Soil stabilisation and ground improvement are geotechnical techniques used to enhance the engineering properties of soil — increasing strength, reducing compressibility, controlling permeability, and improving overall ground performance. These methods are essential when in-situ soils do not meet the requirements of the proposed construction.
What Is Soil Stabilisation?
Soil stabilisation is the process of treating natural soil or fill materials to improve their physical and mechanical properties. Ground improvement covers a broader range of techniques that modify the ground to reduce settlement risk, improve bearing capacity, and control groundwater.
When Is Ground Improvement Needed?
- Soft clays and silts — low bearing capacity, high settlement potential
- Loose sands — liquefaction risk, low bearing capacity
- Expansive clays — shrink/swell problems
- Fill materials — uncontrolled or poorly compacted fill
- Landfill and contaminated ground — ground treatment before reuse
- Slope stabilisation — to prevent landslides or slips
Mechanical Methods
Compaction
The simplest and most common improvement method.
| Technique | Application | Depth |
|---|---|---|
| Vibratory roller | Granular fills, pavement layers | 0–1 m |
| Sheep's foot roller | Cohesive fills, clay layers | 0–1 m |
| Heavy tamping | Granular and cohesive soils | 3–6 m |
Vibro-Compaction
Deep compaction of granular soils using a vibrating probe.
- Suitable for loose to medium dense sands
- Effective to depths of 20–30 m
- Reduces liquefaction risk
- Increases relative density to 70–85%
Dynamic Compaction (Heavy Tamping)
A heavy weight (10–40 tonnes) is dropped from height (10–30 m) in a grid pattern.
- Suitable for granular fills, loose sands, mine wastes
- Effective to depths of 5–10 m
- Low cost per unit area
- Not suitable for sensitive clays
Hydraulic Methods
Preloading / Surcharge
Temporary fill load applied to consolidate soft clay before construction.
- Suitable for soft clays, silts, organic soils
- Accelerates settlement before construction
- May require many months (less with vertical drains)
Vertical Drains (PVDs)
Pre-fabricated vertical drains accelerate consolidation under preload.
- Used with preloading
- Reduces consolidation time from years to months
- Spaced 1–3 m apart in a grid pattern
Dewatering
Lowering the groundwater table improves soil strength and reduces water pressure.
- Suitable for sands, silts, weathered rock
- Temporary or permanent
- Methods include wellpoints, deep wells, ejectors
Chemical Methods
Cement Stabilisation
Portland cement is mixed with soil to increase strength and durability.
| Application | Cement Content | Typical Use |
|---|---|---|
| Pavement subgrade | 2–5% | Improve CBR value |
| Foundation fill | 3–8% | Increase bearing capacity |
| Slope stabilisation | 5–10% | Increase shear strength |
| Soil mixing columns | 10–20% | Deep ground improvement |
Lime Stabilisation
Hydrated lime is added to clay soils to reduce plasticity and reactivity.
- Immediate effect — drying of wet clay (modification)
- Long-term effect — pozzolanic reaction increases strength (stabilisation)
- Typical dosage: 2–6% lime by dry weight
- Most effective for high-plasticity clays
Grouting
| Grout Type | Application | Soil Type |
|---|---|---|
| Cement grout | Void filling, rock fissures | Gravels, fractured rock |
| Chemical grout | Permeation grouting | Sands (fine to medium) |
| Jet grouting | In-situ soil-cement columns | All soil types |
| Compaction grouting | Densification through grout bulb injection | Loose sands, settlement control |
| Polyurethane grout | Rapid void filling, water cutoff | All soil types |
Deep Soil Mixing (DSM)
Mechanical mixing of in-situ soil with cementitious binder using auger-based equipment.
- Creates soil-cement columns or walls
- Effective to depths of 20–50 m
- Suitable for soft clays, peat, organic soils
- Used for foundations, retaining walls, cut-off walls
Reinforcement Methods
Stone Columns (Gravel Columns)
Vertical columns of compacted gravel installed in soft soil.
- Improve bearing capacity (2–4 times improvement)
- Accelerate consolidation
- Reduce liquefaction potential
- Suitable for soft clays and loose sands
Geosynthetics
Geotextiles, geogrids, and geocomposites used for soil reinforcement.
| Application | Geosynthetic | Function |
|---|---|---|
| Steep slopes | Geogrid | Tensile reinforcement |
| Embankments on soft ground | Geotextile | Separation and reinforcement |
| Retaining walls | Geogrid | Mechanically stabilised earth (MSE) |
| Pavement | Geogrid | Base course reinforcement |
| Drainage | Geocomposite | In-plane drainage |
Micro-piles / Pin-piles
Small-diameter piles (100–300 mm) installed through problematic ground to transfer loads to competent bearing strata.
- Suitable for low headroom, restricted access
- Can be installed through existing fill
- Used for foundation underpinning and slope stabilisation
Selection Guide
| Ground Condition | Recommended Method | Alternatives |
|---|---|---|
| Loose sand, liquefaction risk | Vibro-compaction, dynamic compaction | Stone columns, grouting |
| Soft clay, high settlement | Preloading + PVD, deep soil mixing | Stone columns, piles |
| Expansive clay | Lime stabilisation | Moisture barriers, removal & replace |
| Contaminated fill | Cement stabilisation, containment | Excavation and disposal |
| Slope instability | Soil nailing, tieback anchors | Retaining wall, grouting |
| Mine tailings, dredged material | Deep soil mixing, vacuum consolidation | Preloading, dynamic compaction |
Australian Standards
| Standard | Relevance |
|---|---|
| AS 3798-2007 | Earthworks — fill placement and compaction |
| AS 1289 Series | Soil testing methods |
| AS 1726-2017 | Geotechnical site investigations |
| Austroads Guide to Pavement Technology | Pavement subgrade stabilisation |
| AGPT05B | Stabilisation materials and methods |