Trees and footings on Sydney clay, including the risk in removing one

Trees and footings on Sydney clay, including the risk in removing one

A large part of Sydney sits on clay that changes volume with moisture. Wet ground swells, dry ground shrinks, and anything founded in it moves with the season.

Put a mature tree next to a house on that soil and you have added a pump. The tree draws moisture out of the ground within its root zone, and the clay under that part of the footing sits permanently drier than the clay elsewhere.

What that does to a building

The footing on the tree side sits lower because the soil beneath it has shrunk. The footing on the far side does not. The difference between them is what cracks the building.

The signature is fairly consistent. Diagonal cracking that widens as it rises, concentrated on the elevation nearest the tree, often with doors that bind in summer and free up after a wet winter. Internally you tend to see cracking above openings on the same side.

The severity depends on how reactive the clay is, how close the tree sits relative to its mature height, what species it is, and how deep the footing goes. AS 2870 gives guidance on tree proximity for exactly this reason, and a site classification is the starting point for any assessment.

The part that catches people out

The instinct is to remove the tree. On a reactive site that can be the more damaging decision, and the damage arrives in the opposite direction.

Once the tree is gone, nothing is drawing moisture out of that soil any more. The clay slowly rehydrates back toward its natural moisture content and swells. The ground that had settled now lifts, and the footing lifts with it.

That process is slow, often several years, and it can produce heave larger than the original settlement. A building that had reached equilibrium with the tree in place gets a second round of movement in the opposite direction, and the cracking pattern that had stabilised reopens somewhere new.

None of that means the tree should stay. It means the decision needs an assessment before it is made rather than after.

What an assessment looks at

Distance from the trunk to the footing, expressed against the mature height of the species rather than its current height.

The species itself, because water demand varies enormously and a fast growing high demand species at eight metres is a different proposition to a slow growing one at the same distance.

The site classification and the depth of the reactive layer, from a geotechnical investigation.

The existing footing, exposed and measured rather than assumed.

The history. Whether the cracking is seasonal and stable, or progressive.

The options that exist

Leave the tree and strengthen the building. Underpinning to a depth below the influence of the root zone takes the footing out of the moving layer entirely.

Remove the tree and allow for heave. On some sites that means staging the removal, monitoring for a period, and repairing once movement has settled rather than immediately.

Install a root barrier. A vertical barrier between the tree and the building, taken to sufficient depth, limits root extension toward the footing without removing the tree. It is not a universal answer and it needs designing to the site.

Manage moisture. A large share of the cases I inspect are made worse by a leaking stormwater line, a downpipe discharging at the footing, or irrigation on one side only. That is the cheapest thing on this list and it is checked first.

If you are building new near an established tree

Design for it at the start. A deepened edge beam, a stiffened raft, or piles founded below the influence zone all solve it cheaply at design stage and expensively afterwards.

Sydney councils also protect a lot of these trees, so removal is often not available as an option anyway. The design has to work around a tree that is staying, and that is a normal condition rather than a difficult one when it is known before the slab is set out.

We handle this through geotechnical and structural assessment together, because the ground and the footing only make sense read against each other.