Sheet Piling Cofferdams

A cofferdam is a retaining structure, usually temporary, which is utilized to keep water or earth out of an excavation site until the permanent works are constructed. Such struc- tures usually consist of vertical sheet piling walls forming a closed perimeter and braced internally or externally by a sys- tem of structural members or ties.

The bracing system consists of horizontal members called wales, which transfer loads from the sheet piling to compres- sion members called struts, or to external supports called tie backs or anchors.

The safe design of these temporary structures is important, since the safety of workmen or the protection of other struc- tures is almost always involved. The participation of wales and struts in the overall stability of cofferdams must be eval- uated much more closely than the support system of an anchored bulkhead. The sequence of construction regarding the excavation of soil or the pumping out of water from a cof- ferdam will affect the loads on all elements of the system. Maximum loading conditions may occur during the construc- tion phase, or during the placement of permanent work due to removal or relocation of bracing, rather than after the cof- ferdam is completed. In addition, pressures outside the wall may contribute to instability of the floor of the cofferdams that must be evaluated and accounted for in design planning.

Types of sheet piling cofferdams include the following:

• Water Cofferdams. Sheet piling "box" cofferdams are virtu- ally the only means for constructing permanent piers for bridges or other structures in water under dry conditions. Loads on these walls consist of unbalanced pressure from water and submerged earth.

• Land Sited Cofferdams and Trench Retainment Systems. Sheet piling cofferdams are constructed on land for any under- ground construction where workmen and adjacent structures must be protected against collapse of the excavation walls. If the area to be excavated is relatively small in area or the wall- to-wall distance is reasonable, internal cross bracing consist- ing of wales and struts is practical. If the area is large and the walls well apart, internal braces such as sloping raker struts, or external anchors behind the wall must be used. The values of earth pressure and its probable distribution against rigidly braced walls have been found to be at some variance with the pattern obtained by anchored, yielding walls.

This article comes from pilebuck edit released