DKAINAGE 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 fo 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 
