Page 205 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.7

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Assume that a ⁄ 2 -yard hoe with a 36-inch-wide bucket, including side cutters, is digging a ditch
3 feet wide and 6 feet deep in common earth, with no special complications.
This ditch has a width of 1 yard and a depth of 2, so its cross section is 2 square yards. There
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will be 2 cubic yards removed for each lineal yard of digging, or ⁄ 3 yard per lineal foot.
This machine may have a cycle of 13 seconds, and an efficiency hour of 45 minutes. It should
complete 206 cycles per hour.
The soil has a swell factor of .8 (25 percent swell). The bucket averages only four-fifths of a
load in loose yards; that is, its efficiency factor is .8. Multiplying the swell factor by the efficiency
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factor by the ⁄ 2 -yard capacity of the bucket, we have
.8 .8 .5 .32 bank yard per cycle

Multiplying 206 cycles per hour by the .32 bucket load, we have a production of 65.92, say 66, yards
per hour. Since there is 2 cubic yards to each running yard of ditch, the ditching rate is 33 yards or 99
feet (say 100 feet) per hour.
A 30-inch-wide trench with a 30-inch bucket would come out about the same, as what was
gained in handling smaller volume would be lost in poorer bucket efficiency. However, if the
ditch were 12 feet deep, either bucket would probably fill well.

OTHER SHOVEL RIGS

Clamshell. A clamshell ditches best when on the centerline. If the ditch is narrow, the tagline
chains are fastened to one jaw, or for a very wide cut, to both jaws. A ditch of intermediate width
is made with the chains in the one-jaw position, and the soil is taken out in layers.
Connections between ditch sections are made by attaching the tagline chains to both jaws after
completion of the main ditching, and digging the connection from the side. Whole ditches may be
done from the side in this manner, but it is harder to keep on the correct line. The side position is
desirable in deepening an existing ditch, or in digging beside a wall.
Smooth curves may be dug either by frequent readjustments of the position of the shovel, in the
same manner as with a backhoe, or by having someone on the ground twist the bucket into proper
position by pushing it by hand or with a stick as it is about to touch the ground.

Dragline. The dragline is the preferred shovel for ditching in swamps, and for making ditches
with sloped banks when the spoil is to be piled alongside. It works along the centerline of the
ditch, as in Fig. 5.5(A), cutting the bottom and slopes in one operation. If the ditch is too wide for
this, two cuts are made from the sides, as in (B) and (C).
If the fill is to be trucked away, a dragline or a backhoe may be used in this manner. Draglines
may have difficulty digging hard earth that the hoe would move easily.

Front Shovel. The front shovel can dig trenches 4 to 8 feet in depth from the top, or wide trenches
from the inside. A neat ditch may be dug from the top by straddling it, if the soil is very firm or
if support platforms are used. Trenching may also be done beside and parallel to the shovel’s path,
but this involves quite a wide cut in proportion to depth and is difficult to trim.
Interior digging conforms in general patterns to that discussed for basements in the previous
chapter. Partial swing shovels can dig narrower slots than conventional models, as they do not
need space for tail swing, but they cannot load trucks behind them.

Comparisons. The backhoe is the best machine for ditches of moderate depth and width where
boulders or stumps may be encountered. It will break up heavily fractured hard rock and soft or
thin bedded shale, and will dig very hard soils if the bucket teeth are long and sharp. It can dig out