Figure 3-13.-Types of culverts.
a deep V-bottom ditch, the side slope is 3:1, 2:1, or 1:1.
Culverts
The side slope for a trapezoidal ditch is 3:1,2:1, or 1:1.
The flat bottom is generally 2 feet wide but can range
from 1 foot to 6 feet or more.
Interceptor Ditches
The volume of water draining into a roadway ditch
can be decreased by the use of shallow ditches that
extend around the top of the cut and intercept the water
draining from the original ground toward the roadway.
An interceptor ditch shown in figure 3-12 is dug 2 or 3
feet behind the backslope limits. Its size depends on the
original ground slope, runoff area, type of soil and
vegetation, and other factors related to runoff volume.
Diversion Ditches
As it leaves the cut, water from the roadway ditches
cannot be allowed to pond in the ditches or against the
roadway fill. Therefore, diversion ditches are dug to
carry the water away from the roadway to natural drains.
These drains can be rivers, streams, gullies, sinkholes,
natural depressions, or hollows.
Sometimes it is necessary to have the water flow
from one side of the road to the other or have the road
cross a small stream. You do this with cross drains. They
are called culverts if they are 10 feet or less in width.
Over 10 feet wide, they are called bridges. Culverts are
made of many materials, such as corrugated metal,
reinforced concrete, concrete pipe, timber, logs, and
even open-ended oil drums. The type of material
selected is dependent upon various factors including, in
part, the type and life expectancy of the road.
For permanent roads and highways with concrete or
asphalt paving, the most durable of materials, such as
reinforced concrete or concrete pipe, should be used.
Concrete pipe is one of the strongest and most durable
materials used in making culverts. The shell thickness
and length depend on the pipe diameter. (The larger the
diameter, the thicker the shell and longer the section.)
Pipe diameters are nominal inside dimensions. For
semipermanent and temporary roads, the design
engineer may choose to use materials such as those
shown in figure 3-13.
3-13
