Design and Installation of Gabion Structures

A Gabion Structure is a pre-assembled structure prepared from rectangular cages made of mechanically woven double-twisted hexagonal wire mesh filled with rock or stones. Designed especially for erosion control and earth retention, these structures utilize steel and geo-fabric to tie the mass together for maximum strength.

Assembled Gabion Retaining Wall

IRC Standard Reference: Design and installation guidelines are governed by IRC SP:116-2018 and MORTH Section 2500.

1. Technical Design Considerations

As per IRC SP:116, gabion walls are mass gravity structures. Unlike rigid R.C.C. walls, they offer flexibility and high permeability. Stability must be verified for:

• External Stability: Overturning, Sliding, and Bearing Capacity.
• Internal Stability: Rupture of wire mesh and internal shear resistance.
• Global Stability: Overall slope safety using slip circle analysis.

Additional Design Parameters from IRC SP:116-2018:

• Base Width: Typically 0.6 to 0.75 times the wall height for walls 1-6m high.
• Embedment Depth: Sufficient to prevent undermining, often 1.5-2 times expected scour depth.
• Batter Angle: Recommended 6-10 degrees for enhanced stability.
• Porosity: Maximum 40% considered for design; achievable 30-40% with proper filling.
• Factor of Safety: Sliding ≥1.5; Overturning ≥1.5; Bearing Capacity ≥2.0-3.0 depending on conditions.

Typical Gabion Structures with different components

2. Major Components

A) Gabion Box (Wire Mesh)

The mesh is mechanically woven with a double-twist to prevent unravelling if a wire is cut. Typical mesh sizes are 80x100mm or 100x120mm.

Mesh Specifications (per IRC SP:116-2018 & IS 16014):

• Tensile Strength: 350 N/mm² – 550 N/mm² (per IS 280).
• Metallic Coating: Zn + Al alloy (Galfan) or heavy galvanised per IS 4826; better corrosion resistance than plain zinc.
• Elongation: Not less than 10% on 20cm samples.
• Mesh Types: Common 8x10 (80mm) and 10x12 (100mm); Wire dia: Mesh 2.7-3.0mm, Selvedge 3.4-3.9mm, Lacing 2.2mm.
• Optional PVC Coating: Thickness min. 0.4mm; for aggressive environments.

B) Stone Filling

The selection of rock is critical for the stability and durability of the wall.

• Quality: Use hard, angular, unweathered rock.
• Size: Must be 1.5 to 2.5 times the mesh opening "D".
• Void Ratio: Hand-packing is required to keep voids below 30%.

Stone Specifications from IRC SP:116-2018:

• Hard, durable, angular stones; specific gravity ≥2.65.
• Minimum size: 100-150mm for 80-100mm mesh; no stone smaller than mesh opening +25mm.
• Manual placement to minimize voids; smaller stones to fill gaps for lower porosity.

C) Geotextile Filter Media

To prevent soil migration into the stone voids while allowing water to pass, a geotextile is placed behind the gabion facia.

Geotextile Specifications (MORTH Section 700):

• Non-woven geotextile placed behind gabions to act as filter and separator.
• Permeability > soil; aperture size to retain fines while allowing drainage.
• Overlaps min. 300-500mm; secured to prevent displacement.

3. Construction Sequence

Step	Action
1. Foundation	Excavate and compact foundation soil. A small batter (slope) is often provided.
2. Assembly	Unfold gabion units and lace edges with binding wire.
3. Filling	Fill in 300mm layers. Use cross-connecting wires to prevent bulging.
4. Closing	Lace the lid tightly to the top edges and diaphragms.

Additional Construction Guidelines from IRC SP:116-2018:

• Foundation: Compact to ≥95% Proctor density; level and check for weak pockets.
• Assembly: Use lacing wire or rings; connect adjacent units securely.
• Filling: Manual placement preferred; internal stiffeners every 300mm lift.
• Layering: Step back for batter; ensure alignment and no bulging.
• Drainage: Provide weepholes or gravel drains if needed.

Note: For absence of specific soil properties, reference IS 14458-Part 2 for default design values.

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