Page 220 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

5.22 THE WORK

Gradients. The slope or gradient of a drain will depend on the work it has to do. In tidal marshes
and other practically flat swamps, ditches with zero gradient may serve to lower the water level
substantially. In general, water will flow through a flat ditch, but is easily choked by sediment,
growth of weeds, and dirt falling from banks, as the water flowing through it will have little or no
ability to clean it. Too steep a ditch gradient may cause erosion of the bottom, undermining of
banks through stream action, and damage from depositing of mud below the discharge point.
The slope must be adjusted first to the necessities of the situation, and second to the relation
between the amount of water to be carried and the nature of the soil. A bottom gradient between
1-foot drop to 1,000 feet and 2 feet to 100 feet is desirable under most conditions encountered.
Drainage pipes should not be flat, as costs in cleaning out sediment and debris will be very
high. Low gradients can be used when the water is clean, the pipe is short and large enough to
allow personnel to work in it conveniently, or there is a sharp fall at the outlet so that water will
flow rapidly. Generally, the minimum gradient should be 6 inches to 1,000 feet, and the maximum
2 feet in 100 feet for land title, and 10 feet in 100 for tight joint pipe.

Surface Water. Surface drainage may consist of disposal of water from rain or melting snow, or
lowering the water level in ponds, ditches, or swamps. It may use open channels, conduits, or both.
The water is usually led to a natural stream or body of water.
Such drainage may be accomplished by deepening, enlarging, or straightening and protecting
existing streambeds; by digging artificial channels; or by installing underground pipes or tunnels.
There is no definite separation between surface and subsurface drainage as they operate on dif-
ferent parts of the same water mass.

Trenchless Excavation. If it is not practical to ditch to install a drain or diversion pipe, boring
or tunneling may be used. Trenchless excavation construction methods include all the methods of
installing utility systems below grade without direct installation into an open-cut trench. These
methods are differentiated from the large-diameter tunnels by several factors, such as purpose or
use and diameter of excavation. Tunneling is discussed in Chap. 9.
Classification. There are three major categories of trenchless excavation: horizontal earth
boring, pipe jacking, and utility tunneling. Horizontal earth boring includes methods in which
the bore-hole excavation is done by mechanical means without workers inside the bore hole. See
Fig. 5.14. Both pipe jacking and utility tunneling methods require workers inside the bore hole
during the excavation and casing installation process. However, they are differentiated by the
support structure. The pipe is the structure in one, and the other has liners installed for the structure.

FIGURE 5.14 Trenchless excavation with earth auger borer. (Courtesy of ASCE Journal of Construction
Engineering Management.)