DRAINAGE METHODS FOR COUNTY ROADS. 49 
and finished with a light drag. Slips, wheelers, or Fresnos may be 
used with entire success by skillful workmen, and if the heavy grad- 
ing warrants its presence on the job a steam shovel in the hands of 
an expert operator can be used to do all but the finishing, which a 
light drag will do. 
It is impracticable to supply detailed instructions as to the best 
methods for shaping and compacting all kinds of soil, because in 
many cases experienced judgment is the only safe guide. Where the 
soil composing the subgrade consists essentially of sand, for example, 
it may be necessary to wet the subgrade practically to saturation 
before it can be rolled satisfactorily, and in some cases pipe lines 
must be provided along the work to supply water for this purpose. 
On the other hand, clay soils can be rolled effectively only when 
comparatively dry, and when construction work is proceeding during 
wet seasons it may be necessary to provide additional facilities for 
draining the subgrade which would be wholly unnecessary if the work 
were being done under more favorable weather conditions. No mat- 
ter what the circumstances of soil and weather may be, the sub- 
grade, when completed, should conform closely in grade and cross 
section with the requirements of the plans and should present an 
even uniform surface. It also should be as firm and unyielding as 
the conditions will permit, because whatever the type of the founda- 
tion or the road crust, all loads passing over the surface must be 
supported ultimately by the subgrade, and unless this feature be 
sufficient the crust and foundation necessarily must fail. 
Some of the most important details which require careful atten- 
tion in preparing the subgrade, and are perhaps most frequently 
overlooked, are (1) back filling culvert trenches, (2) exclusion of 
vegetable matter from fills, (3) provision for draining any wet- 
weather springs that occur in the subgrade, and (4) the proper 
handling and use of materials contained in old road crusts which are 
to be worked into the new construction. 
The back filling of culvert trenches is a detail requiring the very 
closest attention if subsequent settlement of the subgrade and road 
crust, over the trenches, is to be prevented. Most specifications gov- 
erning this feature of the work make suitable provisions for having 
the back fills over all drainage structures properly tamped and com- 
pacted, but both engineers and contractors frequently have under- 
estimated the cost of doing such work satisfactorily, and as a result 
there has been a strong tendency to permit and to do less compacting 
than the conditions require. The amount of compacting necessary 
varies, of course, with the character of the soil composing the back 
fill. Where the soil contains considerable sand or gravel and is 
moderately moist, it usually may be compacted best by placing the 
'—18— Bull. 724 4 
