Building on reactive clay: design decisions before the soil test

Reactive clay shrinks as it dries and swells as it wets, and the resulting surface movement is what AS 2870 site classifications measure. On a highly or extremely reactive site the ground under a house can move tens of millimetres through a seasonal cycle, and the building has to tolerate that movement rather than resist it.

Most of the design decisions that determine how well a house tolerates it are made before the site classification exists: where the building sits, what the floor level is, how the plan is shaped, which trees stay and where water goes after handover. This sets out those decisions and the evidence available to inform them at sketch stage.

What the classification will tell you, and when

AS 2870 classifies a site by characteristic surface movement (ys), the expected surface rise and fall over a dry-to-wet cycle. Lochaden and Haberfield (2018) give the bands: up to 20 mm is Class S, over 20 to 40 mm is M, over 40 to 60 mm is H1, over 60 to 75 mm is H2, and over 75 mm is Class E. The letter arrives after a geotechnical engineer has drilled, sampled and tested. Details of what sets it are in AS 2870 site classification explained.

Two design consequences follow. Movement of that order exceeds the tolerance of most brittle finishes, and a compliant footing is designed to limit distortion rather than eliminate it. Cameron (2018) notes that designs complying with AS 2870 may still show some movement and distress, usually cosmetic, and that a client seeking better performance can request an upgraded footing design at additional cost.

The decisions below are listed in the order they lock in.

Decision	What it changes	Typically locked at
Footprint position	Exposure to fill, fall and existing trees	Sketch design
Floor level, cut and fill	Retaining, drainage, which footing systems suit	Sketch design to DA
Plan shape and articulation	Where differential movement appears	Design development
Trees retained or added	Design surface movement under AS 2870	DA and landscape plan
Paving, drainage, irrigation	The moisture regime after handover	Construction documentation

Decision 1: where the building sits on the lot

Footprint placement is the least expensive decision to change and is usually made before any ground information exists. Three things are worth establishing first:

• Where fill is likely. Filled gullies, former dam sites and cut-and-fill platforms rarely cover a whole lot. Moving a footprint off deep fill can be the difference between a standard footing and an engineered one.
• Where the fall runs. A building placed across the contour requires more cut, fill or retaining than the same building aligned along it.
• Where mature trees stand. Distance from a tree is a design parameter under AS 2870, not a landscaping preference.

Mapped geology, historical aerial imagery and nearby borehole logs will not give a founding level, but they indicate which part of the lot carries less risk while the plan is still fluid.

Decision 2: floor level, cut and fill

Cutting into a slope exposes stiffer material and reduces fill, and can also bring reactive clay closer to the surface and introduce retaining. Filling to reach a level pad is faster and can place the entire footing on material whose behaviour then has to be accounted for.

The footing system available depends on that choice. Lochaden and Haberfield (2018) describe the waffle raft as founding its beams essentially at ground level, with polystyrene void formers between them, so little excavation occurs during construction. Deciding to fill for level is therefore also a decision about which footing systems remain suitable.

Decision 3: plan shape and where movement will show

Reactive ground deforms the whole platform, so plan geometry determines where the deformation appears:

• Long unbroken masonry walls have the least tolerance for differential movement. Articulation joints exist to accommodate it.
• Re-entrant corners in L, T and U-shaped plans concentrate distortion, so complex footprints on reactive sites generally require more articulation.
• Brittle finishes such as rigid tiling, full-height stone, large fixed glazing and rendered masonry show movement first.
• Mixed construction, with lightweight over part of the plan and masonry over the rest, changes both loading and the flexibility of what sits on the slab.

None of this requires the classification. It requires knowing the site is likely to be reactive, which mapped geology and the climate zone indicate at sketch stage.

Decision 4: trees

AS 2870 includes a method for accounting for the additional soil drying caused by nearby trees, which Cameron (2018) describes as an addition to the design movement rather than a minor adjustment.

The published case histories show the scale. Cameron and Beal (2011) tabulate South East Queensland houses affected by tree drying, including cases at Silkstone and Redbank Plains near Ipswich with design surface movements of 151 mm and 93 mm, both Class E, and a Morayfield case in the Class H range. In Melbourne, Li (2018) calculated a characteristic surface movement of 89 mm at a basaltic clay research site, a Class E result before any tree effect was added.

Geometry matters as well as magnitude. Cameron and Beal (2011) distinguish the drying profile of a single tree from that of a row, group or stand. A tree on one side of a building produces an asymmetric mound rather than the symmetrical one assumed by a standard design. The practical questions at design stage are which trees are being retained, how far they stand from the footprint, whether they form a line along a boundary, and what the landscape plan proposes to add.

Decision 5: water after handover

The moisture regime around a finished house is largely set by architectural and landscape decisions: where paving falls, whether downpipes discharge into a proper system, path widths, the position of garden beds and irrigation against the slab edge, and whether a pool is added later. Reactive soil damage more often follows a change in the moisture regime than a fault in the footing, which is the pattern in most of the cases covered in reactive soils and building damage.

What the engineer needs from the drawings

The footing is designed against the architectural set. A set that supports good design includes the footprint on the survey, proposed floor levels, cut and fill intent, tree positions and sizes with retained trees marked, paving and drainage layout, and the location of heavy or brittle elements.

Desktop ground evidence belongs with it: mapped geology, terrain, nearby bore logs and the likely reactive conditions for the lot. That evidence is not a classification and does not replace the site investigation. It allows the design conversation to start from the ground the building will sit on.

Common questions

Can you build on reactive clay?

Yes. Most of the housing in Adelaide, Melbourne, Sydney and South East Queensland sits on reactive soil. AS 2870 provides standard footing designs for sites up to Class H2, with Class E and Class P sites designed specifically by an engineer.

What does a Class H or E site mean for the design?

It means the building has to tolerate more movement. In practice that means more articulation, care with long masonry runs and brittle finishes, a footing system suited to the platform, and control of drainage and planting around the slab.

How close can a tree be to a house on reactive clay?

There is no single distance. AS 2870 gives a method based on the tree's mature height, its distance from the footing and the reactivity of the site, and groups of trees behave differently from single trees. The calculation belongs to the geotechnical and structural engineers, which is why tree positions and species need to be on the drawings.

Can an architect design before the soil test?

Yes, provided the design is based on a defensible expectation of the ground. Mapped geology, climate zone, terrain and nearby bore records give that expectation, so the classification confirms the approach instead of overturning it.

To see the geology, terrain and nearby ground evidence for a lot at concept stage, generate a desktop report for the address, or see a sample report to see how it reads.

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

• Andrew L.E. Lochaden and Chris M. Haberfield (2018). Assessment of the AS2870 standard designs for residential rafts on reactive clay. Australian Geomechanics, Volume 53, Number 1 (Mar 2018). geomechanics.org.au
• Donald A. Cameron (2018). Dealing with reactive clay soils through a national standard. Australian Geomechanics, Volume 53, Number 1 (Mar 2018). geomechanics.org.au
• D. A. Cameron and N. S. Beal (2011). Estimation of foundation movement and design of footing systems on reactive soils for the effects of trees. Australian Geomechanics, Volume 46, Number 3 (Sep 2011). geomechanics.org.au
• Jie Li (2018). Influence of trees on expansive soils in Melbourne. Australian Geomechanics, Volume 53, Number 1 (Mar 2018). 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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