Box caissons are commonly used in civil construction for underwater foundations such as piers, bridge supports, and other marine structures. Typically constructed from reinforced concrete or steel, these watertight boxes are built on land, floated to the desired location, and then carefully sunk into position.
As infrastructure development becomes more necessary, especially in coastal areas, the use of box caissons is important for upgrading port and harbour systems. With their holistic power of providing secure and robust bases, they play a major role in large construction projects. Explore how Box caissons are shaping modern infrastructure development.
What is a Box Caisson?
A box caisson is a watertight structure used in underwater or wet environment construction projects. It is utilized to make way for stiff platforms for structures like bridges, piers, or seaward establishments. Box caissons are constructed on land using wood, steel, or reinforced concrete materials and then floated into place where they will be used.
Once in position, the caisson is sunk into the water using additional weight, such as sand, concrete, or gravel, to ensure stability. This technique is used to achieve safe and stable buildings under poor conditions, and it forms a critical element of modern civil engineering works.
Applications of Box Caissons in Construction
Some of the common applications of Box Caissons in construction are as follows:
1. Foundations for Bridges and Piers
Box caissons are used primarily to provide stable foundations for bridges and piers, particularly in areas with deep water or poor soil conditions. Their strong design guarantees the structure’s durability even in the most extreme scenario.
2. Construction of Offshore Platforms
Box caissons are the most commonly used to support offshore systems such as oil rig subsystems, wind turbines, and marine equipment. The harsh environment of the sea requires secure foundations to be built, and box caissons are a reliable solution.
3. Harbor and Dock Structures
Box caissons play a crucial role in the construction of quay walls, docks, and jetties. They assist with anchoring harbour and dock structures that are constantly exposed to tidal forces and heavy loads from vessels.
4. Underwater Tunnels and Shafts
The box caissons provide a dry, safe environment for the construction of tunnels or shafts under the water. They facilitate excavation and assembly in flooded regions, making it safer and more efficient.
5. Seawalls and Breakwaters
These structures are useful in the construction of seawalls and breakwaters that help protect coastlines from erosion, wave action, and storms. These structures are built to withstand the most violent forces of nature.
6. Supporting Heavy Coastal Infrastructure
Box caissons serve an essential role in construction by acting as a solid foundation for coastal structures such as cranes, industrial plants, and storage facilities, absorbing the stresses of heavy equipment and ensuring structural integrity.
Advantages and Disadvantage of Box caissons
Advantages
- Affordable: Box caissons can eliminate the need for some extra foundation elements, such as pile caps, which can significantly reduce construction costs under favourable site conditions.
- High Load-Bearing Capacity: Their strong axial and lateral load capacity makes them ideal for supporting heavy structures such as bridges, piers, and offshore platforms.
- Lower Vibration and Noise: Installing box caissons generates less noise and vibration compared to driven piles, making them an environmentally friendly choice.
- Flexibility: Box caissons can be tailored to accommodate a range of site conditions, allowing for versatile design and use.
- Stability: They offer very good resistance to differential settlement and environmental pressure, which better maintains the project structure when installed correctly.
Disadvantages
- Complex Structure: The construction and placement of box caissons require skilled labour and specialized equipment. Errors in execution can impact alignment and structural integrity, potentially affecting the project timeline and quality
- Site Limitations: Box caissons are best suited for sites with stable and relatively firm seabeds. They are not ideal for soft, highly compressible, or contaminated soils, which may affect settlement or sealing during placement.
Construction Process of Box Caissons
Here is a detailed overview of the typical construction procedure:
1. Planning and Design
Before construction begins, engineers assess site conditions, including soil type and water depth, to design the box caisson appropriately. This planning phase is crucial for determining the dimensions and materials needed.
2. Site Preparation
A level-bearing surface is prepared at the construction site. If necessary, soft or loose soil is dredged to create a stable base for the caisson. In some cases, piles may be installed to support the caisson.
3. Fabrication
The box caisson is constructed on land using reinforced concrete or steel. The structure typically consists of vertical walls and a closed bottom, with the top remaining open during initial construction.
4. Transportation
After fabrication, the caisson is floated to its site by towboats. It is secured with mooring cables to prevent movement during installation.
5. Sinking the Caisson
The caisson is gradually submerged in the water by digging soil out from within it. This can be done to continuously clear out water and mud using dredging techniques or pumps to keep the workplace dry.
6. Filling and Sealing
Once the desired depth is reached, the interior of the caisson is filled with sand, gravel, or concrete to add ballast and stability. The top of the caisson is sealed to keep water out.
7. Final Adjustments
Once the caisson is securely in place and filled, any necessary adjustments are made before proceeding with further construction above water, such as building bridge piers or other structures.
Component of Box Caissons
1. Walls
Box caissons may be constructed of reinforced concrete, steel, or timber. This creates a watertight barrel, closed at the bottom and open at the top, surrounded by vertical walls. Walls are designed according to pertinent loads and environmental considerations, including thickness and material.
2. Bottom Slab
The bottom slab is an essential component that contributes to the structural strength and stability of the entirety. It is watertight to keep out water and to hold the weight of the caisson when it is loaded with ballast material.
3. Open Top
The open top allows for easy access during construction and excavation activities. It facilitates the removal of soil from inside the caisson, enabling it to sink into position.
4. Ballast Material
Once positioned, the interior of the box caisson is filled with ballast materials such as sand, gravel, or concrete. This filling helps anchor the caisson and counteracts buoyancy forces from water.
5. Sealing Mechanism
After filling, a sealing mechanism may be implemented at the top to prevent water from entering the caisson and to maintain its structural integrity during construction.
6. Dredge Wells (Optional)
In some designs, dredge wells may be included to facilitate excavation within the caisson while maintaining a dry working environment.
To Wrap Up
Box caissons serve as crucial key elements, offering stability and longevity to a wide range of structures, particularly in adverse underwater and coastal circumstances. They are a staple for bridges, piers, and offshore platforms due to their superior load capabilities, flexibility, and economy.
Though it seems complex, the construction process provides excellent long-term stability and environmental impact. Box caissons continue to be a key component in modern infrastructure development, supporting a wide range of projects and contributing to the growth of coastal and marine infrastructure.