Design of Caissons
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Caissons are large watertight structures used in construction, particularly for
underwater foundations, bridge piers, and dock structures. They provide a dry
working environment underwater and are essential for projects involving deep
foundations in rivers, lakes, or marine environments. The design of caissons
involves careful consideration of structural integrity, buoyancy, stability, and
construction method. There are several types of caissons, each suited to specific
conditions and construction needs.
Types of Caissons
1. Open Caissons

Description: A box-like structure with an open bottom and closed top, often
used in shallow water.

Construction: Typically constructed onshore and then floated to the site, where
they are sunk into place.

Applications: Bridge piers, docks, and similar structures where the caisson can
be lowered into position.
2. Box Caissons

Description: Similar to open caissons but with a closed bottom, creating a
watertight box.

Construction: Prefabricated onshore and floated to the site, then sunk by filling
with water or ballast.

Applications: Used in conditions where it is necessary to have a dry working area
at the bottom.
3. Pneumatic Caissons

Description: A box or cylinder with an airtight chamber where work can be done
under air pressure.

Construction: Workers enter the chamber through airlocks, and compressed air
prevents water from entering.

Applications: Deep foundations where the caisson must penetrate into the
bedrock or through unstable soils.
4. Floating Caissons

Description: Large hollow structures that are floated into position and then sunk
by adding weight.

Construction: Built onshore and towed to the site, where they are flooded and
sunk.

Applications: Large marine projects such as quay walls, breakwaters, and other
large-scale water structures.
Design Considerations
1. Structural Integrity

Materials: Typically constructed from reinforced concrete or steel, chosen for
durability and strength.

Reinforcement: Proper reinforcement is necessary to withstand the loads and
stresses during sinking and operational phases.

Load Bearing: Designed to carry the loads from the superstructure and
distribute them to the foundation soil or bedrock.
2. Buoyancy and Stability

Buoyancy Control: The caisson must be buoyant enough to float during
transportation but can be ballasted to sink into place.

Stability During Sinking: Measures to prevent tilting or rolling during the
sinking process include using guide piles or anchor cables.

Permanent Stability: Once in place, the caisson must remain stable under
operational loads and environmental forces such as currents and waves.
3. Foundation Preparation

Site Investigation: Thorough geotechnical investigation to understand soil
conditions, bearing capacity, and any potential hazards.

Preparation: Depending on soil conditions, preparation may include dredging,
leveling, or even the use of temporary guide piles to ensure correct positioning.
4. Construction Method

Onshore Construction: Prefabrication in dry docks or on slipways allows for
controlled construction environments and quality control.

Transportation: Careful planning of the transportation route and method,
whether floating or towing, to the installation site.

Sinking: Controlled sinking by filling compartments with water or ballast,
ensuring an even descent to avoid tilting.

Sealing: Sealing the base of the caisson to prevent water ingress, often using
grouting or other sealing techniques once the caisson is in place.
5. Environmental and Safety Considerations

Environmental Impact: Minimizing impact on the aquatic environment,
including measures to protect water quality and marine life.

Worker Safety: Ensuring safety protocols for workers, particularly in pneumatic
caissons where air pressure is involved.
Example Design Process for a Box Caisson
1. Initial Design and Planning
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Define the requirements based on the project scope, including dimensions, loadbearing capacity, and site conditions.

Perform a geotechnical survey to understand soil characteristics and water depth.
2. Structural Design

Calculate loads and stresses, including self-weight, live loads from the
superstructure, and environmental loads (wave, current, and seismic).
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Design the caisson walls and base with appropriate reinforcement to handle
these loads.

Ensure watertight integrity, using high-quality concrete mix and waterproofing
measures.
3. Construction Preparation

Prefabricate the caisson in a dry dock with careful attention to alignment and
dimensions.

Install necessary fittings for ballast tanks, guide piles, and other operational
features.
4. Transportation and Sinking

Float the caisson to the site using tugboats or other suitable methods.

Gradually fill the caisson with water or ballast to sink it into position, monitoring
for stability and alignment.

Once in place, seal the base using grout or similar materials to create a watertight
structure.
5. Final Adjustments and Integration

Perform any necessary leveling or adjustments to ensure the caisson is properly
seated on the foundation.

Integrate the caisson with the superstructure, ensuring all joints and connections
are secure.
Conclusion
The design of caissons involves a detailed and multifaceted approach,
considering structural integrity, buoyancy, stability, foundation preparation, and
construction methods. Each type of caisson is suited to specific conditions and
project requirements, and careful planning and execution are essential to ensure
the success and longevity of the structure. By addressing these key
considerations, engineers can create effective and durable caisson foundations
for a wide range of marine and underwater construction projects.