Building structures in the ocean, such as bridge supports or large platforms, requires specialized techniques to create stable foundations in challenging environments. One prominent method for installing structural piers, especially for bridges, is the caisson method.
The Caisson Method for Structural Piers
As noted in engineering practices, piers for bridges are often installed by the caisson method. This technique allows constructors to create a robust foundation deep within the seabed.
What is a Caisson?
A caisson is essentially a hollow boxlike structure. Made from materials like steel or reinforced concrete, it acts as a watertight chamber that can be progressively lowered into the water and through the seafloor.
How the Caisson Method Works
The process involves several key steps:
- Placement: The caisson is typically floated to the desired location or constructed in place above the water.
- Sinking: The structure is gradually sunk downwards through the water.
- Penetration: It continues to sink through the ground beneath the water. This sinking is achieved by excavating material (like soil, mud, or rock) from its interior. Workers or machinery operate inside the caisson, removing the material from the seabed below.
- Anchoring: As material is removed from the bottom, the weight of the caisson (and potentially added weight) drives it deeper until it reaches a suitable bearing stratum – a layer of strong, stable ground capable of supporting the pier's load.
- Permanent Integration: Once the caisson is firmly seated on the bearing stratum, its interior is usually filled with concrete. The structure then ultimately becomes a permanent part of the completed pier, forming the solid base upon which the upper structure (like a bridge deck or platform) is built.
Why Use This Method?
The caisson method is particularly valuable for constructing large, heavy piers in deep water or in locations with soft or unstable seabed conditions. It provides a dry or semi-dry working environment below the waterline, allowing for controlled excavation and foundation construction regardless of the surrounding water depth or tides.
While the caisson method is a common approach for structural piers supporting significant loads like bridges, other methods, such as pile driving (driving long columns deep into the seabed), may be used for different types of piers, including smaller recreational or fishing piers extending from the shore. The specific technique chosen depends on factors like water depth, seabed composition, load requirements, and environmental considerations.
