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Cofferdam - Cellular Cofferdams
A cofferdam is a structure that retains water and soil that allows the enclosed area to be pumped out and excavated dry. Cofferdams are commonly used for construction of bridge piers and other support structures built within water. Cofferdams walls are usually formed from sheet piles that are supported by walers and internal braces, and cross braces. Cofferdams are typically dismantled after permanent works are completed.Since cofferdams are usually constructed within water, the sheet piles are installed using preconstructed templates that permit the correct positioning of each sheet pile from a barge.
Image Above: Steel sheet pile cofferdams for $.3.9 Billion Tapan Zee Bridge in New York, designed by Siefert Associates with DeepEX software program by Deep Excavation
A cofferdam can in essence be constructed in almost any desirable shape. However, for practical and reasons of economy most cofferdams are either rectangular or circular. In general circular cofferdams offer the advantage that they provide a more open excavation over the entire plan area when bracing is provided solely by ring beams. However, installaton of sheet piles in circular cofferdams requires stricter construction controls. Rectangular cofferdams are simpler to construct but typically require more complex internal bracing.
1. Braced cofferdams: They are formed from a single wall of sheet piles that is driven into the ground to form a “box” around the excavation. The sheet piles are then braced on the inside and the interior is dewatered. It is primarily used for bridge piers in shallow water (30 - 35 ft, 9 to 12m depth).
2. Cellular cofferdams: Cellular cofferdams are used only in those circumstances where the excavation size precludes the use of cross-excavation bracing. In this case, the cofferdam must be stable by virtue of its own resistance to lateral forces.
3. Double-walled sheet pile cofferdams: They are cofferdams comprising two parallel rows of sheet piles driven into the ground and connected together by a system of tie rods at one or more levels. The space between the walls is generally filled with granular material such as sand, gravel or broken rock.
Cofferdam wall for the high speed train bridge at the River Po bridge in Italy
COFFERDAM CONSTRUCTION SEQUENCE:
For a typical cofferdam, such as for a bridge pier, the construction procedure generally is:
1. Pre-dredge to remove soil or soft sediments and level the area of the cofferdam
2. Drive temporary support piles for template
3. Temporarily erect bracing frame on the support piles for the template
4. Install steel sheet piles, starting at all four corners and meeting at the center of each side
5. Drive sheet piles to grade
6. Block between bracing frame and sheets, and provide ties for sheet piles at the top as necessary.
7. Excavate inside the grade or slightly below grade, while leaving the cofferdam full of water. Then lower the water inside and progressively install internal bracing as required by the design.
8. Drive piles within the cofferdam (if required).
9. Place rock fill as a leveling and support course.
10. Place tremie concrete seal.
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