30 



BULLETIN 124^ U. S. DEPARTMENT OF AGRICULTUBE. 



in all sections of the country and, when properly constructed, usually 

 have proved satisfactory for a considerable period of time. 

 When such ditches are used it is common practice to place one on 

 each side of the road immediate!}^ 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 Avide 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, ear+h filling. 



Center, dust to 2" in diameter. | 



Bottom, 2" to 4" in diameter. 



.mjij^^A .-'v^WAVAW''--'^^ 



D=2'to3' 



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 imder the side ditches, the 

 stone or gi\avel 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 shoAvn 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. 



