Road Drainage. 447 



must be sufficiently thick to float, on the soft soil beneath, 

 any load which may be driven along it, just as the ice floats 

 its burden. 



549. Place For the Drain. In the narrow roads of eight 

 to sixteen feet, where the water to be removed is that which 

 may be raised by hydrostatic pressure vertically upward 

 beneath the road-bed, the best place for the drain is di- 

 rectly beneath the center of the drive-way. 



Where the main source of the water causing the trouble 

 is an underflow through sands and gravels from adjacent 

 higher lands then the drain should be placed upon the side 

 of the road from which the water comes. 



Where the ground is marshy on all sides, and particu- 

 larly if the road is wide, it may then be necessary to lay 

 two lines of tile, one on each side. 



If springy places occur under or near the road-bed 

 drains must be connected with the spring itself, so as to 

 effectually remove the excess of water. 



550. Fall of the Drain The fall of the drain will usu- 

 ally conform somewhat nearly to the grade of the road-bed, 

 but should not be less than two inches in 100 feet, if this 

 can be secured. It will, however, be necessary sometimes to 

 lay the drain on a slope less than this, even as low as -J 

 an inch in 100 feet. In all cases care should be exercised 

 to lay the tile on a true grade, not allowing them to drop 

 anywhere below or rise above a rigidly maintained grade 

 line. If they are not laid in this manner water will 

 stand in the sags and behind the bends, and in these places 

 the tile may become filled with silt. 



It may sometimes occur that the road is so nearly level 

 that there is no fall for the drain. In such cases it may 

 be necessary to lay the beginning end of the drain nearer 

 the surface of the ground by as much as six or even twelve 

 inches. In this way there could be given a fall of one inch 

 in 100 feet over a distance of 1,200 feet, but of course 



