What an AS 2870 site classification actually tells you

The classes in plain English

• Class A — sand or rock, little or no movement
• Class S — slightly reactive clay
• Class M — moderately reactive clay, common in Brisbane
• Class H1 / H2 — highly reactive clay, stiffer slabs needed
• Class E — extremely reactive clay
• Class P — problem sites: fill, soft soils, trees, slopes or erosion

Why neighbours can differ

Fill placed during subdivision, cut and fill earthworks, different soil depths over rock and nearby trees can all give two adjoining lots different classes.

How it is determined

A geotechnical engineer drills boreholes, logs the soil and tests reactivity in the laboratory. The result is a characteristic surface movement (ys) that sets the class and the footing design.

Class P is not a dead end

Class P simply means the site needs specific engineering — often deeper piers, controlled fill or a stiffer raft. A proper investigation keeps that design economical.