ll be a poor facility for a considerable period each year
in the regions of year-around rainfall. In most localities, roads of
distinctly minor importance are of necessity only earth roads and for
the comparatively small territory they serve and the small amount of
traffic, they probably serve the purpose. For roads of any importance
in the humid areas of the United States, the earth road cannot carry
satisfactorily the traffic of a prosperous and busy community.

CHAPTER VI

SAND-CLAY AND GRAVEL ROADS

In Chapter IV, mention was made of the variation in serviceability of
road surfaces composed of the natural soil existing on the
right-of-way of the road. It has been found that soils of a clayey
nature in which there is a considerable percentage of sand usually
afford a serviceable road surface for light or moderate traffic,
especially in areas where climatic conditions are favorable. A study
of these soils, together with the construction of experimental roads
of various mixtures of sand and clay, has led to a fairly
comprehensive understanding of the principles of construction and
range of capacity of this type of road surface, which is known as the
sand-clay road.

The sand-clay road surface consists of a natural or artificial mixture
of sand and clay, in which the amount of clay is somewhat greater than
sufficient to fill the voids in the dry sand. It may be assumed that
the sand contains 40 per cent of voids and that at least 45 per cent
of clay is required to fill the voids and bind the sand grains
together, because the clay spreads the sand grains apart during the
mixing, thus having the effect of increasing the voids. As a matter of
experiment, it is found to be impractical to secure by available
construction methods mixtures of sufficient uniformity to render it
necessary to exercise great exactness in proportioning the components,
but reasonable care in proportioning the materials is desirable.

Successful utilization of this type of surface requires considerable
study of available materials and investigations of their behavior when
combined. Extensive and exhaustive experiments have been conducted
with sand-clay mixtures in various places where they are widely used
for road surfaces and the following general principles have been
deduced.

=The Binder.=--In the sand-clay road, stability is obtained by
utilizing the bonding properties possessed to some degree by all
soils. Naturally this characte