DRAINAGE METHODS FOR COUNTY ROADS. 47 



be provided by means of a concrete foundation, and under extreme 

 conditions of this sort concrete is practically the only satisfactory 

 foundation material. (Fig. IT.) 



DESIGN AND CONSTRUCTION. 



No matter what type of foundation may be selected for a particular 

 road or section of road it should, of course, be designed and con- 

 structed with a view to meeting adequately the conditions involved. 

 Since these conditions vary greatly, sometimes even on the same proj- 

 . ect, it is manifestly impracticable to standardize either the design or 

 construction of foundations except to a very limited extent. In the 

 following discussion, therefore, no attempt will be made to do riiore 

 than point out the limits of good design for each type of foundation 

 and to describe, in a general way, what are believed to be the best 

 methods of construction. Thus, unavoidably, much must be left to 

 the judgment of those who design and superintend such work. 



Practical experience and observation disclose that the best roads 

 usually have a great variety of foundations. These may have been 



CONCRETE FOUNDATION 



Fig. 17. 



laid at the time of original construction, or added to meet conditions 

 that arise from season to season. For long distances the changes 

 often occur as frequently as every few hundred feet. Rarely are con- 

 structed foundations now laid mile after mile, as was done formerly 

 on the pikes of central Kentucky and elsewhere. The cost of such 

 work is so great that the individual and local treatment has more 

 generally been adopted by highway engineers. 



GRADING AND PREPARING THE SUBGRADE. 



The discussion on bearing power of different soils should be con- 

 sidered in connection with what is said here regarding the proper 

 method of grading a road and preparing the subgrade. The table of 

 bearing powers given on page 41 is based on the assumption that the 

 roadbed and subgrade will be prepared in accordance with some such 

 methods as those described below. 



The shape of the subgrade varies somewhat with different types 

 of foundations or road crusts, but this feature does not need to be 

 especially considered here, because the recommended shape for the 

 subgrade is shown for each type of foundation discussed. Where 



