ristic may be expected to vary widely
with the several types of soil. It is generally considered to be a
common property of clay, but the term clay is a general one that is
often applied to soils differing greatly in physical characteristics
and the term therefore loses its significance in this connection.
Those soils that are properly and technically called clay are
decidedly sticky when wet and are the best materials for sand-clay
construction. Of the clays, those that produce a tough sticky mud are
best. This can be tested by mixing a small quantity into a stiff mud
and molding it into a ball and immersing in water. If the ball retains
its shape for some little time, it is likely to prove a very
satisfactory binder, but, if it becomes plastic and loses its shape,
it will be an inferior binder, as a general rule. The ball clay, as
the former is called, may be of any color common to soils, not
necessarily yellow or reddish as is sometimes supposed. Likewise,
balls of mixtures containing varying percentages of sand and the
binder to be used may be made up and immersed in water. The mixture
that holds its shape longest is of course the best combination of the
materials and indicates the mixture to use in the construction.

An ideal, or even a fairly satisfactory soil for a binder may not
exist in the vicinity of a proposed improvement, and consequently an
inferior binder is frequently the only material available.

Sometimes deposits of clay or gravel contain a considerable percentage
of gypsum which serves as a binder and is particularly effective when
used in combination with clay and sand or gravel.

In many places a soil of the type used for adobe and called "caliche"
may be found and this is an excellent binder for sand or gravel.

=Top-Soil or Natural Mixtures.=--Deposits consisting of a natural
mixture of sand and clay in which the ingredients happen to exist in
about the correct relative proportions for sand-clay road surfaces are
found in many localities. These mixtures are commonly referred to as
top-soil. If the deposits are somewhat deficient either in sand or
clay, they can be utilized if the proper corrections in the
proportions are made during construction. Very satisfactory road
surfaces are sometimes constructed with mixtures that appear to be far
from ideal in composition, but experience and frequent trials are
needed to determine the best way in which to handle these mixtures.

=Sand-Clay Surfaces