d to give a satisfactory mixture, the proper material
is added and mixed in as the work progresses. The surface is finally
smoothed by means of the grader and drag.

=Sand-Clay on Clay or Loam.=--If the existing road is of clay or loam,
ample drainage will be required as discussed in Chapter IV. The
surface may be constructed of a natural sand-clay mixture or of a sand
mixed with the natural soil. If the former, the surface of the
existing road is prepared by grading so as to insure good drainage and
the natural mixture is then deposited and the surface completed as
described in the preceding section.

If the surface is formed by mixing sand with the existing soil, the
sands may be deposited in a layer about six inches thick which will
gradually mix with the soil as the road is used. A second application
of sand may follow in a year or two if it is needed. Such a road
surface will lack uniformity of composition and it seems preferable to
mix the sand with the soil by plowing and discing as previously
described.

=Characteristics.=--Sand-clay road surfaces do not have sufficient
durability for heavily traveled highways, but will be satisfactory for
a moderate amount of traffic. These surfaces have maximum
serviceability when moist, not wet, and consequently are not as
durable in dry climates as in humid areas. They are likely to become
sticky and unstable in continued wet weather and to become friable and
wear into chuck holes in long continued dry weather. At their best,
they are dustless, somewhat resilient and of low tractive resistance.

GRAVEL ROAD SURFACES

[Illustration: Fig. 15.--Cross Sections for Gravel Highways]

=Natural Gravel.=--Gravel is the name given to a material consisting
of a mixture of more or less rounded stones, sand and earthy material,
which is found in natural deposits. These deposits exist in almost
every part of North America, being especially numerous in the
glaciated areas, but by no means confined to them. Gravel deposits
consist of pieces of rock varying in size from those of a cubic yard
or more in volume to the finest stone dust, but with pieces ranging in
size from that which will pass a 3-inch ring down to fine sand
predominating. The larger pieces are usually more or less rounded and
the finer particles may be rounded or may be angular. Many varieties
of rocks are to be found among the gravel pebbles, but the rocks of
igneous origin and possessing a considerable degree of hard