Your land: The truth about soil.


The type of soil you lay your foundations on can have repercussions for your house long after it has been built. Some soils have a greater potential to change volume than others, and this amount of potential needs to be measured with a soil test to make sure your house’s footings are designed in a way that helps protect it from soil movement that might otherwise cause it to sink or buckle.

A brief overview of site classifications and movement based on soil reactivity is listed as;

  • Stable, non-reactive- Most sand and rock sites. Little or no ground movement likely as a result of moisture changes.
  • Slightly reactive clay sites- May experience slight ground movement as a result of moisture changes.
  • Moderately reactive clay or silt sites- May experience moderate ground movement as a result of soil
  • Highly reactive clay sites- May experience a high amount of ground movement as a result of soil conditions and moisture changes.
  • Highly reactive clay sites- May experience very high ground movement as a result of soil conditions and moisture changes.
  • Extremely reactive sites- May experience extreme amounts of ground movement as a result of soil conditions and moisture changes.
  • Problem sites- The ability of the soil to evenly bear a load is very poor. Sites may be classified as ‘Class P’ as a result of mine subsidence, landslip, collapse activity or coastal erosion (e.g. dunes), soft soils with a lack of suitable bearing, cut and/or filled sites, or creep areas.
  • Ground movement as a result of moisture changes may be very severe, and these sites are typically subject to abnormal moisture conditions resulting from things like trees, dams and poor site drainage. If you are building on a Class P site you will need to consult a structural engineer.
  • The ‘D’ – in these classifications refers to ‘deep’ movements in soil due to deep variances in moisture. These classifications are mostly found in dry areas

For all ratings, it’s simply a matter of building a subfloor that’s suitable for the soil type and for the type, shape and size of house you want to build – Your building contractor normally works with an engineer who will be able to figure out what’s most appropriate design for the soil on the site you’re building on.

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