Soil testing for houses –

or “Site Classification” as it is correctly known – is carried out at each building site to estimate the expected soil movement – and hence foundation movement – due to extreme moisture changes. (The ground that supports a house is called a foundation, and the concrete structure that transfers the load to this foundation is the footing system.) There are three methods of site classification given in Australian Standard AS2870 “Residential Slabs and Footings” and these are:

(i) Visual assessment of the performance of existing buildings and knowing the footing type used on those buildings.

(ii) Identification of the soil profile compared with established data of building performance on a similar soil profile.

(iii) Laboratory testing and computation of expected soil movement.

In Victoria, the most common method of site classification is by identifying the soil profile found on the site and comparing it with known building performance on sites with similar soil profiles and climatic conditions.

AS2870 also provides prescribed footing and slab designs, construction methods and suggested methods of foundation maintenance.

The system of site classifications is mainly focused on clay sites that swell and shrink with changes in moisture content as these sites are known to have the most problems. Clay sites (reactive) are classified as S, M, H or E as follows:

S Clays that have not given trouble in the past.

M Moderately reactive clays that may cause minor damage to brick houses on old-style light strip footings. Moderately reactive clays are common in Victoria.

H Highly reactive clays that often damage houses, paths and fences.

E Extremely reactive clays that frequently damage houses even with strong footings. Generally rare in major cities except Adelaide. Other occurrences include outback NSW, Darling Downs, Geelong and Horsham.

Proper maintenance of such clay sites requires that the moisture content of the founding clay for the footings are kept as consistent as possible. Despite proper design, construction and foundation maintenance, some minor cracking can be expected in most masonry walls on reactive clay sites. Cracks up to 3mm in width are considered minor and cracks up to 5mm in width are considered large but crack widths of 15mm to 25mm are not uncommon on reactive clay sites that are poorly maintained.