Classification of Earth Retaining Structures
Earth retaining structures are engineered systems designed to support soil masses at different elevations and prevent lateral soil movement. These structures are extensively used in highways, railways, bridge abutments, basement excavations, waterfront structures, mining operations, and slope stabilization projects.
Need for Earth Retaining Structures
- Support vertical or near-vertical soil faces.
- Prevent landslides and slope failures.
- Facilitate deep excavations in urban areas.
- Provide support for highways and railways.
- Enable construction of basements and underground facilities.
- Protect adjacent structures from soil movement.
Classification of Earth Retaining Structures
According to O'Rourke and Jones (1990), earth retaining structures can be broadly classified into:
- Externally Stabilized Retaining Structures
- Internally Stabilized Retaining Structures
1. Externally Stabilized Retaining Structures
These retaining systems resist earth pressure primarily through their own weight, structural rigidity, external supports, anchors, or bracing systems.
A. In-Situ Walls
In-situ walls are constructed directly in the ground and are commonly used for excavation support.
1. Structural Walls (Cut Walls)
- Sheet Pile Walls
- Soldier Pile and Lagging Walls
- Cast-in-Situ Walls
- Slurry Walls
- Bored Pile Walls
- Contiguous Pile Walls
- Tangent Pile Walls
- Secant Pile Walls
- Non-Contiguous Pile Walls
Sheet Pile Walls
Sheet piles are thin interlocking steel, vinyl, or concrete sections driven into the ground to create a continuous barrier. They are widely used in waterfront structures and temporary excavation support.
Soldier Pile and Lagging Walls
These walls consist of vertical steel H-piles installed at intervals and horizontal lagging placed between them as excavation progresses.
Slurry Walls
Slurry walls are deep reinforced concrete walls constructed in trenches stabilized with bentonite slurry. They provide excellent groundwater cutoff and structural support.
Bored Pile Walls
Bored pile walls consist of reinforced concrete piles installed close together to retain soil.
| Type | Description |
|---|---|
| Contiguous Pile Wall | Piles are placed close together with small gaps. |
| Tangent Pile Wall | Adjacent piles touch each other. |
| Secant Pile Wall | Overlapping piles provide water-tight construction. |
| Non-Contiguous Pile Wall | Piles are spaced apart and used in stable soils. |
2. Chemical Walls (Cut Walls)
Ground improvement techniques that increase soil strength and reduce permeability.
- Jet Grouting
- Deep Soil Mixing
Jet Grouting
High-pressure grout jets are used to mix and strengthen in-situ soil, creating soilcrete columns.
Deep Soil Mixing
Mechanical mixing of cementitious binders with soil produces improved ground having higher strength and stiffness.
B. Braced Excavation Systems
Bracing systems provide additional support to retaining walls during excavation.
- Cross-Lot Bracing
- Rakers
Cross-Lot Bracing
Horizontal struts span across an excavation to resist inward wall movement.
Rakers
Inclined compression members transfer earth pressure to the excavation base.
C. Anchored Systems
Anchors transfer retaining forces into stable soil or rock beyond the failure surface.
- Augered Anchors
- Belled Anchors
- Pressure Injected Anchors
Advantages of Anchored Walls
- Large unobstructed excavation area.
- Reduced wall deflections.
- Suitable for deep excavations.
- Economical in urban environments.
D. Gravity Walls
Gravity walls resist earth pressure primarily through their own weight.
1. Cast-in-Place Concrete Gravity Walls
- Cantilever Retaining Walls
- Counterfort Retaining Walls
- Buttress Retaining Walls
Cantilever Wall
The most common reinforced concrete retaining wall where the stem and base slab act as cantilever elements.
Counterfort Wall
Counterforts connect the stem and base slab, reducing bending moments for high retaining walls.
Buttress Wall
Buttresses are placed on the front side of the wall to increase stability.
2. Modular Gravity Walls
- Masonry Walls
- Crib Walls
- Bin Walls
- Gabion Walls
- Concrete Modular Block Walls
Gabion Walls
Gabions are wire mesh baskets filled with rocks. They provide flexibility, drainage, and erosion control.
2. Internally Stabilized Retaining Structures
Internally stabilized systems improve the strength of the retained soil mass by introducing reinforcing elements within the soil itself.
A. Mechanically Stabilized Earth (MSE) Walls
MSE walls are among the most widely used retaining systems in modern infrastructure projects.
Components of MSE Walls
- Facing Panels
- Reinforcing Strips
- Geogrids
- Geotextiles
- Selected Backfill Material
Types
- Metallic Reinforcement Systems
- Polymeric Reinforcement Systems
- Anchored Earth Systems
- Reinforced Soil Slopes
B. In-Situ Reinforced Walls
These systems reinforce the existing soil mass without removing and replacing it.
1. Soil Nailing
Soil nailing involves installing passive steel bars into a soil slope or excavation face and covering the surface with shotcrete.
Applications
- Road widening projects
- Deep excavations
- Slope stabilization
- Tunnel portals
2. Reticulated Micro-Pile Walls
A network of closely spaced micro-piles forms a reinforced soil-retaining system suitable for unstable slopes.
Hybrid Retaining Walls
Hybrid walls combine characteristics of gravity walls and mechanically stabilized earth systems.
Examples
- Tailed Segmental Walls
- Low Density Fill Walls
These systems provide enhanced stability while reducing construction costs and material requirements.
Comparison of Major Retaining Systems
| Retaining System | Main Resistance Mechanism | Typical Use |
|---|---|---|
| Sheet Pile Wall | Flexural Resistance | Waterfront and Temporary Excavations |
| Soldier Pile Wall | Structural Support | Urban Excavations |
| Slurry Wall | Rigid Concrete Barrier | Deep Basements |
| Gravity Wall | Self Weight | Permanent Retaining Structures |
| MSE Wall | Soil Reinforcement | Highways and Railways |
| Soil Nailed Wall | In-Situ Reinforcement | Slope Stabilization |
Advantages of Earth Retaining Structures
- Efficient land utilization.
- Improved slope stability.
- Protection against erosion.
- Support for deep excavations.
- Enhanced safety of nearby structures.
- Long-term infrastructure stability.
Conclusion
Earth retaining structures form an essential part of geotechnical and civil engineering practice. The selection of a suitable retaining system depends on soil conditions, groundwater level, retained height, loading conditions, available space, construction constraints, and project economics. Modern retaining solutions range from conventional gravity walls to advanced mechanically stabilized earth systems and soil nailing techniques, providing engineers with a wide range of options for safe and economical construction.
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