What a soil report does
A soil report, formally a soil site classification report, tells the structural engineer what to allow for in the slab and footing design, so the footings are strong and stiff enough to resist soil movement, seasonal moisture changes, fill and low soil strength. It is prepared to AS 2870, Residential slabs and footings, by a suitably qualified geotechnical consultant or engineer, and lodged with your building permit documents.
- What the soil is made of: clay, silt, gravel, sand, rock or fill
- Its properties: profile, reactivity, moisture and strength
- Anything affecting its normal moisture
- Any fill, its type and characteristics
- Suitable founding conditions: footing depth and soil strength
What reactivity means
Site classes are based on reactivity, which is how much a soil shrinks and swells as its moisture changes. Gravel and sand have very low reactivity. Most silts lose strength when very wet. Clay is the main cause of footing movement and of most Class P sites. Picture a sponge: it shrinks when dry and swells when wet. Clays behave the same way, from slightly reactive to extremely reactive.
What Class P actually means
Under AS 2870, problem means the soil's normal characteristics on that site have been changed in a way the footing design must allow for. The report writer must give a Class P whenever the soil is not in its normal state.
- Trees and vegetation drying out clay soils
- Existing buildings and paving covering the soil, or leaking services and poor drainage wetting it
- Former trees, buildings or structures whose drying effect has been removed
- Septic tanks, dams or groundwater
- Fill placed over the natural soil
- Mining activity, such as mine shafts and mullock
- Saturated or soft soils that lose bearing strength
- Steep slopes at risk of slipping
Why most house blocks are Class P
The normal site AS 2870 describes is rare in real life. It would be a strip of land with no significant vegetation and no previous human activity. New estates usually have street trees near the building envelope and some subdivision fill. Renovations and extensions almost always have abnormal moisture from the existing house. Shallow rock can cancel out abnormal moisture, because rock does not react to water, but most sites still come back Class P.
What happens next
Your footings are designed by an engineer, with a little more thought than a normal site needs. Our reports include recommendations for that engineer, such as an H1 equivalent slab on a moderately reactive site affected by trees, or a fully suspended slab on bored piers where there is fill or mining. If you are concerned, talk to one of our engineers or your builder.