Underpinning

A different problem to restumping

Restumping deals with supports that have failed. Underpinning deals with ground that has failed.

If your house sits on a concrete slab or a strip footing rather than stumps, there are no stumps to replace. The footing is bearing directly on the soil, and if that soil moves, subsides or loses its capacity to carry load, the footing goes with it and the walls above crack. Replacing anything above ground does not help, because the problem is underneath.

Underpinning extends the footing downward until it reaches ground that will carry the load reliably. In practice that means excavating in sections beneath the existing footing and building new piers below it, so the load transfers down past the unstable layer.

How to tell which one you have

The building type is the first clue and it is usually obvious. A raised timber house with a crawl space and visible posts underneath is a stump house. A house sitting at ground level on a concrete slab is not.

The crack pattern is the second clue. Movement in a slab or footing tends to show as diagonal cracking in brickwork, stepping through the mortar joints, often widening toward the top. Doors and windows go out of square. Cracks that are wider at one end than the other point at rotation rather than simple shrinkage.

Not all cracking is structural. New houses shrink and settle in their first years and hairline cracks in plaster are routine. What warrants attention is cracking that is wide enough to put a coin in, that is getting worse over months, that runs diagonally from window and door corners, or that is accompanied by doors that no longer close.

Why ground moves here

Reactive clay. Through the hinterland and the western suburbs, from Nerang out to Canungra and up to Tamborine, clay content is high. Clay expands when it takes on water and contracts when it dries. A house sitting on it rides that cycle. Where the movement is even across the whole footprint it does relatively little damage. Where one side of the house is shaded and damp and the other bakes in the sun, the differential is what cracks walls.

Fill. Large parts of the coastal strip and the canal estates are built on fill. Well compacted fill is fine. Fill that was placed quickly and not compacted properly keeps consolidating for years, and rarely at the same rate across a whole site.

Water. Leaking pipes, poorly directed stormwater and downpipes discharging next to footings are among the most common causes of localised subsidence anywhere in Australia, and subtropical rainfall makes the consequences arrive faster. Large trees close to a house also draw substantial moisture out of clay in dry spells.

How underpinning is done

Mass concrete piers

The traditional method. Short sections beneath the footing are excavated one at a time, in a sequence that never leaves too much unsupported at once, and each is filled with concrete and dry packed hard against the underside of the existing footing. Slow, labour intensive, and very well proven. Suits sites where competent ground is not far down.

Screw piles

Steel shafts with a helical plate on the end, wound into the ground by machine until they reach the required torque, then bracketed to the footing. Faster, cleaner, much less excavation, and they can reach deeper. Access matters, since the machine needs to get to the wall.

Resin injection

Expanding structural resin injected into the soil under the footing, which compacts the surrounding ground and can lift the footing slightly. Quick and minimally invasive. Suitability depends heavily on soil conditions, so it is not a universal answer.

Which is appropriate is a geotechnical and structural question, not a preference. On a job of any consequence you would expect a soil investigation and a structural engineer's design, with the contractor building to that design rather than choosing the method on the spot.

Get an engineer involved

This is the single most important point on this page. Underpinning is remedial structural work on a building that is already moving. The right scope depends on what the soil is doing at depth, which cannot be determined by looking at a crack from the outside.

A proper process looks like this. Someone monitors the movement, ideally over a period rather than at one moment. A geotechnical investigation establishes the soil profile. A structural engineer designs the remedy. A licensed contractor builds to that design. You end up with documentation that means something if the house is ever sold or if there is ever a dispute.

Be wary of anyone who arrives, looks at a crack, and quotes a pier count on the spot without any of that.

Insurance is worth checking first

Home insurance policies in Australia commonly exclude gradual subsidence and soil movement, but many will respond where the cause is a sudden event or an escape of water from a burst pipe. Since a leaking service is one of the more frequent causes of localised subsidence, it is worth reporting the damage to your insurer and letting them assess it before you commission repairs. Doing the work first can complicate a claim.