DEAINAGE METHODS FOR COUNTY ROADS. 65 



delivered on the work in sizes suitable for handling by one man it can 

 be broken and placed in a foundation of ordinary depth at the rate 

 of from 1 to 3 square yards per laborer per hour. Assuming labor at 

 20 cents per hour and making a reasonable allowance for overhead 

 charges, the cost of setting and wedging an ordinary Telford foun- 

 dation may be estimated at from 10 to 25 cents per square yard ; the 

 variation being controlled by the character of the stone. 



The cost of quarrying and .hauling has been discussed in connection 

 with crushed stone and gravel foundations, and it is evident from 

 those discussions that no definite rules for estimating these items can 

 be stated. In localities where Telford foundations have been used ex- 

 tensively the cost of quarrying generally varies between 40 cents and 

 75 cents per cubic yard, according to the character of the stone and 

 the methods employed, and the contract price for the foundation is 

 between 50 cents and 90 cents per square yard. 



CONCRETE FOUNDATIONS. 



At present concrete foundations are constructed almost exclusively 

 of Portland cement concrete, and are used generally in connection 

 with such types of road crusts as granite block, vitrified brick, wood 

 block, asphalt, and bituminous concrete. The advantages possessed 

 by concrete foundations for these types are : 



1. Properly prepared concrete is more uniformly reliable in distributing the 

 pressure due to concentrated loads than any other material of which road 

 foundations are ordinarily constructed. Concrete foundations are to be pre- 

 ferred therefore, for expensive road crusts such as those mentioned above, 

 which might be damaged seriously by settlement of the foundation. 



2. For all practical purposes as foundations concrete is impervious to water 

 and prevents the surface crust from being damaged by the action of ground 

 water, or surface water from percolating through to the subgrade, where 

 either of these conditions otherwise might occur. 



3. In case of slightly irregular settlement of the subgrade a concrete founda- 

 tion may be sufficiently strong to act as a bridge over depressions which, with 

 other types of foundations, would cause irregularities to develop in the road 

 surface. The extent to which a concrete foundation affords these advantages 

 is controlled by the design of the foundation, the selection of the concrete mate- 

 rials, and the care exercised in construction. Each of these features will be 

 discussed briefly. 



DESIGN. 



Figure 17 shows a typical cross section for a concrete foundation 

 and indicates the limiting dimensions ordinarily used in good prac- 

 tice. A lower minimum depth than 5 inches at the center is used 

 sometimes, but hardly can be justified by experience, except where the 

 surface crust is designed to aid materially in distributing the pres- 

 sure due to concentrated loads, Plate IX shows an example of fail- 

 ure where a sand-filled brick pavement was constructed on a 4-inch 

 66998°— 18— Bull. 724 5 



