So you have found your perfect block of land and cannot wait to build your dream home!
One of the first steps of building a new home is having the soil on the site tested by a geotechnical engineer. Most builders will take care of this for you but some builders will not undertake a soil test of the site at tender stage and instead include an estimated allowance.
In order to avoid an unexpected price hike in the cost of your build make sure your builder undertakes soil testing of your land before you sign a building contract. It’s a good idea to ask your builder upfront.
A soil test assesses the soil type, moisture content, salinity, bearing capacity and possible soil reactivity. Based on the results of the site test your site will be given a ‘soil classification’.
The reactivity of soil is determined by how much the soil on the site is likely to move, expand and contract which usually depends on the changing moisture levels.
Soil classification is graded and is very important, as it will ultimately determine the type of slab, foundation and piering that will be required so that your new home complies with Australian Standard AS 2870/2011 - Residential slabs and footings.
Below is table of the different grades of soil classification:
Site classifications based on soil reactivity
Class A: Stable, non-reactive. Usually very little or no ground movement likely as a result of moisture changes. Generally sand and rock sites.
Class S: Slightly reactive clay sites. Usually experience slight ground movement as a result of moisture changes. Generally clay sites.
Class M: Moderately reactive clay or silt sites. Usually experience moderate ground movement as a result of moisture changes. Generally clay or silt sites.
Class H1: Highly reactive clay sites. Usually experience a high amount of ground movement as a result of moisture changes. Generally clay sites.
Class H2: Highly reactive clay sites. Usually experience very high ground movement as a result of moisture changes. Generally clay sites.
Class E: Extremely reactive sites. Usually experience extreme amounts of ground movement as a result of moisture changes.
Class P: Problem sites. Usually ground movement as a result of moisture changes are very severe and such sites are generally subject to abnormal moisture conditions, for example surrounding trees, dams or poor site drainage. The ability of Class P sites to evenly support load is very poor. Sites may be classified as 'Class P' due to events such as mine subsidence, landslides, collapse activity or coastal erosion. Therefore if you are building on a ‘Class P’ site make sure you consult a qualified structural engineer.
Classes M-D, H1-D, H2-D and E-D: The 'D' in these classifications refers to 'deep' movements in the soil due to deep variances in moisture. Generally these classifications are only found in dry areas.
Obviously, Class A sites will always be the easiest to build on and generally the cheapest, but any good builder should be able to adapt your build to suit any classification, however the costs of building on the different grades of soil classification will vary.