VITRIFIED BRICK AS MATERIAL FOR COUNTRY ROADS. 
11 
of small individual blocks, any one of which might be easily forced 
down, causing unevenness in the surface, if the foundation were poor; 
and since the ability of the pavement to resist wear depends very 
largely on the smoothness of the surface, every reasonable precaution 
should be taken to prevent any unevenness from developing. 
The proper type of foundation depends largely on the material com- 
posing the subgrade and the character of traffic for which the road is 
designed. Where the traffic is comparatively light and the subgrade 
is composed of some firm material which does not readily absorb 
water, a very satisfactory foundation may be constructed of broken 
stone or gravel filled with sand. Where the traffic is comparatively 
heavy, however, or where the material composing the subgrade is 
defective in any way, a monolithic concrete foundation should be 
used. Foundations consisting of a course of brick laid flat upon a 
Fig. 2.— Concrete curb and gutter combined. 
previously compacted layer of gravel or broken stone have also been 
extensively used, and pavements constructed upon foundations of 
this kind, ordinarily called "double-layer" pavements, have in gen- 
eral proved satisfactory, even where the subgrade was composed of 
an inferior material. At the present time, however, such foundations 
can rarely be constructed at less cost than the more durable concrete 
foundations, and they will therefore be given no further consideration 
here. 
Gravel and broken-stone foundations may be spread in one or more 
courses, each of which should be from 5 to 9 inches thick before com- 
pacting. The materials used should conform in the matter of 
physical characteristics to the ordinary requirements for similar 
materials used in constructing macadam roads; that is, the stone or 
gravel should be clean, hard, tough, and durable, and should be 
graded in size between certain reasonable, fixed limits. It should be 
