Differential Settlement And What Causes It

Uniform settlement is rarely a problem. A building that sinks evenly by a small amount takes it in its stride.

Damage comes from differential settlement: one part going down further than another, which forces the structure to distort.

What causes it

A change in the soil profile under the building. A footing on rock at one end and on fill at the other will settle differently, and the change can happen over a few metres. This is the most common cause on Sydney sites, where sandstone can rise or fall steeply under a single footprint.

Uncontrolled or variable fill. Fill placed without compaction control, or fill of varying depth across a site. It continues to compress for years.

Variable loading. A three storey section and a single storey section of the same building apply very different pressures. Same soil, different settlement.

Reactive clay. Shrink swell movement in response to moisture rather than load. Not strictly settlement, and it produces the same distortion. It is seasonal, it is reversible, and on [Sydney clay sites](/blog/reactive-clay-sites-footing-design-nsw) it is the dominant cause of residential cracking.

Trees. A large tree draws moisture from clay and shrinks it locally. Removing a large tree lets the clay re-swell, sometimes for years. Both directions cause movement, and it is covered in [trees and footings on Sydney clay](/blog/trees-and-footings-on-sydney-clay).

Water. A leaking service, poor surface drainage, or a broken stormwater line will soften ground under one part of a building and nowhere else.

Adjacent construction. Excavation, dewatering or new loading next door changes the ground under your footings, which is why [dilapidation reports](/blog/importance-of-dilapidation-reports) matter before it starts.

Reading the cracking

Cracks are the visible output and their pattern indicates the mechanism.

Diagonal cracks running up from a corner, wider at the top, generally indicate that part of the building has dropped. Cracks wider at the bottom generally indicate heave.

Cracking concentrated at openings is expected, because openings are where stress concentrates, and it does not by itself mean the movement is severe.

Doors and windows binding, floors out of level, and gaps opening between elements that used to meet are all part of the same picture, and frequently more informative than the cracks.

What matters most is whether it is still moving. A crack that has been the same width for ten years is a record of something that already happened. A crack that has opened over a season is active, and the cause is still operating. The [brickwork cracking](/blog/cracking-in-brickwork-what-it-means) guide covers how the pattern is read.

Establishing what is happening

Level survey. Floor levels across the building give the shape of the distortion immediately, and they are cheap to take.

Crack monitoring. Gauges read over months. This is the only way to distinguish active from historic, and there is no shortcut.

Geotechnical investigation. Boreholes to establish the actual profile under the building, not the assumed one. Frequently the whole answer appears here.

Drainage inspection. Stormwater and sewer lines checked for leaks. A surprising proportion of settlement cases are a broken pipe.

Documentation review. Original drawings where they exist, and site history: what was here before, what was filled, what was demolished.

Fixing it

The rule is to stop the cause before repairing the effect. Patching cracks over an active mechanism buys a season.

Drainage correction is the cheapest fix and it resolves a meaningful share of cases.

Tree management, carefully, because removal can cause as much movement as retention.

Underpinning where the footings genuinely cannot carry the load or need to reach better material. It is effective and it is the expensive option, and it is covered in [underpinning adjoining structures](/blog/underpinning-adjoining-structures-explained).

Doing nothing, which is the correct answer more often than owners expect. Where movement is historic and the damage is cosmetic, monitoring and repair is a rational outcome.

For a purchaser or an owner

The question is never whether there are cracks. It is what caused them, whether it is still happening, and what it would cost to deal with.

That is a structural and geotechnical assessment, and it is the substance of a [pre-purchase structural inspection](/blog/pre-purchase-structural-inspection).

ACSES provides geotechnical engineering and structural assessment across Sydney. Get in touch.