Guidelines for retaining wall for hill area, Part 2: Design of retaining/breast walls

IS 14458 Part 2: Scope & Key Design References

Scope Highlights (Clause 5.4):

• Standard designs (Table 3) apply to retaining walls < 8 m height and < 120 m² area in low hazard zones.
• Applicable only if allowable bearing capacity > max pressure from Table 3.

Key Design Data

Important Notes on Calculations

• Round off results per IS 2:1960 rules, maintaining significant digits as per standard.
• Experimental determination of soil parameters is preferred over preliminary values.

Summary Table Reference (Example)

flowchart TD
    A[Start: Wall Design] --> B{Height < 8m?}
    B -- Yes --> C{Area < 120 m²?}
    C -- Yes --> D{Low hazard zone?}
    D -- Yes --> E[Use Table 3 standard design]
    D -- No --> F[Full design required]
    C -- No --> F
    B -- No --> F
    F --> G[Determine soil parameters (c, φ) experimentally or use Table 2]
    G --> H[Check bearing capacity > max pressure]
    H --> I[Design wall accordingly]

For detailed values, refer to Table 2 (soil parameters) and Table 3 (standard wall pressures) in IS 14458 Part 2.

IS 14458 Part 2 references multiple Indian Standards (listed in Annex A) that form integral provisions of this code. Key points:

• Referenced Standards: Annex A lists all Indian Standards applicable; these are mandatory for compliance.
• Edition Validity: Editions valid at publication time apply, but users should check for the latest versions.
• Rounding Off: Use IS 2:1960 for rounding test or calculation results, matching the significant digits of specified values.

Commonly Referenced IS Codes (Examples)

Rounding Off (IS 2:1960)

• Round values to the same decimal places as the specified value.
• Example: If specified value = 12.345, round test results to 3 decimals.

flowchart LR
    A[IS 14458 Part 2] --> B[Annex A: Referenced IS Codes]
    B --> C[Design Codes (IS 456, IS 875)]
    B --> D[Detailing Codes (IS 13920)]
    B --> E[Seismic Codes (IS 1893)]
    A --> F[IS 2:1960 Rounding Rules]

Always verify the latest editions of referenced standards for compliance.

IS 14458 Part 2: Retaining Structures in Hill Areas

Types of Retaining Structures (Clause 3.1)

• Breast Wall: A low retaining wall, often used to support soil at hill slopes.
• Retaining Wall: Larger structures designed to hold back soil, typically gravity type.

Key Specifications

• Backfill Top: Horizontal with surcharge of 1.5 t/m² (Clause 1.5).
• Foundation: Wall dimensions must ensure allowable bearing capacity > foundation pressure.
• Protection:
  • 300 mm silty soil with boulders layer to prevent rainwater ingress.
  • Pitching at the toe to prevent erosion.
• Slope: Recommended slope ranges from 1:5 to 1:3 for stability.

Design Considerations

• Ensure the bearing capacity of soil supports the wall load.
• Use gravity wall principles: weight of the wall resists lateral earth pressure.
• Backfill should be compacted with proper drainage to reduce hydrostatic pressure.

Typical Gravity Wall Pressure Calculation

[ P = \frac{1}{2} \gamma H^2 K_a + q K_a H ]

Where:

• (P) = lateral earth pressure
• (\gamma) = unit weight of soil
• (H) = height of wall
• (K_a) = active earth pressure coefficient
• (q) = surcharge load (1.5 t/m²)

flowchart TD
    A[Hill Slope] --> B[Breast Wall]
    A --> C[Retaining Wall]
    B --> D[Supports Soil]
    C --> E[Resists Earth Pressure]
    E --> F[Foundation Bearing Capacity Check]
    F --> G[Backfill with Surcharge 1.5 t/m²]
    G --> H[Drainage and Pitching]

Summary: Use breast or retaining walls with proper foundation and backfill, maintain slope 1:5 to 1:3, and protect against water ingress and erosion per IS 14458 Part 2 guidelines.

IS 14458 Part 2: Bearing Capacity Key Points

1. Allowable Bearing Capacity (Clause 4.1)

• Calculated per IS 6403 using soil test data.
• For non-erodible rocks:
  • Tight joints: max bearing = ½ × unconfined compressive strength (qu)
  • Open joints: max bearing = ⅒ × qu
• For weak/closely jointed rocks, assess by visual and lab tests.
• In absence of soil data, use Table 1 values for preliminary design.
• Rocks bearing capacity per IS 12070.
• Use gabion walls for erodible/weak soils to tolerate settlements.

2. Safe Bearing Capacity Values (Table 1)

3. Preliminary Cohesion (c) and Friction Angle (φ) (Clause 4.

IS 14458 Part 2: Design Criteria Key Points

Forces to Consider (Clause 5.1.1)

• Self-weight of retaining structure
• Live and imposed loads (IS 875 Parts 1-5)
• Earth pressure on wall (IS 1893 for seismic)
• Water pressure from water table/seepage (minimum 30% always considered)
• Seismic forces (except low volume roads)
• Special loads, if any

Soil Strength Characteristics (Table 2, Clause 4.3)

Use experimental values or Table 2 for preliminary design.

Bearing Capacity (Clause 4.1 & Table 1)

Refer IS 6403 for detailed bearing capacity calculations.

Earth Pressure & Loads

• Earth pressure per IS 1893.
• Live load on wall top: 250 to 500 kg/m².
• Water pressure: consider ≥30% even if drainage provided.
• Seismic forces as per IS 1893.
• For roads, surcharge per IRC codes.

Summary Formula for Earth Pressure (Rankine's Approximation):

[ P_a = \frac{1}{2} \gamma H^2 K_a - 2c \sqrt{K_a}

IS 14458 Part 2 provides guidelines for the design of retaining and breast walls, focusing on stability and earth pressure reduction.

Key Specifications & Recommendations

• Negative Batter on Breast Wall Backside (Clause 6.4):

  • Recommended negative batter: up to 1:3 (horizontal:vertical)
  • Effect: Significantly reduces earth pressure on cut slopes.
  • Condition: Effective if founded on rock or firm natural ground.

• Design Approach (Clause 5.2):

  • Retaining and breast walls are designed as rigid walls considering earth pressure and stability.

Standard Dimensions & Foundation Pressures (Table 3 Extract)

Similar values apply for Dry Stone Masonry with slight variations.

Common Design Formulae

• Active Earth Pressure (Rankine):
  [ P_a = \frac{1}{2} \gamma H^2 K_a ] Where:
  • ( \gamma ) = Unit weight of backfill soil
  • ( H ) = Height of wall
  • ( K_a

Depth of Walls (IS 14458 Part 2: Clause 6.1 & 6.4)

• Minimum Depth Below Ground/Terrace Level:
  ≥ 500 mm below side drain in soil or highly jointed rock.
  Foundation must rest on natural firm ground or firm rock for multiple breast walls.

• Negative Batter for Breast Walls (Clause 6.4):

  • Recommended negative batter: 1:3 (Horizontal:Vertical) on cut-slope side.
  • Reduces earth pressure, allowing nominal section breast walls to stabilize slopes.

Key Specifications from Table 3 (Clause 5.4)

• Note: Design walls less than 8 m height and 120 m² area can adopt these standard dimensions if bearing capacity > foundation pressure.

Additional Design Notes

• Backfill top: Horizontal with 1.5 t/m² surcharge (Clause 1.5).
• Select wall dimensions so that allowable bearing capacity > foundation pressure.
• Use 300 mm silty soil with boulders layer to prevent rainwater ingress.
• Provide pitching to prevent toe erosion with slope between 1:5 to 1:3.

graph LR
A[Foundation on Firm Ground] --> B[Minimum Depth 500mm below drain]
B --> C[Retaining / Breast Wall]
C --> D[Negative Batter 1:

Stepping of Base of Wall on Rock Slope (IS 14458 Part 2, Clause 6.2)

• Foundation Stepping:
  Foundation on rock slope shall be stepped as per Fig. 2 (not shown here). Steps follow the rock slope contour to ensure stability and reduce sliding.

• Slope Consideration:
  For steep slopes > 35°, use retaining walls with:

  • Front face nearly vertical
  • Back face inclined
    This reduces wall height and earth pressure.

• Backfill & Drainage:
  Use self-draining backfill to avoid hydrostatic pressure buildup.

• Foundation Design:
  Ensure foundation pressure < allowable bearing capacity of rock.
  Typical surcharge on backfill top: 1.5 t/m² (Clause 1.5).

• Slope of Steps:
  Recommended slope of stepping: 1 in 5 to 1 in 3 (Fig. 3).

Key Specifications Summary

Typical Design Considerations (from Table 3)

Dip of the Base of Wall Towards Hillside (IS 14458 Part 2, Clause 6.3)

• Recommended Dip Ratio: 3 (horizontal) : 1 (vertical) towards the hillside.
• Purpose: Improves seismic performance and significantly increases factor of safety against sliding.
• Effect: Economical design by reducing sliding forces and foundation pressure.

Key Specifications:

Design Considerations:

• Ensure allowable bearing capacity > foundation pressure.
• For rock slopes (steep >35°), use stepped foundation (Fig. 2) with vertical front face and inclined back face.
• Use self-draining backfill to avoid hydrostatic pressure buildup.

Visual Concept (Simplified):

graph LR
A[Base of Wall] -->|Dip 3:1 towards hillside| B[Hillside Slope]
B --> C[Backfill (Horizontal + 1.5 t/m² surcharge)]
C --> D[Pitching (1 in 5 to 1 in 3 slope)]
D --> E[300 mm Silty Soil Layer + Boulders]

Summary:
Adopt a 3:1 dip of the base towards hillside for seismic safety and sliding resistance, combined with proper backfill and pitching as per IS 14458 Part 2 guidelines.

Negative Batter of Backside of Breast Wall (IS 14458 Part 2, Clause 6.4)

• Definition: Breast wall with negative batter means the back face (cut-slope side) slopes inward, reducing earth pressure.
• Recommended Batter: Up to 1:3 (horizontal:vertical) — i.e., for every 3 units vertical, 1 unit horizontal inward slope.
• Effect: Significantly reduces earth pressure, allowing even a nominal section to stabilize slopes if founded on firm ground or rock.

Key Specifications from Table 6.4 & Table 3 (Standard Design)

• Backfill types: Good, Fair, Poor; drainage conditions: Full drainage, low/high pore water pressure.
• Wall heights considered: 3m, 6m, 8m, 10m.

Design Notes:

• Ensure breast wall foundation is on rock or firm natural ground.
• Use negative batter to reduce earth pressure and improve stability.
• Refer to Table 3 for exact dimensions based on soil and drainage conditions.

Visual Concept of Negative Batter:

graph LR
    A[Cut Slope]
    B[Breast Wall]
    C[Backfill]
    A -->|Slope| B
    B -->|Negative Batter 1:3| C
    style B fill:#f9f,stroke:#333,stroke-width:2px

Summary:
Use a negative batter of 1:3 on the backside of breast walls to reduce earth pressure, enabling economical design. Refer to IS 14458 Part 2 tables for sizing based on soil, drainage, and wall height.

IS 14458 Part 2 (1997) – Drainage Plan Key Points

1. Drainage Layer & Protection

• Provide a 300 mm layer of silty soil with boulders below or behind retaining walls to prevent rainwater ingress.
• Use pitching (stone masonry) at the toe of the wall to prevent erosion.

2. Slope Specifications

• Adopt slopes between 1:5 to 1:3 for terraces and drainage paths to ensure controlled runoff.

3. Backfill & Surcharge

• Backfill should be horizontal with a surcharge of 1.5 t/m² (Clause 1.5).
• Ensure backfill material is well-drained to reduce hydrostatic pressure.

4. Wall Dimensions & Foundation Pressure

• Select wall dimensions so that allowable bearing capacity > foundation pressure (Clause 1.5).
• Use soil bearing capacity tables from IS 6403 or relevant codes for foundation design.

Typical Drainage Plan Components:

flowchart LR
    Rainwater --> DrainageLayer[300 mm Silty Soil + Boulders]
    DrainageLayer --> DrainageSlope[Slope 1:5 to 1:3]
    DrainageSlope --> ToeProtection[Stone Pitching]
    Backfill[Backfill with surcharge 1.5 t/m²] --> DrainageLayer
    WallDimensions --> FoundationPressure[Allowable bearing capacity > foundation pressure]

Summary: The drainage plan per IS 14458 Part 2 ensures protection against water ingress and erosion by layering silty soil with boulders, maintaining proper slope, and protecting the toe with pitching, while designing walls for safe foundation pressure.

Erosion Control of Toe of Retaining Walls (IS 14458 Part 2:1997)

Key Specifications:

• Toe Protection Layer:

  • 300 mm thick layer of silty soil with boulders to prevent ingress of rainwater.

• Pitching:

  • Provide pitching at the toe to prevent erosion.

• Slope:

  • Toe slope should be between 1:5 to 1:3 (vertical:horizontal).

• Backfill Top:

  • Horizontal backfill with surcharge of 1.5 t/m².

• Bearing Capacity:

  • Select wall dimensions so that allowable bearing capacity > foundation pressure.

Forces to Consider in Design (Clause 5.1.1):

• Self-weight of retaining structure
• Live and imposed loads (250–500 kg/m² on top width)
• Earth pressure (active/passive)
• Water pressure (including toe side)
• Seismic forces (per IS 1893)
• Special loads if applicable

Soil Strength Parameters (Typical Values from Table 2):

Important Formula for Earth Pressure (Rankine's Active Earth Pressure):

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

Where:

• (P_a) = active earth pressure
• (\gamma) = unit weight of soil
• (H) = height of wall
• (c) = cohesion
• (K_a = \tan^2(45^\circ - \frac{\phi}{2})) = active earth pressure coefficient

Toe Erosion Control Summary:

flowchart LR
    A[Rainwater] --> B

IS 14458 Part 2: Safety Factors & Stability Requirements

Key Safety Factors

• Against Sliding:
  • Static loads: > 1.5
  • Earthquake forces: > 1.0
• Against Overturning:
  • Static loads: > 2.0
• Slip Surface (steep hills):
  • Static: > 1.5
  • Seismic: > 1.0

Note: Live and imposed loads aiding stability are excluded from safety factor calculations.

Bearing Capacity

• Maximum base pressure ≤ allowable bearing capacity ( q_a )
• Calculated per IS 6403 based on soil tests.
• For non-erodible rock:
  • Tight joints: ≤ 0.5 × unconfined compressive strength
  • Open joints: ≤ 0.1 × unconfined compressive strength
• For weak/erodible soils, prefer gabion walls.

Table 1: Safe Bearing Capacities (kN/m²)

IS 14458 Part 2 — Load Considerations for Retaining Walls

1. Allowable Bearing Capacity (Clause 4.1)

• Calculated per IS 6403 using soil test data.
• For non-erodible rocks:
  • Tight joints: ≤ 0.5 × unconfined compressive strength.
  • Open joints: ≤ 0.1 × unconfined compressive strength.
• In absence of soil data, use Table 1 (Safe Bearing Capacities):

2. Design Loads (Clause 5.1.1)

• Consider:
  • Self-weight of structure
  • Live and imposed loads (250–500 kg/m² on top width per IS 875)
  • Earth pressure (IRC & IS 1893 for seismic)
  • Water pressure (minimum 30% even with drainage)
  • Seismic forces (if applicable)
  • Special loads

3. Safety Factors (Clause 2.0 & 1.5)

• Against overturning: > 2.0 (static)
• Against sliding: > 1.5 (static), > 1.0 (with earthquake)
• With earthquake: overall factor of safety > 1.5

4. Soil Strength Parameters (Table 2)

IS 14458 Part 2 references several Indian Standards essential for structural design and construction, but the standard itself does not list formulas or tables directly. Instead, it refers to other IS codes for detailed provisions.

Key Points on Referred Indian Standards:

• Annex A of IS 14458 Part 2 lists all referenced IS codes.
• These referenced standards provide design formulas, material specifications, and construction practices.
• Users should consult the latest editions of these IS codes for updated provisions.

Commonly Referred IS Codes in Structural Engineering:

Example: Basic Load Combination (from IS 875)

[ \text{Design Load} = 1.5 \times \text{Dead Load} + 1.5 \times \text{Live Load} ]

Recommendation:

• Always verify the latest edition of each referred IS code.
• Use the tables and formulas within those IS codes for detailed design.

flowchart LR
    A[IS 14458 Part 2] --> B[Annex A: List of Referred IS Codes]
    B --> C[IS 456 - Concrete Design]
    B --> D[IS 800 - Steel Design]
    B --> E[IS 875 - Loads]
    B --> F[IS 1893 - Earthquake Design]
    B --> G[IS 13920 - Ductile Detailing]

Summary: IS 14458 Part 2 acts as a referencing document; detailed formulas and tables are found in the referred IS codes listed in Annex A.