Building on reactive clay: decisions before the soil test

Reactive clay shrinks as it dries and swells as it wets, and on a highly or extremely reactive site the ground under a house can move tens of millimetres through the seasons. The building has to tolerate that movement rather than resist it, and most of what decides how well it does is settled before the soil test: where the house sits, how the floor level is reached, the shape of the plan, which trees stay and where the water goes.

Richard Drew, a structural engineer writing about footing failures in Melbourne's western suburbs, describes how the last of those goes wrong (Drew 2017). The waffle pod slabs used there on highly reactive sites assume paved ground falling away from the house. But paving and drainage usually sit in a landscaping package outside the building contract, so the house is handed over with bare clay against the walls, sometimes sloping the wrong way. The slab was designed for a site that was never finished. Whether the drainage is in the contract is a design decision like any other.

What will the site classification tell you, and when?

AS 2870-2011 classifies a site by characteristic surface movement (ys), the expected rise and fall of the ground surface over a dry-to-wet cycle: up to 20 mm is Class S, rising through M, H1 and H2 to Class E above 75 mm. The letter arrives after an engineer has drilled, sampled and tested; AS 2870 site classification explained covers what sets it.

A compliant footing limits distortion rather than eliminating it. Cameron (2018) notes that houses built to the Standard may still show some distress, usually cosmetic, and an owner who wants better can pay for an upgraded footing design.

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

Where should the house sit on the lot?

Footprint position is the cheapest decision to change and is usually made before any ground information exists. Three things are worth knowing first:

• Where fill is likely. Filled gullies, old 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 across the contour needs more cut, fill or retaining than the same building along it.
• Where mature trees stand. Distance from a tree is a design input under AS 2870, not a landscaping preference.

Mapped geology, historical aerials and terrain can show which part of the lot is more likely to have been filled or cut while the plan is still fluid. Only the site investigation confirms it.

Should you cut or fill to get a level floor?

Cutting reduces fill but can bring reactive clay closer to the surface and adds retaining. Filling is faster and can put the whole footing on fill. Under clause 2.5.3 of AS 2870-2011, controlled fill up to 0.4 m deep (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 uncontrolled fill makes it Class P unless all footings go through it to natural ground. The detail is in uncontrolled fill: how to tell if a block has it.

The choice also decides which footing suits. A waffle raft's beams sit on the ground surface, formed between polystyrene void formers, so it needs little or no excavation and suits a level pad. A stiffened raft's beams are dug into the ground, 0.3 to 1.1 m deep depending on the class. Lochaden and Haberfield (2018) modelled the Standard's designs for both on an H2 site and found the waffle raft moved about 50 percent more than the stiffened raft; on their assumptions, neither met the Standard's own movement limits. On a slope it matters more: Drew (2017) points out that a waffle slab is usually kept at one level, so the finished ground can end up close to its underside, or leave the underside exposed at the edge, letting runoff in.

How does the plan shape affect cracking?

Reactive ground distorts the whole platform, and plan geometry decides where that shows:

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

None of this needs the classification, only a reasonable expectation that the site is reactive.

Which trees can stay?

AS 2870 includes a method for the extra drying caused by nearby trees, and the case histories show it is not a minor adjustment. Cameron and Beal (2011) tabulate South East Queensland houses affected by tree drying, including a duplex at Silkstone and a house at Redbank Plains near Ipswich with characteristic surface movements of 151 mm and 93 mm, both Class E. In Melbourne, Li (2018) calculated 89 mm at a basaltic clay research site, Class E before any tree effect was added.

Geometry matters too. A row or group dries the ground differently from a single tree, and a tree on one side of a building produces a lopsided mound rather than the symmetrical one a standard design assumes. At design stage the questions are which trees stay, how far they are from the footprint, whether they line a boundary, and what the landscape plan adds.

Who decides where the water goes?

The moisture around a finished house is set mostly by architectural and landscape decisions: where paving falls, whether downpipes connect to a proper system, garden beds and irrigation against the slab edge, and whether a pool comes later. Drew recommends an engineered drainage and paving design built with the house, not left to a later package. Damage more often follows a change in moisture than a fault in the footing, which is the pattern in most of the cases in reactive soils and building damage.

What does the engineer need from the drawings?

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

Desktop ground evidence belongs with it: mapped geology, terrain, nearby bore logs and the reactivity range to expect. That is not a site classification and does not replace the site investigation, which decides the class.

Common questions

Can you build on reactive clay?

Yes. Much of the housing in Adelaide, Melbourne, Sydney and South East Queensland sits on reactive soil. AS 2870-2011 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 reasoned expectation of the ground. Mapped geology, climate zone, terrain and nearby bore records give that expectation. The site classification still comes from the site investigation, and the design should be able to absorb a higher class than expected.

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, Victoria and South Australia.

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
• R. J. Drew (2017). The interplay of site reactivity, design practice and construction procurement in structural failures on expansive clay sites. Australian Geomechanics, 2017 Victoria Symposium. 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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