Understanding Soil Types and Their Construction Implications

The ground beneath a home rarely gets a second thought, yet it has an outsized influence on what your build costs and how it performs over time. Soil moves — it swells when wet and shrinks when dry — and the way your footings and slab are engineered depends almost entirely on how much your particular ground is likely to move. Across much of Melbourne, that movement is significant, which is why two near-identical homes on neighbouring streets can have very different foundation costs.

Understanding soil classification before you commit to a block, or finalise a design, removes one of the most common budget shocks in residential building. At Ravcon we treat the soil report as a foundational document in every sense, and this guide explains what it tells you and why it matters.

Why soil matters in Melbourne

Large parts of Melbourne and Victoria sit on reactive clay soils. Reactive clay expands as it absorbs moisture and contracts as it dries out, and that seasonal heave can be enough to crack a footing system that was not designed for it. Our dry summers and wet winters make this cycle pronounced. A footing system that simply ignored this movement would be at real risk of cracking, so the engineering response is to design footings stiff and deep enough to ride out the soil's behaviour.

How a soil test works

A site classification starts with a geotechnical investigation, usually carried out by drilling one or more boreholes on the block. The geotechnical engineer logs the soil layers, takes samples, and assesses how reactive the ground is along with the depth to stable material and the presence of any fill, rock or moisture. The result is a geotechnical report that assigns your site a class under Australian Standard AS 2870, the standard for residential slabs and footings.

• Boreholes are drilled to sample the soil profile across the block.
• Samples are assessed for reactivity, strength, moisture and any fill or rock.
• The engineer determines the likely ground movement and the depth to stable soil.
• A site class is assigned under AS 2870, which the slab and footing design responds to.

The site classes under AS 2870

AS 2870 sorts sites into classes based on how much the ground is expected to move. As you move down the list, the expected movement — and usually the footing cost — increases. In broad terms, the classes are:

• Class A — stable, non-reactive ground such as sand or rock, with little to no movement. Rare in much of Melbourne.
• Class S — slightly reactive clay or silt, with only minor expected movement.
• Class M — moderately reactive clay or silt, with moderate movement. Very common across Melbourne.
• Class H1 — highly reactive clay, with high movement requiring stiffer footings.
• Class H2 — very highly reactive clay, with even greater movement and more substantial footings.
• Class E — extremely reactive sites, where ground movement is the most severe.
• Class P — problem sites that cannot be classified normally, due to fill, soft soil, landslip, mine subsidence or other complications requiring specific engineering.

Many Melbourne blocks land somewhere between Class M and Class H2. A Class P site is not necessarily a deal-breaker, but it does mean a tailored engineering solution — and a cost — that needs to be understood before you proceed.

How soil drives footing design and cost

Your site class directly informs the footing system the engineer designs. The more reactive the soil, the deeper and stiffer the footings need to be to keep the home stable through wet and dry seasons. This can mean a thicker waffle or raft slab, deeper edge beams, more steel reinforcement, or piers driven down to stable ground. Each step up in reactivity typically adds to both the materials and the labour.

1. 01The geotechnical report assigns a site class for your block.
2. 02A structural engineer designs a slab and footing system to suit that class.
3. 03Higher reactivity means deeper, stiffer footings and more reinforcement.
4. 04The footing design feeds directly into the fixed price for your build.

Because the footing system is engineered to the soil, it is one of the few build costs you cannot value-engineer away — it is doing essential work. The smartest approach is to know your site class as early as possible so the figure is in your budget from day one rather than as a mid-project surprise.

Knowing what lies beneath turns an unpredictable cost into a planned one. If you are assessing a block or finalising a design, the Ravcon team can help you read a soil report and understand exactly how it shapes your slab, your footings and your budget.