Dispersive soils are clay soils that deflocculate and erode rapidly when exposed to water. They present significant risks to earthworks, embankments, and hydraulic structures if not properly identified and managed.
What Are Dispersive Soils?
Dispersive soils contain high levels of exchangeable sodium, which causes the clay particles to repel each other when in contact with water rather than staying aggregated. Instead of behaving as stable lumps, dispersive clays break down into individual particles that are easily carried away by flowing water.
Why It Matters
- Embankment piping — dispersive clays in dam cores can erode internally (piping failure)
- Channel erosion — dispersive soils in canal banks erode rapidly
- Gully erosion — pipes and tunnels form within the soil, leading to collapse
- Fill failures — dispersive clay in engineered fill can erode internally
Australian Context
Dispersive soils are found across Australia, notably in:
- NSW — Hunter Valley, Central West, Riverina
- QLD — Darling Downs, Brigalow Belt
- VIC — Northern Plains, Murray River zone
- SA — Murray Basin, Yorke Peninsula
- WA — Wheatbelt region, Swan Coastal Plain
Many dam failures in Australia have been attributed to dispersive soil conditions in embankments.
Laboratory Tests for Dispersion
Emerson Class Number (AS 1289.3.8.1)
The simplest and most widely used field/laboratory test. A soil crumb is placed in distilled water and observed:
| Class | Behaviour | Interpretation |
|---|---|---|
| Class 1 | Complete dispersion — clay colloid forms around the crumb | Highly dispersive |
| Class 2 | Partial dispersion — slight cloudiness | Dispersive |
| Class 3 | No dispersion — crumb remains intact but swells | Non-dispersive |
| Class 4 | No dispersion — crumb remains intact without swelling | Non-dispersive |
| Class 5 | No reaction — crumb slakes to a heap of fine sand/silt | Non-dispersive |
| Class 6 | No reaction — crumb slakes to a heap of fine sand/silt with slight turbidity | Non-dispersive (borderline) |
| Class 7 | No reaction — no slaking, crumb remains firm | Non-dispersive |
| Class 8 | No reaction — no slaking, slight turbidity | Non-dispersive (borderline) |
Interpretation:
- Classes 1–2: Dispersive — requires management
- Classes 3–4: Non-dispersive
- Classes 5–8: Non-dispersive
Pinhole Dispersion Test (AS 1289.3.8.2)
A more definitive test. Water flows through a 1.0 mm diameter hole in a compacted soil specimen under controlled head.
| Classification | Flow Rate | Effluent Colour | Hole Size After Test |
|---|---|---|---|
| D1 — Highly dispersive | High | Very cloudy (suspended clay) | > 2× original diameter |
| D2 — Moderately dispersive | Moderate | Cloudy | Enlarged |
| ND1 — Non-dispersive | Low | Clear | Slightly enlarged |
| ND2 — Non-dispersive | Very low | Clear | Unchanged |
| ND3 — Non-dispersive | Negligible | Clear | Unchanged |
Exchangeable Sodium Percentage (ESP)
The ESP measures the proportion of sodium in the soil's cation exchange capacity:
$$ ESP = \frac{\text{Exchangeable Na}^+}{\text{CEC}} \times 100\% $$| ESP (%) | Dispersion Risk |
|---|---|
| < 6 | Low |
| 6–10 | Moderate |
| 10–15 | High |
| > 15 | Very high |
Sodium Adsorption Ratio (SAR)
SAR measures sodium in relation to calcium and magnesium in the soil solution:
$$ SAR = \frac{Na^+}{\sqrt{\frac{Ca^{2+} + Mg^{2+}}{2}}} $$| SAR | Dispersion Risk |
|---|---|
| < 3 | Low |
| 3–6 | Moderate |
| > 6 | High |
Test Selection Guide
| Test | Cost | Time | Reliability | Use |
|---|---|---|---|---|
| Emerson Class | Low | 1 hour | Screening | Initial assessment |
| Pinhole Dispersion | Moderate | 1–2 days | High | Definitive classification |
| ESP | Moderate | 3–5 days | High | Quantitative measure |
| SAR | Moderate | 3–5 days | High | Correlates with ESP |
Management of Dispersive Soils
Treatment Methods
| Method | Application | Effectiveness |
|---|---|---|
| Gypsum (CaSO₄·2H₂O) application | Surface treatment or layer mixing | Highly effective — calcium replaces sodium |
| Lime (CaO or Ca(OH)₂) treatment | Soil stabilisation | Highly effective — also improves strength |
| Cement stabilisation | Structural fill | Effective — also increases strength |
| Chemical amendments | Polymer additives | Variable — product-specific |
Application Rates
| Amendment | Typical Rate | Application |
|---|---|---|
| Gypsum (powdered) | 5–15 kg/m³ | Mix into fill or spread and incorporate |
| Hydrated lime | 2–6% by dry weight | Mix into fill |
| Agricultural lime | 5–20% by dry weight | Mix into fill (slower reaction) |
Design Considerations for Dispersive Soils
- Embankment core — use treated (non-dispersive) material or install filter zones
- Channels — line with non-dispersive material or use protective linings
- Earth dam spillways — ensure adequate energy dissipation to prevent erosion
- Drainage — filters designed to retain dispersive particles (geotextile filters)
- Compaction — dispersive soils should be compacted wet of optimum to reduce permeability
Australian Standards
| Standard | Title |
|---|---|
| AS 1289.3.8.1 | Determination of the Emerson class number of a soil |
| AS 1289.3.8.2 | Determination of the dispersion characteristics of a soil — Pinhole dispersion test |
| AS 1289.3.8.3 | Determination of the dispersion characteristics of a soil — Soil classification test |
| AS 1726-2017 | Geotechnical site investigations |
| AS 3798-2007 | Earthworks (fill material suitability) |
| ANCOLD Guidelines | Earth dam design — dispersive soil management |
| SCA Guidelines | Soil Conservation Authority — dispersive soil identification |