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
