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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

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