lot if discovered before settlement.
For builders.
- Quote in writing that uncontrolled fill, if discovered, triggers a variation. Without this, you wear the cost when the geotech finds fill.
- Get the soil report at quote stage, not after contract. Reading the soil report should be a builder-stage activity before pricing.
- Discuss with the client. Owners who placed fill themselves often don’t disclose because they didn’t know it mattered.
Also known as: non-engineered fill, unsupervised fill, historical fill.
Category: Site preparation / earthworks / classification.
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Last updated: 2026-05-14. Verified: 2026-05-14.