The demand for land and space is becoming a major challenge in many cities and countries worldwide. Even maritime centers are not immune to this issue. The rise in international trading has led to an increase in commercial maritime traffic and activities in seaports, causing a surge in demand for effective use of ports and harbors. To meet this demand, construction activities have been focused on expanding existing port facilities.
Ports and harbors form the fundamental infrastructure that enables marine traffic, facilitating vessel construction, protecting ships against wave action, and supporting loading and unloading activities. They play a crucial role in promoting international commerce.
Caissons are versatile construction elements that can be used in various marine works and harbor constructions, including ports, breakwaters, wharves, berthing facilities and docks, dry docks, slipways, fishing ports, and marinas. By using caissons, engineers and construction professionals can enhance the efficiency and effectiveness of these structures and improve their durability against harsh environmental conditions.
Floating Caissons Fabrication
To build a caisson, a specific sequence is followed, starting with the slab. The process of building the slab involves assembling the reinforcement cage on an auxiliary floating platform. Once the cage is ready, it is transported to the floating dock. A sliding form is then placed on top of the reinforcement cage, and the slab is poured as a single, solid piece. This ensures that the slab is constructed in a precise and efficient manner, setting the foundation for the rest of the caisson construction process.
The construction process of the caissons involves building the upper part of the structure in increments of one meter, using a sliding form. This process includes placing reinforcement, sliding forms, and pouring and vibrating concrete. The cycle repeats until the caisson reaches its total height.
After completing the fabrication of the caisson, a special set of supporting and locking bars are removed to release it from the floating dock. Once released, the caisson floats by itself and is guided by cables from the shore and tow-boats to its final location. At this stage, the cylinder cavities are filled with granular material using auxiliary floating platforms that carry both the material and a special crane to transfer it.
To finish the filling operation on top of the caisson, tractors, dozers, loaders, and trucks are employed. Meanwhile, in the floating dock, the fabrication of the next caisson begins.
Step 1 – Construction of Hand-dug Caisson
The Hand-dug Caisson was a piling method commonly used in the past. However, in Hong Kong, it has been banned and is no longer allowed. Despite this, it is still important to understand some key information about this method.
Installation of Hand-dug Caisson
To begin the caisson construction process, the position and size of the caisson must be set out. The next step involves excavating one meter into the ground and then erecting a caisson lining steel form. After that, the caisson lining is created using concrete.
Once the caisson lining has been constructed, an excavation platform is erected on top of the caisson center. The following day, the caisson lining steel form is dismantled. This process of excavating one meter into the ground, erecting the caisson lining steel form, and creating the concrete caisson lining is repeated until the bedrock is reached.
Once the bedrock is reached, a bellout is excavated into it until the required level is achieved. The caisson reinforcement is then fixed, and the caisson bottom is cleaned. An installation of a concreting chute follows, and finally, the caisson heart is concreted until it reaches the required level.
Step 2 – Preventive Measures
To prevent excessive settlement of adjacent buildings or pavement and stabilize any unstable layers of subsoil that may be encountered during caisson excavation, grouting can be employed as a preventive measure. This method involves injecting a material, such as cement, into the subsoil to strengthen it and prevent further instability. By doing so, the subsoil is made more able to support the weight of the caisson and any structures built upon it. Additionally, grouting helps to mitigate the effects of dewatering, which can cause further settlement and damage to surrounding structures. Overall, grouting is a valuable technique in ensuring the stability and safety of excavation sites and adjacent structures.
Step 3 – Monitoring
To ensure the safety of adjacent structures during the construction of a caisson, certain precautionary measures and limiting criteria should be implemented and monitored continuously.
Firstly, the level of checkpoints should be closely monitored, and any settlement of building structures and road pavement must not exceed 10 mm and 25 mm, respectively. Regular monitoring will help to identify any adverse effects on these structures and allow for prompt corrective action to be taken.
Secondly, standpipes piezometer should be installed before the caisson excavation to monitor the ground water table drawdown. The drawdown must not exceed a specific value, which will be determined beforehand. This monitoring is crucial as excessive drawdown can cause soil instability and other problems that can compromise the safety of surrounding structures.
Finally, it is essential to maintain accurate records of all monitoring results throughout the construction period. Duplicate copies of these records must be submitted to the Consultants on a regular basis, and they should also be kept on site for inspection at all times. This record-keeping is critical for assessing the effectiveness of the precautionary measures and ensuring that any issues are identified and addressed promptly.
Caisson Sinking
There are two approaches to lining a well with caissons, and one of them involves excavating an unlined well and then inserting the caissons. This technique closely resembles the in-situ lining method. However, it poses similar safety hazards without the advantage of a tight grout seal around the perimeter of the well. Furthermore, if the borehole is not straight or uniform, complications may arise. This method also necessitates additional backfilling, making it less appealing.
Digging a Caisson Lined Well
When starting a construction project that involves excavation, there are two main approaches that can be taken. The first is to dig a starter hole before placing the bottom caisson on the ground, while the second is to place the caisson directly on the ground. In either case, it is essential to ensure that the first few sections are laid out straight and level. The reason for this is that while the upper sections can help keep the lining straight as the hole progresses, it is crucial to have a straight starting point.
In the case where pre-cast sections are used, it is vital to make sure that these sections are secured together. If the sections are simply stacked, there is a risk that the bottom caisson or the cutting ring may fall out of place during the progression of the stack. This can be a highly dangerous situation and may be impossible to recover from. Therefore, it is essential to ensure that the sections are secured together to prevent any accidents or damage from occurring. By taking the necessary precautions, one can ensure that the excavation process progresses smoothly and safely, resulting in a successful project outcome.
Finishing a Caisson Lined Well
When constructing a caisson, it is important to fill the bottom with 7-10 cm of gravel to allow for good flow up to the well while also preventing fines from moving up. To increase the inflow, the bottom sections of the caisson are often made perforated.
However, there are differing opinions on whether or not to use a perforated or porous material on the sections under the water table. Some sources recommend always using such material, while others argue that the joints between the sections themselves will allow for substantial flow. In the latter case, modifying the bottom sections with holes or porous concrete may not be worth the decrease in strength of the caisson.