Building a new house or extending an existing one?
Most local councils throughout Australia will require you (or your builder, architect or engineer) to provide to them a classification (loosely called a ‘soil test’) of your house site.
The purpose of the test is to describe the soil conditions of the house site so your engineer, architect, or builder can design your footings accordingly.
The classification is done according to Australian Standard 2870 – 2011 Residential slabs and footings.
Section 2 of AS2870 – 2011 concerns classification of the building site: “…..classification is based on the expected ground surface movement and the depth to which this movement extends.” (Clause 2.1.1). A note to the Clause states “Site classification may require consideration of factors beyond the boundaries of the subject site.”
- Normal sites are those “where ground movement is predominantly due to soil reactivity under normal moisture conditions” (Clause 2.1.2). They are classified as Class A (rock or sand; little or no ground movement), or Class S, M, H1, H2, or E depending on the extent (slight, medium, high or extreme) of ground movement from moisture changes. At these sites, “foundation moisture variations are caused by seasonal and regular climatic effects, effect of the building and subdivision, and normal garden conditions without abnormal moisture conditions” (Clause 1.3.2). They are maintained such that the original site classification remains valid and abnormal conditions do not develop (Clause 1.3.1(b)).
- Other sites “with inadequate bearing strength or where ground movement may be significantly affected by factors other than reactive soil movements due to normal moisture conditions shall be classified as Class P” (Clause 2.1.3). These sites “include soft or unstable foundations such as soft clay or silt or loose sand, landslip, mine subsidence, collapsing soils and soils subject to erosion, reactive sites subject to abnormal moisture conditions and sites that cannot be classified” (Clause 2.1.3) as Normal sites.
The attached checklist (PDF) may help you navigate the site classification process.
Site investigations might or might not need to include investigation, mapping, test pitting by an excavator, hand or mechanical augering, and soil sampling. Shrink-swell or other testing of clayey soil samples in a laboratory may also be useful to help classify the site.
Some house site assessments are straightforward, but others may need more detailed testing.
House footings for some site classifications require certification by a suitable qualified engineer.
In some instances, your local Council may also require a geotechnical report or a domestic wastewater assessment, or both. Because all of these tests usually involve similar types of investigations (eg. test pitting, augering or drilling, and soil testing) it saves money if they are done at the same time.
Some consultants may not be experienced in all of these fields.