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 Xo. 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 : 
