Guidelines for Design and Installation of Gabion Structures

Scope of IRC:SP:116-2018 (Gabion Structures)

• Covers specifications for gabion and revet mattresses, including:

  • Types of wire mesh (mechanically woven, double-twisted hexagonal)
  • Polymeric coatings and wire diameters (e.g., mesh wire dia: 2.20 mm, coating thickness nominal 0.50 mm)
  • Typical gabion sizes (e.g., 4×1×1 m with 3 diaphragms)
  • Tolerances: Length/Width ±5%, Height ≤0.3 m +10%, >0.3 m +5%

• Filter Media:

  • Nonwoven needle punched or thermally bonded geotextiles (Polyester/Polypropylene)
  • Strength requirements (MARV) vary by installation severity (harsh, moderate, less severe)

• Geotextile UV Stability: ≥70% retained strength after 500 hours exposure

• Subsurface Drainage Geotextile:

  • Permittivity and opening size depend on soil fines content (passing 0.075 mm sieve)

flowchart TD
    A[Scope] --> B[Gabion Mesh & Sizes]
    A --> C[Filter Media Specs]
    A --> D[Geotextile Strength & UV Stability]
    A --> E[Subsurface Drainage Requirements]

This scope ensures durable, stable gabion retaining structures with proper material and installation standards.

Technical Specifications of Gabion Mesh (IRC SP 116)

1. Wire Material and Mechanical Properties

• Tensile Strength: 350 - 550 N/mm² (IS 280)
• Elongation: ≥ 10% on 20 cm sample
• Metallic Coating: Zinc or Zn-Al alloy per IS 4826
• Adhesion: No flaking/cracking when wrapped 6 turns on mandrel (4× wire diameter) (EN 10244-1)

2. Metallic Coating Mass (Table 1)

3. PVC Coating (Clause 20.6)

• Tensile Strength: ≥ 20.6 MPa (IS 13360/ISO 527)
• Elongation at Break: ≥ 200%
• Hardness: Shore D 50-60 (IS 13360/ISO 868)
• Salt Spray Resistance: No effect after 3000 h (IS 13360/ISO 9227)
• Coating Thickness: Nominal 0.5 mm, Min 0.4 mm

4. Mesh Opening Sizes (Table 2)

IRC SP 116 - Mechanical Strength & Testing Requirements Summary

1. Wire Specifications (Clause 4.1)

• Tensile Strength: 350 - 550 N/mm² (IS 280)
• Elongation: ≥ 10% on 20 cm sample
• Metallic Coating: Zinc/Zn-Al alloy per IS 4826, with minimum coating mass (g/m²) as below:

2. Testing Methods & Acceptance (Clause 4.1e, Table 5)

• Tensile Strength Test: Specimen ~0.8 m x 0.5 m, load applied parallel and perpendicular to twist axis.
• Punch Test: Circular ram (350 mm diameter), load rate ≤ 220 N/s.
• Pull-Apart Test: Panels joined by wire fasteners, loaded until failure or 50 mm opening.

Design Principles & Specifications for Gabion and Revet Mattresses (IRC SP 116)

1. Design Principles (Clause 7)

• Thickness of Mattress: Determined based on hydraulic forces and soil conditions.
• Tractive Force Theory: Used to calculate the forces exerted by flowing water on the mattress.
• Safety Checks: Against sliding, overturning, bearing capacity, internal failure, and global failure.
• Launching Apron Design: To prevent undermining during installation.
• Special Cases: Installation underwater, on loose rock, or replaced soil.

2. Technical Specifications of Mesh (Clause 4.1)

• Wire Tensile Strength: 350–550 N/mm² (IS 280).
• Elongation: Minimum 10%.
• Metallic Coating: Zinc or Zn-Al alloy coatings per IS 4826 with corrosion resistance tested by ISO 6988 and ISO 9227.
• Adhesion: Zinc coating must not flake/crack when bent (EN 10244-1).

3. Key Formula: Tractive Force (τ)

[ \tau = \gamma \cdot R \cdot S ]

• τ: Tractive force (N/m²)
• γ: Unit weight of water (kN/m³)
• R: Hydraulic radius (m)
• S: Energy slope (m/m)

flowchart TD
    A[Flowing Water] --> B[Exerts Tractive Force]
    B --> C[Gabion/Revet Mattress]
    C --> D[Resists Sliding & Sc

Safety Against Sliding (IRC SP 116 - Clause 6.10)

Key Specifications:

• Minimum Factor of Safety (FOS) against Sliding:
  • Static Case: 1.5
  • Seismic Case: 1.125

Basic Formula for Sliding Stability:

[ FOS = \frac{\text{Resisting Forces}}{\text{Driving Forces}} \geq 1.5 \quad (\text{static}) ]

Where:

• Resisting Forces mainly include frictional resistance at the base and passive earth pressure.
• Driving Forces are lateral earth pressure and surcharge loads.

Typical Calculation Steps:

1. Calculate lateral earth pressure ( P ) acting on the wall.
2. Calculate frictional resistance ( R_f = \mu \times N ), where:
   • ( \mu ) = coefficient of friction between base and foundation soil
   • ( N ) = normal force (usually the weight of the wall + surcharge)
3. Factor of Safety against sliding: [ FOS = \frac{R_f}{P} ]

Table: Minimum Factors of Safety for Stability Checks (Excerpt)

Diagram: Sliding Forces on Retaining Wall

flowchart LR
    A[Lateral Earth Pressure (Driving Force)] -->|Pushes| Wall[Retaining Wall]
    Wall -->|Frictional Resistance (Resisting Force)| Base[Wall Base]
    Base -->|Normal Force (Weight + Surcharge)| Ground[Foundation Soil]

Summary:
Ensure the resisting frictional force at the base exceeds the lateral driving forces by at least 1.5 times (static). Use soil friction coefficients and wall weight to compute resisting forces. Check seismic reductions accordingly.

Safety Against Bearing (IRC SP 116 - Clause 6.12)

• Normal Force (N): Resultant of vertical and horizontal forces including moments.

• Eccentricity (e):
  [ e = \frac{d}{B} ]
  where ( d = \frac{M_r - M_{oh}}{N} ),
  (M_r) = resultant moment, (M_{oh}) = horizontal moment, (N) = normal force, (B) = base width.

• Bearing Pressure Distribution (linear):
  [ q_{max} = \frac{N}{B} \left(1 + \frac{6e}{B}\right) ] [ q_{min} = \frac{N}{B} \left(1 - \frac{6e}{B}\right) ]

• Design Checks:

  • Eccentricity ( e \leq \frac{B}{6} ) to ensure resultant lies within middle third of base.
  • Maximum bearing pressure ( q_{max} \leq q_{allow} ) (allowable bearing capacity of soil).
  • Factor of Safety (FS):
    [ FS = \frac{q_{ult}}{q_{max}} \quad \text{(should be > 1.5 typically)} ]

• Foundation Depth Recommendations:

  • Non-cohesive soil: minimum 0.5 m for low walls; increase for high walls.
  • Cohesive soil: embedment > 0.5 m + 300-500 mm granular base layer recommended.

• Notes:

  • Use lightweight backfill (e.g., flyash) if bearing pressure exceeds soil capacity.
  • Estimate settlements and compensate height accordingly.

flowchart LR
    A[Calculate Forces: N, Mr, Moh] --> B[Compute eccentricity e = d/B]
    B --> C[Check e ≤ B/6]
    C --> D[Calculate qmax and qmin]
    D --> E{Is qmax ≤ qallow?}
    E -- Yes --> F[Design is Safe]
    E -- No --> G[Modify design: Increase base, use lightweight fill, or

Safety Against Internal Failure (IRC SP 116 - Clause 6.13)

Key Points:

• Internal failure occurs due to internal stresses from active earth thrust and surcharge loads.
• For Gabion walls, each layer must be checked for sliding relative to adjacent layers (Fig. 21).
• Calculate shear stress (τ) and maximum normal stress (σ_max) at the section using force and moment equilibrium.

Formulas:

Let:

• B = width of gabion layer
• T = shear force at the section
• N = normal force at the section
• d = lever arm or distance related to moment equilibrium

[ \tau = \frac{T}{B} ]

[ \sigma_{max} = \frac{N}{B} ]

Design Considerations:

• Allowable shear and normal stresses depend on gabion mesh properties (tensile strength, punch resistance).
• Mesh confinement adds apparent cohesion and increases friction angle.
• Use laboratory test data or experiments to determine allowable stresses.
• Gabion walls are flexible and permeable, tolerating settlements without loss of integrity.
• Assess settlement based on foundation soil properties.
• Evaluate deformation based on normal stress and mesh/fill properties.

flowchart LR
    A[Active Thrust + Surcharge] --> B[Internal Stresses in Gabion Layers]
    B --> C[Calculate Shear (T) and Normal (N) Forces]
    C --> D[Compute Shear Stress τ = T/B]
    C --> E[Compute Normal Stress σ_max = N/B]
    D & E --> F[Compare with Allowable Stresses from Mesh Properties]
    F --> G[Check Safety Against Internal Failure]

Summary: Ensure each gabion layer resists sliding and internal stresses within mesh limits, using equilibrium forces and mesh test data for safe design.

IRC SP 116: Safety Against Overall / Global Failure (Clause 6.14)

Gabion walls may fail globally along an external failure surface, especially when:

• Foundation soils have layers with varying strength.
• Downhill slope under the wall is inclined below horizontal.

Key Points:

• Failure Surface: Usually assumed circular.
• Analysis Method: Limit equilibrium methods, notably Bishop’s Simplified Method of Slices, are used.
• Factor of Safety (FoS): Calculated by dividing soil mass into slices and checking equilibrium.
• Critical Surface: The surface with the lowest FoS is the most critical failure surface.
• Computational Aid: Computer programs are recommended due to complexity.

Bishop’s Simplified Method Formula:

[ F = \frac{\sum \left( c' , l + (W - u , l) \tan \phi' \right) / \cos \alpha}{\sum W \sin \alpha} ]

Where:

• (c') = effective cohesion
• (l) = length of slice base
• (W) = weight of slice
• (u) = pore water pressure
• (\phi') = effective angle of internal friction
• (\alpha) = inclination of slice base

Design Recommendations:

• Use stable slopes for gabion/revet mattress placement.
• Stabilize slopes if instability is present before applying gabions.
• Consider river parameters (velocity, gradient, flood discharge, soil type, water levels, geometry).

flowchart LR
    A[Soil Mass] --> B[Divide into Slices]
    B --> C[Calculate Forces on Each Slice]
    C --> D[Apply Bishop's Method]
    D --> E[Compute Factor of Safety]
    E --> F{FoS < 1?}
    F -- Yes --> G[Global Failure Likely]
    F -- No --> H[Stable Structure]

For detailed design, refer to IRC SP 116:2018, Clause 6.14 and use software tools for slice method analysis.

Backfill Material and Extent as per IRC SP 116

Extent of Backfill (Clause 6.7)

• Backfill shall extend up to the 45° line from the toe of the gabion.
• The backfill surface must meet the top surface level of the gabion structure.
• Use non-woven geotextile between backfill and gabion to prevent soil migration.

graph LR
A[Toe of Gabion] --> B[45° line]
B --> C[Backfill extent]
C --> D[Top surface level of Gabion]

Backfill Material Specifications (Clause 4.4.1 & Table 12)

• Use soil types classified as per IS 1498.
• Typical unit dry weight ranges (g/cm³):

Key Points:

• Select well-draining, stable soils (typically granular soils) for backfill.
• Avoid expansive or highly compressible soils.
• Ensure proper compaction to achieve design density.

This ensures structural stability and durability of the gabion retaining system.

IRC SP 116: Installation Procedures - Key Formulas, Tables & Specs

1. Installation Conditions & Subgrade Preparation

Note: Larger depressions must be filled or a working table placed.

2. Geotextile Strength Requirements (Table 7)

MARV = Minimum Average Roll Value (average - 2 std dev)

3. Puncture Strength by ASTM D 6241 (Table 8)

4. UV Stability Requirements (Table 10)

IRC SP 116: Underwater Installation of Gabion Structures

Though IRC SP 116 does not provide explicit formulas, the key guidelines for underwater installation of gabions are:

Installation Sequence:

• Site Preparation: Remove loose debris and level the bed.
• Placement: Use crane or barge to lower gabions gently to avoid damage.
• Positioning: Ensure gabions are placed tightly without gaps.
• Anchoring: Use anchoring methods if high flow velocities exist.
• Filling: Fill with stones underwater or pre-filled onshore depending on site conditions.

Key Specifications:

• Gabion Size: Typically 1m x 1m x 0.3m or as per design.
• Wire Mesh: Zinc-coated or PVC coated wire with minimum diameter 2.7 mm.
• Stone Size: 40-80 mm angular stones for filling.
• Overlap: 10-15 cm overlap between gabions for stability.
• Safety Factor: Minimum 1.5 against sliding and overturning.

Stability Check (Simplified):

[ \text{Factor of Safety (FS)} = \frac{\text{Resisting Forces}}{\text{Driving Forces}} \geq 1.5 ]

Where resisting forces include weight of gabion + friction, and driving forces include water flow and uplift.

flowchart TD
    A[Site Preparation] --> B[Lower Gabion]
    B --> C[Position & Align]
    C --> D[Anchor if needed]
    D --> E[Fill with stones]
    E --> F[Check Stability]

This sequence ensures proper underwater gabion installation per IRC SP 116 guidelines.

Handling and Placement of Gabion Units (IRC SP 116)

• Weight of Gabion Box:
  For a 1.5 x 1 x 1 m gabion box:
  [ \text{Weight} = 1.5 \times 16.9 = 25.35 \text{ kN} \approx 2.5 \text{ ton} ]
  (Weight of filled gabion should match calculated weight.)

• Filling & Tie Wire Placement:

  • For 1 m high gabions: Fill to 1/3 height, fix tie wires (Fig. 32). Repeat at 2/3 height.
  • For 0.5 m high gabions: One tie wire row at half height.
  • Filled layer height difference between adjoining cells ≤ 300 mm. Overfill by 25-50 mm to allow settlement (Fig. 33).

• Formwork for Facia:
  Use MS pipe/frame formwork to control bulging and achieve uniform dimension (Fig. 34). Place large flat stones on exposed faces for aesthetics.

• Closing & Joining Gabions:
  Lids folded back and laced sequentially to front, side panels, and diaphragms (Fig. 35). Upper layers connected to lower layers along edges.

• Backfill & Compaction:

  • Place non-woven geotextile on inner face before backfilling (Fig. 36).
  • Structural fill compacted to ≥ 95% modified Proctor density.
  • Use 8-10 ton vibro-roller for main compaction; within 1.5 m of face, use 1-ton vibratory plate or walk-behind rollers.

Summary Table: Tie Wire Placement for Gabion Height

flowchart TD
    A[Start: Gabion Box Placement] --> B[Fill to 1/3 height]
    B --> C[Fix Tie Wires]
    C

Causes of Failure and Quality Control in Gabion Retaining Walls (IRC SP 116)

Causes of Failure (Clause 8.1)

• Improper foundation preparation
• Inadequate compaction or replacement of soil
• Poor quality or size of rock fill
• Incorrect installation sequence
• Insufficient protection against sliding, overturning, bearing failure, or internal failure

Stability Checks & Minimum Factors of Safety (Table 15)

Permissible Shear Stress for Gabion Mattress

[ T_c = K_s \times C_s \times (Y_s - Y_w) \times d_{50} ]

• (T_c): Permissible shear stress
• (C_s = 0.10) (shield for rock-filled revet mattress)
• (K_s): Reduction factor based on slope and soil friction angle
• (Y_s): Unit weight of rock fill (22-26 kN/m³)
• (Y_w): Unit weight of water
• (d_{50}): Median diameter of rock fill

Quality Control Checklist (Clauses 8.4 & 8.5)

• Site preparation and foundation inspection
• Correct sequence of underwater installation
• Proper lifting and placement techniques
• Verification of rock fill size and quality
• Ensuring layer compaction and alignment

flowchart TD
    A[Causes of Failure] --> B[Foundation Issues]
    A --> C[Poor Material Quality]
    A --> D[Improper Installation]
    A --> E[Inadequate Stability Checks]

    F[Quality Control] --> G[Site Preparation]
    F --> H[Material Inspection]
    F --> I[Installation Sequence]
    F --> J[Post-Installation Checks]

Design for Seismic and Surcharge Loads (IRC SP 116)

1. Earth Pressure & Surcharge Load:

• Live load surcharge = Equivalent to 1.2 m earth wall (IRC:6).
• Earth pressure force applied at H/3 from base (Coulomb's theory).
• For gabion walls (flexible), consider only 50% dynamic increment of earth pressure.
• Peak horizontal acceleration coefficient A as per seismic zone (IS 1893).

2. Stability Checks & Factors of Safety:

3. Design Notes:

• Wall top width ≥ 0.5 m, width increases with depth, max increment 1.0 m per step.
• Check all failure modes: overall, sliding, overturning, bearing, internal.
• Seismic design considers inertial effect on earth wedge only; vertical acceleration neglected.

Key Formula for Earth Pressure (Coulomb's Theory):

[ P_a = \frac{1}{2} \gamma H^2 K_a - q K_a H ]

• (P_a) = Active earth pressure
• (\gamma) = Unit weight of soil
• (H) = Height of wall
• (K_a) = Active earth pressure coefficient (from Coulomb’s theory)
• (q) = Uniform surcharge (live load surcharge)

flowchart TD
    A[Seismic Zone] --> B[Select Peak Horizontal Acceleration (A)]
    B --> C[Calculate Dynamic Earth Pressure]
    C --> D[Apply 50% Dynamic Increment]
    D --> E[Combine with Static Earth Pressure]
    E --> F[Check Stability Modes]
    F --> G{Satisfy FOS?}
    G -- Yes --> H[Finalize Design]
    G -- No -->

IRC SP 116 (2018) Key Appendices & Reference Tables Summary

1. Gabion Mattress Thickness vs Water Velocity (Annexure II, Clause 7.2)

• Critical Velocity: Max velocity with no stone movement.
• Limiting Velocity: Velocity causing minor deformation without affecting discharge.

2. Minimum Mattress Thickness by Bank Soil & Velocity

3. Filter Media Geotextile Strength (MARV) (Clause 4.3, Table 7)

| Installation Condition | Grab Strength (N) |