DRAINAGE METHODS FOR COUNTY ROADS. 57 



wearing action of traffic by some 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 abatfed 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 3|^ inches for a 6 or 8 inch 

 foundation course. If the foundation is 4 inches, the screen usually 

 is specified to have meshes 2^ 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 rhomboheclral cleavage. If much tabular rock comes 

 through in the tailingSj 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 preferajbly 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 



