Text F Miscellaneous Types of Dams

A timber crib dam is made of timber bolted into cribs and filled with rock. The life of a timber dam varies from 10 to 40 years depending upon climatic conditions and the type of timber used. Cedar, redwood and cypress are the most durable timbers. Where there is an adequate supply of timber, crib dams are quite inexpensive and generally satisfactory for heights not exceeding about 20 ft.

Cofferdams are temporary structures used to divert water from an area where the construction operations are to be conducted. A cofferdam must be relatively low in cost but as watertight as possible. The simplest kind of cofferdam is a single wall sheet-pile type, sometimes strengthened by an earth fill on both sides. A single wall sheet-pile cofferdam can be used only in very shallow water. For depths over about 5 ft cellular cofferdams are commonly used. These consist of circular sheet-pile cells filled with earth. A mixture of sand and clay is the best filler since clay itself tends to wash out through the gaps between the piles while sand is too permeable. Timber-crib dams are sometimes used as cofferdams. The simplest embankment often requires too much room because of the large base width necessary for an embankment of any height.