Page 215 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.17

Sloped Banks. The most satisfactory bank protection for a country ditch is a stable slope and a
good cover of vegetation. This can be reinforced on the outside of bends and other places subject
to strong current action, by placing large boulders, walls, riprap, or light piling holding wire or
brush mats.
Stable slopes vary in steepness with the character of the soil. Loess may stand indefinitely in
vertical cliffs, while certain types of clay may slump if the slope is 1 on 6. Generally, it can be
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said that slopes of 1 on 2 ⁄ 2 or less are advisable if the soil contains much clay or silt; if there is
movement of groundwater through it toward the ditch; if there is considerable drainage of surface
water running down its face; or if it is in layers that dip toward the ditch.
Vegetation. If trees or bushes are to be planted or allowed to grow, the slope can be steeper than
if it is to be kept in grass. Trees have greater holding power, and maintenance of grass may require
the use of mowing machines, whose ability to work on side slopes may determine the grade.
However, trees will interfere with access for cleaning.
If the trench is partly or wholly in barren soil, topsoil may have to be spread on the banks to
encourage vegetation, although some plants generally can be found that will grow well on the sub-
soil if encouraged with lime or fertilizer.
Banks of permanently wet ditches may be strengthened by laying willow poles or logs up and
down the bank, 2 to 4 feet apart. They should be settled well into the soil for their full length, with
the lower end in water or bottom mud, and staked or wired in place.
These poles should grow tops and a continuous root mattress.
Bottom scour may be largely prevented by keeping a gentle gradient. A grass lining will help
prevent scour in a permanent ditch, as shown in Fig. 5.10. If part of the trench is too steep, check
dams may be built.
This localizes the fall of the water at the erosion-resistant aprons of the dams. Good results can
sometimes be obtained with plank dams, or even with heavily anchored brush mats.

FIGURE 5.10 Grass-lined ditch. (Courtesy of North American Green.)