We’re often questioned by clients about the amount of dollars seemingly needing to be spent before the design process even starts on your new dream home, including the expense of a site survey and soil report.
There’s one key reason why we won’t do any work before these two items are completed, and that’s because they tell us everything we need to know about the site and its geotechnical conditions before we start designing your new home or extension.
You may decide to do a site survey and soil test before you even buy a block so you know exactly what you’re dealing with – importantly, dollars spent up front may save you thousands down the track, or spare you a few headaches from unexpected surprises!
In any case it’s always advisable to know the results of a soil test prior to signing your building contract so you can rest assured your builder has included costs based on designing around the specific soil classification for your block.
You might ask why you’d need an individual report on a block in a suburban development which all look identical, but conditions do vary surprisingly from block to adjoining block, so it’s essential to undertake soil testing up front.
Here’s the technical part…
During a soil test, geotechnical engineers use specialised equipment to drill and collect soil samples around the site. The samples are then analysed to determine the soil reactivity, which is the amount the soil may contract, expand and move, and also its weight bearing capacity.
The soil conditions are then classified according to Australian Standard 2870 into the following categories:
· Class A – stable, non-reactive: the possibility of very little or no ground movement as a result of moisture change (often sandy and rocky sites)
· Class S – the possibility of slight ground movement (often clay sites)
· Class M – the possibility of moderate ground movement (often clay or silt sites)
· Class H1 and H2 – the possibility of high ground movement (often clay sites)
· Class E – the possibility of extreme ground movement
· Class P – problem sites: ground movement as a result of moisture change may be very severe
Generally, class A and S soils are fairly straightforward to build on, and don’t require structural engineering modifications to an otherwise basic slab. However, the other classifications will need deeper or reinforced footings, and/or more intensive drainage infrastructure, which of course add dollars to the overall cost in both structural engineering fees and the build itself.
For P sites, you’ll definitely need a structural engineer and a more in-depth geotechnical report to assess the site further before you even start designing, as the home will require specialist engineering design and foundations/footings. But with an upfront soil test at least you will know this from the outset and will be able to design and budget accordingly rather than get a nasty surprise later on!
Ultimately, you need a soil test not only because it’s a legal requirement but because it will save you time and money, avoiding delays and budget overruns. Furthermore, your builder will be able to design for the conditions and cost the project accordingly.