DRAINAGE METHODS FOR COUNTY BO ADS. 45 



local. Such sections as develop difficulties of this sort should be 

 treated individually in order to get the best generally good results at 

 lowest cost. 



FOUNDATION TYPES. 



The types of foundations in common use for country roads are 

 (1) gravel, (2) crushed stone, (3) V-drains, constructed of field 

 stones, (4) stone base, (5) Telford, and (6) Portland-cement con- 

 crete. Selected earth, sand, and cinders also have been used fre- 

 quently to improve the supporting power of subgrades, but since the 

 addition of such materials makes no essential difference in the gen- 

 eral method of preparing the subgrade, they will not be considered 

 as foundation materials. Eoacl foundations have been constructed 

 of brick, boards, corduroy, and many other special materials, 

 and such construction is no doubt sometimes justified, although it is 

 usually of a makeshift nature. In these cases the problem generally 

 is concerned with employing local materials which may be secured 

 at small cost and is one to be solved on the ground or after all the 

 affectina: conditions have been considered. 



GRAVEL FOUNDATION 



Fig. 14. 



From what has been said it is evident that no very exact rules can 

 be stated for determining what, if any, type of foundation should be 

 selected as best for a general case. Foundation troubles are com- 

 monly local and peculiar ajid yield most readily to special treatment. 

 The considerations that should influence the judgment in making a 

 selection for a particular case are efficiency, and availability of mats- 

 rials, and cost of construction. It is possible to design foundations 

 of any type mentioned above which would be efficient for any given 

 condition of subgrade and type of road crust, but considerations of 

 economy usually will serve to reduce the number of practicable types, 

 for any particular case, to not more than two or three at most. 



In general, gravel foundations are adapted to cases where the road 

 crust also is to be of gravel or some other simple type, or where only 

 slight additional stability is required for the subgrade to support 

 concentrated loads on the road surface without being deformed. 

 But, if the gravel possesses high cementing value, it may be used 

 satisfactorily for foundations where crushed stone or concrete other- 

 wise would be required. (Fig, 14.) 



Crushed-Btone foundations have been used for practically all types 

 of road crusts where the subgrade conditions are favorable and a 



