Guidelines for retaining wall for hill area, Part 1: Selection of type of wall

IS 14458 Part 1: Scope & Key Specifications for Retaining Walls

This part of IS 14458 provides guidelines for selecting retaining walls to stabilize hill slopes based on site conditions.

Key Points:

• Purpose: Selection of retaining walls for slope stability in hill areas.
• Design Basis: Assume surcharge of 2 T/m² on roads.
• Wall Types Covered: Timber Crib, Dry Stone, Banded Dry Stone/Masonry, Cement Masonry, Gabion, Reinforced Earth, Breast/Revetment Walls.
• Slope Angles: Suitable walls vary by hill slope angle (e.g., Cement Masonry & Gabion for 30°–60°).
• Foundation: Step foundations if rock encountered; 0.5–1 m depth below drain; inward dip varies (1:3 to horizontal).
• Drainage: Provide 15 cm gravel layer for clayey foundations.
• Backfill: Use granular backfill compacted in layers (<15 cm).
• Durability & Flexibility:
  • Timber Crib: Least durable, ecologically unacceptable.
  • Reinforced Earth: Very flexible, suitable for large height (3–25 m).
  • Cement Masonry/Gabion: Durable, suitable for high walls.

Table: Selection of Retaining Walls (Summary)

Classification of Retaining Walls (IS 14458 Part 1)

Types of Retaining Walls (Clause 2.1 & Fig. 1)

• Gravity walls: Rely on self-weight to resist earth pressure.
• Tie back walls: Anchored by earth anchors in backfill.
• Driven cantilever walls: Piles driven into ground, cantilever action.
• Reinforced earth walls: Soil reinforced with geogrids or straps.

Selection Guidelines (Clause 3.1.14 & Tables)

IS 14458 Part 1: Selection of Type of Walls - Key Points

1. Retaining Walls Selection (Table 1, Clause 3.1.14)

IS 14458 Part 1: Design Considerations for Retaining Walls in Hill Areas

Key Points from the Code and Engineering Practice:

• Earthquake Considerations (Clause 3.1.2):

  • Avoid excessively high retaining walls by using alternative road/terrace geometries.
  • Design walls with a base dip towards the hillside to reduce base width and improve seismic stability.

• Economic and Stability Considerations:

  • Cutting into the hill is often more economical than adding fill with retaining walls.
  • Breast walls are costlier but necessary for long-term slope stability and environmental protection.

Design Guidelines Summary:

Typical Design Formulae:

• Earth Pressure (Seismic):
  [ P = K_a \gamma H + K_e \gamma H ]
  Where:

  • (K_a) = active earth pressure coefficient
  • (K_e) = seismic earth pressure coefficient (from IS 1893)
  • (\gamma) = unit weight of soil
  • (H) = height of wall

• Stability Checks:

  • Sliding, overturning, and bearing capacity as per IS 456 and IS 1893.

flowchart LR
    A[Hill Slope] --> B[Cut Slope]
    B --> C[Retaining Wall]
    C --> D[Base Dip Towards Hill]
    C --> E[Reduced Base Width]
    C --> F[Seismic Load Consideration]
    F --> G[Use IS 1893 for Earthquake Coefficients]

Summary: Use geometric design to minimize wall height, dip the base towards the hill to reduce base width, and apply seismic earth pressure coefficients as per IS 1893 for safe and economical retaining wall design in hill areas.

IS 14458 Part 1: Construction Guidelines for Various Wall Types

Key Specifications from Table 1 (Retaining Walls)

Drainage and Backfill Requirements (IS 14458 Part 1)

Key Points from Clause 3.1.6:

• Proper drainage behind retaining walls is essential to avoid hydrostatic pressure.
• Improper backfill and drainage lead to complex conditions not covered in normal design.
• Provide adequate drainage layers (e.g., gravel or rubble) and weep holes.

Backfill Specifications:

• Use granular backfill compacted in layers ≤ 15 cm for good drainage.
• Avoid weathered or fine-grained soils behind walls.
• For gabion walls, compact granular backfill and specify max/min stone sizes.
• For cement masonry walls, provide 50 cm rubble backing for drainage.

Drainage Provisions:

• Weep holes: Typically 15 cm × 15 cm at 1–2 m c/c (center to center).
• Drainage layer: Minimum 0.5 m thickness below foundation.
• For seepage problems, provide a drainage layer or geosynthetic filters.

Typical Backfill and Drainage Setup (Example for Cement Masonry Wall):

Summary Diagram (Drainage Behind Retaining Wall)

flowchart LR
    Wall[Retaining Wall]
    Backfill[Granular Backfill]
    DrainLayer[Drainage Layer (Gravel/Rubble)]
    WeepHoles[Weep Holes @ 1-2 m c/c]
    Foundation[Foundation]

    Wall --> Backfill --> DrainLayer --> Foundation
    Backfill --> WeepHoles

References:

• Clause 3.1.6 (Drainage importance)
• Table 1 (Backfill and foundation depth)
• General practice: Use well-graded granular backfill and provide drainage to relieve hydrostatic pressure.

IS 14458 Part 1 – Seismic & Earthquake Considerations for Retaining Walls

Key Points from Clauses:

• Clause 3.1.2:

  • High walls in seismic zones lead to excessive dimensions; prefer alternative geometry (roads/terraces).
  • Base dip towards hillside reduces base width, affecting seismic stability.

• Clause 3.1.14 & Table 1:

  • Suitable walls for hill slopes (30°-60°): Cement masonry, RCC, Gabion walls for high walls & volumes.
  • Foundation dip, batter slopes, and base widths vary by wall type for seismic safety.

Table 1: Selection of Retaining Walls (Summary)

IS 14458 Part 1 focuses on retaining walls for hill areas with environmental and ecological considerations embedded in design guidelines rather than explicit formulas.

Key Specifications & Environmental Aspects:

• Wall Types & Materials:

  • Pack stone with bond stones; minimum stone size specified.
  • Cement masonry bands (1:6 mix), 0.5 m thick at 3 m c/c.
  • Weep holes: 15 x 15 cm at 1.5–2 m c/c with 1:10 grading for drainage.
  • Step in front face: 20–50 cm wide.
  • Vertical single drum walls up to 0.7 m height; anchor on sides.
  • Revetment walls: uniform thickness 0.5–0.75 m with batter 2:1 or steeper.

• Environmental Measures:

  • Use gabion walls for poor foundation/seepage; flexible to settlements.
  • Implement check drains, turfing, benching, crack sealing in one season.
  • Curing difficult due to water scarcity; design assumes well-drained backfill.
  • Detailed design required for walls >6 m or poor foundations.
  • Preventive measures collectively more effective than single solutions.

Summary Table: Wall Selection (Clause 2.2)

Drainage & Stability Formula (General Guidance):

• Weep hole spacing:
  [ S = 1.5 \text{ to } 2.0, m ]
• Weep hole size:
  [ 15 \times 15, cm ]
• Backfill grading: 1:10 slope for drainage.

Conceptual Diagram: Retaining Wall with Environmental Measures

flowchart TD
    A[Retaining Wall] --> B[Backfill with good drainage]
    A --> C[Weep Holes (15x15 cm @1.5-2m)]
    A --> D[Gabion Wall

Handling Excavated Material (IS 14458 Part 1)

Clause 3.1.7 Summary:

• Excavated material from hill cutting during retaining wall construction must be disposed of at suitable identified sites.
• Avoid loss of topsoil; preserve it for vegetation and ecological balance.

Key Specifications for Retaining Walls (Clause 3.1.14 & Table 1)

IS 14458 Part 1: Limitations & Special Cases for Retaining Walls

Key Specifications from Clause 3.1.14 & Tables

Limitations & Special Cases

• Hill slope angle suitability: Cement masonry, RCC, and Gabion walls are preferred for slopes between 30° to 60°.
• Foundation: Stepped foundations recommended if rock encountered; minimum drainage layer of 15 cm gravel for clayey soils.
• Earthquake Considerations: Non-ductile walls (e.g., dry stone) are more susceptible to seismic damage; reinforced earth walls are flexible and better for seismic zones.
• Surcharge: Assume 2 T/m² surcharge for road load in design.
• **

IS 14458 Part 1: Annex A - Committee Composition & Key Tables for Retaining Walls

Annex A: Committee Composition (CED 56)

• Chairman: Dr. Gopal Ranjan (University of Roorkee)
• Members: Experts from PWD Jammu & Kashmir, Indian Institute of Remote Sensing, National Buildings Construction Corporation, Central Water Commission, IIT Delhi, IRC, Central Building Research Institute, Geological Survey of India, Ministry of Railways, and others.
• Includes alternates for most members ensuring broad technical representation.

Key Table: Selection of Retaining Walls (Clause 3.1)