In our pursuit of excellence in caisson construction, we prioritize advanced methodologies to create and construct caissons, ensuring structural integrity from the foundation up. This focus on innovation extends to the efficient movement of caissons between construction phases, using our state-of-the-art skidding transfer system. Our approach not only emphasizes the precision of caisson construction processes but also integrates seamless launching and moving, underscoring our comprehensive expertise in delivering complete caisson solutions.
If you are undertaking a construction project that is taking place on, or in, water, you will likely need a caisson construction. A caisson is a watertight structure that is used as an anchor for a foundation. It is also commonly known as a caisson foundation.
Caissons are prefabricated objects or hollow boxes generally made of concrete. The material used will rely heavily on the function of the caisson and how it’s being constructed. Caissons are often used in the construction of bridge piers, concrete dams, and other structures that require a foundation beneath the water surface. For example, bridge towers or floating wind turbines to be mounted on a caisson. In the case of floating concrete construction, we have invented a slipform method called the Mega Gantry Slipform which provides stronger gantry structures than normal and more efficient reinforcements.
A caisson is a critical component of many construction projects because it provides stability to the structure. In cases where the ground can shift under the heavy weight of a new structure, a caisson can be used to distribute the weight of the load evenly. The concrete essentially provides an even foundation and it becomes an integral part of the permanent structure.
A caisson can be required in various conditions, including when:
Caisson foundation construction can be done using several types of foundations. Some common types of foundations include box, open, excavated, sheeted, pneumatic, and floating caissons. It’s worth noting that caissons are easily adaptable to various site conditions.
The entire construction process involves pre-construction, construction, and post-construction procedures.
Caissons can either be produced on land (via gantry slipforming, mega gantry slipforming or traditional slipforming) and launched out into the water (with skidding technology), or manufactured on a dry dock (with gantry slipform technology) and lowered directly into the water.
While caissons are somewhat similar to pile foundations, their installation is different. There are several ways in which a caisson foundation construction can be installed. For example, some caissons are built and installed on-site, but others are built off-site and then moved to their installation site. When a caisson construction is built off-site, it’s generally floated to the installation site.
The placement and type of the caisson foundation will determine the installation process. In some cases, the placement location will need preparation. For example, a cofferdam is sometimes used to clear an area before a caisson can be implemented.
Once the caisson foundation has been lowered to the seabed, filling material is poured into the caisson and the tops are sealed.
USE OF GRAVITY CAISSONS FOR PORT DEVELOPMENT
Wharf front structures for container ports are designed to withstand the vertical forces arising from container handling operations and horizontal loads due to vessel berthing and mooring operations for both long and short terms.
Pile decks were previously used as wharf frontage structures but gravity caissons are now commonly used.
The reasons for the use of gravity caissons are partly due to land constraints and the lack of natural linear coastlines required for today’s large container ships. Installed caissons form a perimeter retaining structure to withstand the forces due to the reclaimed fill materials and operational loads behind, creating additional land and fixed berths for container ships to dock.
FABRICATION OF GRAVITY CAISSONS
The caissons are prefabricated at a nearby site under a protected factory-controlled environment, ensuring consistency of concrete quality and strength and providing a conductive working environment for the employees.
Caissons can either be produced on land (via gantry slipforming, mega gantry slipforming or traditional slipforming) and launched out into the water (with skidding technology), or manufactured on a dry dock (with gantry slipform technology) and lowered directly into the water.
GRAVITY CAISSON DESIGN
Caissons are usually designed with open cells to facilitate transportation as they can float in place once the foundation bed is ready for the installation of the caissons. The open cells of the caissons are then filled with material after the caissons are placed in place to increase the total weight of the caissons for stability.
The caisson system consists of the caisson foundation, caisson structure, caisson infill, reclamation fill behind, scour protection. Subsequent wharf decks and accessories form part of the future wharf for port operations.
The caisson height is determined by the final platform level requirement and the seabed level for container ship operation. The caisson width is determined based on stability and other design checks while the caisson length is determined by the capacity of the floating dock used for transporting the caisson from the casting yard.
The caisson structural system is designed to withstand various load cases which include loads during construction when the caisson is transported from land to sea and when it is filled, backfilled and preloaded, as well as the loads during the operational stage due to port equipment and ship berthing and mooring.
STRUCTURAL DESIGN
The caisson is designed as a normal reinforced concrete structure with sufficient structural strength to withstand the stresses under all possible loading conditions meeting the requirements for both ultimate limit state and serviceability limit state as specified in the relevant codes.
The caisson structure consists of bottom slab, front wall, back wall, side wall, internal transverse walls and internal longitudinal walls. The caisson structure is assessed under loading conditions at various stages such as floating, sinking, construction and operational stages and analyzed for loads and effects are specified to obtain the most severe combinations and envelopes of stress resultants on each structural member.
The sizes of the reinforced concrete members are evaluated for the proposed design and sufficient steel reinforcements are provided for both bending and shear. The caisson base slab is designed with sufficient thickness and reinforcement to allow the loads to be transferred through the caisson structures and finally to the respective base slabs. The main structural elements, i.e. walls and slabs, are effectively tied together in longitudinal, transverse and vertical directions to provide sufficient restraint and stability to the structures. In addition to the caisson structure, other structural parts such as the corbel are also designed accordingly.
GEOTECHNICAL DESIGN
The overall stability of the caisson structure is of utmost importance in caisson design as the consequence of caisson failure will be catastrophic. Overall stability design checks such as caisson sliding, overturning, deep seated failure and bearing failure are carried out to ensure that the geotechnical ultimate limit states of caissons meet the minimum requirements specified for the project. These stability checks are carried out to examine the different load combinations at different stages during the life cycle of the caisson. The minimum design requirements for the various stability checks for all load scenarios used in the design are as follows.
In addition to the ultimate limit states, the serviceability limit state of the caisson system during port operations must be ensured to meet the settlement requirement, in particular the differential settlement. Strict constraints are placed on the wharf crane tracks to ensure smooth movements of the wharf crane without compromising container handling at the wharf.
In order to meet the strict total and differential settlement of the caissons, it is of great importance to ensure that the caissons rest on competent foundations and that the caissons are preloaded.
CAISSON FOUNDATION
The caisson foundation is provided to satisfy not only the overall stability of the caisson structure but also the stringent settlement requirements for caissons. Therefore, soft soil under the caisson is first dredged from the seabed to a competent layer to form the sand key trench. The depth of the sand key varies across the alignment of the wharf in accordance with the ground conditions.
Good quality sand is then filled into the dredged trench. The filled sand key is then well compacted by vibro compaction to achieve a minimum relative density of 75% to increase rigidity and reduce caisson movements. The quality of the compaction is confirmed by the in situ cone penetration test (CPT) to meet the corresponding minimum cone tip resistance.
Durable and strong rocks are then placed on top of the sand layer, compacted and leveled to form the rock pile on which the caisson is placed. This rock pile layer will act as a flat surface for easy installation and as a load distribution from the caisson base to the foundation layer.
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