DEAINAGE 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 ma.j 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 

 66998°— 18— Bull. 724 4 



