Designing on reactive clay: the decisions an architect makes before the soil test

By the time a site classification lands on an architect's desk, most of the decisions it affects have been made. The footprint is placed, the floor levels are set, the plan shape is resolved and the client has fallen in love with the elevation. The classification then arrives as a constraint on a design that was drawn without it.

That sequence is normal, and on uniform ground it costs nothing. On reactive clay it costs plenty, because the letter on the report is not a materials specification. It is a prediction of how much the ground under the building will rise and fall, and a handful of design decisions made months earlier either work with that movement or fight it.

The letter is a movement, and the design has to tolerate it

AS 2870 classifies residential sites by characteristic surface movement (ys), the expected surface rise and fall over a dry-to-wet cycle. Lochaden and Haberfield (2018) list the bands: up to 20 mm is Class S, above 20 to 40 mm is M, above 40 to 60 mm is H1, above 60 to 75 mm is H2, and beyond 75 mm is Class E. Those are not small numbers in architectural terms. A Class E site can move more than the tolerance of most brittle finishes, and the footing system is designed to keep the building within a defined range of distortion, not to hold it perfectly still.

That distinction is written into the Standard. Cameron (2018) notes that designs complying with AS 2870 are expected to experience some movement and distress, usually cosmetic, and that a client wanting a higher standard of performance can request an upgraded footing design at additional cost. An architect who understands this early can offer the client a real choice. One who meets it at handover is managing a complaint. Our explainer on what decides the AS 2870 letter covers the classification itself in detail.

Decision 1: where the building sits on the lot

Footprint placement is the cheapest lever an architect has, and it is usually pulled before any ground information exists. Three things on a lot are worth knowing before the plan is fixed:

• Where the fill is. Filled gullies, old dam sites and cut-and-fill platforms rarely cover a whole lot. Moving a footprint off the deep fill can be the difference between a standard footing and an engineered one.
• Where the fall is. A building placed across the contour needs more cut, more fill or more retaining than the same building turned to sit along it.
• Where the significant trees are. Distance from a mature tree is a design parameter, not a landscape preference.

Mapped geology, historical aerial imagery and nearby borehole logs will not give you a founding level, but they will tell you which half of the lot is the better bet before the plan hardens.

Decision 2: floor level, cut and fill

Setting the floor level is where architecture meets earthworks. Cutting into the slope exposes stiffer material and reduces fill, but it can also expose reactive clay closer to the surface and create retaining. Filling to reach a level pad is fast and can put the entire footing on material whose behaviour must then be accounted for.

Waffle rafts are relevant here because of how they sit. Lochaden and Haberfield (2018) describe the waffle raft as founding its beams essentially at ground level, with the voids formed by polystyrene pods, so little excavation happens during construction. It is an efficient system on the right site, and it is unforgiving of a platform that was made rather than found. A decision to fill for level is therefore a decision about which footing systems remain available.

Decision 3: plan shape and where the movement will show

Reactive ground deforms the whole platform, and the building has to accommodate the shape of the deformation. That makes plan geometry a structural question:

• Long, unbroken masonry walls have the least tolerance for differential movement, and articulation joints exist to give them somewhere to move.
• Re-entrant corners on L, T and U-shaped plans concentrate distortion, which is why complex footprints on reactive sites usually need more articulation, not less.
• Brittle finishes (rigid tiling, full-height stone, large fixed glazing, rendered masonry) show movement first, and their placement is an architectural choice.
• Lightweight construction over part of the plan changes both loading and the flexibility of what sits above the slab.

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

Decision 4: trees, and what they do to the mound

Trees are the design input architects control most directly and consider last. AS 2870 carries a method for accounting for the extra drying caused by nearby trees, and Cameron (2018) describes it as an addition to the design movement rather than a footnote to it.

The published case histories are blunt about the scale. Cameron and Beal (2011) tabulate South East Queensland houses affected by tree drying, including cases at Silkstone and Redbank Plains near Ipswich where design surface movements of 151 mm and 93 mm put the sites in 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.

The geometry matters as much as the number. Cameron and Beal (2011) distinguish the drying profile of a single tree from that of a row, group or stand, and a tree on one side of a building produces an asymmetric mound rather than the symmetrical one a standard design assumes. For an architect, that turns into practical questions: which trees are staying, how far from the footprint, are they in a line along a boundary, and is the client's landscape plan about to add more.

Decision 5: what happens to water after handover

The moisture regime around a finished house is set largely by architectural and landscape decisions: where paving falls, whether the downpipes discharge into a proper system, how wide the paths are, where garden beds and irrigation sit against the slab edge, and whether the client will later add a pool. Reactive-soil damage most often follows a change in the moisture regime rather than a fault in the footing, which is the pattern behind most of the cases in our piece on reactive soils and building damage.

What to hand the engineer, and when

The engineer designs the footing, but they design it against your drawings. The set that produces a good design contains the footprint on the survey, the proposed floor levels, the cut and fill intent, tree positions and sizes with the ones being retained marked, the paving and drainage layout, and any heavy or brittle elements. Add the desktop evidence for the site: the mapped geology, terrain, nearby bore logs and the likely reactive ground conditions. It is not a classification, and it never substitutes for the site investigation, but it lets the design conversation start from the ground the building is going on.

Common questions

Can an architect design before the soil test comes back?

Yes, and most do. What matters is designing with a defensible expectation of the ground, so that the classification confirms the approach rather than overturning it. Mapped geology, climate zone, terrain and nearby bore records give that expectation.

Does a Class H or E site restrict the design?

It restricts tolerance, not architecture. Higher movement means more articulation, more care with brittle finishes and long masonry runs, and a footing system chosen for the platform. It does not rule out a design; it changes what the design has to accommodate.

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

There is no universal distance. AS 2870 gives a method for allowing for tree drying based on the tree's mature height, its distance from the footing and the site's reactivity, and groups of trees behave differently from single ones. It is a calculation for the geotechnical and structural engineers, which is why tree positions belong in the drawings you give them.

At concept stage, when the footprint and levels are still cheap to move, generate a desktop report for the address to see the geology, terrain and nearby ground evidence for the lot, 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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