DRAINAGE METHODS FOR COUNTY ROADS. 57 
wearing action of traffic by seme suitable surface crust, but these 
requirements would not be sufficient for cases where stone from the 
foundation eventually might form a part of the wearing surface. 
In the case of water-bound macadam roads, for example, the so- 
called foundation course often is considered as a part of the wearing 
crust, because ultimately the stone of this course is expected to be- 
come the surface as the real wearing course is abated by traffic. Or 
it is expected that scarifying the road periodically when it is neces- 
sary to add new material will bring some of the lower course to the 
surface. This criticism, would be applicable, of course, in less de- 
gree where it is purposed to maintain the road surface by bituminous 
treatments, or for water-bound macadam roads maintained properly 
with a distinct wearing course and with a foundation not essentially 
a part of the macadam crust. 
The proper size of the stone for use in a crushed-stone foundation 
depends, first, on the depth of the foundation and, second, on the 
quality of the stone used. In order to have a locking or wedging 
effect under the roller it is essential that the stone be considerably 
less in longest dimension than the proposed thickness of the course. 
In general, no stone is used larger than 4 or 4^ inches, and the maxi- 
mum screen specified usually is 3 or 3J inches for a 6 or 8 inch 
foundation course. If the foundation is 4 inches, the screen usually 
is specified to have meshes 2J or 3 inches in diameter. Soft or thinly 
stratified rocks that are likely to break into tabular fragments are 
used quite generally in larger sizes than a hard, amorphous rock 
with cubical or rhombohedral cleavage. If much tabular rock comes 
through in the tailings, it should be re-run through a lower crusher 
to produce more fine stone, as such material usually is short on the 
low sizes. As in the case of gravel, it is customary to fix the size by 
specifying upper and lower limiting screens which the material shall 
pass and be retained upon, respectively, and by providing that the 
stone shall be well graded in size between the limits specified. Where 
the stone is hard and tough and does not crush to any extent under 
the roller it is impracticable to make the successive layers of the 
foundation more than 4 or 5 inches in compacted depth, because it 
is very difficult to compact a greater depth satisfactorily. In such 
cases the particles of stone preferably should be of such size that all 
will pass a screen having circular openings about 2 \ or 3 inches in 
diameter. If, on the other hand, the stone is moderately soft and 
brittle, the compacted depth of the foundation courses sometimes 
may be made as much as 5 or 6 inches and the upper limiting screen 
may have correspondingly larger openings. The % lower limiting 
screen should, in general, be such that the amount of fine particles 
left in the screen product will be no more than sufficient to fill the 
voids and produce a dense, well compacted layer when the stone is 
