n some way if the material
is of a durable nature, regardless of other characteristics. The
serviceability of a gravel road will depend largely on how nearly the
gravel approaches the ideal, but variations in the manipulations will
do much to overcome deficiencies in materials.

=Thickness of Layer.=--The thickness of the layer of gravel required
depends both upon the type of soil upon which it is placed and the
nature of the traffic to which the road will be subjected. Gravel
surfaces should not ordinarily be constructed on highways carrying
heavy truck traffic, but if gross loads of three or four tons are the
heaviest anticipated, the gravel will be reasonably stable. On such
roads, a layer of well compacted gravel ten inches thick will support
the loads if a well drained earth foundation is provided. If but
little truck traffic is anticipated and loads up to three tons on
steel tires are the average, a layer 8 inches thick will be
sufficient. In dry climates, a layer six inches thick will be
adequate if it can be kept from raveling.

On secondary roads, carrying principally farm-to-market traffic, and
not a great volume of that, the above thicknesses may be reduced about
one-fourth.

The exact thickness needed for any particular road is a matter for
special study on account of the variations in the gravels and in the
supporting power of the soil upon which they are placed.

PLACING GRAVEL

=Preparation of the Road.=--The roadway that is to be surfaced with
gravel is first brought to the desired grade and cross section. It
would be advantageous if this could be done a year before the gravel
is placed so that no settlement of the earth foundation would occur
after the gravel surface is completed. But if that is impractical, the
grading may be done just prior to placing the gravel, providing
appropriate methods are adopted for securing compacted fills.

=Trench Method.=--Two distinct methods of placing the gravel are in
general use, known as the trench method and the surface or feather
edge method respectively. The method to adopt for any particular road
will depend largely on certain conditions that will be explained
later.

In the trench method, a trench of the proper width and depth for
receiving the gravel is excavated in the earth road surface and the
gravel is placed therein.

The trench is formed by plowing a few furrows and scraping out the
loosened earth with a blade grader. The loose material is g