Page 257 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.59

weight causes the water to enter the sand columns and rise into the drains. If the columns are properly
spaced, sufficient water will be removed to stabilize the mud sufficiently to carry the intended load.
Part of the fill may be placed on the swamp, then the holes made by sinking a hollow-walled
pipe by jetting with water and compressed air, or by driving a hollow pile with a detachable head.
In either case, when the tube has reached the bottom of the soft layer, it is filled with sand and
pulled, leaving the sand in the hole. Hole diameters of 16 to 24 inches are commonly used, with
spacing varying between 8 and 22 feet.
If the tube is pulled by conventional methods, the sand may stick to it and be raised sufficiently
to allow mud to enter crevices in it and interrupt the drainage. This may be avoided by attaching
an airtight head to the tube after the sand has been placed, and pumping compressed air between
the head and the sand. This raises the tube but exerts an equal downward force against the sand
and holds it in place and together.
The tops of the columns may be drained by spreading a blanket of clean gravel or sand, a foot
or more in depth, over the whole area; or by connecting the columns with tile or rubble drains. If
the fill is all gravel, a drainage layer may not be needed.
Settling may be still further speeded by placing more fill than will be required for final grade.
The extra weight will squeeze the water out of the mud more quickly. When settlement is judged
to be complete, the extra fill is removed.

EXCAVATIONS

It is desirable to remove surface water and groundwater from areas to be excavated, but the cost
may exceed the advantages gained.
Water may be removed naturally by seasonal change, or artificially by diversion, draining,
siphoning, or pumping.
Seasonal Lowering. The seasonal decline in the groundwater level may be quite considerable in
areas having dry summers. Some places that are so wet in winter and spring as to be very expensive
to work, may become dry to depths of 5 to 30 feet. Permanent swamps may develop crusts suffi-
cient to allow movement of machinery.
Such changes are not uniform, as a wet season may keep water levels abnormally high while
an exceptional drought will cause extreme or unseasonable lowering.
When it can be arranged, it is obviously good practice to undertake wet excavation in a dry period,
as any reduction in either mud or unwanted water will reduce costs. The economy is greatest in
work in marshy areas and in shallow excavations, but may be noticeable even in deep work.

PUMPING

Dewatering of excavations commonly requires the use of pumps.
If the water is small in volume or contains a heavy load of mud or other solids, a diaphragm pump is
preferred. A centrifugal pump is needed for larger quantities. A centrifugal pump should be placed as near
the water level as possible. More energy is used pumping water over a high bank than over a low one, but
the total lift may be largely determined by the job. However, these pumps will push the water more effi-
ciently than they will pull it, and much better output will be obtained by keeping the suction line short.
More details about the pumps can be found in Chap. 21.

Inlet Protection. The low end of the inlet pipe should be fitted with a screen that will prevent
entry of any object large enough to plug the pipe or damage the pump. Water containing leaves or
other fibrous matter will clog such a screen readily, and may make necessary the placing of an
1
1
outer screen of ⁄4- or ⁄2-inch mesh, or some similar wire. This outer screen is best located far
enough from the inlet that water going through it will not have force enough to hold rubbish
against it to block it. See Fig. 5.50.