Cofferdams

They are connected to each other by a pipeline system that heads up at a water pump. Well point engineers determine the groundwater level and the direction of flow of the groundwater, and the well point system is placed so as to cut off the flow into the construction area. Well pointing requires highly specialized personnel and expensive equipment. Cofferdams The cofferdam is a temporary structure, usually built in place, and tight enough so that the water can be pumped out of the structure and kept out while construction on the foundations is in progress. Common cofferdam types are earthen, steel sheeting, wooden sheathing, and crib. Figure 10-30 shows a cofferdam under construction. An earthen cofferdam is built by dumping earth fill into the water, shaped to surround the construction area without encroaching upon it. Because swiftly moving currents can carry the material away, earthen cofferdams are limited to sluggish waterways where the velocities do not exceed 5 feet per second. Use is also limited to shallow waters; the quantities of material required in deep waters would be excessive due to the flat slopes to which the earth settles when deposited in the water. For this reason, the earthen type is commonly combined with another type, such as sheathing or cribbing, to reduce the quantities of earthwork. Steel is commonly used for cofferdam construct- ion. Sheet piling is manufactured in many interlocking designs and in many weights and shapes for varying load conditions. The piling is driven as sheeting in a row to forma relatively tight structure surrounding the construction area. This pile wall is supported in several ways. It may be supported by a framework of stringers and struts. A cofferdam wall can consist of a double row of piles tied together with heavy steel ties and filled with earth. This can square, rectangular, circular, or oval shape for stability around the construction area. Wooden sheathing, instead of steel, is similarly used in cofferdam constructions. Interlocking timber sheathing is driven as a single wall and supported by stringers and cross struts between walls, or it is driven in double rows as a wall. The sheathing in each row is connected and tied with braces. Wooden or concrete cribbing may be used in cofferdam construction. The cribbing offers stability Figure 10-30.-Cofferdam under construction. 10-16