Reactive clay and slab heave explained

Many Australian houses sit on clay soils that move with the weather. Most of the time the footings cope. But when one part of the ground gets much wetter or drier than the rest, a concrete slab can lift, drop or crack, and the walls above it follow. Here's what's happening underneath, and how to tell harmless slab cracks from the ones that need an engineer.

Reactive clay and slab heave: quick facts

Reactive clay

Swells when wet, shrinks when dry

Slab heave

Part of a slab lifting as moisture builds up beneath it

Site classes

A, S, M, H1, H2, E and P under AS 2870

Common in

Many Melbourne suburbs and much of metropolitan Adelaide

Main triggers

Leaks, poor drainage, downpipes and trees

Our crack report

From $450 + GST for a typical house

What is reactive clay (reactive soil)?

Reactive soil is soil whose volume changes with its moisture content, and reactive clay is the most common kind. Clay swells as it takes up water and shrinks as it dries, so the ground under a house rises and falls through the year. Cracks on reactive clay often open and close with the seasons for the same reason.

The movement is rarely even, and that's the real problem. Soil under the middle of a slab tends to change slowly, while soil at the edges wets and dries with rain, gardens, downpipes and trees. The difference across the slab, more than the movement itself, is what bends slabs and cracks walls.

AS 2870 site classes: how reactive is your site?

AS 2870, the Australian Standard for residential slabs and footings, groups sites by how much the ground surface could move as the soil wets and dries. For reactive sites the measure is the characteristic surface movement, written y_s, in millimetres. The higher the class, the more movement the footings have to cope with.

For a new house, the site is usually classified from a soil investigation, and the footings are designed to suit. For an existing house, the original classification may be on the building plans, and an engineer may recommend a fresh soil investigation if the house has moved.

Reactive clay in Melbourne and Adelaide

Reactive clay is common in both cities, but soils change from suburb to suburb and sometimes between neighbouring blocks. Treat the notes below as general background: only a soil test tells you about a particular site.

Melbourne

Many Melbourne suburbs sit on reactive clay, including the clays on the basalt plains to the north and west of the city. On these soils, cracks that open and close with the seasons are a familiar pattern, and a leak or a large tree near the house can make the movement worse. See our structural engineers in Melbourne.

Adelaide

Much of metropolitan Adelaide sits on reactive clay, and many sites are classed H1, H2 or E: highly or extremely reactive. That makes moisture control around the house, prompt leak repairs and care with trees especially important. See our structural engineers in Adelaide.

Reactive clays occur in other parts of Australia too, so the same principles apply wherever you live. We inspect slabs and footings across all our service areas.

What is slab heave?

Slab heave is part of a concrete slab lifting, usually because the reactive clay under it has taken up water and swollen. Engineers describe it by where the lift is.

Edge heave

The edges of the slab lift relative to the middle, for example after a wet period or where water ponds against the house.

Centre heave

The middle of the slab sits high relative to the edges, for example when moisture builds up under the slab from a leak, or when the edges dry out and drop.

These terms describe the shape the slab has taken. The cause, and whether the slab is still moving, has to be confirmed on site, often with floor level readings and sometimes a plumbing test to rule out a leak under the slab.

Slab heave vs settlement: what's the difference?

Both crack slabs and walls, and they can look alike at first. The difference is direction, and it changes the fix.

Heave and settlement can even happen in the same house, for example when the edges dry and drop while a leak lifts the middle. That's why one level reading or one crack rarely tells the whole story. For movement across the whole house, read house subsidence and movement, and for what underpinning can and can't do, see what is underpinning?

Are concrete slab cracks serious?

Most fine cracks in concrete slabs aren't structural. Concrete shrinks as it dries, so fine cracks are common in slabs of every age. Slab cracks matter more when they:

• Keep widening or lengthening over weeks or seasons
• Have one side higher than the other, so you can feel a step across the crack
• Crack floor tiles in a line above them
• Come with other signs, such as sticking doors, wall cracks or floors out of level
• Sit near a wet area, a known leak or a downpipe

Engineers assess slab damage separately from wall cracks, looking at crack width and how far the floor has moved out of level. Wall cracks above a moving slab are rated against the AS 2870 damage categories explained in our guide to cracks in walls.

How do you protect a slab on reactive clay?

You can't change the clay, but you can keep the moisture under and around your slab steadier. These habits do most of the work.

When should you call an engineer about slab cracks or heave?

• A slab crack is widening, or one side has lifted or dropped
• Doors start jamming or floors start to feel out of level
• Wall cracks appear above or beside the slab crack
• You suspect a leak under or beside the slab
• You've been quoted for underpinning, resin injection or slab jacking
• You're buying a house on reactive clay with cracked floors or walls

Buying on reactive clay? A pre-purchase structural inspection checks for signs of slab and footing movement before you commit, and our guide to buying a house with foundation problems explains what to weigh up. If a newer home's slab is cracking and you're dealing with the builder, a building defect report records the damage and its likely cause.