Before you build your own home, it is imperative that you understand soil classification. Soil type can not only present constraints when it comes to your artistic vision, but also make a significant dent in your finances. For this reason, we ALWAYS recommend conducting a soil test BEFORE purchasing a property.
Why Soil Type Matters
There are a range of problems that can occur when it comes to the foundations of your house. Nearby trees can cause a problem through root intrusion. Large underground rocks may need to be removed. Previously filled dams may impact where you can place your building envelope. You can find some property issues listed on the Section 32 (click here to read more about the Section 32).
Soil types classify soil according to its reactivity to moisture. When moisture content changes, soil can shift and change volume. If the soil reacts a lot when moisture content changes, then it is considered to be “highly reactive”. Soils with a high clay content are more likely to be reactive.
Red clay soil (with a visiter!)
The way to determine the soil type in your building envelope, is through a soil test (hopefully conducted before land purchase).
What is a Soil Test
Soil testing technicians use a machine to drill into the ground, in several locations within your building envelope. They remove soil samples, which they then take back for testing in a laboratory. The resulting report will state how reactive the soil is.
A soil classification test is easy to organise when you are purchasing a new property. You can make the land purchase conditional upon the results of the soil report. If you have already purchased the land without a soil report, most engineers can organise a soil test for you.
Soil Classifications
Class A
This class encompasses most sandy and rocky sites. There is little to no ground movement associated with moisture fluctuations. The ideal building site would have a class A soil type, with no large rocks to be removed. Large rocks or reefs of rocks can cause problems for foundations, and removing them can cost thousands of dollars.
Class S
A class S site is slightly reactive clay soil. There is minimal movement resulting from fluctuations in moisture, and is considered a good site to build on.
Class M
Made up of silt or clay, this soil type is moderately reactive- meaning there is a medium level of movement from moisture variances. It can still be a good site for construction , but you must take care to follow the engineer’s design for the foundations.
Digging stump holes in Class M soil
Class H
Class H is a highly reactive clay soil type, which experiences significant movement due to moisture fluctuations. You can still build on it, but there will be additional costs associated with the foundations. Engineers will take the soil type into consideration when designing the foundations, and their requirements must be strictly adhered to.
This soil type is broken into two soil classifications- H1 (experiences high movement) and H2 (experiences VERY high movement).
Class E
This clay soil type is extremely reactive. Stumps in class E soil tend to sink into the ground over time. Therefore it is common, in areas with this soil type, for home maintenance to include a jack up. It is not a replacement of stumps, but rather lifting the house off the stumps and fixing blocks to the top of the existing stump. This repositions the house so it is level.
While a good engineer can design foundations for almost any soil, we advise against building in this soil class if possible.
Class P
Class P means problem site. Sites that fall under a class P are unstable, prone to erosion and severe ground movement. They are areas that could have been a filled dam, or on top of mines. They could be dunes, prone to coastal errosion. This is not a site you want to build on but if you do, it could be quite expensive.
The D Inclusion
Dry, hard ground in Summer
If you see a ‘D’ next to your soil class (as in M-D or H1-D), it is referring to deep movements in the soil. These movements are due to deep variances in moisture. This is due to seasonal fluctuations in moisture, that can occur in predominantly dry areas (in the Central Victorian regions of Bendigo and Shepparton, for example). These areas go for long periods with very little rainfall, and then become saturated with rain in the cooler months.
Saturated ground in cooler months
Finally…
Arming yourself with as much knowledge as possible BEFORE you buy a block of land (and certainly before you start the build) can save you time, money and stress. Hopefully this article will help you understand what sort of soil types you should look for in a property. If you have already purchased a block, then know that good engineers are skilled at designing foundations specific to your property, so all is not lost! Good luck!
References
Building Your Straw Bale Home- Brian Hodge, 2006
https://anewhouse.com.au/2020/06/soil-classification/
https://build.com.au/building-reactive-soil-sites
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