58 BULLETIN 724, U. S. DEPARTMENT OF AGRICULTURE. 
spread and rolled. Ordinarily a screen having circular openings 
from J to \ inch in diameter should prove satisfactory for moder- 
ately brittle stone, while in other cases, where the stone is hard and 
tough, it may be permissible to omit the lower limiting screen 
altogether. 
Not infrequently it is desirable to screen out certain sized par- 
ticles from the product of the crusher for use in the road crust or 
for some other purpose, and in such cases only the larger sizes, with 
perhaps a part of the dust or screenings, are available for use in the 
foundation. With some varieties of stone this arrangement may 
not be objectionable because the large particles will wedge in to- 
gether under the roller to form a fairly dense, compact foundation. 
But with other varieties, such as trap rock, for example, the large- 
particles may not lock very readily and it may be necessary to fill 
the voids with either stone screenings, gravel, or coarse sand while 
the rolling is in progress, in order to aid in wedging the crushed 
stone particles firmly in place. 
The compacted thickness of crushed-stone^ foundations varies in 
practice from a minimum of about 3 inches up to a maximum of 
about 9 or 10 inches. The small thickness of 3 inches should be used 
only for light-traffic roads where the subgrade conditions usually are 
favorable or where the road crust is capable of distributing the pres- 
sure under concentrated loads to a considerable extent, while the 
maximum thickness of 10 inches would be required seldom except 
where the surface crust is designed to withstand only the wear of 
traffic and the subgrade is poor. In the past the usual thickness has 
been about 6 inches, but under the heavy traffic which prevails at 
present on many of our improved roads the 6-inch foundation fre- 
quently has proved insufficient. This is shown by numerous partial 
or total failures of expensive road crusts, such as brick and bitumi- 
nous concrete, clue to foundation failure. It is believed that with 
average subgrade conditions and road crusts of the kinds above men- 
tioned, the thickness of crushed-stone foundation should, in general, 
be considerably increased above the common practice of to-day. 
The numjber of courses in which the stone should be spread and 
compacted depends on the depth of stone that can be compacted 
satisfactorily in one course, and, as explained above, this varies with 
the quality of the stone used. Ordinarily not mor'e than two courses 
are necessary, unless the foundation is to be of unusual depth. 
In some easels it has been found of advantage to use a crushed- 
stone foundation under a gravel wearing course. A recent design 
for heavy traffic where unusual speed of construction and immediate 
service were essential called for an 8-inch compacted quarry-run base 
and a 4-inch compacted gravel top. This maele use of a relatively 
cheap but unsatisfactory macadam stone and a relatively expensive 
