46 



BULLETIN 124^ U. S. DEPARTMENT OF AGRICULTURE. 



good qiialit}' of stone is aA'ailable. Such foundations usually are 

 well adapted for road crusts of gravel, macadam, bituminous con- 

 crete, and, where the traffic is onl}^ moderate, for brick. (Fig. 15.) 



The V-drain foundation is adapted to locations where field stones 

 are readily available and where drainage of the subgi^ade is an essen- 

 tial function of the foundation. 



Stone base is a modification of Telford where quarry spalls and 

 broken stone are used, as in V-drains, but wdiere the subgrade is 

 shaped as for Telford. Such stone should be laid on the subgrade so 

 as to bear firmly and not rock, but need not be broken, coursed, and 

 wedged so carefullv as Telford. It is finished with fine material. 



CRUSHED STONE FOUNDATION 



Fig. 15. 



like gravel, small stone, or sand, and rolled. Its use is similar to that 

 of Telford. 



Telford foundations have been used seldom except with macadam 

 or gravel crusts. The reason for this is that the surface of a Telford 

 foundation is irregular and such materials as gravel or crushed 

 stone are well adapted to correcting the irregularities. Such advan- 

 tage as Telford has over crushed stone foundations lies in the fact 

 that since the individual stones are placed by hand larger stones are 

 used that have greater bearing areas and give better distribution of 

 pressure on the subgrade. This is an important consideration where 



2'to8"- 



TELFORD FOUNDATION 



Fig. 1G. 



the subgrade is not very stalile and the necessary rolling of a crushed 

 stone foundation might jorocluce a wavy surface. (Fig. 16.) 



Concrete foundations are well adapted for use where the road crust 

 consists of brick, asplialt, bituminous concrete, or any other material 

 whose integrity and stability can be maintained only when extreme 

 rigidity is supplied b^^ the foundation. In addition to their rigidity, 

 concrete foundations, when properly constructed, present fewer un- 

 certainties due to lack of uniformity in materials and workmanship 

 than any other type, and on that account are greatly to be preferred 

 where the road crust is of an expensive type. Insurance against fail- 

 ure due to lack of uniformity in the subgrade can also, perhaps, best 



