DRAINAGE METHODS FOE COUNTY ROADS. 39 
COST OF V-DKAINS. 
The most expensive feature of constructing a V-clrain 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 shape 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 mean 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 subgrade- 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 concentrated pres- 
