AS 2870 Site Classifications Explained

What is an AS 2870 site classification?

An AS 2870 site classification is a geotechnical rating that indicates how much the soil on a building site is expected to move (shrink and swell) due to moisture changes. In Australia, residential concrete slabs and footings must be designed according to the Australian Standard AS 2870. The classification tells the structural engineer how stiff and deep the footings must be to prevent the house from cracking.

What do the AS 2870 site classes mean?

Site classes are not a "pass or fail" grade; they simply describe the reactivity of the clay. The standard classifications are:

• Class A (Stable): Mostly sand or rock. Little to no ground movement expected.
• Class S (Slightly Reactive): Mild clay. Minor ground movement expected.
• Class M (Moderately Reactive): The most common classification in Sydney. Expect moderate ground movement requiring a stiffer raft slab and deeper edge beams.
• Class H1 / H2 (Highly Reactive): Highly reactive clay with significant movement. Footing design will dictate a large portion of the slab cost.
• Class E (Extremely Reactive): Severe ground movement. The site is still buildable, but demands a heavily engineered, deep footing system (often piers) to ride out the movement.
• Class P (Problem Site): A site with abnormal conditions, such as uncontrolled fill, mine subsidence, landslip, or severe tree root influence. Class P sites require custom engineering design outside the standard AS 2870 tables.

Why did my concrete slab crack?

If a concrete slab cracks significantly, it is rarely because the concrete mix was faulty. In most cases, the slab cracked because the reactive clay underneath it expanded (when wet) or contracted (when dry) more than the footing system was designed to handle. This is why obtaining an accurate AS 2870 site classification before engineering the slab is critical.