Nelson Bustamante: Floating Caissons – The Constructor

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In many urban centers worldwide, the demand for land and space is rapidly outpacing supply, a trend not exempting 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, thus playing a pivotal role in fostering international commerce.

Various marine works and harbor constructions necessitate the utilization of caissons, including but not limited to:

  1. Ports
  2. Breakwaters
  3. Wharves
  4. Berthing Facilities and Docks
  5. Dry Docks and Slipways
  6. Fishing Ports and Marinas
  7. Floating Caissons Fabrication

Floating Caissons Fabrication

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

Floating caisson fabrication

After the slab is ready, the construction of the upper part of the caissons begins, ascending in increments of one meter using the sliding form Each of these increments includes: placing the reinforcement, sliding the forms, and pouring and vibrating the concrete. This sequence is repeated until the total height of the caisson is reached.

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

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 In the floating dock, operations begin for the fabrication of the next caisson

Step 1 – Construction of Hand-dug Caisson

  • Hand-dug Caisson is one of piling methods in the past, however, it is almost banned in Hong Kong. Therefore, we should know something about it and there are some notes about it.

Installation of Hand-dug Caisson

  • Set out caisson position and size.
  • Excavate one meter into ground.
  • Erect caisson lining steel form.
  • Concrete caisson lining.
  • Erect excavation platform on top of caisson centre.
  • Dismantle caisson lining steel form on the next day.
  • Repeat step 2 to 6 (excluding step 5) until bedrock.
  • Excavate bellout into bedrock until required level.
  • Fix caisson reinforcement.
  • Clean caisson bottom.
  • Install concreting chute.
  • Concrete caisson heart until required level.

Step 2 – Preventive Measures

  • In order to stabilise any unstable layers of subsoil which may be encountered during caisson excavation and prevent excessive settlement to the adjoining building/pavement due to the effect of dewatering, grouting is one of methods can be adopted as a preventive measure before caisson excavation.

Step 3 – Monitoring

  • In order to ensure no adverse effects imposed on the adjoining structures during caisson construction, the following precautionary measures and limiting criteria, to be monitored throughout the construction period, are recommended
  • a. The level of checkpoints should be monitored regularly and the measured settlement of building structures and road pavement must not exceed 10 mm and 25 mm respectively.
  • b. Standpipes piezometer shall be installed before the caisson excavation for monitoring of the ground water table drawdown which shall in no case exceeding the specific value.
  • c. During the construction period, duplicate copies of all monitoring results shall be submitted to the Consultants on a regularly basis and be kept available on site for inspection at all times.

Caisson Sinking

There are two methods for lining a well with caissons. The first method involves digging an unlined well and then lowering the caissons into place. This is very similar to the in-situ method of lining.

It has all the same problems of safety but without of the benefits of a tight grout seal along the edge of the well. Problems also arise if the hole is not straight and uniform. The additional backfilling that is required also makes the method undesirable.

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 important to make sure the first few segments start out strait and level. As the hole progresses the upper sections will keep the lining strait, but it must be straight to begin with.

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 it 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 say 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

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