What Are Caissons and Caisson Foundations?

Several types of caisson foundation are used across foundation and marine construction:

• Open caissons have open tops and bases. Workers or mechanical equipment excavate material from within the shaft, allowing the unit to descend progressively. A cutting edge assists penetration, and the base is sealed after the required level has been reached.
• A box caisson has a closed base and is usually constructed in a controlled facility. The unit can float during transport before it is lowered onto a prepared riverbed or seabed and sunk into place by controlled ballasting.
• A pneumatic caisson contains a sealed working chamber near its base. Compressed air restricts water entry so excavation can continue in wet ground, although pressure exposure and access create specialist health and safety requirements.
• A drilled caisson is a large-diameter bored shaft formed by boring into soil or rock. Reinforcement is inserted before the excavation is filled with concrete, creating a cast-in-place structural element.
• Suction caissons are skirted steel units used in suitable seabed soils. A controlled pressure difference assists penetration and helps form an offshore foundation or anchor.

The term floating caissons refers to box-shaped units that are buoyant during transport. They are not left floating in service; once positioned, they are lowered onto their prepared foundation. Concrete caissons and steel systems can therefore belong to different categories according to their form and installation method.

The construction process begins with ground investigation, structural design and selection of a suitable system. Boreholes, laboratory testing and marine surveys can establish soil strength, groundwater, bedrock level, seabed profile and possible obstructions. Designers then set dimensions, founding criteria, reinforcement and construction-stage controls.

For an open caisson, the initial section is built around a cutting edge. As teams excavate from inside, self-weight helps the unit descend. Further wall sections may be added above, and survey checks track position and verticality. At the founding level, the base can be cleaned, sealed and filled to create the required permanent arrangement.

Box units follow another sequence. Contractors may prefabricate the structure in a dry dock or casting basin, launch it, tow it to site and lower it onto a prepared bed. Ballasting, marine conditions and temporary stability require close control while the caisson is sunk into place.

Drilled caissons are typically constructed using rotary drilling equipment to form a cylindrical bore. Temporary casing or drilling support fluid may stabilise the excavation where necessary. After cleaning and inspection, a reinforcement cage is placed, and concrete is installed using a method suited to groundwater and bore stability.

Pneumatic construction adds an access shaft, air locks and a pressurised working chamber. Because personnel may work under compressed air, this method requires specialist procedures and is considered only where its technical case is justified.

Caissons are suited to projects that combine high loads with difficult access, deep water or variable near-surface ground. Examples of caisson applications include:

• Bridge pier foundations within rivers, estuaries or navigation channels;
• Quay walls, dock structures, breakwaters and marine terminals;
• Offshore energy foundations, anchors and selected platform structures;
• Deep shafts and major substructures for rail or infrastructure schemes;
• Waterfront buildings and other heavily loaded structures near water.

Underwater foundations often need a method that limits dependence on a large open excavation. A caisson can provide both temporary support during installation and part of the permanent load path. However, some schemes still use cofferdams to create a protected work area around a foundation or to support adjacent construction.

Marine construction places additional demands on planning. Tides, waves, currents, navigation, scour and environmental restrictions may influence when and how a unit is installed. A prefabricated caisson also needs a viable float-out route between its fabrication area and final location.

Historic foundations may combine caissons with a masonry pier or abutment. Modern caisson construction more often uses reinforced concrete or fabricated steel, with the selected material responding to structural demand, exposure and installation requirements.

Foundation performance depends on more than depth. Designers assess end bearing, shaft resistance, settlement and lateral response, together with uplift, overturning and construction-stage loading. A caisson supporting a bridge pier may also need to resist current, debris, vessel collision or braking forces transferred from the deck.

Ground conditions influence every stage. Weak deposits can offer limited support during sinking, while dense layers, boulders or sloping bedrock may obstruct penetration. Uneven excavation can cause tilt; conversely, high wall friction may prevent an open unit from descending as intended. The design should provide monitoring criteria and planned corrective measures.

For box foundations, bearing-surface preparation affects alignment and load distribution. Scour protection may be needed where flowing water can remove material around the base. Floating stability must also be checked during launch, towing and lowering, before the permanent condition has been established.

Material selection reflects exposure and structural demand. Reinforced concrete needs suitable cover, joint detailing and durability provisions, particularly in tidal or chemically aggressive water. Steel caisson technology requires corrosion protection and fabrication details that account for lifting, transport and installation stresses.