48 BULLETIN *?24, U. S. ■ DEPARTMENT OF AGRICULTURE. 
the subgrade supports the road crust directly 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 thoroughly. 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 
foundations generally can be cut with plow and grading machine 
