What is a Caisson?

A caisson is a watertight chamber or structure that is used for various underwater construction, inspection, or repair purposes related to utility infrastructure. It is designed to create a dry work environment below the water surface, allowing workers to carry out tasks in a submerged or partially submerged area.

The term “caisson” originated from the French word “caisse,” meaning box or chest, which accurately describes its box-like structure. Caissons have been used for centuries in civil engineering and marine construction projects, particularly for the construction of bridges, piers, and underwater tunnels.

The primary function of a caisson is to provide a dry working space in underwater or partially submerged areas. It is typically constructed on land or in a dry environment and then floated or sunk into position at the desired location. Once in place, the caisson is sealed to prevent water from entering and creating a dry, controlled environment inside.

Caissons can be open-bottomed or closed-bottomed depending on the specific project requirements. Open-bottomed caissons are commonly used for bridge pier construction, where the bottom is left open, allowing the caisson to sink by displacing water or by filling it with sand or other ballast material. Closed-bottomed caissons, on the other hand, are sealed at the bottom, creating an airtight chamber.

When a caisson is positioned at the desired location, it is typically weighted down to ensure stability and prevent buoyancy. Various methods can be employed for this purpose, such as using gravity, adding additional ballast material, or pumping out water from the chamber.

Once the caisson is secure and stable, workers can enter the chamber through airlocks or manholes. They can perform tasks such as underwater welding, concrete pouring, inspection of submerged infrastructure, or repair work. The caisson provides a controlled and dry environment, allowing workers to carry out their tasks efficiently and safely.

Caissons can vary in size and shape depending on the specific project requirements. They are often made of reinforced concrete or steel to withstand water pressure and provide structural integrity. Modern caissons may also incorporate advanced technologies for monitoring and controlling various environmental factors inside the chamber, such as temperature, humidity, and air quality.

A caisson is a watertight structure used for underwater construction, inspection, or repair of utility infrastructure. It enables workers to work in a dry and controlled environment beneath the water surface, facilitating various tasks related to civil engineering and marine construction projects.

Additional Details About Caissons

1. Types of Caissons:
   • Pneumatic Caissons: These are caissons that are sealed at the top and have compressed air pumped into them to expel water and create a dry working environment. Workers inside these caissons breathe air under pressure, which helps to keep water out and maintain a stable atmosphere.
   • Box Caissons: Box caissons are rectangular or square-shaped structures used for bridge pier construction. They are open at the bottom and are sunk into place by filling them with ballast material or by using other methods of controlled sinking.
   • Floating Caissons: Floating caissons are used for temporary construction purposes. They are built on land, floated into position using pontoons, and then ballasted to sink into the desired location. These caissons can be refloated and moved to other locations as needed.
   • Cofferdams: Although not technically caissons, cofferdams are temporary watertight enclosures used to create a dry workspace around a construction site, such as bridge piers. They are often constructed by driving sheet piles into the ground and then pumping out the water from the enclosed area.
2. Construction Process:
   • Preparation: The construction of a caisson begins with planning and designing the structure based on the project requirements. Factors such as water depth, soil conditions, and the intended purpose of the caisson are considered during this phase.
   • Fabrication: Caissons are typically constructed on land or in a dry environment to facilitate efficient construction. They are made from materials such as reinforced concrete or steel and are designed to withstand the water pressure and other environmental forces.
   • Positioning: Once the caisson is ready, it is transported to the site and positioned at the desired location. This process can involve floating the caisson into place or sinking it using ballast material, depending on the type of caisson being used.
   • Sealing: The caisson is sealed to prevent water from entering the working chamber. This involves ensuring watertight connections at the joints, using sealing materials, and employing various sealing techniques to maintain the integrity of the structure.
   • Ballasting: To ensure stability and prevent buoyancy, the caisson is weighted down using ballast material, such as concrete or stone. Additional measures like using gravity or pumping out water from the chamber may also be employed to ensure stability.
   • Work Operations: Once the caisson is secured and stable, workers can enter the chamber through airlocks or manholes. They can perform various tasks such as welding, concrete pouring, inspection, or repair work in the dry environment provided by the caisson.
   • Deconstruction: After the completion of the required work, the caisson may be removed or deconstructed depending on the project requirements. This can involve refloating a floating caisson, removing the ballast material, or dismantling the caisson structure.
3. Safety Considerations:
   • Working in caissons can be challenging and potentially hazardous due to the confined space, underwater conditions, and the use of compressed air in pneumatic caissons. Proper safety measures must be implemented, including training workers on confined space entry, providing ventilation, and monitoring for any signs of decompression sickness or other health risks associated with working in pressurized environments.
   • Continuous monitoring of air quality, pressure differentials, and other environmental factors is crucial to ensure worker safety and prevent accidents such as flooding or structural failures.
   • Adequate emergency response plans, including rescue procedures and medical support, should be in place to address any unforeseen emergencies that may arise during caisson operations.

Caissons play a vital role in the construction and maintenance of underwater utility infrastructure, enabling engineers and workers to perform tasks efficiently and safely in submerged or partially submerged environments.