204 



ENGINEERING ON THE FARM 



of a high road grade across wet land is only a makeshift 

 for drainage, that a good tile or open ditch would remove 

 the need of the grade, and that water standing along a 

 road grade saturates the foundation of the road to the level 

 of the standing water and then is raised by capillarity to 

 the surface of the road and in the end destroys the road 

 regardless of its surface covering. 



Cross section. Grading is always needed to a certain 

 extent to give the roadbed the proper shape and to make 

 the most economical use of the covering. The cross section 

 of the road will be determined by the local conditions, such 

 as the volume of traffic and the materials for construction. 

 The forms of the cross section have changed somewhat, owing 

 to the increased number of motor cars used. Since a high 



Fig. 177. Cross section of the English earth road of an early date. 

 Note the great convexity to keep it dry. 



crown nas a tendency to cause side-slipping when it is wet, 

 crowns that were formerly regarded as satisfactory are now 

 made much flatter to accommodate high-speed automobiles. 

 At one time drainage was the controlling element in the 

 degree of rise that was given to the center of the road above 



Fig. 178. Cross section of a Roman military road 



its edges. This is illustrated by Figure 177. In the Roman 

 road there was a permanent base with a rock covering, and, 

 as it could not be injured to any great extent by moisture 



