Guidelines for Design of Causeways and Submersible Bridges as per IRC Code: SP:82-2008

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Causeways and submersible bridges play a crucial role in providing connectivity over seasonal rivers and streams where constructing high-level bridges may not be economically feasible. The Indian Roads Congress (IRC) has laid out specific guidelines for designing these structures in IRC:SP:82-2008 - Guidelines for Design of Causeways and Submersible Bridges. These guidelines ensure cost-effective and sustainable solutions for road infrastructure in flood-prone regions.

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Scope of IRC:SP:82-2008

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The guidelines in IRC:SP:82-2008 cover the planning, design, and construction of causeways and submersible bridges on various categories of roads, including:

• State Highways (SH)
• Major District Roads (MDR)
• Other District Roads (ODR)
• Village Roads (VR)

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Key Definitions

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Before diving into design considerations, let's understand some critical definitions:

• Bridge: A structure spanning more than 6m to carry traffic over an obstruction.
• High-Level Bridge: A bridge that remains above the highest flood level (HFL).
• Submersible Bridge: A bridge designed to be overtopped during floods but retains structural stability.
• Causeway: A paved submersible structure that allows floodwaters to pass over or through it.
• Ford: A shallow, unpaved river crossing used in dry conditions.
• Afflux: The rise in upstream water levels due to the obstruction caused by the structure.

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Design Considerations for Causeways and Submersible Bridges

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1. Hydrology and Hydraulics

The hydrological study should include:

• Catchment area and rainfall data
• Estimation of design flood levels
• Determination of scour depth
• Impact assessment of afflux on upstream and downstream areas

2. Waterway and Deck Level Fixation

• Waterway Calculation: The total waterway should ensure the safe passage of floods with minimal obstruction.
• Deck Level Fixation:
  • The deck level should allow limited overtopping during high floods.
  • A clearance of 200mm is maintained for submersible bridges to allow vehicle movement during ordinary flood levels (OFL).
  • The deck should not be higher than approach roads to prevent breaches.

3. Structural Design

The design of submersible bridges must account for:

• Foundation Types: Raft, well, or pile foundations based on soil conditions.
• Superstructure Type: RCC slabs, arch structures, or vented causeways depending on site conditions.
• Span Arrangement: The number of spans should be minimized to reduce obstructions and maintenance requirements.

4. Safety Features and Protection Works

• Scour Protection: Aprons, launching aprons, and cut-off walls should be used to prevent scouring.
• Warning Signs: Flood gauges and cautionary signs should be installed at appropriate locations.
• Anchorage Systems: Deck slabs must be anchored to resist water currents.

5. Geometric Standards

• Carriageway Width:
  • Single Lane: 6.8m (Plain/Rolling Terrain), 5.5m (Mountainous Terrain)
  • Two Lanes: 7.5m in all terrains
• Approach Road Alignment:
  • A minimum straight approach of 30m before the structure.
  • Horizontal curves should have a minimum radius of 60m in plains and 30m in hilly areas.
• Design Speed:
  • Recommended at 35 km/h for approach roads.

6. Construction and Maintenance

• Materials Used: Concrete, stone masonry, RCC pipes for vented causeways.
• Periodic Maintenance: Regular desilting of vents, repairing embankment erosion, and maintaining protection works.
• Debris Management: Provision of debris arresters to prevent clogging.

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Conclusion

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The IRC:SP:82-2008 provides a comprehensive framework for designing causeways and submersible bridges, ensuring safety, cost-effectiveness, and durability. These guidelines are essential for engineers and policymakers working on rural and urban road connectivity projects. By adhering to these standards, we can ensure reliable infrastructure that withstands seasonal floods while minimizing maintenance costs.

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