Page 229 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.31

To determine the size of a culvert, the drainage area is measured or estimated. Topographic or
airplane maps are particularly useful for this purpose. The number of acres, or the next-higher figure,
is selected in the left-hand column. The figure opposite this acreage, in the vertical column whose
description best fits the area in question, is taken and multiplied by the rainfall rate shown for the
locality by the appropriate map.
This will give the culvert area in square feet. To obtain the diameter of the proper size of round
pipe, use the formula

area
Diameter 2
(twice the square root of the area divided by 3.14).
The indicated size should be increased if full culvert capacity may not be available, or any
local conditions (such as abnormally intense rainfall, or extremely steep and nonabsorbent slopes)
indicate the need.
Even generously designed culverts may be inadequate for exceptional storms, as it is seldom
economically practical to provide for them.

Sidewalls or Headwalls. Sidewalls serve to hold embankments from falling into inlet or outlet
channels; to direct water into and away from the passage or barrel; to reduce turbulence and pre-
vent undercutting of the embankment; to support the ends of the culvert, and to hold pipe sections
against separating inside the fill.
The wall requirement is reduced by lengthening the pipe, as in Fig. 5.21, top. Pipe resting on the
original grade, or projecting clear of the bank as in Fig. 5.22, does not usually need a wall at the
outlet.
A sidewall is usually of reinforced concrete but may be of stone, wood, or metal. It may be of
heavy construction and firmly founded to resist movement in any direction; or it may be light and
superficial, so that any settlement will affect it to the same extent as the pipe.
It is most convenient to place wall footings before laying the end pipes.
Metal headwalls (Fig. 5.22), are fastened to corrugated pipe by standard couplings. They can
be removed and reused if the culvert is lengthened.
Alignment. Culvert barrels should be straight under most circumstances.
It is usually desirable to use the original channel for the culvert passage and to have the culvert
cross the road at right angles to the centerline.
These two objectives are often in conflict.
The original channel may be undesirable if it is crooked, crosses at a sharp angle or skew,
shows rock ridges, is made up of soft mud, or has a strong flow of water. In such cases it may be more
economical to dig a trench nearby, lay the pipe, and then divert the stream into it.
Right-angle alignment may be ignored if the natural channel is diagonal, and can be easily pre-
pared for the culvert; or when excessive trenching is required to bring the stream straight across.
Referring to Fig. 5.23(C), it will be seen that a slight change in stream alignment under the
road can lead to a considerable amount of digging on the side, that the new stream channel will
be out of balance, and it may require mats or revetments to protect the outside banks of the curves.
On the other hand, changes may involve comparatively little excavating and produce more sat-
isfactory channels than the original.
If good alignment between stream and culvert cannot be obtained on both sides, the upstream
side should be favored. When the capacity of the culvert is heavily taxed by a storm, it is advan-
tageous to get the water into the culvert smoothly.

Gradient. It is desirable to lay the culvert on the floor of the natural channel, on the original
ground surface, or in a smoothly dug ditch. This gives firmer support than fresh fill. Inequalities
in channel or ground are smoothed by cutting off ridges and tamping fill into hollows.
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The passage should have at least a ⁄ 2 percent slope, but 2 to 4 percent is preferable. It should
not be over 8 or 10 percent, because of probable erosion of the bottom of the lining. The gradient