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Laying The Foundation: Caissons Explained
This article includes interactive multimedia features that explain caisson construction. The Flash 6 player is required to view these presentations. If features do not appear, you must download the Flash 6 player.

 

Fall, 2002 - A major highlight in the construction of long-span bridges is the building of towers, the tallest and most recognizable components of the structure. It is the towers, after all, that characterize a bridge and burn its image into our memories, whether it is the great stone arches of the Brooklyn Bridge or the bright red art deco of the Golden Gate.

Equally as fascinating, but far less visible, is the process of building the massive underwater foundations on which these towers rest, structures called caissons.

What exactly is a caisson and how are they built? Much can be learned by studying the word itself. Caisson is derived from the French word caisse, meaning "box". Strictly speaking, a caisson is defined as a box used for underwater construction. Originally, these boxes were open-bottom structures; compressed air was pumped into the box to force out the water and to keep it from re-entering through the bottom. Workers labored inside this pressurized box to remove soil and other unsuitable material by hand. As this material was removed, the box would sink into the ground and down to the founding material -- the bedrock, clay or other earth material on which the caisson would rest.

The two caissons for the two main towers of the Greenville Bridge will begin as short, open-topped boxes of steel and concrete. In their completed form, they will be columns of concrete 175 and 205 feet (53m and 62m) tall that will reach from far below the river bottom to just above the water line. Unlike the pneumatic (pressurized air) caissons of old, these caissons will be open dredged, meaning river bottom material will be removed through the open top of the long caisson box. As soil and other material is removed, the caissons will sink into the river bottom to their planned elevations.

To understand how these caissons are being built, we will first look at the various components used in their construction.

 

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.

Caissons will form the foundations for Pier 37, off the Arkansas shore of the river, and Pier 38, just off the Mississippi side. Both piers are located approximately 600 feet (183m) from their respective shores. The river is 70 to 80 feet (21 to 24m) deep at these locations. (See the plan and elevation diagram on the Technical Documents page)

 

CONSTRUCTION TIMETABLE

The cutting edge for Pier 37 was floated into position on April 10, 2002, with the cutting edge for Pier 38 following on June 26, 2002. View photos for both the Pier 37 and Pier 38 cutting edge placements in our Photo Album.

Each caisson will take 12 to 16 months to complete, with 6 to 8 months of concrete placement needed for the caissons to reach river bottom and another 6 to 8 months for them to reach their planned elevations below the bottom of the river.


Selected questions regarding construction of the Greenville Bridge are periodically answered on our
Ask The Engineer
page. Submit your questions by e-mail to learn@greenvillebridge.com.