Melbourne Geology (Simplified)

If you’ve ever hired a builder or plumber to work on your property, you might have been told at one point or another: “Your house is moving.” We’ve actually had clients who have heard- and misunderstood- this phrase so often that they genuinely believed their house would end up on the next block!

If a professional has ever uttered this phrase about your house, there’s no need to worry. It’s very common for footings to experience some extent of movements, also known as “differential foundation movements,” and this movement is rarely lateral.

Reactive soil, common in Melbourne, can cause differential foundation movements, which can become serious from time to time.

how and why reactive soil moves

“Differential foundation movements” occur because the foundation which supports footings is just soil. Soil in Melbourne has a clayey texture and is made up of microscopic particles that interlock with each other to form plates. With clay, water collects between the plates, which is why it’s always moist. While clayey foundation soil can certainly support your home’s footings, it can also be “reactive,” due to its moist consistency. “Reactivity” is the word used to describe the tendency of foundation soil to change its volume (shrink or swell) with a variation of moisture content.

Melbourne geology graph

When clay soil absorbs extra water, the water floats between the clay particles and pushes them apart, causing the soil to expand. As the soil swells, it applies upward pressure to the footing causing ‘heave’ to occur. Conversely, when clay soil loses water, it shrinks, which causes footing to be pulled down and ultimately resulting in “settlement.” Therefore, the more “reactive” the soils is, the more your home’s footings will move.

black soil

Learn more about effect of trees on footing Here.

Now you know that if a professional ever tells you that “your house is moving,” all he means is that you have reactive foundation soil. Whilst slight footing movements are completely normal, excessive differential footing movements should be taken seriously and assessed properly by forensic engineers.

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