DRAINAGE METHODS FOR COUNTY ROADS. 63 
used it is permissible sometimes to make the minimum width as low 
as 1^ inches, but for most conditions this minimum should be pre- 
ferably not less than about 3 or 4 inches. The maximum allowable 
width is controlled by much the same considerations as the maximum 
length, and in general, should not be greater than about 9 or 10 
inches. 
CROSS SECTION. 
The cross section of a Telford foundation usually is shaped in one 
of three, ways: (1) The sub grade may be crowned to conform with 
the surface of the finished road and the depth of Telford made uni- 
form for all parts of the section; (2) the subgrade may be flat and 
the depth of the Telford increased from sides to center so as to> give 
the required crown; (3) both subgrade and Telford may be given a 
flat cross section and the required crown secured by increasing the 
depth of the surface crust from sides to center. 
The first method, which is the one most commonly employed at 
present, has the advantages that the stones do not need to be sorted 
so as to secure a uniformly varying depth and that any tendency of 
the subgrade to settle irregularly will be counterbalanced in a meas- 
ure by the arch action of the tightly wedged Telford. 
The second method was the one employed originally and, while 
it is more expensive than the first, the extra depth it affords at the 
center may be of especial advantage where the subgrade consists of 
soil that is hard to drain. If the side ditches are not sufficiently 
deep to lower the ground water level below the frost line at the 
center of the road, an additional depth of Telford at this point evi- 
dently would decrease the likelihood of heaving due to freezing of 
the subgrade. 
The third method affords practically the same safeguard against 
frost as the second, but is subject to the objection that the slightest 
irregular settlement of the subgrade will weaken, rather than 
strengthen, the mechanical bond between the different Telford stones 
and thus will increase the tendency for irregularities to develop. 
Figure 16 shows a typical cross section for a Telford foundation, 
and Plate VIII is a photograph illustrating the method of construc- 
tion. 
DEPTH. 
The depth of Telford foundations varies in common practice from 
6 to 8 inches. Where subgrade conditions are extremely unfavorable 
it may be advisable to increase the depth above 8 inches, but under 
no ordinary circumstances should a depth less than about 6 inches 
be employed, because when the depth is less than this it is hardly 
practicable to bond the individual stones together securely by wedg- 
ing and rolling. 
