Floating Caissons
by Luis H. Martinez & Ismael Rodriguez

   
 

Objectives

The objective of this paper is to provide a summary of the prefabricating operations for building floating caissons used in the construction of docks and harbors. A description of the technology, the processes involved and a simulation model are presented

Floating Caissons

In certain cities and even countries the demand for land and space is rapidly exceeding the supply. Maritime centers do not escape from this reality, the continuous expansion of commercial maritime traffic and activities in seaports due to increases in international trading has generated an increased demand for an effective use of ports and harbors. Construction activities have been oriented to the expansion of existing facilities

Port and harbor facilities form the infrastructure that makes marine traffic possible, facilitating the construction of vessels, its protection against wave action and, its loading and unloading activities. In other words, they play an important role in facilitating international commerce

As mentioned above, a solution to this supply problem has been the expansion of ports. One of the methods that allows a fast paced construction of docks is based on the use of floating caissons. The floating caissons are prefabricated concrete box-like elements with cylinder cavities or cells that are built with the help of a special equipment named "Floating Docks." The floating caissons dimensions are customized to each project requirements within certain limits. The floating dock fabrication equipment studied in this report is a proprietary technology owned by DRAGADOS (Fig. 1).

The different marine works and harbor constructions in which these caissons can be used include:

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

Prefabricating Operation

The prefabricating operation of the caissons starts with the concrete preparation. Two sets of equipment are used simultaneously covering each half of the operation. The operation sequence is as follows: A mixer prepares the concrete batch that is then poured into a hopper that feeds a concrete pump. The concrete is then pumped to a delivery pipe that is installed above the working deck (Fig. 2).

Floating Caissons fabrication (16348 bytes)
Fig. 2: 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

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 (Fig. 3).

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Fig. 3: Floating Caissons Fabrication

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 (Fig. 4). 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 (Fig. 5).

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Fig. 4: Floating Caissons Fabrication

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Fig. 5: Floating Caissons Fabrication

Using Floating Caissons for the Expansion of the Port of Valencia

This report is based on the caisson fabrication operations performed by DRAGADOS for the expansion of the Port of Valencia in Spain. Valencia is located 352 km southwest of Madrid in the Azahar Coast of the Mediterranean Sea (Fig. 6).

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Fig. 6: Port of Valencia Expansion

The project involved the construction of 3,805 m of breakwaters and 996 m of wharves. The new area created between these two structures was of 120 Ha. (297 Acres). Almost half of Purdue's total area of 650 acres. This new area required more than 17,000,000 m3 of dirt to be filled.

The 996 m of wharves were constructed using 26 floating caissons Each floating caisson had the following dimensions: 42 25 m (138.61 ft) long, 15.65 m (51.35 ft) width, 16.5 m (54.13 ft) height, its concrete volume was of 2,857m3 (3,737 cy), required 116, 27 metric tons of rebar and had an approximate weight of 10,560 metric tons. The caisson consisted of a concrete solid slab of 0.6m (1.97 ft) height, followed by a concrete parallel piped with 62 cylinder cavities, 3 8m (12.47ft) diameter, to reduce its weight (Fig. 7). DRAGADOS has fabricated a total of almost 1,000 of these caissons during the last 50 years. Their experience has allowed them to successfully develop and continuously improve this technology.

Floating Caissons sketch (58236 bytes)
Fig. 7: Floating Caissons - Longitudinal Section

Analysis of the Main Processes

By observing the prefabricating operation above described and considering the durations and resources involved, four processes or cycles were determined to be relevant for the operation's production, these were:

  1. Concrete Mixing and Pumping
  2. Slab Pouring
  3. Upper Caisson Pouring
  4. Retrieval of Support Bars


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Besides the traditional materials required in concrete construction, the resources involved in the fabrication operation include:

  • The Floating Dock that holds two Concrete Delivery Pipes and a Crane
  • Two Auxiliary Floating Platforms
  • Batch Plant
  • Two Concrete Pumps
  • Gantry Crane
  • Loaders
  • Air Compressors
  • Pneumatic Pumps
  • 34 Workmen per 3 Shifts

Of the four main processes being considered the Upper Caisson Pouring was chosen for study It was the most relevant processes, considering the amount of work involved It represented 85% of the concrete being poured and an estimated 74% of the fabricating operation's time.

The analysis of the process cycle was conducted when the third caisson was under fabrication. The recorded total fabrication cycle for the two first caissons averaged 153 hr. 39 hr above the established 114 hr when construction was planned. Considering that each day had a production cost of PTA 2.5M (US$18,700) improvements needed to be made.

A flow unit of 1 cubic meter of concrete was established for modeling purposes. Since this process cycle depended on the concrete mixing and pumping, cycle capacities were checked to make sure that they did not represent a bottleneck The capacities obtained were:

Concrete Mixing Process: Plant 65 m3/hr 
Pumping Process: Pump 1  53 m3/hr
Pump 2  31 m3/hr

The Upper Caisson Pouring process registered a productivity of 35.7 m3/hr, very much below the 65 m3/hr that is possible to produce and pump.

Development of the CYCLONE Model

Considerations:

  1. The upper caisson is poured in layers of 25 cm (38 m3). For this volume each pump must discharge 19 m3 of concrete, equivalent to 12.7 batches.
  2. The concrete delivery pipes, have to interrupt their pouring process 12 times each, due to the steel beams of the working deck that restrict their free movement. This means that each delivery pipe will discharge approximately 1.1 batches before changing its position.
  3. The concrete plant prepares batches that are directed to each pump as needed.

The model developed for this operation is shown in Fig. 9 with the PROSIDYC simulation reports in the following pages. The results of these runs will be the basis for comparisons between proposed alternatives.


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Proposed Alternatives

  • Alternative 1: Double the capacity of the pump hoppers allowing them to receive each, two batches from the concrete plant. The results and reports from this alternative can be seen in Fig. 10.


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  • Alternative 2: Consists of alternative 1 plus the reduction in the number of Delivery Pipe positioning from 12 to 8. This could be done by modifying the shape of the working deck structure beams. Instead of using a beam with a rectangular section, a V section could be used in order to have a large area to pour concrete without repositioning the delivery pipe. The results and reports from this alternative can be seen in Fig. 11.


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Results

The productivity obtained in each of the models are the following, considering 100% and 90% time efficiency:

Time Efficiency: 100% 90%
Original Situation 34 m3/h 31 m3/h
Alternative 1 42 m3/h 37 m3/h
Alternative 2 47 m3/h 42 m3/h

Conclusions and Recommendations

  1. The productivity of the process was increased in 24% by doubling the capacity of the pump hoppers.
  2. Alternative 2 considered the double capacity of alternative 1 and also a modification in the working deck structure, by changing the section of its beams, from a rectangular shape to a V shape. Allowing a larger pouring area which reduced the number of positions for the delivery pipes. This modifications showed an increase in the productivity of 38% when compared to the original process or 14% over Alternative 1.
  3. A cost analysis of the structural modification to the working deck is recommended, comparing the cost benefits that could be received by an improvement of only 14% of the productivity, to the cost of such structural modifications.
  4. In the models used in this analysis, two pumps with different capacities were considered. This created an uneven concrete assignment from the plant which translated in an uneven concrete pouring task for each area. The use of two equal capacity pumps should be studied.
  5. The capacity of both pumps is superior to the capacity of the plant, thus a further study is recommended to optimize such relation.