ROADS AND BRIDGES 



3 2I 



should be so chosen for the upper part of the substructure 

 that the gravel or other surfacing material may have a 

 dry, firm bed to lie on. By this means, the amount of 

 surfacing materials needed will often be reduced and 

 the expense of a good road be made less. Placing the 

 materials systematically, and carefully tramping each 

 load, is necessary in some cases, but where the grade 

 may be given a year in which to settle before placing a 

 permanent surface, it may easily be leveled down with 

 a reversible road machine 

 before the final surfacing 

 is laid. 



The crowns and side 

 ditches. The form of the 

 cross-section differs with 



,1 f . j * Figure 207. The solid line shows the 



the Surfacing materials USed rough roadbed as left by the slush, Fresno 



. . ., . t . or wheel scraper. The dotted line shows 



and With SOme Other COndl- the graded and rounded surface after It is 



dressed up with the reversible road ma- 



tions. In case of loose chine - 



gravel and of clay the steepness of the crown should 



vary from 24 inch to i^ inches to the foot. 



Where the center of the road is made up of materials 

 which will be compacted or rapidly worn by travel, the 



slope from center to side is 

 made considerable. Where 

 the surface is hard and 

 durable, thus forming a 

 perfect watershed, the road 

 be more nearly level 



it is 



over in this 

 tion. In some earth 

 which are built very broad, 



because of the large amount of earth from large drainage 

 ditches at either side used in draining nearly level lands, 

 it is necessary to use special means of draining the center 

 of the roadway. In Figure 208 is shown how "top 



Figure 208. Heavy side ditches with outer 

 banks 54 feet apart, with 20-foot roadway 

 and small side ditches, A and B, at top of 

 high grade, with grass seeded on sides of 

 grades A, C and B, D. Often in very heavy 

 clay roads these grassed sides can be used 

 when the roadway, A, B, is wet and soft. 



