Textvrt of Road Materials. 451 



size of the pieces of material used, the layers being thicker 

 the coarser the material. With crushed rock having 

 pieces 2 to 2 1-2 inches in diameter the layers will need to 

 be 3 to 4 inches thick ; with smaller pieces the layers should 

 be thinner. If thicker layers than these are made the ef- 

 fect will be the formation of a closely packed crust, a lit- 

 tle thicker than the diameter of the material used, over a 

 loose and open structure below. 



The hardest and best earth road can be built only by 

 spreading the material on very uniformly in thin layers 

 and thoroughly compacting each layer before the next is 

 put in place; the thickness of these layers should be 2 

 inches and less, rather than more. 



558. Uniformity of Size of Material Used. It is impossi- 

 ble to crush rock into sizes varying all the way from fine 

 dust to pieces 1.5 inches in diameter and then use this ma- 

 terial unsorted to make a solid, unyielding road. The 

 materials when laid down at once with all sizes mixed will 

 not pack so as not to work up loose with the travel upon it ; 

 and this is the main reason why more solid roads cannot 

 be built from earth. 



Crushed rock must be carefully separated into nearly 

 uniform sizes by means of screens and the different grades 

 applied to the road in layers. 



When a layer is made of only a single size of pieces 

 these may be brought together by packing so that all touch 

 and press firmly against one another. If now a grade is 

 used of smaller pieces such as will work readily into the 

 pores left between the angles of the larger ones, pressing 

 hard upon all sides, a still more stable layer will be formed. 

 If it were practicable to follow this method step by step 

 there would be reproduced a nearly solid rock from the 

 fragments made and the most substantial of roads built. 



559. Shape of Fragments. The shape of the materials 

 used in road building has important bearings on the quality 

 of the road. The best form is that which approaches most 



