Uncontrolled fill: how to tell before you buy, and what it does to your footing

Fill is soil or rock that was placed by people rather than laid down by nature. On a building block it is the single most common reason a soil test comes back as a "problem site" instead of a clean letter, and the most common reason a slab allowance blows out after the contract is signed. The frustrating part is that fill almost always leaves a trace in the records before anyone drills.

This guide is for builders and developers: what fill is, why "controlled" and "uncontrolled" matter so much, what fill does to a footing even when it was placed well, and how to spot it from your desk.

Controlled vs uncontrolled, in one paragraph

Controlled (engineered) fill was placed in layers, compacted to a specification, tested as it went in and documented. A footing can be designed to sit on it, with care. Uncontrolled fill is everything else: a gully pushed in with a dozer, an old dam backfilled, demolition rubble, topsoil stripped off one part of a site and dumped on another, or simply fill with no records. Nobody can say how dense it is, how deep it goes or what is in it, so a footing cannot rely on it. That is the trigger for Class P.

Why even good fill changes the slab

This is the part that surprises people. Hargreaves (2005), reviewing twenty years of AS 2870 practice in South East Queensland, explains that natural clay has a cracked, crumbly top layer that absorbs some of the seasonal movement. Compacted clay fill has no such layer. In his worked examples the same clay, once placed as fill, produced surface movement roughly 50 to 66 percent higher than it did in the ground. His warning is practical: the soil test often describes the natural soil, and the slab designer never hears that the top metre of the pad is compacted reactive fill with its own heave potential. If you are on a cut-and-fill estate, ask whether the classification covers the fill.

Adjusting a classification for reactive clay fill is now an explicit step in the Standard's method. It is not an exotic case; it is the normal condition on an earthworked estate.

Why fill keeps moving (and why retests argue)

Deep fill settles under its own weight and under the house. On a rehabilitated basalt quarry at Niddrie in Melbourne's north-west, where houses were to sit on engineered fill up to about 35 m thick, monitoring showed the fill settled by roughly 1 to 1.5 percent of its own depth, most of it while the fill was still being placed and little ongoing movement nine months after the earthworks finished (Colls et al. 2010). That is what well-engineered deep fill looks like: real movement, but front-loaded and measured.

Uncontrolled fill gives you neither the measurement nor the timing. A Monash study of a stiffened raft on non-engineered fill in the Melbourne suburb of Chelsea found that the simplified "soft spot" design method commonly used for such sites could not predict how the raft behaved in the field (Schult et al. 2007). In other words, the shortcut designs do not rescue an unknown fill.

There is a further wrinkle worth knowing if you ever end up in a dispute. Burman et al. (2008) reviewed seven cases where clay fills were retested some time after construction and came back below specification. The usual conclusion is that the earthworks were done badly. Their evidence, from multiple reputable testers, is that compacted clay fill is not inert: it can lose apparent density with time and moisture, so a failed retest years later does not by itself prove the fill was placed badly. Records from the time of placement matter more than a retest.

What it costs to fix

If the fill is shallow and controlled, the footing may only need a modest adjustment. If it is deep or uncontrolled, the options are piers or screw piles through the fill to natural ground, removing and re-compacting it, or ground improvement. On a large contaminated uncontrolled fill site at Moorebank in Sydney the engineers weighed "dig out and replace" against compacting the fill in place with a heavy impact roller and chose the roller, validated with load and penetration testing (Yang et al. 2020). That is an industrial-scale example, but the choice is the same one a builder faces on a filled block: pay to go through it, pay to replace it, or pay to prove it. Each costs more than knowing about the fill before you priced the job.

How to tell from your desk

Almost all fill shows up in existing records:

• Historical aerial imagery. The gully, farm dam, creek line or quarry that was there in the 1970s and is a flat pad today. Multiple dates let you see when it was filled.
• Elevation. A flat platform cut into a slope has a cut side and a fill side. Contours and a cross-section show which side of the lot is which.
• Geology and soils mapping. Floodplain and swamp deposits, which are the ground most often built up with fill, and the reactive clay units most likely to have been used as fill.
• Nearby bore logs. Whether neighbouring investigations logged fill, and how deep.
• Planning overlays and records. Former land uses, flood extent, and in some areas a register of filling or earthworks approvals.

None of that is a soil test. It tells you whether to expect one, what to brief it to look for, and whether to budget for an engineered footing before the contract is fixed. Our guide to what a geotechnical desktop study covers explains how these pieces fit together.

Common questions

What is uncontrolled fill? Fill placed without a compaction specification, testing or records, so its density, depth and contents are unknown. A footing cannot rely on it, which usually makes the site Class P under AS 2870.

Does controlled fill mean a standard slab? Not automatically. Compacted reactive clay fill can move more than the same clay in its natural state, and the classification should account for it. 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, nearby bore logs and planning records together flag filled gullies, dams, cut-and-fill platforms and floodplain ground. A field investigation then confirms depth and quality.

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See the historical imagery, elevation cross-section, geology and nearby bore logs for a lot before you allow for the slab. Generate a desktop report for the address or see a sample report.

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

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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