DRAINAGE METHODS FOE 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 subgrade 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. 



