Page 214 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

5.16 THE WORK

Digging a straight trench from a dredge or barge takes experience. Anchors and winch lines
must be so arranged that the barge can both pull itself across the stream, and keep or regain position
against pressure of the current. Alignment is checked by surveying instruments that may be on the
barge, on the shore, or in both places.
Many streams have sufficient current to fill in a trench as fast as it is dug. At low-water periods it
is usually possible to block off a substantial part of the stream channel with a fill or jetty, trench
in it or just downstream from it, lay the pipe, and remove the fill. The operation is then repeated in
another section. Erosion at the ends of the fill may be severe.
The jetty may be built from one bank to the stream center, and then from the other bank to con-
nect, or a dragline may build itself an island that it moves by digging at the rear and filling the front.
The pipe may be assembled in sections as trench space becomes available; or the whole crossing
may be put together at one time, dragged and floated across the water, anchored in approximate
position, and sunk into the trench as sections of it are completed. Floating and sinking of the pipe
are regulated by the amount of air or water in it, and/or by floats and weights.
Work of this type is subject to disastrous damage if the stream floods. If there is a little warning,
equipment can be pulled back from shore-based fills, but a dragline on its own island is likely to
be lost. A wire rope connection should be kept to ensure getting back the crew if the water rises
too suddenly to permit rescue by boat, and to help locate and salvage the machine if it capsizes.
Floating pipe may be pulled back to shore.
Because of flood danger, all possible preparations are made in advance, and the crossing
pushed as rapidly as possible once it is started. The urgency depends largely on the past behavior
of the stream at that time of year, but few streams are ever entirely secure against flooding.
Small Streams. It is seldom practical to ditch directly across a brook without taking precautions
to keep it out of the trench. This may be done by digging a sump hole upstream, and using a pump
or pumps to move the water across the ditch line and back into the stream.
A less expensive method is to confine the stream flow to pipes, and work under them.
Sufficient pipe to accommodate the expected flow is laid in the stream across the ditch line.
Sectional corrugated pipe should have seams filled with mastic to prevent drip leaks.
Dams are then built across the brook above and below the ditch area, confining the flow to the
pipes. There may have to be two sets of dams, a first pair of light temporary ones toward the pipe
ends to permit drying out areas where the regular dams can be built of selected soil, carefully
tamped around the pipes. Bentonite might be added to the soil to improve water resistance. One
dam may be made wide enough to serve as a road for machinery.
The ditch is then dug in the regular manner, under and on each side of the pipes. Water oozing
into the ditch can be pumped out, diverted by well pumping, or blocked off by grouting.
When the pipe has been laid in the trench and the path or road across the stream is no longer needed,
the dams are dug away and the trench backfilled, and the pipes are lifted out for reuse elsewhere.
If stream flow increases beyond pipe capacity, it will overflow the dams, fill the trench, and
stop work. After the flow has subsided, the dams are repaired and the trench is pumped out.
Redigging to remove washed-in soil may be necessary.

PERMANENT DITCHES

When a ditch is to be left open permanently, its sides usually must be protected by masonry or rot-
resistant sheet piling, or sloped back far enough that they will not slump, cave, or wash into the
bottom. If a large volume of water may flow through the trench at any time, the bottom should
also be protected against erosion, unless the gradient is so flat, or the water so burdened with silt,
that cutting will not occur.
Ditches with low gradients, or which carry dirty water, must be cleaned out periodically by a
dragline shovel or other excavator. Masonry, riprap, and in particular vertical stone walls interfere
with machine digging and are liable to be damaged. This should be borne in mind in designing
any artificial protection.