Reactive clay and how it decides your footing

Reactive clay and how it decides your footing

A large share of the residential structural problems I see trace back to one thing: a footing that was designed for a less reactive site than the one it was built on.

Reactive clay changes volume with moisture. It swells when wet and shrinks when it dries. That movement is seasonal, it is measured in tens of millimetres on the more reactive sites, and it is entirely predictable if someone bothers to classify the ground first.

The classifications and what they mean

Sites are classified by how much surface movement the soil is expected to produce.

Class A is stable, typically sand or rock, with negligible movement. Class S is slightly reactive. Class M is moderately reactive. Class H1 and H2 are highly reactive, with H2 producing substantial movement. Class E is extremely reactive. Class P covers problem sites, which includes fill, soft soils, landslip and mine subsidence, and is a separate conversation.

Each step up the scale requires a stiffer footing system, because the design approach is to make the footing rigid enough that the building moves as one unit rather than distorting.

Riding movement rather than resisting it

This is the part that surprises people. You do not stop reactive clay from moving. The forces involved are far larger than any economical footing would resist.

Instead, the footing is stiffened so that when the ground moves differentially under it, the structure above tilts slightly rather than cracking. A stiffened raft with adequately sized beams distributes that movement. A shallow strip footing on the same site does not, and the building relieves the stress through the brickwork.

That is why diagonal cracking in walls is so frequently a footing story rather than a wall story.

Moisture is the variable you can control

Since movement is driven by moisture change, the practical mitigations are all about keeping moisture stable around the footing.

Drainage that takes water away from the building rather than releasing it at the perimeter. Downpipes connected to a system rather than discharging at the slab edge. Paving graded away. Plumbing leaks fixed promptly, because a slow leak under a slab produces localised heave that looks like a structural failure.

Trees are the other half. A large tree drawing moisture from clay near a footing causes localised shrinkage, and removing a mature tree near a building on reactive clay causes the opposite as the soil rehydrates over several years. Neither is a reason to avoid trees. Both are reasons to consider them in the design.

Classification is cheap, remediation is not

A site classification involves boreholes and testing and is a small line item. Underpinning a house that has distorted because the footing was under-designed is not a small line item.

On any residential project in Sydney, the geotechnical investigation should happen before the footing is designed, not after the slab has cracked. The investigation also picks up fill, which is common on subdivided sites and which changes the design entirely.

What this means for a developer

Budget the classification early and design to what it returns. A project priced on a Class S assumption that turns out to be H2 has a real cost difference in the footing system, and that difference is far better discovered at feasibility than at construction.