An In-Depth Overview of Floating Caissons: Fabrication, Sinking, Digging, and Finishing

 

In many urban centers worldwide, the demand for land and space is rapidly outpacing supply, a trend that does not exclude maritime hubs. The relentless expansion of commercial maritime traffic and activities in seaports, driven by the surge in international trading, has precipitated a heightened need for efficient port and harbor utilization. Consequently, construction endeavors have been predominantly geared towards expanding existing facilities to meet burgeoning demands.

Port and harbor infrastructures are quintessential for enabling marine traffic, facilitating vessel construction, shielding against wave action, and facilitating loading and unloading operations. These infrastructures play a pivotal role in fostering international commerce.

Various marine works and harbor constructions necessitate the utilization of floating caissons. These include:

• Ports
• Breakwaters
• Wharves
• Berthing Facilities and Docks
• Dry Docks and Slipways
• Fishing Ports and Marinas

Floating Caissons Fabrication

Initial Construction

The fabrication of each floating caisson begins in an ascending sequence starting with the slab. The slab reinforcement cage is initially assembled on an auxiliary floating platform, then moved to the floating dock. A sliding form is placed, and the slab is poured as a monolithic element.

Incremental Construction

After the slab is ready, the construction of the upper part of the caisson begins, ascending in increments of one meter using the sliding form. Each increment involves:

• Placing the reinforcement
• Sliding the forms
• Pouring and vibrating the concrete

This sequence is repeated until the total height of the caisson is reached.

Release and Transportation

Upon completion of the caisson fabrication, a special set of supporting and locking bars are removed, allowing the release of the caisson from the floating dock. The caisson then floats by itself and is guided with the help of cables from the shore and tow-boats to its final location. Upon reaching the final position, the cylinder cavities begin to be filled with granular material.

Filling Operation

This operation is performed by auxiliary floating platforms that carry both the material and a special crane to transfer the material. Tractors, dozers, loaders, and trucks help finish the filling operation on top of the caisson. Meanwhile, in the floating dock, operations begin for the fabrication of the next caisson.

Step-by-Step Process of Hand-Dug Caisson Construction

Step 1 – Construction of Hand-dug Caisson

Although hand-dug caissons are a piling method from the past, it is almost banned in places like Hong Kong. However, understanding its construction process provides valuable insight:

Installation Process

1. Set out the caisson position and size.
2. Excavate one meter into the ground.
3. Erect the caisson lining steel form.
4. Concrete the caisson lining.
5. Erect an excavation platform on top of the caisson center.
6. Dismantle the caisson lining steel form the next day.
7. Repeat steps 2 to 6 (excluding step 5) until reaching bedrock.
8. Excavate the bellout into the bedrock until the required level.
9. Fix the caisson reinforcement.
10. Clean the caisson bottom.
11. Install the concreting chute.
12. Concrete the caisson heart until the required level.

Step 2 – Preventive Measures

To stabilize any unstable layers of subsoil encountered during caisson excavation and to prevent excessive settlement of adjoining buildings or pavements due to dewatering, grouting is adopted as a preventive measure before caisson excavation.

Step 3 – Monitoring

To ensure no adverse effects are imposed on adjoining structures during caisson construction, the following precautionary measures and limiting criteria should be monitored throughout the construction period:

• The level of checkpoints should be monitored regularly, with the measured settlement of building structures and road pavements not exceeding 10 mm and 25 mm respectively.
• Standpipes piezometers should be installed before the caisson excavation to monitor the groundwater table drawdown, which should not exceed specific values.
• During the construction period, duplicate copies of all monitoring results should be submitted to the consultants regularly and be kept available on-site for inspection at all times.

Caisson Sinking

There are two methods for lining a well with caissons: digging an unlined well and lowering the caissons into place, or using a pre-cast method. Both methods have their own sets of challenges.

Digging a Caisson Lined Well

A starter hole can be dug first, or the bottom caisson can be placed directly on the ground. It is crucial to ensure that the first few segments start out straight and level. As the hole progresses, the upper sections will keep the lining straight, but it must be straight from the beginning.

If separate pre-cast sections are used, it is important that the sections are secured together. If the sections are simply stacked, the bottom caisson, or the cutting ring, can fall out of place while the stack progresses. This is potentially dangerous and can be impossible to recover from.

Finishing a Caisson Lined Well

When the bottom of the caisson reaches the required depth, the bottom should be filled with 7-10 cm of gravel to allow good flow up to the well but prevent fines from moving up. To increase the inflow, the bottom sections are often perforated.

Some sources recommend always using a perforated or porous material on the sections under the water table. Others argue that joints between the sections will allow substantial flow and modifying the bottom sections with holes or porous concrete is not worth the decrease in strength.

Summary

The use of floating caissons in port and harbor construction is a testament to the advancements in engineering and construction techniques. These structures are integral to the efficient operation of marine facilities, ensuring the smooth handling of the ever-increasing maritime traffic. Understanding the detailed processes involved in the fabrication, sinking, digging, and finishing of caissons provides a comprehensive overview of their significance in modern infrastructure development. 

By incorporating the best practices and preventive measures, the construction of caissons can be executed safely and effectively, contributing to the robustness and resilience of maritime hubs worldwide.