Soil, Reactivity, and Why Drainage Is a Structural Detail (AS 2870)
AS 2870 classifies residential sites based on expected ground movement. In practice, the building’s long-term performance is often determined by how well the site manages water. Most slab and footing distress is water-driven, not “bad concrete”.
1. Site Classes (AS 2870 Overview)
- Class A: little or no ground movement (sand/rock).
- Class S: slightly reactive clay.
- Class M / H / E: moderately to extremely reactive clays (movement increases).
- Class P: problem sites (fill, soft soils, landslip, collapsing soils, etc.).
Builder warning: “It looks like sand” is not a geotech report. Adjacent lots can vary significantly.
2. The Geotechnical Report: What to Look For
- Depth of reactive zone: tells you how deep moisture variation drives movement.
- Fill and uncontrolled material: triggers Class P behaviour and settlement risk.
- Groundwater: influences excavation stability and long-term moisture conditions.
- Recommendations: footing type, slab design assumptions, drainage notes, pier requirements.
3. Moisture Management: The Real Job
Reactive clays move when moisture changes. The most common causes of moisture variation are:
- poor surface drainage (water ponding at slab edge),
- downpipes not connected to legal point of discharge,
- garden beds and irrigation against the slab,
- leaking services (often slow, hidden leaks),
- trees and removal of trees (root zone moisture changes).
Practical rule: Treat drainage and downpipes as structural protection, not landscaping.
4. Footing Systems (What They’re Trying to Achieve)
- Raft slabs: spread loads and provide stiffness across potential differential movement.
- Stiffened slabs: increase edge and internal beam stiffness for reactive sites.
- Piers / piles: transfer load to more stable strata, bypassing poor near-surface soils.
5. Diagnosing Movement On Site
Typical signs of differential movement include:
- stair-step cracking in masonry,
- doors out of square,
- cracking radiating from slab corners,
- gaps at skirtings/cornices that change seasonally.
Before blaming “bad build”, check drainage and moisture sources. Fixing the cause is usually more important than patching the symptom.