Design of Temporary Cofferdams to Facilitate Dam Rehabilitation Projects

Abstract Only - It is common practice for the design engineer of a dam rehabilitation to leave design of temporary cofferdams to the contractor who is performing the repair work. Often a stamped cofferdam design is required as part of the contractor’s Control of Water submittals. Varying information is provided for use in the cofferdam design ranging from watershed models and chance of design storm exceedance to specific cofferdam design height elevations.
This paper will include three case studies illustrating various design methods for temporary cofferdams for control of water during construction. The paper will present cases that include temporary sheetpile cofferdams, earthen berm cofferdams, and concrete block cofferdams. Depending on the height of water to be retained and the flow rate, each option offers pros and cons and various design strategies are employed.
Sheetpile cofferdams are often appropriate when there are larger volumes of water to control or when seepage is a main concern. They may be driven in single rows or for cases where rock is near the surface, a double row of sheetpiles connected with threaded tie rods and the annular spaced filled with shot rock or ballast may be used. In cases where significant earthwork is required and the existing on-site soils are suitable, an earthen berm may be the most cost-effective solution. An example will be illustrated where a new labyrinth spillway was constructed in the center of an existing embankment dam and a horseshoe shaped temporary earthen cofferdam was built utilizing soil from the excavation for the proposed spillway. Often dam rehabilitation projects require repair of select elements while others must remain intact and in service. The third case study will discuss the use of concrete bin blocks to control water on existing slabs and concrete spillways. In each of the case studies, various design elements will be discussed as well as details such as tie-ins to the existing structure.