DBAIXAGE METHODS FOE COUNTY EOADS. 53 
GRAVEL FOUNDATIONS. 
Gravel for use in road foundations should consist of sound, dura- 
ble pebbles mixed with sufficient cementing material to bond the 
individual particles together firmly when the gravel is spread and 
rolled. Clay is the only cementing material carried by most gravels 
as they occur in nature, though there are a few deposits that contain 
other cementing agents, such as iron oxide, lime carbonate, etc. 
Foundations bonded with the latter materials are much more reliable 
than those in which the binder is composed essentially of clay. The 
ordinary gravel foundation with clay binder is used in general only 
where the wearing crust is to be of gravel also, while if the binder 
consists of some stable mineral cement, properly constructed gravel 
foundations may be employed with brick, bituminous concrete, or 
other highly improved types of road crusts. 
It is customary to require that gravel used in foundations shall 
contain from 60 to To per cent, by weight, of pebbles ranging in size 
between such as will be just retained on a % or 14 i ncn screen and 
such as will just pass a 2-J or 3 inch screen, and that the remainder 
shall consist of sand combined with some suitable cementing mate- 
rial. Where clay is the cementing material it should not comprise 
less than about 15 per cent nor more than about 20 per cent of the 
total weight. It is important that the gravel be evenly graded in 
size from fine to coarse and that the cementing material be uniformly 
distributed through the mass,, because poor grading or poor distribu- 
tion of the clay content evidently would tend to produce unequal 
density of the foundation. 
It very often happens that, in order to be made suitable for use in 
a road foundation, natural gravel must be screened to remove a part 
of either the fine or coarse material so as to keep the different per- 
centages within the limits mentioned above. It also is necessary 
sometimes to wash the gravel in order to remove excessive clay, but 
this process is expensive, and it generally is practicable to secure the 
desired results in some other way. When it is necessary to add clay 
it should be done as the gravel is placed by spreading a uniform layer 
of clay over each course of gravel and mixing the two together by 
harrowing. 
If this is necessary, it must be done with the utmost care and great 
thoroughness. TThen it is realized that one load of clay to a hundred 
of gravel increases the clay content approximately 1 per cent, it is 
seen how difficult it may be to raise the clay a few per cent through- 
out an entire job. Almost always too much is added, and the dis- 
tribution is exceedingly uneven. To obtain good results the clay 
should be as dry as possible, well pulverized, and distributed broad- 
cast with shovels just as screenings are on a macadam road. This 
