Caisson Foundation

Caisson Foundation

WHAT IS CAISSONS?

• It’s a prefabricated hollow box or cylinder

• It is sunk into the ground to some desired depth and then filled with concrete thus forming a foundation.

• Most often used in the construction of bridge piers & other structures that require foundation beneath rivers & other bodies of water

• This is because caissons can be floated to the job site and sunk into place

• Basically it is similar in form to pile  foundation but installed using different way

• used when soil of adequate bearing strength is found below surface layers of weak materials such as fill or peat

• It’s a form of deep foundation which are

• constructed above ground level, then sunk to  the required level by excavating or dredging material from within the caisson

TYPES OF CAISSONS

• Box Caissons

• Excavated Caissons

• Floating Caissons

• Open Caissons

• Pneumatic Caissons

ADVANTAGES

• Economics

• Minimizes pile cap needs

• Slightly less noise and reduced vibrations

• Easily adaptable to varying site conditions

DISADVANTAGES

• Extremely sensitive to construction procedures

• Not good for contaminated sites

THE PROCESS: BUILDING A CAISSON

• After some initial form work and concrete pours, the cutting edge is floated to the breakwater by towboat and fastened to the caisson guide. Concrete is placed (poured) into steel forms built up along the perimeter of the box. With every concrete placement, the box becomes heavier and sinks into the water along the caisson guide.

• Forms are also built inside the box around the air domes and concrete is placed in between. The resulting open tubes above the air domes are called dredge wells

• When the caisson finally touches the river bottom, the air domes are removed and earth is excavated through the long dredge well tubes, as shown in the animation below. The caisson sinks into the river bottom. Excavation continues until the caisson sinks to its predetermined depth

• As a final step, concrete is placed (poured) into the bottom 30 feet of the hollow dredge wells and the tops are sealed.