A site soil classification to AS2870 (Residential Slabs and Footings) will generally be necessary for most residential dwellings. The site soil classification is a method of classifying a site’s soil characteristics so that it is possible to use standard foundation and slab designs given in AS2870.

The main feature considered in a site classification is the reactivity and amount of clay in the soil profile. Clay expands when it absorbs moisture and shrinks when it dries out. The more clay or more reactive the clay the more the surface of the site will move with subsequent moisture changes.

The soil / clay in the upper layers is more important than in lower layers as the moisture content of the soil becomes more stable the deeper in the ground it is. That is soil at the very top may be saturated in the morning and dry in the afternoon whereas soil say 2m down may have a very constant moisture content and only experience drying out in a severe drought.

The type of clay is important eg. clays derived from basalt rock are generally more reactive or moisture sensitive than clays derived from granite rocks.

Climate also has an effect as the frequency and amount of moisture (rain) entering the soil varies directly with the climate. Soils in coastal climates receive rain regularly and the moisture variations may only be significant down to say 1.5m depth whereas in semi-arid regions there may be moisture variations down to say 4m.

The main site classifications are

  • Class A – Most sand and rock sites with little or no ground movement from moisture changes.
  • Class S – Slightly reactive clay sites with only slight ground movement from moisture changes.
  • Class M- Moderately reactive clay or silt sites, which can experience moderate ground movement from moisture changes.
  • Class H- Highly reactive clay sites, which can experience high ground movement from moisture changes.
  • Class E- Extremely reactive sites, which can experience extreme ground movement from moisture changes.
  • Class P – Certain conditions make standard designs inappropriate without special consideration. Such conditions classed as P include soft soils, mine subsidence, land slip, uncontrolled fill and collapsing soils. Class P does not signify and particular severity of problem, but rather that a site is disqualified from other classes and therefore requires special consideration, therefore in theory class P should be classed as the ‘everything else class’.

Tasman Engineering Consultants can classify your site as part of the structural design process.