46 BULLETIN 463, U. S. DEPARTMENT OF AGRICULTURE. 



SELECTION OF GRAVEL. 



Ordinarily the selection of gravel for use in road surfacing must 

 be confined to local materials which are or can be made suitable for 

 that purpose. Because of high freight costs it seldom happens 

 that road gravel of even the very best quality is transported by rail 

 a greater distance than about 100 miles from the source of supply, 

 and in the vast majority of cases it is hauled directly from pit to 

 road in wagons. Since the quality of gravel varies to a consider- 

 able extent, sometimes in different parts of the same pit, it is essen- 

 tial that local road officials, who frequently are charged with the 

 selection of such material, should have some knowledge of the rela- 

 tion between the physical characteristics of gravel and its wearing 

 qualities when placed in a road surface. While such knowledge can 

 best be gained by actual experience with different kinds of gravel, 

 there are certain general requirements which may be stated for the 

 guidance of the inexperienced. For example, the following excerpt 

 from Bulletin No. 2 of the Michigan State highway department pre- 

 sents in very concise form the characteristics which gravel for road 

 surfacing should possess, in the judgment of the State highway 

 commissioner, in order to meet the Michigan conditions. 



Next to proper drainage, the most important thing in building gravel roads 

 is to secure a good quality of gravel. Authorities have differed as to the re- 

 quirements of suitable road gravels, most of them, in my opinion, placing too 

 much stress on the immediate packing qualities. Indeed, the average town- 

 ship commissioner and farmers generally have become so imbued with the idea 

 that it is necessary to use a gravel that will pack quickly that they have 

 almost lost sight of the fact that the only thing which makes a gravel road 

 better than an earth road is the pebbles, real stones, that it contains and is 

 dependent upon to bear up traffic and resist wear. 



The most common material sought after for the binder in gravel roads is 

 clay. But, considering all kinds of weather, it is probably the poorest cement- 

 ing material we have. If present, much in excess of 10 per cent of the mass, 

 it will make mud whenever there is a prolonged wet spell, and especially when 

 frost is coming out of the ground in the spring. Ideal clay gravels contain 

 only enough clay to coat the pebbles, with no free lumps. Such gravels are 

 excellent for the first layer on sandy soils, but sand gravels are much better 

 for the first layer on clay and loamy soils. 



Gravels that come from the pit with the pebbles cemented together, even 

 though they contain no clay, will recement in the road and become harder than 

 they were in the pit. Tests of specimens of this kind always show that there 

 is much lime present and usually some iron, both of which are excellent 

 cementing materials. Briefly, the experience of the State highway department 

 warrants the statement that there are few, if any, bank gravels in Michigan 

 that do not contain enough limestone and other soft pebbles which grind up 

 under traffic to furnish sufficient binder to cause them to consolidate in a few 

 months' time, if separated from the surplus sand and earth, and properly 

 treated after applying to the road. 



In accordance with these suggestions, gravels are considered valuable for 

 road purposes: in the following order : 



