EAETH, SAND-CLAY, AND GRAVEL ROADS. 7 



DRAINAGE. 



The subject of road drainage has been discussed at considerable 

 length in other bulletins of the Office of Public Roads and Rural 

 Engineering, and therefore will be given only brief mention here. 



Effective drainage usually should be the very first consideration in 

 connection with the location and design of any road. This state- 

 ment requires no explanation, because the action of water in chang- 

 ing clay into mud and in causing all kinds of soils, except sand, to 

 give way when a load is applied, is familiar to every person living in 



SIDE DITCH 



ROADWAY 



id 



Fig. 2. 



a humid climate. The following summary supplies a few suggestions 

 as to how water may best be removed from a road bed : 



1. The road surface should be crowned so as to shed water off 

 to the side ditches as rapidly as it falls on the road. The typical 

 cross sections (shown in figs. 15, 20, and 22) indicate the amounts 

 of crown advisable under different conditions. These figures are 

 supplemented by a discussion on page 9. 



2. Wherever the road is in an excavation, suitable side ditches or 

 gutters should be provided along the sides so that the water may 



SIDE DITCH 



S/o/xJlL 



Fig. 3. 



be conducted to some point where it may be turned off from the 

 road. Figures 2, 3, and 4 show typical cross sections for earth side 

 ditches. The cross section shown in figure 2 is suitable for steep 

 grades where the depth of water in the side ditches must be kept low 

 in order to prevent washing of the soil. Figure 3 shows a modified 

 section which gives a greater capacity for the same widths of ditch 

 than the cross section shown in figure 2, but which can be employed 

 only on comparatively flat grades if washing is to be prevented. 

 Figure 4 shows a cross section adapted especially to flat grades where 

 a considerable volume of water must be carried. Ditches of the 



