EARTH, SAND-CLAY, AND GRAVEL ROADS. 17 
qualities of the clay. Some varieties of clay, when compressed into 
a ball and placed in water, will continue in the original shape for 
a considerable time, even if thoroughly saturated and softened, 
while compressed balls made from other varieties will slake or break 
down almost immediately upon being placed in water. Clays of the 
first-mentioned variety are called "nonslaking clays," and of the 
latter, " slaking clays." It has been observed that slaking clays pro- 
duce more mud in wet weather and more dust in dry weather than 
those of the nonslaking variety. This characteristic of clays is fur- 
ther considered in the discussion of sand-clay roads appearing in 
another part of this bulletin, where methods for testing clay are 
suggested. 
Most clays, as they occur in nature, contain more or less sand or 
gravel, which, as pointed out above, usually has a stabilizing effect by 
making the clay more easily drained and increasing its bearing 
power when wet. In fact, a good quality of clay mixed with coarse 
sand in such proportions that the clay fills the voids in the sand and 
cements the individual particles together produces a soil which pos- 
sesses the good qualities of both materials and which overcomes, to 
a great extent, the objections to each. 
SAND. 
Sand is composed of granular particles of mineral or stone which 
occur in nature and which will pass a J-inch mesh screen. The 
J-inch mesh screen is fixed arbitrarily as the dividing line between 
sand and gravel and is generally, though not universally, accepted 
as such. Nearly all sand consists essentially of quartz grains that 
are very hard and durable. But there is no coherence betweeu 
the different grains, and therefore soils composed principally of 
sand are unstable, except when confined in some way. If properly 
confined and protected from undermining, sand foundations may, 
under favorable climatic conditions, prove entirely satisfactory for 
almost any type of road crust, but it is practically impossible, under 
any circumstances, to make a satisfactory road surface of sand alone. 
Dry sand offers almost as great resistance to traffic as mud, and except 
in very wet seasons sandy roads are likely to dry out to a considerable 
depth. The reason for this is that the capillary power of sand is 
comparatively low and the ground-water elevation may be lowered 
practically to the prevailing level of surface streams in extended dry 
seasons. 
Sand roads are at their best when they are kept moist, and for 
this reason they should be designed with a view to retaining moisture 
in the sand rather than to effective drainage, as is the case with clay 
61531°— Bull. 463—17 2 
