DRAINAGE METHODS FOE COUNTY ROADS. 
31 
TILE DRAINS. 
Tile drains are, in general, much superior to either open ditches 
or ditches filled with stone for subdraining roads, and though gen- 
erally more expensive than either open or stone drains, generally can 
be constructed at comparatively small cost. These drains, sometimes 
called French drains, are designed as shown in figure 11a. The depth 
of stone or gravel fill placed around and above the pipe varies accord- 
ing to the soil and ground water conditions. In some instances the 
stone fill is carried to the surface, in others it is omitted entirely. If 
a complete interception of underground flow is required in highly 
stratified material, it is best to fill the trench entirely with stone or 
gravel, but this is rarely necessary and when unnecessary is a waste 
of material. In any case, precautions should be taken to prevent 
FRENCH DRAIN 
D-ZW 
Inside Diameter 
Fig. 11a. 
silt from entering the pipe or clogging the stone fill. It usually is 
effective to lay tarred paper strips or pieces of broken tile over and 
around the joints when the trench is to be back filled with earth. If 
stone or gravel fill is used the same arrangement of sizes should pre- 
vail as for stone drain already described, care being taken to have an 
unbroken layer of sand on top of the finest stone. 
Practice varies with regard to location of such drains. Ordi- 
narily, one line of tile laid immediately under one of the side 
ditches and at a depth of 3 or 4 feet below the center of the road is 
sufficient, but where the soil is unusually retentive, two lines may be 
necessary. In some cases a single line of tile laid along the center 
of the road has proved satisfactory in retentive soils. 
Where, on account of the design of the side ditches, or for other 
reasons, it is not desirable to place a tile drain under the ditch or 
