Types of Bridge Foundations and Design Criteria

Introduction

Bridge foundations play a crucial role in bridge engineering as they provide the necessary support and stability to the structure. The design of bridge foundations involves considering various factors such as the type of soil, load capacity, settlement analysis, and stability against overturning and sliding. In this topic, we will explore the different types of bridge foundations and their general design criteria.

Types of Bridge Foundations and General Design Criteria

Shallow Foundations

Shallow foundations are typically used when the soil near the surface has sufficient bearing capacity to support the bridge structure. There are two main types of shallow foundations:

1. Spread Footings

Spread footings, also known as pad footings, are used to distribute the load of the bridge over a larger area of soil. They are commonly used for smaller bridges and structures.

1. Mat Foundations

Mat foundations, also known as raft foundations, are used when the soil has low bearing capacity or when the loads from the bridge are unevenly distributed. They provide a larger contact area with the soil, reducing the stress on the underlying soil.

The design criteria for shallow foundations include:

• Soil bearing capacity: The soil must have sufficient strength to support the loads from the bridge.
• Settlement analysis: The settlement of the foundation should be within acceptable limits to prevent structural damage.
• Stability against overturning and sliding: The foundation must be stable and resistant to lateral movement.

Deep Foundations

Deep foundations are used when the soil near the surface is not suitable for supporting the bridge structure. They transfer the load of the bridge to deeper, more competent layers of soil or rock. The two main types of deep foundations are:

1. Piles

Piles are long, slender structural elements that are driven or drilled into the ground. They transfer the load of the bridge through skin friction and/or end bearing. Piles can be made of various materials such as concrete, steel, or timber.

1. Wells and Pneumatic Caissons

Wells and pneumatic caissons are large, watertight structures that are sunk into the ground. They are commonly used in situations where the soil is below the water table. Wells are open at the bottom, while pneumatic caissons are sealed and pressurized during construction.

The design criteria for deep foundations include:

• Load capacity analysis: The capacity of the deep foundation to support the loads from the bridge must be determined.
• Settlement analysis: The settlement of the foundation should be within acceptable limits to prevent structural damage.
• Lateral stability analysis: The deep foundation must be stable and resistant to lateral movement.

Piles

Piles are a type of deep foundation that are commonly used in bridge construction. They are long, slender structural elements that are driven or drilled into the ground. Piles transfer the load of the bridge through skin friction along the sides of the pile and/or end bearing at the pile tip.

There are three main types of piles:

1. Driven Piles

Driven piles are installed by hammering them into the ground using a pile driver. They can be made of various materials such as concrete, steel, or timber. Driven piles are commonly used in situations where the soil is suitable for driving.

1. Bored Piles

Bored piles are installed by drilling a hole into the ground and then placing reinforcement and concrete into the hole. They are commonly used in situations where the soil is too hard or too dense for driving.

1. Screw Piles

Screw piles, also known as helical piles, are installed by rotating them into the ground. They have helical blades that help to screw the pile into the soil. Screw piles are commonly used in situations where the soil is soft or where there are environmental restrictions on noise and vibration.

The design criteria for piles include:

• Load capacity analysis: The capacity of the pile to support the loads from the bridge must be determined.
• Settlement analysis: The settlement of the pile should be within acceptable limits to prevent structural damage.
• Lateral stability analysis: The pile must be stable and resistant to lateral movement.

Wells and Pneumatic Caissons

Wells and pneumatic caissons are deep foundations that are commonly used in situations where the soil is below the water table. They are large, watertight structures that are sunk into the ground.

There are three main types of wells and pneumatic caissons:

1. Open Caissons

Open caissons are large, box-like structures that are open at the bottom. They are sunk into the ground by excavating the soil from within the caisson. Open caissons are commonly used in situations where the soil is stable and self-supporting.

1. Box Caissons

Box caissons are similar to open caissons, but they are closed at the bottom. They are sunk into the ground by excavating the soil from within the caisson. Box caissons are commonly used in situations where the soil is soft or where there is a high water table.

1. Pneumatic Caissons

Pneumatic caissons are similar to box caissons, but they are sealed and pressurized during construction. The pressurized air inside the caisson prevents water and soil from entering the working chamber. Pneumatic caissons are commonly used in situations where the soil is below the water table and where the excavation depth is significant.

The design criteria for wells and pneumatic caissons include:

• Load capacity analysis: The capacity of the well or caisson to support the loads from the bridge must be determined.
• Settlement analysis: The settlement of the well or caisson should be within acceptable limits to prevent structural damage.
• Lateral stability analysis: The well or caisson must be stable and resistant to lateral movement.

River Training Works

River training works are structures that are built to control and manage the flow of rivers. They are often used in bridge engineering to protect the bridge from erosion and to maintain a stable river channel.

There are three main types of river training works:

1. Groynes

Groynes are structures that are built perpendicular to the riverbank. They are used to trap sediment and prevent it from being carried away by the river. Groynes help to stabilize the river channel and reduce erosion.

1. Spur Dikes

Spur dikes are structures that are built diagonally across the river. They are used to redirect the flow of water and prevent erosion along the riverbank. Spur dikes help to maintain a stable river channel and protect the bridge from scour.

1. River Training Walls

River training walls are structures that are built parallel to the riverbank. They are used to guide the flow of water and prevent erosion along the riverbank. River training walls help to maintain a stable river channel and protect the bridge from scour.

The design criteria for river training works include:

• Hydraulic analysis: The flow of water in the river must be analyzed to determine the design parameters for the training works.
• Stability analysis: The stability of the training works must be analyzed to ensure they can withstand the forces exerted by the river.
• Scour protection measures: Measures must be taken to protect the bridge foundations from scour, which is the erosion of soil around the foundations caused by the flow of water.

Real-world Applications and Examples

To better understand the different types of bridge foundations and their design criteria, let's look at some real-world applications and examples.

• Example 1: Bridge A

Bridge A is a small pedestrian bridge that spans a river. It has shallow spread footings as its foundation. The design criteria for the shallow foundations of Bridge A include analyzing the soil bearing capacity, settlement analysis, and stability against overturning and sliding.

• Example 2: Bridge B

Bridge B is a large highway bridge that spans a deep river. It has deep pile foundations. The design criteria for the pile foundations of Bridge B include analyzing the load capacity, settlement analysis, and lateral stability.

• Example 3: Bridge C

Bridge C is a bridge that spans a river with a high water table. It has pneumatic caissons as its foundation. The design criteria for the caisson foundations of Bridge C include analyzing the load capacity, settlement analysis, and lateral stability.

Advantages and Disadvantages of Different Types of Bridge Foundations

Each type of bridge foundation has its own advantages and disadvantages. Let's explore them:

• Advantages of Shallow Foundations:

  • Shallow foundations are generally less expensive and easier to construct compared to deep foundations.
  • They are suitable for sites with stable soil near the surface.
  • Shallow foundations have a smaller environmental impact as they require less excavation.

• Advantages of Deep Foundations:

  • Deep foundations can support heavier loads and are suitable for sites with weak or unstable soil near the surface.
  • They can be used in areas with high water tables or where the soil is below the water table.
  • Deep foundations can be installed in a variety of soil conditions.

• Disadvantages of Shallow Foundations:

  • Shallow foundations may not be suitable for sites with weak or unstable soil near the surface.
  • They have limited load-bearing capacity compared to deep foundations.
  • Shallow foundations may be more susceptible to settlement and lateral movement.

• Disadvantages of Deep Foundations:

  • Deep foundations are generally more expensive and time-consuming to construct compared to shallow foundations.
  • They may require specialized equipment and expertise for installation.
  • Deep foundations can have a larger environmental impact due to the excavation and construction processes.

Conclusion

In conclusion, bridge foundations are an essential component of bridge engineering. The design of bridge foundations involves considering various factors such as soil characteristics, load capacity, settlement analysis, and stability. Shallow foundations, including spread footings and mat foundations, are used when the soil near the surface has sufficient bearing capacity. Deep foundations, including piles and wells/pneumatic caissons, are used when the soil near the surface is not suitable for supporting the bridge structure. River training works, such as groynes, spur dikes, and river training walls, are used to control and manage the flow of rivers and protect the bridge from erosion. Each type of bridge foundation has its own advantages and disadvantages, and the choice of foundation type depends on the site-specific conditions and requirements.

Summary

Bridge foundations are crucial for providing support and stability to bridge structures. Shallow foundations, such as spread footings and mat foundations, are used when the soil near the surface has sufficient bearing capacity. Deep foundations, such as piles and wells/pneumatic caissons, are used when the soil near the surface is not suitable for supporting the bridge structure. River training works, such as groynes, spur dikes, and river training walls, are used to control and manage the flow of rivers and protect the bridge from erosion. Each type of bridge foundation has its own advantages and disadvantages, and the choice of foundation type depends on the site-specific conditions and requirements.

Analogy

Building a bridge is like building a house. The foundation of the house is crucial for providing support and stability to the structure. Similarly, the foundation of a bridge is essential for supporting the bridge and ensuring its structural integrity. Just as different types of foundations are used for different types of houses, different types of bridge foundations are used based on the site-specific conditions and requirements.