48 BULLETIN '■<24, U. S. DEPARTMENT OF AGRICULTUEE. 



the siibgrade supports the road crust directlj^ the cross section may- 

 be either flat or crowned to conform with the finished surface. The 

 flat cross section generally is preferable, because when this shape 

 is used the crown of the road will give a greater thickness of crust 

 at the center where greater strength is desirable. The cross section 

 of course may have any desired crown less than that of the finished 

 surface. 



The most important features to consider in forming a roadbed 

 upon which any type of pavement is to be constructed are (1) ade- 

 quate and effective drainage, (2) firmness and stability, and (3) uni- 

 formity in grade and cross section. 



The question of drainage has been discussed separately in this 

 bulletin and it is therefore unnecessary to give this feature further 

 attention, except perhaps to emphasize its importance. This prob- 

 ably can be done best by calling attention to the fact that many 

 earth roads have been converted from almost impassable stretches 

 of mud into excellent driveways for all but extremely bad weather 

 conditions by merely making proper provision for drainage with 

 no other improvement whatever. 



The second requirement, firmness and stability, can be secured 

 only after effective drainage has been provided. Soils which absorb 

 moisture readily can not always be drained properly during wet 

 seasons, and should not be permitted to form a part of the roadbed 

 except where full allowance for their reduced bearing power has 

 been made in designing the foundation or road crust. While it is 

 true that for some types of roads foundations are necessary only 

 where soil conditions are unfavorable, it nevertheless is advisable to 

 improve the unfavorable conditions so far as practicable by selecting 

 the best available material for use in the subgrade, even when the 

 very best type of foundation is to be employed. In order that the 

 subgrade may be unyielding it is also necessary for the entire road- 

 bed to be compacted thoroughl}^ In forming embankments the ma- 

 terial should be spread in comparatively thin layers and each layer 

 should be compacted by rolling or by properly directing traffic over 

 it during construction. For this purpose a power roller weighing 

 about 10 tons or a horse roller of less weight may be used. When 

 the grading is substantially completed the subgrade in both excava- 

 tion and .embankment should be brought to its final shape by any 

 suitable devices, and rolled. 



Until recent years the preparation of subgrade usually has been 

 done by hand with pick and shovel, and in localities where labor is 

 cheap still may be done so; but quicker and less expensive methods 

 may be used to advantage now. If the grade is to be flat, a drag of 

 railroad iron often is satisfactory. Telford, V-drain, and trench 

 foujidations generally can be cut with plow and grading machine 



