DRAINAGE METHODS FOE COUNTY EOADS. 39 



COST OF V-DRAIJJifS. 



The most expensive feature of constructing a V-drain foundation 

 is to collect and haul the field stones. The conditions affecting this 

 part of the work seldom can be compared for different projects, even 

 in a general way, and any cost data that might be given here would 

 be more likely to prove misleading than otherwise. After the stones 

 have been collected and hauled, the additional work required to put 

 the foundation in shap3 usually is very slight, and its cost should not 

 exceed a few cents per square yard. The cost of preparing the sub- 

 grade to receive the stones is discussed on page 51. 



FOUNDATIONS. 



In order to avoid confusion in the following discussion, the term 

 foundation will be used to designate a specially constructed artificial 

 base upon which the road surface or crust rests, and the term sub- 

 grade will be understood to msan that part of the roadbed, composed 

 of natural earth or other similar material, which supports either the 

 foundation, or in the absence of a foundation, the road crust. This 

 distinction is purely arbitrary and is made simply for convenience. 



The purposes of a foundation are, first, to aid in distributing the 

 pressure due to wheel loads concentrated on the road surface so that 

 the intensity of pressure on the subgi'ade- will not exceed its safe 

 unit bearing power, and second, to prevent, if possible, the crust of the 

 road from being disturbed by the upward pressure of the subgrade 

 due to the action of frost, ground water, or other disturbing influence. 

 The relative extent to which these respective purposes should govern 

 in designing a foundation depends on the character of the road crust 

 and of the material composing the subgrade as well as on climatic 

 conditions. For example, in the case of a pavement consisting of 

 comparatively small blocks, such as vitrified brick, bonded to each 

 other very slightly or not at all, an entire wheel load might be con- 

 centrated on only a few square inches of foundation, in which case 

 it would be necessary for the foundation to take care of practically 

 the entire distribution of the load. On the other hand, a monolithic 

 pavement or a pavement in which the individual units are well 

 bonded together might distribute the wheel loads sufficiently to ob- 

 viate all necessity for further distribution by the foundation and if 

 adequate drainage were provided and there were no danger of heav- 

 ing of the subgrade, the foundation might be omitted entirely. 



LOADS. 



The heaviest vehicle that ordinarily has passed over a public 

 highway heretofore is the 15-ton traction engine. Such engines 

 usually are so designed that the rear axle carries about two-thirds 

 of the total weight, which arrangement gives a conoentrated pres- 



