Shallow Foundations for Overhead Transmission Line Towers (Pad & Chimney Foundations)

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Abstract

Shallow foundations are one of the most commonly adopted foundation systems for Overhead Transmission Line (OHTL) towers, particularly in areas where soil bearing capacity is adequate and uplift forces are within manageable limits. Among shallow foundations, Pad & Chimney Foundations are widely used for suspension and light angle towers due to their simplicity, cost-effectiveness, and reliable performance.
This article presents a comprehensive technical discussion on Pad & Chimney Foundations, covering design philosophy, load transfer mechanisms, geotechnical and structural verification, construction practices, limitations, and a practical case study based on internationally recognized standards.

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1. Introduction

In overhead transmission line projects, the foundation system plays a critical role in ensuring the long-term safety and stability of transmission towers. Shallow foundations are preferred when competent soil layers are available near ground level and excessive settlement or shear failure can be avoided.
Pad & Chimney Foundations, a widely used type of shallow foundation for OHTL towers, provide an efficient solution where vertical, horizontal, and uplift loads can be safely transferred to the soil.

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2. What Are Shallow Foundations in OHTL Projects?

A shallow foundation is defined as a foundation where the depth of embedment is relatively small compared to its width. In OHTL applications, shallow foundations are commonly used when:

• Soil bearing capacity is sufficient at shallow depth
• Groundwater table is relatively low
• Tower loads are moderate
• Construction speed and economy are important

Common shallow foundation types for transmission towers include:

• Pad foundations
• Pad & Chimney foundations
• Combined footings (for special cases)

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3. Pad & Chimney Foundation – General Description

A Pad & Chimney Foundation is a specific type of shallow foundation used extensively for lattice steel transmission towers.

Main Components

• Concrete Pad (Footing):
  Distributes vertical and horizontal loads from the tower leg to the supporting soil.
• Chimney (Pedestal):
  A vertical concrete member that connects the pad to the tower leg, providing required elevation and embedment.

Each tower leg is typically supported by an individual isolated foundation, although combined foundations may be used depending on tower geometry and loading.

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4. Structural Design Philosophy

The design of Pad & Chimney Foundations is based on the following key principles:

• Soil contact pressure must remain within allowable bearing capacity
• Adequate safety against sliding and overturning
• Control of total and differential settlement
• Sufficient resistance against uplift forces caused by conductor tension and wind loads

All checks are performed using appropriate load combinations and safety factors as specified in relevant standards.

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5. Loading Conditions and Load Combinations

The following loads are considered in shallow foundation design for OHTL towers:

• Vertical Loads:
  Self-weight of tower, conductors, insulators, and accessories
• Horizontal Loads:
  Conductor tension and wind loads acting on tower and conductors
• Uplift Loads:
  Particularly critical for angle, tension, and dead-end towers

Typical design cases include:

• Normal operating condition
• Broken wire condition
• Extreme wind condition

These are evaluated in accordance with IEC 60826, IEEE 691, and EN 50341.

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6. Geotechnical Design Considerations

6.1 Bearing Capacity

Soil bearing capacity is determined using analytical methods or in-situ tests such as SPT and CPT. A conservative margin is maintained by ensuring working stresses are significantly below the allowable bearing pressure.

6.2 Settlement Control

Both total and differential settlements are checked. Excessive settlement can introduce secondary stresses in the tower structure and must be limited within acceptable values for transmission lines.

6.3 Uplift Resistance

Uplift resistance in Pad & Chimney Foundations is achieved through:

• Self-weight of concrete foundation
• Weight of overlying backfill soil
• Side friction along the chimney
• Adequate embedment depth

IFC DRAWING FOR FOUNDATION DETAILS FOR SUSPENSION TOWER TYPE USA

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7. Construction Considerations

Proper construction practices are essential for the performance of shallow foundations:

• Excavation to the specified founding level without disturbing natural soil
• Protection of excavation from water ingress
• Well-sealed formwork to prevent cement slurry loss
• Compliance with concrete mix design, compaction, and curing requirements
• Accurate alignment and leveling of tower legs or anchor bolts

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8. Advantages and Limitations

8.1 Advantages

• Simple and fast construction
• Economical compared to deep foundations
• Easy inspection and quality control
• Widely applicable in favorable soil conditions

8.2 Limitations

• Not suitable for weak or highly compressible soils
• Limited resistance to very high uplift forces
• Performance may be affected by high groundwater levels

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9. Practical Case Study

9.1 Project Overview

A 400 kV overhead transmission line project was constructed in an area with medium-dense sandy soil. Pad & Chimney Foundations were selected for suspension and light angle towers.

9.2 Geotechnical Conditions

• Allowable bearing capacity: 250–300 kN/m²
• Groundwater depth: greater than 6.0 m

9.3 Design Inputs

• Tower type: Lattice steel suspension tower
• Vertical load per leg: ~ 850 kN
• Horizontal load: ~ 220 kN
• Maximum uplift load: ~ 180 kN
• Embedment depth: 2.5 m

9.4 Adopted Foundation System

Each tower leg was supported by:

• Square concrete pad: 3.5 m × 3.5 m × 0.9 m
• Chimney height: 1.2 m

9.5 Design Verification

• Bearing pressure < 60% of allowable capacity
• Total settlement < 15 mm
• Sliding and overturning safety factor > 1.5
• Uplift safety factor > 2.0

9.6 Construction and Performance

Construction was carried out under strict supervision. Concrete strength tests at 7 and 28 days met all design requirements, confirming the suitability of shallow foundations for the site.

IFC DRAWING FOR FOUNDATION DETAILS FOR SUSPENSION TOWER TYPE UAT

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10. Conclusion

Pad & Chimney Foundations are a reliable and economical shallow foundation solution for OHTL towers when soil conditions are favorable. Their successful application depends on accurate geotechnical investigation, proper load assessment, rigorous design checks, and strict construction quality control. When designed and executed correctly, they ensure long-term safety and performance of overhead transmission line systems.

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11. References

• IEC 60826 – Design criteria of overhead transmission lines
• IEEE Std 691 – Guide for Transmission Structure Foundation Design
• EN 50341 – Overhead electrical lines exceeding AC 1 kV
• Bowles, J.E. Foundation Analysis and Design
• Das, B.M. Principles of Foundation Engineering

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