Unsaturated soil mechanics deals with the behaviour of soils where the pore spaces contain both water and air.
Most geotechnical problems in Australia involve unsaturated conditions, particularly in arid and semi-arid regions where the water table is deep below the ground surface.
What Are Unsaturated Soils?
An unsaturated soil has three phases: solid particles, water, and air. The presence of air creates pore water tension (suction) that significantly influences the soil's strength, stiffness, and volume change behaviour.
Key Differences from Saturated Soil Mechanics
| Aspect | Saturated Soil | Unsaturated Soil |
|---|---|---|
| Pores | Fully filled with water | Water and air coexist |
| Effective stress | σ' = σ - u_w | σ' = (σ - u_a) + χ(u_a - u_w) |
| Shear strength | Depends on effective stress | Depends on net stress + suction |
| Volume change | Consolidation | Swelling/collapse on wetting |
| Permeability | Constant for given void ratio | Varies with degree of saturation |
Soil Suction
Soil suction is the negative pore water pressure that holds water in the soil pores. It has two components:
| Component | Description | Measurement |
|---|---|---|
| Matric suction (u_a - u_w) | Capillary and adsorptive forces within the soil matrix | Tensiometer, filter paper, axis translation |
| Osmotic suction | Dissolved salts in pore water | Electrical conductivity, filter paper |
| Total suction | Sum of matric + osmotic | Thermocouple psychrometer, filter paper |
Soil Water Characteristic Curve (SWCC)
The SWCC (also called the Soil-Water Retention Curve) describes the relationship between soil suction and water content (or degree of saturation). It is the fundamental constitutive relationship for unsaturated soil behaviour.
Typical SWCC Shape:
- Low suction (< 100 kPa) — large pores drain first
- Intermediate suction (100–1,000 kPa) — intermediate pores
- High suction (> 1,000 kPa) — small pores, adsorbed water
| Soil Type | Air Entry Value (kPa) | Residual Suction (kPa) |
|---|---|---|
| Sand | 2–10 | 10–100 |
| Silt | 10–50 | 100–500 |
| Clay | 50–500 | 500–10,000+ |
Laboratory Testing Methods
SWCC Measurement
| Method | Suction Range | Test Duration | Standard |
|---|---|---|---|
| Axis Translation (Tempe Cell) | 0–500 kPa | 1–4 weeks | ASTM D6836 |
| Axis Translation (Pressure Plate) | 0–1,500 kPa | 2–8 weeks | ASTM D6836 |
| Filter Paper Method | 100–10,000+ kPa | 7–14 days | ASTM D5298 |
| Chilled Mirror Hygrometer | 1,000–300,000 kPa | 1–2 hours | ASTM D6836 |
| Tensiometer | 0–100 kPa | Continuous | Field and lab |
Unsaturated Triaxial Testing
Measures shear strength and volume change under controlled suction conditions using the axis translation technique with a high air entry (HAE) ceramic disc.
| Test Type | Parameters | Application |
|---|---|---|
| Unsaturated CD | c', φ', φᵇ (suction angle) | Long-term drained strength |
| Unsaturated CU | Undrained strength with suction | Short-term loading |
Constant Water Content (CW) Tests
For simulating rapid loading of unsaturated soils where drainage cannot occur.
Applications in Australia
Expansive Clay Behaviour
Reactive clay soils in Australia repeatedly wet and dry with seasonal climate cycles. Understanding unsaturated soil mechanics is essential for:
- AS 2870 site classification — predicted surface movement (Yₛ) based on suction change
- Foundation movement — heave/shrinkage prediction
- Slab design — reinforcement and articulation requirements
Collapse Settlement
Some unsaturated soils (typically silty sands or low-density fills) undergo collapse on wetting — a sudden, large volume reduction without additional load. Common in:
- Aeolian (wind-blown) sands — parts of SA and WA
- Colluvial and alluvial fans — foothills of NSW and VIC
- Uncontrolled fill — collapsing on first wetting after construction
Slope Stability
Rainfall-induced landslides are controlled by changes in soil suction:
- During dry periods, suction maintains slope stability
- Rainfall infiltration reduces suction, decreasing shear strength
- The wet season failure mechanism is a classic unsaturated soil problem
Pavement Design
- Subgrade suction affects pavement performance
- CBR and stiffness depend on moisture condition
- Pavement design must consider the equilibrium suction under sealed pavements
Mine Waste and Tailings
- Cover system design for tailings storage facilities
- Vertical moisture barriers
- Capillary break design
Key Parameters for Engineering Design
| Parameter | Symbol | How Determined | Use |
|---|---|---|---|
| Air entry value | ψaev | SWCC | Onset of desaturation |
| Residual suction | ψr | SWCC | Below this, water is immobile |
| Saturated permeability | ks | Lab test | Upper bound of hydraulic conductivity |
| Unsaturated permeability | k(ψ) | SWCC + ks model | Water flow in unsaturated zone |
| Suction strength angle | φᵇ | Unsaturated triaxial | Suction contribution to strength |
| Collapse index | Icp | Double oedometer | Collapse potential on wetting |
Numerical Modelling
Software capable of unsaturated flow and coupled stress analysis:
- SEEP/W (GeoStudio) — unsaturated seepage modelling
- SVFlux — variably saturated flow
- FLAC — coupled hydro-mechanical analysis
- Plaxis 2D/3D — unsaturated soil models (Barcelona Basic Model)
- UNSAT-H — unsaturated flow in soils
Frequently Asked Questions
Why can't we just use saturated soil mechanics?
Saturated soil mechanics significantly underestimates the strength and overestimates the compressibility of unsaturated soils. This leads to overly conservative (expensive) designs — or in some cases (expansive soils), unconservative designs.
Do Australian standards address unsaturated soil?
AS 2870 addresses reactive soils using an empirical approach based on the shrink/swell index and design suction profiles (climate zones). However, most Australian standards do not explicitly use unsaturated soil mechanics — practical design relies on empirical correlations.
Is unsaturated soil testing expensive?
Yes — unsaturated triaxial tests are more costly than standard triaxial tests (typically 2–3× more) due to the specialised equipment and longer test durations. SWCC testing costs $500–$2,000 per curve depending on suction range.
When is unsaturated testing necessary?
- Sites with reactive clays (AS 2870 classifications M–E)
- Collapse-susceptible soils
- Rainfall-triggered landslide assessment
- Cover system design for mine waste
- Nuclear waste storage or deep geological disposal