Soil is a three-phase material consisting of solids (mineral particles), water, and air. The proportions and interactions between these phases determine the engineering behaviour of the soil.
Understanding soil properties allows engineers to:
- Classify and describe soils for design purposes
- Predict settlement, strength, and permeability
- Design foundations, retaining walls, slopes, and pavements
- Specify compaction and ground improvement requirements
2. Physical Properties (Index Properties)
2.1 Water Content (Moisture Content)
$$ w = \frac{M_w}{M_s} \times 100\% $$Where $M_w$ = mass of water, $M_s$ = mass of solids.
Measured by oven-drying method (AS 1289.2.1.1): a soil sample is weighed, dried at 105–110°C for 24 hours, and reweighed.
| Soil Type | Typical Water Content |
|---|---|
| Dry sand | 0–5% |
| Moist sand | 5–15% |
| Saturated sand | 15–30% |
| Low plasticity clay | 15–30% |
| High plasticity clay | 30–100%+ |
| Peat / organic soils | 100–500%+ |
2.2 Bulk Density and Dry Density
$$ \rho = \frac{M}{V} \quad \text{(bulk density)} $$ $$ \rho_d = \frac{M_s}{V} \quad \text{(dry density)} $$Measured by:
- Core cutter method (AS 1289.5.1.1) — for cohesive soils
- Sand replacement method (AS 1289.5.3.1) — for granular soils
- Nuclear density gauge (AS 1289.5.8.1) — rapid field measurement
2.3 Specific Gravity
$$ G_s = \frac{M_s}{V_s \rho_w} $$Specific gravity of soil solids is measured using a density bottle (AS 1289.3.5.1).
| Soil Mineral | Specific Gravity ($G_s$) |
|---|---|
| Quartz | 2.65 |
| Feldspar | 2.55–2.75 |
| Kaolinite | 2.60–2.65 |
| Illite | 2.70–2.80 |
| Montmorillonite | 2.65–2.80 |
| Organic matter | 1.0–2.0 |
2.4 Void Ratio and Porosity
$$ e = \frac{V_v}{V_s} \quad \text{(void ratio)} $$ $$ n = \frac{V_v}{V} \times 100\% \quad \text{(porosity)} $$Relationship: $n = \frac{e}{1+e}$
| Soil Type | Typical Void Ratio ($e$) |
|---|---|
| Dense sand | 0.35–0.50 |
| Loose sand | 0.50–0.85 |
| Soft clay | 1.0–2.5 |
| Stiff clay | 0.50–1.0 |
| Peat | 3.0–15.0 |
2.5 Degree of Saturation
$$ S = \frac{V_w}{V_v} \times 100\% $$| Saturation | Description |
|---|---|
| 0% | Dry |
| 0–50% | Partially saturated |
| 50–80% | Moist / damp |
| 80–100% | Nearly saturated |
| 100% | Fully saturated |
2.6 Unit Weight Relationships
Dry unit weight: $$ \gamma_d = \frac{G_s \gamma_w}{1+e} $$
Saturated unit weight: $$ \gamma_{sat} = \frac{(G_s+e)\gamma_w}{1+e} $$
Submerged (buoyant) unit weight: $$ \gamma' = \gamma_{sat} - \gamma_w $$
3. Key Phase Relationships
The three-phase diagram is the foundation for all soil property calculations:
| Relationship | Formula | Notes |
|---|---|---|
| Void ratio from porosity | $e = \frac{n}{1-n}$ | — |
| Dry density from bulk density | $\rho_d = \frac{\rho}{1+w}$ | — |
| Saturated density | $\rho_{sat} = \frac{G_s + e}{1+e} \rho_w$ | S = 100% |
| Air void content | $A_v = \frac{V_a}{V} \times 100\%$ | — |
4. Grain Size Distribution
4.1 Sieve Analysis (AS 1289.3.6.1)
Determines the distribution of particle sizes for coarse-grained soils (gravel and sand fractions) using a stack of standard sieves.
Australian standard sieve sizes:
| Sieve Size | Retained Material |
|---|---|
| 63 mm | Cobbles |
| 19 mm | Coarse gravel |
| 6.7 mm | Medium gravel |
| 2.36 mm | Fine gravel |
| 425 μm | Coarse sand |
| 75 μm | Fine sand (silt/clay boundary) |
4.2 Hydrometer Analysis (AS 1289.3.6.3)
Determines the particle size distribution of fine-grained soils (silt and clay) using Stokes' Law of sedimentation.
4.3 Key Grain Size Parameters
| Parameter | Definition | Engineering Significance |
|---|---|---|
| $D_{10}$ | Particle size at 10% passing (effective size) | Permeability correlation (Hazen's formula) |
| $D_{30}$ | Particle size at 30% passing | Used in uniformity/curvature calculations |
| $D_{60}$ | Particle size at 60% passing | Used in uniformity coefficient |
| $C_u$ | $D_{60}/D_{10}$ (uniformity coefficient) | $C_u > 6$ = well-graded sand; $C_u < 4$ = uniform |
| $C_c$ | $D_{30}^2/(D_{60} \times D_{10})$ (coefficient of curvature) | 1–3 indicates well-graded |
| Fines content | % passing 75 μm | Critical for frost susceptibility, liquefaction |
Fines classification:
| Fines Content | Description |
|---|---|
| < 5% | Insufficient fines to affect behaviour |
| 5–12% | Intermediate — may influence behaviour |
| > 12% | Fines-dominated behaviour |
5. Consistency and Plasticity (Fine-Grained Soils)
5.1 Atterberg Limits
Measured according to AS 1289.3.x series:
| Limit | Test Method | Description |
|---|---|---|
| Liquid Limit (LL) | AS 1289.3.1.1 (Casagrande cup) or AS 1289.3.9.1 (cone penetrometer) | Water content at which soil transitions from plastic to liquid state |
| Plastic Limit (PL) | AS 1289.3.2.1 | Water content at which soil crumbles when rolled into 3 mm threads |
| Shrinkage Limit (SL) | AS 1289.3.3.1 | Water content below which no further volume reduction occurs on drying |
5.2 Plasticity Index
$$ PI = LL - PL $$| PI | Soil Description | Engineering Behaviour |
|---|---|---|
| 0 | Non-plastic (NP) | Silt or sand — negligible cohesion |
| 1–7 | Low plasticity | Slightly plastic — some cohesion |
| 7–17 | Medium plasticity | Moderately plastic — significant shrink/swell potential |
| 17–35 | High plasticity | Highly plastic — reactive soil |
| 35+ | Very high plasticity | Extremely reactive — problematic for foundations |
5.3 Consistency Index
$$ I_c = \frac{LL - w}{PI} $$| $I_c$ | In-situ Consistency |
|---|---|
| 0 | Liquid |
| 0–0.25 | Very soft |
| 0.25–0.50 | Soft |
| 0.50–0.75 | Firm |
| 0.75–1.0 | Stiff |
| > 1.0 | Very stiff / hard |
6. Engineering Properties
6.1 Permeability (Hydraulic Conductivity)
Measured by constant head test (AS 1289.6.7.1) for granular soils or falling head test (AS 1289.6.7.2) for fine-grained soils.
| Soil Type | Permeability $k$ (m/s) | Drainage |
|---|---|---|
| Clean gravel | $10^{-1}$ to $10^{-2}$ | Very good |
| Clean sand | $10^{-2}$ to $10^{-4}$ | Good to moderate |
| Silty sand | $10^{-4}$ to $10^{-6}$ | Poor |
| Clay | $10^{-8}$ to $10^{-11}$ | Practically impervious |
Hazen's formula (for clean sands): $k = C \times D_{10}^2$
Where $C$ ≈ 0.01–0.015 and $D_{10}$ is in mm.
6.2 Compressibility and Consolidation
Compression index ($C_c$): Slope of the virgin compression line on e–log $\sigma'$ plot
$$ C_c = 0.009(LL - 10) \quad \text{(Terzaghi and Peck correlation)} $$Swell index ($C_s$): Slope of the unloading/reloading line — typically $C_s \approx 0.1$ to $0.2 C_c$
Coefficient of consolidation ($c_v$): Rate of consolidation (Casagrande's log-time method or Taylor's √t method)
6.3 Shear Strength
Mohr-Coulomb failure criterion:
$$ \tau_f = c' + \sigma'_n \tan\phi' $$Where:
- $\tau_f$ = shear strength at failure
- $c'$ = effective cohesion
- $\sigma'_n$ = effective normal stress on failure plane
- $\phi'$ = effective friction angle
Typical shear strength parameters:
| Soil Type | $c'$ (kPa) | $\phi'$ (°) |
|---|---|---|
| Clean sand | 0 | 30–38 |
| Silty sand | 0–5 | 28–35 |
| Sandy clay | 5–20 | 22–30 |
| Silty clay | 10–30 | 18–27 |
| Stiff clay | 20–80 | 15–25 |
| Soft clay | 0–15 | 15–22 |
6.4 California Bearing Ratio (CBR)
Measure of subgrade strength for pavement design (AS 1289.6.1.1):
| CBR Value | Subgrade Classification |
|---|---|
| < 2% | Very poor — requires stabilisation |
| 2–5% | Poor — thick pavement required |
| 5–10% | Fair — typical |
| 10–20% | Good |
| 20–50% | Very good |
| > 50% | Excellent — crushed rock |
7. Compaction Properties
Standard Proctor (AS 1289.5.1.1) vs Modified Proctor (AS 1289.5.1.1):
| Parameter | Standard Proctor | Modified Proctor |
|---|---|---|
| Rammer weight | 2.5 kg | 4.5 kg |
| Drop height | 300 mm | 450 mm |
| Layers | 3 | 5 |
| Blows per layer | 25 | 25 |
| Energy | 596 kJ/m³ | 2703 kJ/m³ |
Typical compaction values:
| Soil Type | Max Dry Density ($\rho_{dmax}$) | Optimum Moisture Content (OMC) |
|---|---|---|
| Sandy gravel | 2.10–2.30 t/m³ | 6–10% |
| Well-graded sand | 1.90–2.10 t/m³ | 9–14% |
| Silty sand | 1.80–2.00 t/m³ | 12–16% |
| Clayey sand | 1.70–1.90 t/m³ | 13–18% |
| Clay | 1.55–1.75 t/m³ | 18–25% |
8. Key Australian Standards
| Standard | Title | Relevance |
|---|---|---|
| AS 1289 | Soil Testing Methods (entire series) | All standardised test methods |
| AS 1726 | Geotechnical Site Investigations | Sampling, logging, field testing |
| AS 2870 | Residential Slabs and Footings | Site classification using soil properties |
| AS 3798 | Guidelines on Earthworks | Compaction specifications |
| AS 4678 | Earth Retaining Structures | Soil properties for design |
| AS 2159 | Piling — Design and Installation | Geotechnical parameters for piles |
9. Common Correlations
Strength-to-SPT Correlation (for cohesionless soils)
| SPT N-value | Relative Density | $\phi'$ (approximate) |
|---|---|---|
| 0–4 | Very loose | < 28° |
| 4–10 | Loose | 28–30° |
| 10–30 | Medium dense | 30–35° |
| 30–50 | Dense | 35–40° |
| > 50 | Very dense | > 40° |
Undrained Shear Strength (for cohesive soils)
| Consistency | $s_u$ (kPa) | SPT N-value (approximate) |
|---|---|---|
| Very soft | < 12 | < 2 |
| Soft | 12–25 | 2–5 |
| Firm | 25–50 | 5–10 |
| Stiff | 50–100 | 10–20 |
| Very stiff | 100–200 | 20–40 |
| Hard | > 200 | > 40 |