Why do foundations sink

Why do foundations sink?

If you’ve spotted large cracks in the walls of your home or gaps appearing between the skirting and floors, there may be a chance the building and the ground beneath it are sinking.

Working with a qualified structural engineer to understand the causes of subsidence (the technical term for sinking) is usually the first step to addressing and repairing the problem.

Subsidence can occur across specific areas (large or small) over a lengthy period of time or as an immediate reaction to nearby activity or natural events.

What causes foundations to sink?

Foundations sink for a number of reasons. All involve a change in the ground, which in turn generates movement of the soil on which the foundations rest.

These changes may be due to:
  • Droughts, which dry out the ground and cause reactive clay soils to shrink. In dry conditions, tree roots can suck the remaining moisture from the ground and cause further settlement.
  • Earthquakes shaking the ground, often resulting in liquefaction of saturated soil. When this happens, the soil weakens and goes into a semi-liquid state allowing the building’s footings to sink. Once the shaking stops, this ‘liquid’ reverts back to a solid.
  • Renovations or nearby construction may change the condition of the ground, particularly if excavation is taking place. Often, a poorly supported excavation can allow nearby soil to slip, causing adjoining footings to sink. Heavy traffic and the vibration of machinery can also move or displace the soil.
  • Different footing systems (such as those found in very old buildings or in homes with extensions or alterations) perform differently. In reactive clay soils, deeper footings move less seasonally than shallow footings. An addition or extension alongside an existing home will settle differently to the original building, with most of the settlement of new footings typically occurring in the first five years after construction.
  • Water that floods the ground, changing the soil conditions. This may be a result of washaways from broken pipes, poorly compacted fill or sloping foundations (causing water to pool). In sandy soils, the finer grains are removed and the larger grains settle down. In clay soils, over-saturation causes the soil to weaken and the footing to settle down through the soil.

Different types of soil

Different types of soil react differently to environmental influences. Depending on how the soil changes, it will impact how the structures built on it move. For example:

  • Clay is particularly prone to contracting in drought conditions and expanding in wet conditions, causing the ground and building to move.
  • Gravel and stony grounds are more likely to collapse or move when nearby ground is excavated,  causing foundations to shift and subside.
  • Sandy soils are more easily displaced by water. The finer particles are washed out of the soil matrix, causing the bridging action between the larger soil particles to collapse. As a result, the building above subsides.

Soils are classified by how reactive they are. This refers to the amount that soil is likely to expand or contract in changing moisture conditions. You can read more about building on reactive and non-reactive soils here.

The Foundation Maintenance and Footing Performance guide from Australia’s CSIRO provides expert advice to homeowners on identifying the causes of movement in a building, including an explanation of the different types of soil.

What to do if your foundations are sinking?

We always recommend addressing the problem of sinking foundations as soon as you notice the warning signs to minimise further damage. Mainmark specialises in professionally repairing sinking building  foundations with non-invasive technologies to minimise disruption. Call the Mainmark team on 1800 623 312 (Australia) or 0800 873 835 (New Zealand) to discuss your options.

James OGradyBy James O’Grady

James O’Grady is the Sales and Business Development Manager at Mainmark. He is a civil engineer with 25 years’ experience in structural engineering, construction materials and ground treatment.