Soil Tests, Site Classification & Footings
Why your block's soil matters — how site classification under AS 2870 works in the Northern Rivers NSW, from reactive clays to sand, and what it means for footings.
Before a slab or footing is designed, the ground it sits on has to be understood — and across the Northern Rivers the ground varies enormously, from reactive plateau clays to coastal sand to floodplain silt. A soil test and site classification turn that variation into a number the engineer designs to. This guide explains what they are and why they matter to your build.
It is general guidance rather than advice for your specific block. When you are ready to talk through your own site, get in touch and we will arrange the testing and explain what it means for your footings.
What site classification means
A geotechnical engineer takes soil samples — a borehole or two — and classifies the site under Australian Standard AS 2870, the residential slabs and footings standard. Classes run from stable sand and rock (Class A) through increasingly reactive clays (Class M, H1, H2) to problem sites (Class P) with deep fill, soft ground or other issues. The class describes how much the ground is likely to move with moisture, and that directly drives how the footing system is designed.
Why it matters across the Northern Rivers
Reactive soils that shrink and swell with the seasons are common on the hinterland plateaus around Alstonville and Wollongbar; coastal blocks bring sand and sometimes acid-sulfate soils; floodplain sites carry soft silts and fill. Each needs a different footing response, from a standard slab to a stiffened raft or piered system. Skipping or guessing the classification is how slabs crack later. We arrange the test early so the footing is engineered to your actual ground and quoted accurately, rather than discovered to be inadequate on site.
Questions
- Do I really need a soil test?
- Yes — a soil test and site classification under AS 2870 are a normal, necessary part of designing footings for any new home or major addition. They tell the engineer how the ground behaves so the footing suits it. Skipping the test risks an under-designed slab and cracking down the track.
- What is a reactive site?
- A reactive site is one where the soil — usually clay — shrinks and swells noticeably as its moisture changes with the seasons, which can move a poorly designed slab. Reactive clays are common on the Northern Rivers hinterland plateaus. A reactive classification means the footing is designed to be stiffer to cope with that movement.