Page 221 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.23

FIGURE 5.15 Trenchless excavation with earth boring machine. (Courtesy of Bor-It Mfg. Co. Inc.)

Earth Boring Methods. The horizontal auger earth boring method uses the process of simul-
taneously jacking the casing through the earth while removing the spoil inside the encasement, as
shown in Fig. 5.15. There is the slurry rotary drilling method, which uses drill bits and tubing with
fluid to remove the spoil earth, instead of augers and cutting heads. The drilling fluid is a bentonite
slurry, water, or air which aids in the removal of spoil. This method of horizontal directional
drilling is discussed in greater detail in Chap. 20.
There is also the compaction method which compresses and displaces the earth surrounding
the casing. This method is restricted to relatively small lines, perhaps 2 to 6 inches in diameter in
compressible soil. Another is the water-jetting method, which uses the principle of soil liquefaction
to make the bore hole. This is similar to methods for sinking pipe in the ground vertically. There
are several other methods for horizontal earth boring, but these should be enough to give an idea
of trenchless excavation.

Siphons. If a drainage line is to be used only occasionally, the expense of ditching or tunneling
may be avoided by use of a siphon. This is an airtight pipe or stiff hose across the water barrier,
with one end in the water to be drained, the other at a lower level (Fig. 5.16). Maximum rise above
water level is about 25 feet. Lower lifts work better.
When the siphon pipe has been filled with water, that which is between the high point and the
lower or discharge end moves down the pipe by gravity, while atmospheric pressure, acting through
the pond water, pushes the shorter and lighter column of water after it. This water is in turn renewed
from the pond so that movement continues until the water level drops sufficiently to outbalance
the suction, or to allow air to enter the pipe; air enters through leaks or through the discharge end,
or water rises around the outlet to the same level as the intake.
The rate of flow will depend chiefly on the drop between the top of the water being drained and
the point where the water loses contact with the outlet end of the siphon. As a pond is drained and its
level drops, the flow will become slower.