As a part of an application for a Building Permit for a new home, a Soil Test report (Geotechnical report) will be required to be included to verify that the foundation design documented on the working drawings is correct for the conditions relating to your site.
Your designer will have compiled a suitable footing system, whether for a concrete slab or stumps and timber flooring, based on the applicable Australian Standard AS2870, to suit your particular site as a result of the recommendation of the soil report.
A soil test is undertaken by drilling 2 or 3 small diameter bore holes within the area that the home is to be built and then from the soil profiles obtained a site classification and foundation depth will be nominated by the Geotechnical firm.
The standard requires that all sites be classified basically as either -
Class A - Most sand & rock sites with little or no ground movement from moisture changes.
Class S - Slightly reactive clay sites, which may experience only slight ground movement from moisture changes.
Class M - Moderately reactive clay or silt sites, which may experience moderate ground movement from moisture changes.
Class H1 - Highly reactive clay sites, which may experience high ground movement from moisture changes.
Class H2 - Highly reactive clay sites, which may experience very high ground movement from moisture changes.
Class E - Extremely reactive sites, which may experience extreme ground movement from moisture changes.
Class P - Sites which include soft or unstable foundations such as soft clay or silt or loose sands, landslip, mine subsidence, collapsing soils and soils subject to erosion, reactive sites subject to abnormal moisture conditions.
Designs for footing systems for Class A to Class H sites can be adopted by your building designer directly from the Australian Standard.
Designs for Class E & P sites must be designed by a suitably qualified engineer.
All site assessments should be done after any 'cut and fill' operations are carried out as an alteration to the finished levels, other than removing the top litter, will require a new assesment to be undertaken.
What does this mean to you as a home builder.
Fundamentally footings on a Class A site are cheaper than those on a Class H site, with a more substantial increase in difference when a Class E or P classification.
For example a brick veneer home with a concrete slab on a Class A site is required to have edge beams a minimum 300mm deep around the perimeter.
The same home on a Class H2 site is required to have edge beams, & internal stiffening beams at 4 metre centres, 600mm deep. There is also an increase in the steel reinforcing required, which again adds to the cost.
Similarly, a strip footing used in conjunction with stumps on a solid brick home on a Class A site is 300mm deep by 400mm wide; on a Class M site 900 deep by 400 wide.
As material prices will vary throughout Australia, it is not possible to give exact cost variations, but it is clear that the type of site you build on could impact substantially on your budget. In a worst case scenario the required foundation type and depth could add many thousands of dollars to your project.
Classifications can vary within a small area.
For example - Cut and fill operations during road and drainage construction could leave your site with deep uncontrolled fill. The adjoining site may be entirely on natural soil.
Removal of large trees during estate development, can create very localised variations.
In rural subdivisions, filled dam sites could possibly be a major problem.
For those people looking to purchase land, I would recommend requesting a soil report and using the findings as part of your decision making process. For those people with land, engage a firm to compile a soil report before you start designing your new home. You and your designer will then be able to make any necessary adjustments at the start of the design process.
The soil test should be carried out at the proposed building site level, therefore on sloping sites and the like any cutting or levelling of the building area should be carried out prior to the test. - A soil test carried out prior to land purchase may have to be redone if there is subsequent significant site works but the initial cost may well be money well spent