Uncontrolled Fill and Class P Sites in Adelaide: What to Compare Before You Brief the Geotechnical Engineer
Uncontrolled Fill and Class P Sites in Adelaide: What to Compare Before You Brief the Geotechnical Engineer
General information for South Australian landowners only. This is not engineering, geotechnical, legal, tax, valuation or planning advice. Route ground conditions, site classification and footing design to a geotechnical engineer and a structural engineer, the structural documents your building consent needs to a building surveyor or certifier, what fill material is made of to an environmental consultant, cost to a quantity surveyor (QS), levels and boundaries to a licensed surveyor, contracts and site-condition clauses to an SA property lawyer or conveyancer, and zone and division questions to a qualified planning consultant. Australian Standards are reissued and state design standards are updated, so confirm the current versions and the current position for your own parcel with your own advisers before you rely on anything here.
Same street, same clay, two different footing designs
Picture two blocks side by side in an older Adelaide suburb. The same geology under both, the same reactive clay, the same aspect. Both owners take a concept plan for one dwelling into two, and both send a geotechnical engineer to site.
One report comes back with a footing system a local builder has detailed many times before. The other comes back as a problem site, with a footing system that has to be worked out specifically for that piece of ground.
In this picture the difference is not the soil type; by construction of the example, the geology under both blocks is the same. What separates them is what has been done to the ground. On one of them, before either owner bought, a low corner at the back had been levelled off with whatever material was on hand: spoil from an excavation down the road, a demolished outbuilding, garden waste, broken concrete. Grass grew over it. Today that yard looks exactly like the neighbouring one.
Soil has properties, and it also has a history. The properties are the same either side of the fence. The history is not.
Where that history sits is changing. As infill densification pushes more division into established suburbs, more new allotments and building envelopes are being set down on the footprint of a house already demolished once, and on the parts of old back yards that were levelled to make them usable — the tightening of residential infill rules in SA covers the planning side of that shift. Those are among the places where previously placed material can turn up. That is a judgement about where infill tends to land rather than a measured statistic, and on any given block only an investigation settles it.
Four things you can compare before anyone goes to site
You are about to brief a geotechnical engineer. There is a comparison you can run yourself first, on material already published, without engaging anyone.
1. Your own yard, in two aerial photographs taken years apart. On the blocks Cyberate PM has helped coordinate, this is often the comparison that raises the first question. Find your address in a historical aerial image and set it beside a current one. Look for where the original house and its outbuildings stood, where the old driveway ran, and whether a dam, a pit, a watercourse or an obvious low area has since become flat lawn. South Australia has published historical Greater Adelaide aerial imagery as open data (source: data.sa.gov.au), the Commonwealth's historical aerial photography can be searched and downloaded online (source: Geoscience Australia, Historical Aerial Photo Explorer), and both of those are free to view. Beyond the free datasets, the state's wider aerial photography archive is held by DEW Mapland, which supplies imagery on order (source: Mapland) — that one is a request, not a download. Anything that was a hole and is now level is worth a question.
2. Older contours against today's ground. Put an older topographic contour over a current set of survey levels and you are asking one thing: does the ground today sit above where the old surface ran? Where it appears to, that is a difference worth putting to the engineer — it may be placed material, or it may be a difference in survey datum or accuracy between the two sets of levels. Reading levels off a plan and relating them to a boundary is a licensed surveyor's work, not a job for a screenshot.
3. Which side of the neighbouring retaining wall is the high side. A retaining wall holds back ground that is higher on one side than the other. If the high side is yours, the ground on your side is higher than the ground next door — which may be natural fall, or may be material somebody placed and had to hold there. Which one it is, is a question for your geotechnical engineer, not something the wall itself answers. Retaining walls carry their own ownership and boundary questions, set out in retaining walls, boundaries and sloping blocks in Adelaide.
4. The yard against the street and against both neighbours. Compare the level at the rear of the property with the kerb out the front and with the ground either side. A yard that steps up noticeably behind the house, or a rear boundary sitting above the laneway behind it, is worth asking about.
None of these four is a finding, and they can all be innocent — natural ground falls and rises, and old photographs are hard to read. Whether there is fill, how deep it goes and where it stops is a question for a geotechnical engineer, answered on whatever investigation they judge the site needs. What the comparison buys you is the ability to point: this part of the yard has a history, and I want it covered.
The dividing line is not whether there is fill — it is what the engineer can verify about it
Fill by itself is not the problem. On the account geotechnical practice gives, fill placed deliberately, in layers, compacted, tested as it goes, with those test results retained, is described as controlled fill, and later design can be based on it. Fill placed without that supervision and that documentation is described as uncontrolled fill — whatever it is made of, and however solid it feels underfoot. Real sites are rarely as tidy as the two labels, and which description fits the material on your own block is your geotechnical engineer's assessment against the standards in force, not something the labels settle on their own.
As geotechnical practice describes it, the reference for that placement-and-testing regime is AS 3798, the Standards Australia guideline on earthworks for commercial and residential developments, and the treatment of residential sites containing uncontrolled fill sits in the site classification standard, AS 2870. Both are paid standards, reissued from time to time, so the wording that applies is for your geotechnical engineer to confirm against the edition in force.
In South Australia, the engineering requirements published as design standards for land division through the PlanSA system include earthworks among their technical content, which is where the controlled-fill expectation — compaction witnessed and tested to AS 3798, with the records retained — sits on the division side (source: PlanSA — Design Standards). What that bears on for your own division is for your civil engineer and your planning consultant to read against the current published version.
Where no compaction record exists, there is nothing for a later design to verify the material against, and engineering practice generally treats it as unverified rather than as proven. The material might be excellent. If nobody can demonstrate that, a later design has less to lean on. But records are one of the things weighed rather than the whole test: the description that applies follows the standard in force as your geotechnical engineer applies it to the material actually in front of them, alongside what it is made of, how thick it is and what the rest of the site is doing. How much the absence of records matters, and what the engineer does about it, is theirs to judge on your own site.
Two entirely separate lines of investigation open when fill is found, and this article follows only one. The investigation this article is about is a geotechnical investigation: it is produced by a geotechnical engineer working to AS 2870 and AS 3798, its criteria include the thickness of the fill layer and the compaction records behind it — alongside the nature of the material itself and the other site conditions the engineer weighs — and where it goes is the structural design for your building consent. Whether that material carries a contamination issue — what it is made of in environmental terms — is a different question again, answered by an environmental consultant in a separate report, and it is set out in site contamination assessment for development in Adelaide.
Class P: not a worse grade, but a site outside the scale — as geotechnical practice describes it
Geotechnical practice describes Class P not as another rung on the reactivity scale but as the site stepping outside that scale — and that is the part that is easy to read backwards.
The lettered site classes are a scale, running from A at the least reactive end through to E at the most reactive; the classes in between carry their own letters rather than following the alphabet. They describe how much the ground is expected to move with seasonal moisture change. Where a site sits on that scale, the class gives the engineers a recognised starting point to design from — the footing that ends up on the drawings is still selected for that site and that building, which is why two blocks of the same class do not automatically get the same footing.
Class P is not the next step along that run. It is what the classification system reaches for when the site does not sit on the scale at all — conditions abnormal or uncertain enough that a class cannot carry the design, and the footing system is worked out for that specific site instead. Uncontrolled fill is one of the situations that puts a site there, on the account geotechnical practice gives; confirm that against the current edition with your own engineer.
The consequence is not a grade dearer. It is that the design cannot take the class as its starting point, because there is no class — so the footing system is determined for that site, by your engineers, on what the investigation in front of them found. The cost stops being a matter of reading across from a band and becomes a matter of pricing a scheme drawn for your block.
A site classification report is normally an input to the structural design that sits behind a building consent, and which structural documents your consent needs is something your building surveyor or certifier will tell you for your own job.
Two questions run in parallel here, and they are worth keeping apart. Reactive clay, site classification and footings in Adelaide answers how difficult this ground is by nature. This article answers what has been done to it. On plenty of Adelaide blocks both are in play and both come from the same engineer — but only one changes with where you decide to put the house.
The variable that may still be open: where the lines and the houses go
You have a concept plan. The boundary between the two allotments is a line on it. The two building envelopes are rectangles on it. At this moment, all three can be moved by redrawing them.
On a block where the fill occupies one part of the yard and not the rest, the position of the new dwelling and of the allotment line is one of the few inputs still open to redrawing — and it stays that way only while the plan is a drawing. Moving it is not free, and it is not only a footing question. Shifting an envelope or a boundary redraws setbacks, driveways, private open space, solar access and service runs; it changes what each allotment can hold, and it can send drawings, survey work and the approval path back around. What the move does is trade one set of costs for another — it may avoid an earthworks or deep-footing stage while adding design, survey and yield consequences. Whether that trade comes out ahead depends on the extent and depth of the fill and on what the move costs everywhere else; that comparison is for your engineers, your designer, your surveyor and your QS. Once a layout is lodged and an approval has been granted against it, changing the dwelling position or the boundary generally means going back to the design stage — what that involves for your own application is a question for your planning consultant.
The rest of the site — the depth of the fill, the reactivity of the clay, the shape of the block — is the condition you start from, not the condition you are stuck with. Fill can be dug out and replaced, founded through, or otherwise dealt with by engineered works, and ground can be treated; what you inherit is the starting point, and what the site ends up as is the outcome of a scheme your engineers design and a QS prices. Layout is different only in that it is the input you can still change with a pencil.
That does not mean the move is available on every block. Whether the new dwelling can be placed clear of the fill at all depends on the extent and depth of the fill, the shape of the parcel, and what the setbacks and access leave you — a judgment your geotechnical engineer and your designer make together, not one an owner can make from an aerial photograph.
One position deserves particular attention: the footprint of the house that was demolished. Old footings, backfilled service trenches, a filled cellar or an old rainwater tank pit can all sit within that footprint — and on an infill site it is often close to where somebody wants to put a new slab.
Three ways forward on a filled site, and the question to put in your brief
If the report comes back describing uncontrolled fill under a proposed building position, there are broadly three directions. The table below deliberately uses relative bands rather than figures.
Remove the fill and replace it as controlled fill — When it is on the table: The fill is shallow and confined enough to dig out, with room to work without undermining neighbouring ground; Relative magnitude against a conventional footing scheme: Same order to clearly higher, rising with the depth and extent of the material; Relative effect on the program: Adds a separate earthworks stage ahead of footings, with its own testing and records; Who decides: Geotechnical engineer scopes it, structural engineer accepts the result, QS prices it
Found through the fill on piles or another deep footing system — When it is on the table: The fill is deep or extensive, and competent ground can be reached beneath it; Relative magnitude against a conventional footing scheme: Clearly higher, and higher again with the depth to founding level; Relative effect on the program: Generally sits inside the footing stage rather than beside it, with its own plant and access needs; Who decides: Geotechnical and structural engineers together, QS prices the scheme
Move the building — and where possible the allotment line — onto natural ground — When it is on the table: There is natural ground on the block that can still take a compliant envelope; Relative magnitude against a conventional footing scheme: Can be the same order as a conventional scheme where it removes the need for the other two; the saving is not automatic and depends on what the move costs elsewhere; Relative effect on the program: Generally avoids adding an earthworks or deep-footing stage while the plan is still a drawing; after approval it sends the layout back to the design stage, and the net effect is for your engineers, designer and QS to weigh; Who decides: Geotechnical engineer on where natural ground is, designer and planning consultant on whether the moved envelope works, QS on the comparison
Every row is expressed in relative terms on purpose. The figures come from a quantity surveyor pricing the actual scheme your engineers put on paper, and the trade-off between the three rows is judged by the geotechnical and structural engineers together with that QS — not settled from a table. Digging fill out near a boundary can also pull questions into the job that were not there before — retaining design back to your geotechnical and structural engineers, and the neighbour's position back to your SA property lawyer or conveyancer, with the ownership and boundary side of it set out in retaining walls, boundaries and sloping blocks in Adelaide.
Until those figures exist, foundation uncertainty belongs in a feasibility study as an allowance with a stated range and a stated assumption behind it, priced by a QS against the engineer's scheme — not as a single number an owner has picked to make the sheet balance.
Which brings this back to the brief you are writing. There is one question in it worth putting plainly to your geotechnical engineer:
Will the investigation cover the actual positions of the two proposed houses, rather than a single point in the middle of the site?
The reason to ask is straightforward. A single point in the centre of a block returns information about that point. A fill boundary can run between it and the place a slab will actually sit, and a report can be entirely accurate about a piece of ground nobody is building on. Where the investigation goes, how many locations it covers and how deep it runs are the engineer's decisions, made on the site in front of them and on the standards they work to — they may want more than the building positions, or something different again. What is yours to supply is the information those decisions rest on: name the proposed dwelling positions in the brief, and ask how the investigation will treat them.
Cyberate PM does none of this work. We do not classify sites, drill or log boreholes, design footings, assess contamination or price anything. We coordinate: that the comparison you have run is in front of the engineer rather than in your head, that the brief names the proposed building positions rather than the site in general, that the report reaches the structural engineer and the building surveyor in a form each can use, and that if the answer comes back as a filled site, the question could the house move instead gets asked while the plan can still be redrawn.
If you are writing that brief now and want one set of assumptions held across the engineers, the designer and the QS, talk to us.
Australian Standards are reissued and withdrawn, and South Australia's design standards for land division are updated over time. This article reflects publicly available material at the time of writing and is general information only — confirm the current versions with your geotechnical engineer, your structural engineer and your building surveyor or certifier before relying on anything here.
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