Caisson FoundationWHAT IS CAISSONS?Its a prefabricated hollow box or cylinderIt 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 waterThis is because caissons can be floated to the job site and sunk into placeBasically it is similar in form to pile foundation but installed using different wayused when soil of adequate bearing strength is found below surface layers of weak materials such as fill or peatIts a form of deep foundation which areconstructed above ground level, then sunk to the required level by excavating or dredging material from within the caissonTYPES OF CAISSONSBox CaissonsExcavated CaissonsFloating CaissonsOpen CaissonsPneumatic CaissonsADVANTAGESEconomicsMinimizes pile cap needsSlightly less noise and reduced vibrationsEasily adaptable to varying site conditionsDISADVANTAGESExtremely sensitive to construction proceduresNot good for contaminated sitesTHE PROCESS: BUILDING A CAISSONAfter 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 wellsWhen 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 depthAs a final step, concrete is placed (poured) into the bottom 30 feet of the hollow dredge wells and the tops are sealed.