Uncontrolled fill: how to tell if a block has it

You can usually tell whether a block is likely to have fill before you buy, from historical aerial imagery, elevation, geology mapping and planning records that show a filled gully, dam, quarry or cut-and-fill platform. Only a site investigation on the lot confirms whether fill is there, how deep it is and whether it was controlled. It matters because under AS 2870-2011 uncontrolled fill makes a site Class P (a problem site needing an engineer's design) unless every footing goes through the fill to natural ground.

What is the difference between controlled and uncontrolled fill?

The material can be identical. The difference is how it went in and whether anyone can prove it.

The former basalt quarry at Niddrie, about 15 km north-west of Melbourne's CBD, had both. When quarrying stopped in 1976 it left a pit up to 40 m deep with a groundwater lake in the bottom, and about 2.4 million cubic metres of overburden stockpiled in and around it, up to 30 m deep. That was uncontrolled fill. To build a housing estate there, every stockpile was dug up in 2002 and 2003, sorted, blended, brought to the right moisture and put back as compacted, tested fill up to about 35 m deep (Colls et al. 2010). Same rock and clay; now controlled fill, with records.

Controlled (engineered) fill is placed in layers, compacted to a specification, tested as it goes in and documented. Uncontrolled fill is everything else: a gully pushed in with a dozer, a backfilled dam, demolition rubble, or any fill without records. Nobody knows how dense or deep it is, so a footing cannot rely on it.

How much fill does AS 2870 allow before a site is Class P?

Clause 2.5.3 of AS 2870-2011 sets three rules:

• Controlled fill up to 0.4 m deep (0.8 m if it is sand): the site is classified as the natural site was before filling.
• Controlled fill deeper than 0.4 m that is not sand: the site is Class P, although an engineer may reclassify it by engineering principles, which means calculating the movement for the actual profile including the fill.
• Uncontrolled fill: the site is Class P, unless all footings are founded on natural soil through the fill, for example on piers or deepened beams.

The commentary adds a limit on reclassification: a filled site must not end up in a less severe class than the natural ground would have been, unless the controlled fill is non-reactive and deeper than 1 m or half of Hs (the depth to which the soil dries and wets seasonally), whichever is greater. Class P on its own says little, so the report should state why the site is P. More in Class P problem sites.

At Niddrie, under the 1996 edition then in force, houses on the natural basalt clay nearby were typically Class H, and the developer aimed for lots on the deep fill to be reclassified from P to H as well. That shaped the earthworks: fill under shallow house footings had to be low in clay, and clay-rich material went where houses would be piled to the old quarry floor.

Why does compacted clay fill move more than natural clay?

Natural clay has a cracked top layer that absorbs some of the seasonal swelling and shrinking. Freshly compacted clay fill has none, so the calculation gives it no cracked zone. Hargreaves (2005), looking back on twenty years of AS 2870 in South East Queensland, worked examples where 1.2 m of cut and 1.2 m of compacted clay fill gave 50 to 66 percent more predicted surface movement than the same clay left in the ground.

He also explained how it gets missed. On a single house lot, the compaction tester rarely certifies the fill as good enough to build on without piers, so it is treated as uncontrolled and the engineer designs piers. The report describes the natural soil, and the designer may never hear that the compacted clay fill can heave. Ask whether the classification covers the fill.

Does fill keep settling after it is placed?

Yes. At Niddrie, settlement plates in the fill recorded about 1 to 1.5 percent of its depth, most of it while more fill was going on top, and little after September 2004, nine months after the bulk earthworks finished (Colls et al. 2010). That is well-engineered deep fill: real movement, but measured, and mostly early.

Uncontrolled fill offers neither the measurement nor the early finish. A Monash study of a raft slab on non-engineered fill in the Melbourne suburb of Chelsea found the "soft spot" design shortcut commonly used on such sites could not predict how the slab behaved (Schult et al. 2007).

Can a later test prove the fill was placed badly?

Not on its own. Burman, Mostyn and Piccolo (2008) collected seven NSW cases where clay fill passed during construction and tested lower later. At the Brewarrina flood levees, rebuilt in 2001 and 2002 with about 75,000 cubic metres of clay, every control test passed. Taken at face value, retests then showed compaction dropping by about 5 percent within months. One set was so low it ended up before the NSW Supreme Court, which rejected it because of how the testing was done. The authors' view is that retests deserve no automatic preference over tests taken at placement, so the earthworks records matter. Fill on a building block covers what those records look like.

What are the options if a block has uncontrolled fill?

Go through it, replace it, or improve it and prove it: piers or screw piles to natural ground, digging it out and re-placing it as controlled fill, or compacting it in place. At a large contaminated fill site at Moorebank in Sydney, engineers chose an impact roller over dig-out-and-replace and proved the result with load and penetration tests (Yang et al. 2020). Which option fits is decided after the investigation has measured the fill.

How can you tell from the records whether a block has fill?

Most filling leaves a trace: a gully, dam or quarry on old aerial photos that is a flat pad today; a level platform cut into a slope; floodplain or mapped made ground on the geology; former land uses and estate earthworks records. Fill in a nearby bore log shows filling happened in the area, not whether this lot was filled or how deep. Only holes on the lot measure that.

None of that is a soil test or a site classification; the site investigation decides both. The records tell you whether to expect fill, where to point the investigation, and whether to allow for an engineered footing before the contract is fixed.

Common questions

What is uncontrolled fill? Fill placed without a compaction specification, testing or records, so its density, depth and contents are unknown. Under AS 2870-2011 it makes the site Class P unless all footings are founded on natural soil through the fill.

Does controlled fill mean a standard slab? Only if it is shallow. Controlled fill up to 0.4 m (0.8 m for sand) is classified as the natural site before filling. Deeper controlled fill other than sand makes the site Class P unless an engineer reclassifies it by calculation, and compacted reactive clay fill can move more than the same clay in its natural state. Ask whether the soil test covered the fill as well as the natural ground.

How do I find out if a block has fill before buying? Historical aerial imagery, elevation and contours, geology and soil mapping, and planning and estate records flag filled gullies, dams, cut-and-fill platforms and floodplain ground. Only a field investigation on the lot confirms whether fill is present, how deep it is and whether it was controlled.

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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
• Colls, S., Finlayson, J. and Goad, D. (2010). Settlement behaviour of deep engineered fill, former basalt quarry, Niddrie, Victoria. Australian Geomechanics, Vol 45 No 1. geomechanics.org.au
• Hargreaves, B. (2005). 20 years of AS 2870 in South East Queensland: a personal view. Australian Geomechanics, Vol 40 No 3. geomechanics.org.au
• Schult, M., Haque, A. and Ranjith, P. G. (2007). Designing stiffened raft footing on non-engineered fill. Australian Geomechanics, Vol 42 No 2. geomechanics.org.au
• Yang, Q. J., Succar, A., McIlquham, J. and Chen, K. (2020). Ground improvement of contaminated uncontrolled fill using impact roller compaction. Australian Geomechanics, Vol 55 No 3. 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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