es from 24 to
30 feet for main roads. The headwall to the culvert should not be a
monument, but should be no higher than needed to prevent vehicles from
leaving the roadway at the culvert.

=Farm Entrance Culverts.=--At farm entrances, culverts are required to
carry the farm driveway across the side ditch of the road. These
culverts are usually about 16 feet along, and should be of a size
adequate to take the flow of the side ditch. The farm entrance culvert
should be of such design that it can be easily removed to permit
cleaning out the ditches with a road grader.

TYPES OF CULVERTS

Culverts constructed of concrete and poured in place are called box
culverts because of the rectangular form of the cross section.
Culverts of pre-cast pipe are known as pipe culverts. Several forms of
pipe culvert are in general use.

[Illustration: Fig. 3.--Typical Concrete Box Culvert]

=Metal Pipe.=--These may be of cast iron, steel or wrought iron. The
cast iron pipe is very durable but expensive and heavy to handle and
is not widely used in highway construction. Steel pipe has been
employed to a limited extent but its durability is questioned. At
least it is known that the pipe made from uncoated, light sheet steel
is not very durable. Sheet iron and sheets made from alloy iron
coated with spelter have been extensively used and seem to be durable,
especially when laid deep enough to eliminate possibility of damage
from heavy loads. To insure reasonable resistance to corrosion, the
metal sheets should be coated with at least one and one-half ounces of
spelter per square foot of sheet and the sheets should not be lighter
than 16 gauge for small sizes and should be heavier for the larger
sizes.

=Clay and Cement Concrete Pipe.=--The ordinary burned clay bell and
spigot pipe that is employed for sewer construction is sometimes used
for culverts. It must be very carefully bedded, preferably on a
concrete cradle and the joints filled with cement mortar. Culverts of
this type have a tendency to break under unusual loads, such as
traction engines or trucks. They may be damaged by the pressure from
freezing water, particularly when successive freezing and thawing
results in the culvert filling with mushy snow, which subsequently
freezes.

=Concrete Pipe.=--Reinforced concrete pipe is a satisfactory material
for culverts, if the pipe is properly designed. The pipe should be
carefully laid on a firm earth bed with earth carefully b