Reactive soils and building damage: causes and what to check

Reactive soils damage buildings when the clay under the footings swells as it wets and shrinks as it dries by different amounts in different places, bending the slab or footing and cracking the walls and floors above it. Most serious damage traces to moisture change beyond the normal seasons, a site classified too low, or fill, rather than to ordinary seasons on a correctly designed footing.

Russell Brown, who has investigated failed houses on Melbourne's basalt clays for 40 years, describes how these combine in a composite of jobs in Northcote, Footscray and nearby suburbs (Brown 2017). An old weatherboard house got a brick veneer extension on a raft slab. Eight mature fruit trees came out and most of the yard was paved, cutting the area that could soak up rain from about 60 percent of the lot to under 20 percent. Earthenware drains around the old wet areas had been leaking for something like 30 years and were left as they were. No soil moisture was measured, and no borehole went near the wet areas.

The back of the new slab rose about 80 mm. The junction with the old house dropped about 90 mm. The repair cost more than the original building work, and the owners' legal bill alone came to five times what screw piles would have cost at the start.

How does reactive clay move a house?

Reactive clay contains minerals such as smectite that take up water and swell, then give it up and shrink. If the whole house rose and fell evenly, little would crack. Damage comes from one part of the footing moving more than another. In a dry summer the perimeter dries and drops while the middle stays moist, bending the slab into a dome (edge drying). Heavy rain, a leak or ponding against a wall does the opposite and dishes it (edge wetting). Over the years after construction, moisture builds up under the middle of the slab and the centre rises (centre heave). And a tree, a leaking pipe or a wet trench moves one part of the site while the rest stays put.

AS 2870-2011, the Standard for residential slabs and footings, designs for the normal seasonal version of these patterns, with the class letter setting how much movement the footing must cope with (how the class is worked out).

What does reactive soil damage look like?

Movement shows first at a building's weak points: diagonal cracks from the corners of windows and doors, cracked mortar joints, gaps at cornices, doors that stick in one season and not the next, and in worse cases floors visibly out of level. Cracks that open and close with the seasons point to reactive clay rather than one-off settlement.

AS 2870 grades wall damage by crack width in its Appendix C:

Damage category	Typical damage	Approximate crack width
0, negligible	Hairline cracks	under 0.1 mm
1, very slight	Fine cracks that do not need repair	under 1 mm
2, slight	Cracks noticeable but easily filled; doors and windows stick slightly	under 5 mm
3, moderate	Cracks repairable, some wall sections may need replacing; doors and windows stick; service pipes can fracture	5 to 15 mm
4, severe	Extensive repair, sections of wall broken out and replaced; frames distort; walls lean or bulge	15 to 25 mm

A separate scale for concrete floors also looks at how far the slab is out of level.

How much movement is the Standard designed to allow?

A footing built to AS 2870 is designed to limit damage, not prevent all movement. On a normal, maintained site the Standard expects usually no damage, a low incidence of Category 1 and an occasional incidence of Category 2, with design movement set to have less than a 5 percent chance of being exceeded over 50 years.

Cameron (2018) explains the trade: the aim is to cut the community cost of damage at a price people can afford, so some cosmetic cracking is accepted, and an owner who wants better can pay for an upgraded design. Drew (2017) doubts new-home buyers see it that way. He reports cracks of 2 to 5 mm, "slight" under the Standard, are not unusual in near-new houses on highly reactive sites in Melbourne's west. Category 3 or worse usually points to something outside the design assumptions.

What causes most reactive soil damage?

Water in leaks and service trenches. Brown (2017) compared two units in one development of five, each the same distance from the same trees. Unit A showed 22 mm of differential movement. Unit C showed 188 mm: it also had a failed downpipe and a very wet sewer trench. His conclusion is that extreme wetting moves the ground far more than extreme drying. With dye tests on the plumbing, he found water running into the site along service trenches from outside the lot in close to every case of heave. Trees rarely affect footings in the first two or three years, so very early damage points to plumbing, poorly compacted fill or building practice.

Trees. Roots dry the clay deeper and further than the seasons alone, and when a tree goes, the dried zone slowly wets up and swells. Cameron and Beal (2011) reviewed damaged houses in South East Queensland, at Redbank Plains, Morayfield, Loganholme, Toowoomba and Logan Village among others, with distortion found four to six years after building. At Toowoomba a large eucalypt had stood where the house went and was cut down just before construction. Their work informed Appendix H of AS 2870-2011, the method for designing footings near trees. The Standard's maintenance guidance (Appendix B) suggests keeping new trees at least three-quarters of their mature height from the house on Class M sites, one times on H1 and H2, and one and a half times on E, with more for groups or rows.

Drainage and landscaping left undone. AS 2870 relies on the ground falling away from the house (at least 50 mm over the first metre in Appendix B) and drainage finished by the end of construction. Drew (2017) describes what often happens in Melbourne's western suburbs instead: paving and drainage sit in a landscaping package outside the building contract, the house is handed over with bare clay against the walls, and water runs into the sand layer under the waffle pod slab, typically 385 mm deep on highly reactive sites. He recommends engineered drainage built with the house and, without it, waffle slabs at least 460 mm deep for articulated masonry veneer on H2 sites, and probably H1.

A class that was too low. Delaney (2005) reports that an insurer's review of claims after the 1989 Newcastle earthquake found reactive soils were the main cause in 51 percent of them, and that houses built to AS 2870 were rarely damaged where the right class had been adopted and large trees near the house avoided. Boreholes that miss part of the lot, untested reactivity or the wrong climate depth can all produce a class that is too low.

Fill. Uncompacted fill, or clay fill placed without the cracked upper layer that natural ground has, moves differently from the natural profile beside it. AS 2870 makes most filled sites Class P (needing engineered design) for this reason; see Class P triggers.

What should you check before building on reactive clay?

1. The classification and its basis. How many boreholes, how deep, whether the clay was tested or moisture measured, which climate depth (Hs) was used, and whether trees, fill or old structures were recorded.
2. Trees on the lot and next door. Those staying, those being removed and those planned. Trees on the adjacent site count under the Standard.
3. Fill and site history. Old dams, filled gullies, earlier buildings, old drains and cut-and-fill platforms change the class or make the site P.
4. The drainage design. Whether it is in the building contract, who builds it and when. On M to E sites the Standard requires drainage controlled from the start of site work and completed by the end of construction.
5. Plumbing and trench details. The Standard requires service trenches near the house to be capped with compacted clay so water cannot run along them under the slab, and on H1 to E sites, lagging or sleeves where pipes pass through footings.
6. A maintenance plan. Garden beds and irrigation against the walls, downpipes discharging at the footing and new planting all change the moisture under the slab after handover.

What should you do if cracks appear?

Record before you repair. Note each crack's location, width (a crack-width card is cheap) and date, photograph it with a scale, and repeat over several months so the seasonal pattern shows. An investigating engineer will typically survey floor levels, drill to compare soil moisture with the expected profile, and test the stormwater and sewer lines (Brown 2017).

AS 2870's guidance (Appendix B) treats Category 1 and 2 damage as normal maintenance: stabilise the moisture around the house (fix leaks, regrade, move garden beds and irrigation back, manage trees), then repair or disguise the cracks. For wider cracks, find the cause, stabilise the moisture, then repair the masonry with added articulation. Underpinning is a last resort, generally to be avoided on reactive clay and only after a proper engineering appraisal.

Common questions

Is some cracking normal on a reactive clay site? Yes. AS 2870 expects usually no damage, a low incidence of Category 1 (fine cracks under 1 mm) and an occasional incidence of Category 2 (cracks under 5 mm that are easily filled) on a normal, well-maintained site.

What is the most common cause of reactive soil damage? Moisture change beyond the normal seasonal cycle: trees close to the house or recently removed, leaking pipes and wet service trenches, and drainage that was never built or not maintained. A site classified lower than the ground deserves is another common factor.

Should a house on reactive clay be underpinned when it cracks? Usually not as a first step. AS 2870's guidance is to investigate the cause, stabilise the moisture conditions and repair the cracks, with underpinning kept as a last resort after an engineering appraisal.

Can I find out whether a site has reactive clay before buying? Mapped geology, soil landscape mapping, nearby bore logs and the climate zone show the reactivity range to expect. Only a site investigation gives the classification the footing is designed to.

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To see the mapped geology, soils, nearby bore records and site history for a lot before the investigation is booked, generate a desktop report for the address or see a sample report.

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Sources

• Brown, R. I. (2017). Methods of investigation and repair of light construction on reactive soil not complying with expectations. Australian Geomechanics, 2017 Victoria Symposium. geomechanics.org.au
• Cameron, D. A. (2018). Dealing with reactive clay soils through a national standard. Australian Geomechanics, Vol 53 No 1. geomechanics.org.au
• Cameron, D. A. and Beal, N. S. (2011). Estimation of foundation movement and design of footing systems on reactive soils for the effects of trees. Australian Geomechanics, Vol 46 No 3. geomechanics.org.au
• Delaney, M. G. (2005). An overview of engineering geology and geotechnical challenges in the Newcastle Region. Australian Geomechanics, Vol 40 No 1. geomechanics.org.au
• Drew, R. J. (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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