458 Farm Mechanics. 



There are several important features which a good road 

 gravel must possess: 



1. There must be one prevailing size of pebble in suffi- 

 cient quantity so that when thoroughly rolled they press 

 against one another. 



2. There must be enough of the finer sizes of coarse sand 

 and fine gravel to fill the voids between the coarser gravel. 



3. There must be enough of fine loam to fill the voids 

 between the coarse sand and fine gravel and retain a suffi- 

 cient amount of water to bind the sand grains together and 

 prevent their rolling. 



4. The coarse and fine gravel and the sand must be made 

 up of more or less angular fragments in order that flat 

 faces of rock may set together and thus lessen the danger 

 of rolling and of crushing under the weight of the load. 



It is not possible to give specific, concise directions for 

 identifying a good road gravel, but a man who has seen 

 and worked with it readily recognizes it. 



570. Clean White Gravel Not Suitable. It will be appar- 

 ent at once that the several characteristics which have been 

 pointed out are not likely often to occur together in just 

 the right ratios ; and so there will be all possible gradations 

 from the ideal gravels to those which will not answer at all. 

 Indeed it must be said that most gravel beds have had the 

 finer materials so completely washed out that only clean 

 sand and gravel remains ; and when this is true it is useless 

 to try to make a road with it. Such materials can only be 

 used to temper a road which is too clayey in its texture, 

 by reducing its water capacity. 



571. Texture of Gravels Altered by Crushing and Screen- 

 ing. It happens in the majority of cases that much of the 

 gravel is too large and too rounded to permit close packing 

 and fast binding. When this is true much better qualities 

 may be secured by using either the crusher or the screen 

 or both together, one form of which is represented in Fig. 

 216. It will be at once apparent that where much of the 



