Page 216 - Moving the Earth_ The Workbook of Excavation
P. 216
DITCHING AND DEWATERING
5.18 THE WORK
FIGURE 5.11 “W” ditch.
Spoil Arrangements. The spoil piles from a ditch are apt to interfere with local surface
drainage. If the land is flat, spoil may be piled on both sides, but frequent breaks should be made
in the windrows so that water will not pond behind them. If the ditch cuts across a slope, these breaks
need be made in the upper pile only. Until the ditch slope is protected by vegetation, gullies may
form at these spots unless the area is protected by pipe, flumes, or stone.
The “W” ditch seen in Fig. 5.11 is a double ditch, separated by sufficient space to provide disposal
area for the spoil. This eliminates any blocking of drainage on either side, and allows maintenance
of field grade to the ditch edge.
If the depth of the ditch is determined by the flow capacity required, two can be shallower than
one. This construction is only slightly more expensive than the single drainageway, although it
usually takes more land out of production.
If depth is determined by a flow gradient, a W ditch will about double the amount of excavation
required. If the depth is considerable, the additional spoil is likely to damage an excessive area.
Whenever possible, the spoil piles from permanent ditches should be rounded off so as to permit
easy access to the ditch and to make them less prominent in the landscape. This should be done
before they are overgrown by trees.
PIPE AND CONDUIT TRENCHES
Most trenches are dug for the purpose of burying pipes or conduits. Conduits, and pipes for gas
and water supply, run at more or less fixed depth below the surface. Sewers, storm drains, and
other gravity-flow pipes must maintain a minimum gradient from source to outlet or booster
pump, and will have a variable depth below an irregular surface,
Fixed Depth. In cold-winter areas, water pipes are laid below the frost line in the ground.
Conduit and wires are laid only deep enough to protect them against accident. In either case, depth
may be increased under sharp ridges so as to provide smooth vertical curves.
In fields, the most important menace is the moldboard plow, which penetrates 8 or 10 inches.
There is a chance that a subsoil plow, with a penetration of 18 inches to 2 feet, might be used; in
addition, land even on gentle slopes gradually washes away, and the surface may be lowered sev-
eral inches during the life of the conduit. Depths of 2 to 4 feet are usual, and are figured from the
surface of the ground regardless of slope.
Gravity Systems. Close supervision is required to ensure accurate digging for a sewer or other
gravity system. A number of methods are used to keep ongrade, following the surveying methods
discussed in Chap. 2.