30 
BULLETIN 724, U. S. DEPARTMENT OF AGRICULTURE. 
in all sections of the country and, when properly constructed, usually 
have proved satisfactory for a considerable period of time. 
TThen such ditches are used it is common, practice to place one" on 
each side of the road immediately under the ditches. They are ex- 
cavated to a depth of from 2 to 3 feet with vertical sides, and usually 
are made about 18 inches wide at the top or just wide enough for a 
man to work in them conveniently. To be effective they must have 
a uniform grade and be carried to a suitable outlet. 
While common practice in this regard is as cited, it is not the best. 
Unless great care is taken in grading the material with which the 
ditch is filled, there is a marked tendency in all heavy storms for 
the stone-filled trenches to flush full of water heavily laden with silt 
and become clogged. A much better design is to lay one ditch down 
the center of the road, sufficiently deep to lower the water table 
enough at each edge and to cover it with the surfacing material. In 
STONE DRAIN 
Top, earth filling. 
Center, dust to 2" in diameter, 
Bottom, 2" to 4" in diameter. 
D- 2'te 3' 
Fig. 11. 
this way, the amount of storm water it accumulates by percolation is 
a minimum and it gathers no silt. 
The stone or gravel used for filling these ditches should preferably 
be of a uniform, size so that the percentage of voids will be high. A 
material consisting of rather large particles also is to be preferred 
because the individuals voids in such material are large and are not 
so easily filled by deposits of silt. 
If the subdrainage ditches are placed under the side ditches, the 
stone or gravel fill should be graded finer for about 6 to 10 inches 
near the surface, until the top 2 or 3 inches are sand, over which the 
natural soil and silt will tend to form a covering and so retard the 
filling and clogging of the lower stone. This grading of material is 
entirely unnecessary if the ditch is located under the surfacing. 
Figure 11 shows a typical cross section for a ditch of this kind. In 
the cross section large stones of a uniform size are shown in the 
lower part of the ditch and finer material in the upper part. This is 
a very effective arrangement, as the fine material near the top pre- 
vents silt from being carried down by the surface water and de- 
posited in the interstices between the stone in the bottom layer. 
