Code of practice for design and construction of foundations for transmission line towers and poles

IS 4091: Scope & Key Specifications Summary

1. Scope (Clause 2.0)

• Defines terms related to under-reamed piles used in foundations.
• Covers design, construction, and load capacities for under-reamed piles in various soil conditions.

2. Key Notes on Design & Construction

• Clear cover for longitudinal bars: 4 cm (may be curtailed near pile toe).
• Steel reinforcement: Required for piles under pull/lateral thrust.
• Overload allowance: 10% overload permitted if pile load capacity is slightly short.
• Safe load for pile groups: Sum of individual pile safe loads.
• Water presence during concreting: Reduce safe load to 75% if boreholes are full of subsoil water.
• Minimum pile diameter in underwater boring: 25 cm.
• Multi-under-reamed piles: Upper bulb center depth ≥ 2 × bulb diameter.
• High load piles: Increase shaft diameter or reinforcement above top bulb.

3. Load & Footing Classification (Table 5.1.6)

4. Safe Load Table for Under-Reamed Piles (Excerpt)

IS 4091: Definitions & Key Specifications

Key Definitions (Clause 2.0 & Notes)

• Longitudinal Bars Cover: Normally 4 cm clear cover; may be curtailed/eliminated near pile toe depending on stress (Note 2).
• Steel for Under-reamed Piles: Provide steel for piles under pull/lateral thrust (Note 3).
• Overload Allowance: 10% overload permitted if pile load is slightly less than required (Note 6).
• Safe Load for Pile Groups: Safe load = individual pile safe load × number of piles (Note 7).
• Water in Bore Holes: Reduce safe load to 75% if bore holes are full of subsoil water during concreting (Note 8).
• Minimum Pile Size Underwater: 25 cm diameter recommended for underwater boring/under-reaming in sandy soils (Note 9).
• Multi-Under-reamed Piles: Upper bulb center depth ≥ 2 × bulb diameter (Note 10).
• High Load Piles: Increase shaft diameter or reinforcement above top under-ream for high loads (Note 13).

Safe Load Table for Vertical Under-reamed Piles (Clause 5.1.10)

IS 4091: Key Information for Design & Construction of Transmission Tower Foundations

1. Information Required (Clause 3.1)

• Route map: Layout of towers with topography and nearby settlements.
• Soil data: Trial borings or pits sections.
• Tower layout: General arrangement details.
• Loads: Magnitude, direction at tower base (normal & broken wire).
• Special info: Wind direction, frost depth, seismic data (per IS 1893).
• Performance review: Similar local structures.
• Max deformation: Allowed at tower base.

2. Concrete & Reinforcement (Clause 4.3)

• Follow IS 456:1978 for materials, mixing, and quality control.
• Use relevant cement and steel specs (IS 269, IS 383, IS 432, IS 1786, etc.).

3. Design Notes (Clause 3.5)

• Longitudinal bars cover: 4 cm clear.
• Steel for piles: Adequate for pull/lateral thrust.
• Load allowance: 10% overload permissible.
• Group piles: Safe load = sum of individual safe loads.
• Water presence: Reduce safe load to 75% if borehole full of subsoil water.
• Pile diameter: Minimum 25 cm in underwater boring.
• Under-reamed piles: Bulb center depth ≥ 2× bulb diameter.
• Broken wire condition: Increase design loads by 50%.

4. Footing Classification & Soil Reaction (Table 5.1.10.2)

5. Safe Load for Under-Reamed Piles (Excerpt from Table 5.1.10)

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IS 4091: Materials - Key Specifications & Tables

Concrete (Clause 4.3)

• Materials, mixing, and quality control per IS 456-1978.
• Cement types per relevant IS standards:
  • Ordinary & Rapid-hardening Portland cement
  • Blast furnace slag cement
  • Portland pozzolana cement
  • Supersulphated cement
  • High strength & rapid-hardening Portland cement

Steel (Clause 4.2)

• Steel types conforming to:
  • Mild and medium tensile steel: IS 432 (Part I & II)-1966
  • Hot rolled mild, medium tensile & high yield strength steel: IS 1139-1966
  • Cold twisted steel bars: IS 1786-1979

Reinforcement Cover

• Longitudinal bars: Clear cover = 4 cm (Clause 3.5 Note 2)

Table: Safe Load for Vertical Under-Reamed Piles (Clause 5.1.10)

IS 4091: General Design Criteria for Footings in Soils

Key Design Criteria (Clause 5.1 & Notes from Clause 3.5)

• Concrete & Reinforcement: Follow IS 456-1978 for concrete materials, mixing, and quality control.
• Reinforcement Cover: Longitudinal bars should have a minimum clear cover of 4 cm.
• Steel Specifications: Use mild/medium tensile steel bars or deformed bars as per relevant IS codes (IS 432, IS 1786).
• Safe Load Calculations:
  • Safe load for a pile group = safe load per pile × number of piles.
  • Reduce safe load to 75% if boreholes contain subsoil water during concreting.
  • Allow 10% overload on piles slightly short of required load.
• Pile Dimensions:
  • Minimum diameter for underwater boring & under-reaming in sandy soils: 25 cm.
  • Depth of upper bulb in multi-under-reamed piles ≥ 2 × bulb diameter.
• Load Increase for Broken Wire Condition: Increase safe load by 50% for transmission tower footings.
• Multi-Under-Reamed Piles:
  • Additional bulbs add 50% of load capacity per bulb.
  • Shaft diameter/reinforcement may be increased for very high loads.

Design Inputs (Clause 3.1)

• Site topography and soil data.
• Tower layout and load details (normal & broken wire conditions).
• Environmental factors (wind, frost, earthquake per IS 1893).
• Maximum allowable base deformation.

Summary Table: Safe Load Adjustments

flowchart TD
    A[Site Investigation] --> B[Soil Data & Topography]
    B --> C[Load Calculation]
    C --> D[Safe Load per Pile]
    D --> E[Adjust for Water in Borehole?]
    E -->|Yes| F[Reduce to 75%]
    E

IS 4091: Details of Foundation Construction for Transmission-Line Towers

Key Specifications & References:

• Scope: Concrete foundations with anchor bolts grouted into rock (Clause 1.1).
• Excluded: Grillage, brick/masonry footings, anchor plates, and prestressed concrete foundations (Clause 1.2).
• Footing Depth: Must comply with relevant IS codes:
  • IS 1080-1980 (General foundations)
  • IS 1904-1978 (Foundation design for soil)
  • IS 2911 (Part I/II/III) - Pile foundations (Clause 5.4.2).

Essential Data for Design (Clause 3.1):

• Route map & tower layout
• Soil investigation (borings/pits)
• Load details (normal & broken wire conditions)
• Environmental data (wind, frost, earthquake per IS 1893-1975)
• Performance of similar structures locally
• Maximum allowable base deformation

Typical Foundation Design Considerations:

• Depth of footing (d): Must exceed frost penetration depth and ensure adequate bearing capacity.
• Anchor bolt embedment length (L): Usually 10-15 times bolt diameter for proper load transfer.
• Concrete grade: As per IS 456, minimum M20 recommended for foundations.
• Safety factors: Follow IS 456 and relevant IS codes for load factors and material strengths.

Summary Table: Foundation Depth Reference

flowchart TD
    A[Start: Foundation Design] --> B[Collect Site Data]
    B --> C[Soil Investigation]
    B --> D[Load & Environmental Data]
    C --> E[Select Foundation Type]
    D --> E
    E --> F[Determine Footing Depth & Size]
    F --> G[Design Anchor Bolts]
    G --> H[Check IS Code Compliance]
    H --> I[Construction & Quality Control]

For detailed design, always cross-check with IS 2911 for piles and

IS 4091: Protective Measures for Foundations — Key Points

1. Footing Depth & Type (Clause 5.4.2 & Table 5.1.10.2)

• Depth per IS 1080, IS 1904, IS 2911 series.
• Footing classes based on load type:

2. Under-Reamed Pile Safe Loads (Clause 5.1.10, Table excerpt)

3. Design Inputs (Clause 3.1)

• Site topography & soil borings.
• Tower layout & load magnitudes (normal & broken wire).
• Environmental data: wind, frost, seismic (per IS 1893).
• Maximum allowable base deformation.

Summary Diagram: Footing Types vs Load

IS 4091 - Special Considerations for Seismic Zones

1. Seismic Footing Design

   • Clause 5.7.1: Footings in seismic zones must comply with IS 1893-1975 provisions for earthquake-resistant design.
   • Key factors: lateral forces, overturning moments, uplift, and soil-structure interaction under seismic loads.

2. Footing Classification (Table 5.1.10.2)

   Class	Load Type	Structure Type	Recommended Footing Type	Soil Reaction Type
   A	Heavy uplift, light shear	Wide base towers	Enlarged (under-reamed) base	Weight of earth on enlarged base
   B	Heavy overturning moments	Poles/columns with narrow footings	With/without enlarged base or piles	Lateral resistance + soil pressure on base
   C	Heavy downward load	Heavy electrical equipment	Enlarged base, under-reamed, or pile groups	Allowable soil pressure + shaft resistance

3. Under-Reamed Pile Safe Loads (Clause 5.1.10)

   • Diameter, reinforcement, bearing, uplift, and lateral thrust capacities are tabulated.
   • Example for 30 cm pile:
     • Bearing load (single under-reamed): 16 t
     • Uplift resistance: 8 t
     • Lateral thrust: 2.0 t

4. Design Inputs (Clause 3.1)

   • Soil data, load magnitudes (normal & broken wire), seismic data per IS 1893, wind direction, frost depth.
   • Layout and deformation limits must be considered.

Summary Diagram: Footing Types vs Load & Soil Reaction

graph TD
    A[Heavy uplift, light shear] -->|Wide base towers| B[Enlarged/Under-reamed base]
    C[Heavy overturning moments] -->|Poles/Columns narrow footing| D[With/without enlarged base or piles]
    E[Heavy downward load] -->|Heavy equipment| F[Enlarged base or pile groups]
    B --> G[Weight of earth on base]
    D --> H[Lateral

Design Modifications for Alignment Deviations (IS 4091)

Key Points from Clause 5.7.6:

• Deviations up to 2°: No special design modifications needed.
• Deviations > 2°: Foundation design must be modified to accommodate increased moments and forces due to misalignment.

Design Considerations:

• Increased bending moments and lateral forces on foundations.
• Reinforcement and pile design must be checked for additional stresses.
• For under-reamed piles, provide adequate longitudinal and ring reinforcement (Clause 5.1.10 & Notes).

Safe Load & Reinforcement Table for Under-Reamed Piles (Clause 5.1.10):

Additional Specifications:

• Increase pile shaft diameter or reinforcement for high loads (Note 13).
• Allow 10% overload on piles close to required load (Note 6).
• For multiple bulbs, increase capacity by 50% per additional bulb (Note 12).
• Use 75% safe load if borehole is full of water during concre

IS 4091 - Concreting and Quality Control Key Points

1. Concrete Materials & Quality Control (Clause 4.3 & 4.1)

• Concrete materials, mixing, and quality control follow IS 456:1978.
• Cement types allowed:
  • Ordinary & Rapid-hardening Portland Cement (IS 269, IS 8041)
  • Blast Furnace Slag Cement (IS 455)
  • Portland Pozzolana Cement (IS 1489)
  • Supersulphated Cement (IS 6909)
  • High Strength Ordinary Portland Cement (IS 8112)

2. Concreting Practice (Clause 5.8)

• Follow IS 456:1978 for concreting methods.
• Clear cover for longitudinal bars: 4 cm minimum.
• Reinforcement may be curtailed/eliminated near pile toe based on stress.
• For underwater concreting by tremie in 25-30 cm piles, use equivalent reinforcement (single angle iron centrally).
• Allow 10% overload on piles close to design load.
• Safe load for pile groups = safe load per pile × number of piles.
• Reduce safe load by 25% if boreholes are full of subsoil water during concreting.
• Minimum pile diameter for underwater boring/under-reaming in sandy soils: 25 cm.

3. Safe Load Adjustments

4. Reinforcement Specifications

• Follow IS codes for steel bars:
  • Mild & medium tensile steel bars (IS 432 Part I)
  • Cold twisted steel bars (IS 1786)
  • Hot rolled deformed bars (IS 1786 revised)

Summary Diagram: Concreting Quality Control Flow

flowchart TD
    A[Material Selection] --> B[Mixing as per IS 456]
    B --> C[Reinforcement Placement]
    C --> D[Concreting Method]
    D --> E[Quality Checks]
    E --> F[Load Testing & Adjustments]

References:

• IS

IS 4091: Safety and Structural Stability - Key Points

Structural Safety (Clause 5.4)

• Safety against sliding, overturning, and footing stability follows IS 1904-1978.
• Longitudinal bars: 4 cm clear cover; may be curtailed near the toe based on stresses.
• For under-reamed piles under pull/lateral thrust, adequate steel reinforcement is mandatory.
• Load allowance: 10% overload permitted if pile capacity is slightly short.
• For pile groups, safe load = sum of individual safe loads.
• Reduce safe load to 75% if boreholes are full of water during concreting.
• Minimum pile diameter for underwater boring in sandy soils: 25 cm.
• Multi-under-reamed piles: upper bulb center depth ≥ 2 × bulb diameter.
• Increase capacity by 50% for broken wire condition (transmission towers).
• For multi-bulb piles, add 50% of single bulb load for each extra bulb.

Table: Safe Loads for Vertical Under-Reamed Piles (Clause 5.1.10)

IS 4091: References to Related Standards - Key Formulas, Tables & Specs

Related Standards Reference:

• Rounding off values as per IS 2:1960.
• Reinforcement cover: minimum 4 cm clear cover for longitudinal bars.
• Overload allowance: 10% on piles slightly short of required load.
• Safe load for pile groups = safe load per pile × number of piles.
• Water presence during concreting reduces safe load to 75% if borehole is full of subsoil water.

Important Tables & Specifications

Table 1: Loading & Footing Classification (Clause 5.1.6)

Table 2: Safe Loads for Vertical Under-reamed Piles (Clause 5.1.10)