Fill on a building block: what the records show

Public records can show whether a building block is likely to have been filled before anyone drills: historical aerial imagery, old and current elevation, geology mapping, land-use history and estate earthworks records between them reveal most filled gullies, dams, quarries and cut-and-fill platforms. What they cannot do is confirm fill is there, measure how deep it is, or say whether it was compacted properly. That takes boreholes or test pits on the lot.

The best record a buyer can hope for looks like this: on one outer Sydney residential estate, up to 12 m of fill went in between 2002 and 2004 under Level 1 control (a geotechnical engineer on site through the filling), with 432 density tests and anything that failed reworked or removed (Burman et al. 2008). Most filled ground has nothing like that, so the job is reading the evidence that does exist. What fill means for the footing under AS 2870-2011, including when it makes a site Class P, is in uncontrolled fill: how to tell if a block has it.

Which records show that a block has been filled?

No single dataset settles it. Read together, these usually show whether fill is a real possibility and which part of the lot it is most likely under.

Record	What it can show	Where it runs out
Historical aerial imagery	A gully, dam, creek line, quarry or stockpile that is now a flat pad, and roughly when it was filled	Cannot show depth or what the fill is made of
Elevation and contours	Flat platforms on a slope, ground sitting unnaturally high or level, cut and fill sides of a pad	Shows today's surface, not the original one, unless older survey exists
Geology mapping	Made ground mapped directly as "fill" near tips, canals and reclaimed land; floodplain and swamp units often built up	Mapped at a scale far coarser than a house lot
Land-use history	Former tips, quarries, brickworks, industrial sites, old waterways, reclaimed foreshore	Says fill is likely, not where on the lot
Estate earthworks records	Lot-by-lot fill depths and compaction testing on newer estates	Only exists where the developer kept and releases it
Nearby bore logs	Whether fill was logged in the area	Says nothing about fill depth on this lot

What do imagery, elevation and mapping each add?

Historical imagery usually says the most. Compare several dates, back to the 1950s or 1960s where the state archive holds them, and look for what disappears: a farm dam, a drainage line, a gully, a quarry pit, a stockpile. The date also tells you whether the filling happened before earthworks control was routine.

Elevation from LiDAR (laser-scanned ground levels) shows the shape of the ground today. A level pad cut into a slope has a cut side and a fill side, and a cross-section shows which is which and roughly how much earth was moved. Older contours, where they exist, show where the ground has been raised. Neither gives a measured fill depth.

Geology and land-use mapping sometimes show made ground directly, around old quarries, tips, canals and reclaimed land. Elsewhere the clue is a mismatch, such as a level pad on a mapped floodplain. Mapping also protects you from a bad log. In a Gold Coast case, the first investigation of a cracked house logged residual clay; the geological and soil maps put the estate on the Nerang River floodplain, where the natural soil is alluvium, and hand-augered holes found up to 2.2 m of fill (Tayler 2009). The full story is in what a geotechnical desktop study covers.

What records exist on a new estate?

Sloping estates are often benched into terraces, so one lot can be part cut and part fill. On newer estates the developer's geotechnical engineer often supervises the earthworks and reports lot by lot. Under AS 3798 (the standard for earthworks on residential and commercial developments) this is often Level 1 inspection and testing, where the engineer is on site during filling and signs off on compaction. Ask the developer or seller for the earthworks completion report and the fill depth plan for the lot. If they exist, they are better evidence than anything else here.

Those records carry weight. Burman, Mostyn and Piccolo (2008) reviewed seven NSW cases where fill passed during construction and tested lower after completion (on the outer Sydney estate, the developer's own test pits came in about 2 percent below), and argued a later retest deserves no automatic preference over tests taken at the time. Check that the report says where on your lot the tests were taken: the same authors found construction tests usually could not be located afterwards, because positions were recorded as areas and heights as layers rather than surveyed levels.

What can nearby bore logs tell you about fill?

Bore logs near the lot can show that fill was met in the area. They cannot tell you whether this lot was filled or how deep. Fill depth changes sharply between neighbouring lots, along the edge of a filled gully and across a cut-and-fill pad, so only a hole on the lot measures it.

When is the desktop evidence enough?

For screening, scoping and pricing risk, often. It does not classify the site and never replaces the site investigation. Where the records point to fill, or cannot rule it out, brief the investigation to find it: holes on the likely fill side, taken through any fill into natural ground, and logged for what the fill is and its condition. The desktop tells you where to look; the site investigation decides what is there.

Common questions

Can a desktop report confirm there is no fill on my block? No. Confirming fill, or its absence, needs boreholes or test pits. A desktop study flags the likelihood of fill from historical imagery, terrain, geology, land-use history and estate records, so you know where the investigation should look.

Does fill always mean a Class P site? No. Under AS 2870-2011 controlled fill up to 0.4 m deep (0.8 m for sand) is classified as the natural site was before filling. Deeper controlled fill other than sand makes the site Class P unless an engineer reclassifies it by calculation, and uncontrolled fill makes it Class P unless all footings go through it to natural ground.

I am buying in a new estate on a slope - should I check for fill? Yes. Sloping estates are usually built on cut-and-fill terraces, and a house can sit partly on cut and partly on fill. Ask the developer for the earthworks completion report and the fill depth plan for your lot before you design, and use the elevation and imagery to see where cut and fill are likely to meet.

See the records for your address

Generate a desktop report for the address or see a sample report to see the historical imagery, elevation, geology and nearby bore logs for a lot.

LayeredGeo compiles geology, soils, groundwater and site data into an automated geotechnical desktop report for any address in Queensland, New South Wales, Victoria and South Australia.

Sources

• Burman, B. C., Mostyn, G. and Piccolo, D. (2008). Experiences with post-construction retesting of engineered clay fills. Australian Geomechanics, Vol 43 No 4. geomechanics.org.au
• Tayler, J. (2009). Forensic investigation of subsidence to dwelling, December 2007. Australian Geomechanics, Vol 44 No 1. geomechanics.org.au

About this article. Published by LayeredGeo and written from the published research cited in the Sources section above. It is general information about how property and ground conditions are assessed in Australia, not engineering, planning, legal or financial advice, and it is not specific to any property. Check anything that matters against the source dataset or a suitably qualified professional before you rely on it. If you spot something wrong, tell us at hello@layeredgeo.com.au and we will fix it.

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