The Basics of Caisson Foundation - All You Need To Know

Every structure stands on a foundation that transfers load to the ground. Broadly, the types of foundations fall into two groups: shallow foundations and deep foundations. When surface soils are weak, deep solutions are chosen. One proven deep-foundation option is the Caisson Foundation, designed for difficult conditions such as soft soil and the presence of groundwater, especially where underwater work is required.

What is a Caisson Foundation?

A Caisson Foundation is a watertight, hollow box or cylinder constructed above ground, then placed to depth and filled with concrete. Once set in place, it becomes part of the permanent works and provides a solid foundation for the superstructure.

In marine and riverside settings, a caisson also acts as a water-retaining shell that creates a dry, stable workspace below water level. A prefabricated box or cylinder is sunk to the required depth and then filled to resist heavy loads, making it especially suited to harbors, docks, and bridge construction where other options may be impractical.

Why Caisson Foundations Matter

Caissons are built as large hollow units that are lowered to design depth and filled to develop the mass and stiffness needed to carry significant loads while limiting water ingress into the work zone. By creating a dry, reinforced chamber, they improve stability and reduce the risk of movement from soil shifts. Compared with piles, caissons offer characteristics that are advantageous for specific sites and loading conditions, particularly in deep water or fast-flowing rivers.

A Look into the Mechanism of Caisson Foundation

“Caisson” comes from the French word for “box.” In operation, the shell acts as an impermeable, bottom-open chamber that can be placed underwater while keeping the interior dry. Trapped air initially resists water entry; to control pressure, a platform above the caisson connects via a tube that supplies compressed air. Pressure equalization keeps water out so work can continue.

After stabilization, crews excavate soil from the base. Removing soil can allow water to rush in, so a water-filled pipe is used. Air pressure inside the caisson and from outside holds the water column in the pipe, allowing a bucket to be lowered to collect soil and lift it to the surface. The process repeats in stages so the caisson sinks progressively to the target depth, remaining stable and watertight as the excavation advances.

Types of Caisson Foundation

Below is an overview of common variants used in practice:

• Box Caissons: Heavy, watertight boxes open at the top. They are floated to position, sunk, and filled with brickwork or concrete.

• Excavated Caissons: Cylindrical shells placed in a pre-dug location and filled to stabilize the foundation.

• Floating Caissons: Prefabricated boxes with cylindrical cavities that are easy to transport and install; often used where towing and set-down are preferred.

• Open Caissons: Open at both ends; suited to deep manholes, jetties, and tunnel access on soft ground where continued sinking is required.

• Pneumatic Caissons: Large waterproof boxes operated under air pressure for underwater work, enabling dry, safe excavation at depth.

• Monolithic Caissons: Large single-piece units cast as one element where high rigidity and size are required.

Construction Process: Step-by-Step

Design and execution require precision and experienced teams. A typical workflow is as follows:

1. Design

   Engineers design the caisson foundation based on structural loads and site conditions, including soil profile and water level.

2. Fabrication on Land

   The caisson is fabricated above ground. A cutting edge is fixed to its bottom perimeter to promote controlled sinking as soil is removed.

3. Site Preparation and Marking

   The installation zone is surveyed, marked, and secured for alignment, clearances, and access.

4. Transport to Location

   A towboat or equivalent moves the partially completed unit to the site when marine transport is needed.

5. Guiding and Positioning

   A guide frame or similar arrangement holds line and level while the caisson is lowered gradually.

6. Initial Ballasting

   Concrete is placed in stages to add weight so the unit begins to sink in a controlled manner toward design depth.

7. Pressure Control and Dry Workspace

   For underwater operations, compressed air lines and access tubes are connected. Air pressure is regulated to keep the interior dry.

8. Progressive Excavation

   Crews excavate through dredge wells or working chambers. As soil is removed, the caisson sinks further, with the guide frame maintaining alignment.

9. Reaching Design Depth

   Excavation continues until the specified elevation and bearing stratum are achieved. Position and plumb are re-verified.

10. Base Preparation and Concrete Filling

    The base is cleaned and prepared. Concrete Filling of the lower dredge wells and chamber is carried out to seal the bottom and develop bearing capacity.

11. Final Fill and Closure

    Remaining voids are filled and tops are sealed to complete the shell. The unit now functions as a solid foundation element.

12. Connection to Superstructure

    Caps, piers, or other uprights are constructed and connected so that loads pass efficiently from the structure into the caisson.

What are the advantages and disadvantages of caisson foundation?

While caisson foundations have several advantages, they also come with some cons. Here are a few points to consider:

Typical Applications

Caissons are widely used for bridge construction, docks, breakwaters, harbor works, and dam-related structures. Their ability to create a dry environment below water level makes them suitable for areas where excavation and placement must be controlled in saturated conditions, while delivering a solid foundation for heavy, permanent loads.

Good Practice Considerations

• Confirm soil and groundwater parameters early to choose the correct caisson type and size.

• Plan pressure systems and access routes for safe working in pneumatic or compressed-air conditions.

• Sequence concrete filling to maintain stability during sinking and after reaching depth.

• Maintain strict inspection regimes for alignment, elevation, and sealing.

• Integrate detailing for long-term durability in aggressive marine exposures.

When surface soils are weak or work must proceed below water, the caisson foundation offers a dependable deep-foundation solution. Built as a watertight unit, sunk to depth, and stabilized through concrete filling, it creates a controlled environment for excavation and then serves as a solid foundation for heavy, permanent works.