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.
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