156 AGRICULTURAL ENGINEERING 



extent in the gravel. On the other hand, it has some 

 undesirable characteristics. It loses its binding power when 

 dry, and is susceptible to the action of frost. In many cases 

 other kinds of binders are used. Stone dust has excellent 

 cementing properties and is considered better than clay, 

 but is more expensive. As will be explained in the chapter 

 on stone roads, automobiles have introduced many new 

 problems in connection with road construction. Many 

 forms of binders and dust preventives are being experi- 

 mented with. Bitumen, tar, crude oils, and chlorides are 

 used to hold the gravel together. 



Drainage. A good gravel road must be thoroughly 

 underdrained if it is to be satisfactory. The method of 

 draining does not differ materially from that described for 

 earth roads. Many mistakes have been made by those 



Fiqst Class 



Oeciioi in Cut 



Fig. 82. Cross section of gravel road. (Iowa Highway Commission.) 



having the matter of road construction in hand, by applying 

 surfacing material to a road which needed under drainage 

 badly, and so the material did not produce the results which 

 were hoped for. The ground water coming up from below 

 softened the surface, and the gravel was forced down into 

 the earth until it entirely disappeared. Gravel roads should 

 have sufficient amount of crown or lateral slope to secure the 

 rapid drainage of all surface water to the side ditches. The 

 amount of slope is usually given as 3^ to 1 inch to one foot of 

 width. 



Surface Construction. There are two general methods of 

 surfacing roads with gravel. The cheapest method is known 



