Shrink/swell testing measures the volume change potential of clay soils when their moisture content changes. Under AS 2870, this is an important parameter for site classification, as reactive clay soils can cause significant foundation movement and structural damage to buildings.
What Are Reactive Soils?
Reactive soils (also called expansive soils) are clay soils that undergo significant volume changes with variations in moisture content. When wet, these soils swell (increase in volume, pushing foundations upward). When dry, they shrink (contract, causing foundations to settle or crack).
In Australia, reactive clay soils are widespread, particularly in:
- South East Queensland — high-plasticity basaltic and alluvial clays
- Western Sydney — Bringelly Shale and Wianamatta Group clays
- Melbourne — basaltic clay soils in the west and north
- Adelaide — highly reactive clay plains
- Perth — alluvial and estuarine clay deposits
How Shrink/Swell Is Measured
The Shrink/Swell Index (Iss)
The Shrink/Swell Index is determined in accordance with AS 1289.7.1.1 and is expressed as:
$$ I_{ss} = \frac{\Delta V}{V_0 \times \Delta w} $$Where:
- ΔV = change in specimen volume
- V₀ = original specimen volume
- Δw = change in moisture content
The Iss value represents the percentage volume change per unit change in moisture content.
Laboratory Procedure
| Step | Description |
|---|---|
| 1 | A soil sample is compacted in a standard mould |
| 2 | The sample is saturated and the initial volume and moisture are recorded |
| 3 | The sample is oven-dried to remove all moisture |
| 4 | The final volume and moisture are recorded |
| 5 | Iss is calculated from the volume change per unit moisture change |
AS 2870 Site Classification Based on Shrink/Swell
The Iss value directly determines the AS 2870 site class:
| Site Class | Iss (%) | Yₛ (mm) | Typical Movement |
|---|---|---|---|
| S (Slightly reactive) | < 1.0 | 0–20 mm | Low |
| M (Moderately reactive) | 1.0 – 1.7 | 20–40 mm | Moderate |
| H1 (Highly reactive) | 1.7 – 2.5 | 40–60 mm | High |
| H2 (Highly reactive) | 2.5 – 3.2 | 60–75 mm | Very high |
| E (Extremely reactive) | > 3.2 | > 75 mm | Extreme |
Where Yₛ = characteristic surface movement (estimated heave/shrinkage).
Factors Affecting Reactivity
| Factor | Effect on Shrink/Swell |
|---|---|
| Clay content | Higher clay content → higher reactivity |
| Clay mineralogy | Montmorillonite (smectite) > illite > kaolinite |
| Moisture variation | Greater seasonal change → more movement |
| Vegetation | Trees draw moisture → increased shrinkage near trees |
| Depth to water table | Shallow water table reduces net movement |
| Climate | Prolonged dry/wet cycles → greatest movement |
Australian Clay Reactivity
| State | Common Reactive Clays | Typical Reactivity |
|---|---|---|
| QLD | Basaltic clay, alluvial clay | M to H1 |
| NSW | Wianamatta shale, alluvial clay | H1 to H2 |
| VIC | Basaltic clay (Werribee, Keilor) | M to H1 |
| SA | Adelaide plains clay | H1 to E |
| WA | Swan Coastal Plain clay | S to M |
Managing Reactive Soils
Foundation Design Solutions
| Method | Application |
|---|---|
| Stiffened raft slab | Standard solution — deeper beams through reactive layer |
| Waffle raft slab | Lightweight alternative for moderately reactive sites |
| Stiffened strip footing | For less reactive soils |
| Pier and beam | Deep piers founded below active zone |
| Pole house | Elevated structure for highly reactive or sloping sites |
Moisture Management
- Ensure site drainage directs water away from foundations
- Install irrigation systems uniformly (avoid concentrated watering)
- Maintain consistent vegetation patterns (avoid large trees near footings)
- Install root barriers near established trees
Ground Improvement
- Lime stabilisation — reduces plasticity and reactivity
- Cement stabilisation — improves strength, reduces reactivity
- Pre-wetting / controlled flooding — pre-swell before foundation construction
- Moisture barriers — vertical and horizontal barriers to limit moisture change
Australian Standards
| Standard | Title |
|---|---|
| AS 1289.7.1.1 | Determination of the shrinkage index of a soil — Shrink/swell index |
| AS 2870-2011 | Residential slabs and footings — site classification |
| AS 1289.3.3.1 | Determination of the plasticity index — Atterberg limits |
| AS 1289.3.4.1 | Determination of the linear shrinkage of a soil |
Frequently Asked Questions
Is shrink/swell testing included in standard site classification?
Many geotechnical providers include shrink/swell testing as part of their standard site classification report. Ideal Geotech includes it at no extra cost.
Can I build on highly reactive soil?
Yes, but it requires an engineered foundation design suited to the site class (typically a stiffened raft or pier and beam system).
How do trees affect reactive soils?
Trees can extract significant moisture from the soil, causing localised shrinkage. This can result in differential foundation movement. The AS 2870 design accounts for this in the design suction profile for reactive sites.
Can reactivity be reduced?
Lime treatment is effective for reducing the shrink/swell potential of clay soils, often allowing a lower site classification and more economical footing design.