One of the first and most important processes to undertake in the design of any new structure is geotechnical testing (testing the engineering parameters of the soil). In this instance we’ll talk about residential structures and the Site Classification (also referred to as soil test) process.

A Site Classification is a relatively simple test that classifies a site due largely to its reactivity. Reactivity is a term used to describe how much the soil will shrink when dried and swell when wet.

Classes of Reactivity.

AS-2870 Residential Slabs and Footings, breaks reactivity into a number of classes:

A (Stable, 0mm) – Sand / rock sites

S (Slightly Reactive, 1mm – 20mm) – Slight ground movement with moisture changes

M (Moderately Reactive, 21mm – 40mm) –  Moderate ground movement with moisture changes

H1 (Highly Reactive, 41mm – 60mm ) – High ground movement with moisture changes

H2 (Highly Reactive, 61mm – 75mm) – Very high ground movement with moisture changes

E (Extremely Reactive, >75mm) – Extreme ground movement with moisture changes

Conducting a Site Classification.

AS-2870 Residential Slabs and Footings, provides a list of requirements that need to be met when conducting a site classification.  We wont go into the details here, but the general procedure is:

  1. Drill bore holes to rock or 1800mm minimum (varies depending on location) and log soil profile
  2. Take an undisturbed sample (where possible) of soil if it is reactive (no need to sample sand or rock sites)
  3. Split sample into 2 parts in the laboratory and dry one section and swell the other section
  4. Determine shrink / swell index and calculate the characteristic surface movement (ys) for the site
  5. Classify the site as per above based on the ys.

Weighing soil sample before drying

Problem Sites.

If there are soft soils or fill on the site it is likely the classifier will class the site as a P or Problem Site. This is to highlight that there may be an issue with the site other than reactivity that needs to be considered during the slab / footing design process.

What we’ve learnt.

So what’s the take home message from all of this, well there are a few.

  1. Getting the site classification correct is vitally important to ensuring the slab and footings perform adequately for the life of the building
  2. A site classification should be conducted using shrink/swell testing as this method of testing is much more reliable than other quicker forms of testing – ask you soil tester what they are doing
  3. Heavier / deeper clay normally equals higher reactivity
  4. Generally, the more reactive the site the more expensive the slab and footing system will be
  5. Clays only shrink and swell with changes in soil moisture, keep trees and gardens away from the house on reactive sites and maintain good drainage around the house.


Drill rig performing testing for site classification