DEAINAGE METHODS FOR COUNTY ROADS. 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 75 per cent, by weight, of pebbles ranging in size 

 between such as will be just retained on a % or ^ inch screen and 

 such as will just pass a 2^ 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. When 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 



