Engineering and Construction

Steel sheet piling is widely used for the construction of a cofferdam, which is defined as a temporary structure designed to support the sides of an excavation and to exclude water from the excavation. Tomlinson (1986) presents an in-depth discussion of the different types of cofferdams and construction techniques. For example, the first step in the construction of a bridge foundation could be the installation of a large circular cofferdam. Then the river water would be pumped out from inside the cofferdam and the river bottom would be excavated to the desired bearing strata for the bridge foundation.

In general, the following loads could be exerted on the cofferdam:

1. Hydrostatic groundwater pressures located outside the cofferdam. If the cofferdam is constructed in a river or other body of water, then the hydrostatic water pressure should be based on the anticipated high water level.
2. Hydrostatic pressure of water inside the cofferdam if the water level outside falls below the interior level during initial pumping of water from the cofferdam. This condition could cause bursting of the cofferdam by interlock tension.
3. Earth pressure outside the cofferdam.
4. Other loads, such as surcharge loads, wave pressures, and earthquake loading.


Based on these loading conditions, the cofferdam must have an adequate factor of safety for the following conditions:

1. Heave of the soil located at the bottom of the cofferdam.
2. Sliding of the cofferdam along its base.
3. Overturning and tilting of the cofferdam.
4. The inward or outward yielding of the cofferdam (i.e., maximum moment and shear in the sheet piling and interlock tension).
5. Piping of soil that could undermine the cofferdam or lead to sudden flooding of the interior of the cofferdam.

An important consideration in the design of cofferdams is the ability of the sheet pile wall to deflect. If struts are used that brace the opposite sides of the excavation (i.e., braced excavation), the deflection of the sheet pile wall will be restricted. The next section will discuss the earth pressures exerted on braced excavations. If the sheet pile wall is able to deform, such as by using raking braces, then the deflections of the wall during construction will permit mobilization of active pressures in accordance with the previous discussion. The USS Steel Sheet Piling Design Manual (1984) and NAVFAC DM-7.2 (1982) present a further discussion of the design of cofferdams.