58 BULLETIN 724, U. S. DEPAETMENT OP AGEICULTUEE. 



spread and rolled. Ordinarily a screen having circular openings 

 from I to -^- inch in diameter should prove satisfactory for moder- 

 atelv brittle stone, while in other cases, where the stone is hard and 

 tough, it may be permissible to omit the lower limiting screen 

 altogether. 



Xot infrequentl}^ 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 j)articles 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 1 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 subgTacle 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, due 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 considerabl}^ increased above the common practice of to-day. 



The numjber of courses in whioh 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 cascb 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 made use of a relatively 

 cheap but unsatisfactory macadam stone and a relatively expensive 



