20 BULLETIN 724, U. S. DEPARTMENT OF AGRICULTURE. 



even surface. In designing gutters it is well to bear in mind the 

 f ollo-wing points : 



1. It is not usually necessary to extend a gutter entirely to the top of a 

 grade, because, as examples cited indicate, there usually is not water enough 

 flowing in the upper part of the side ditch to cause erosion. 



2. It is not usually economical to vary the size of the gutters on long 

 grades so that they will have only the required capacity at all points ; but 

 changes may and should be made if any considerable change occurs in the 

 required capacity. 



3. All gutters should be sufficiently strong to withstand the impact caused 

 by an occasional vehicle being driven into them, but unless they are capable 

 of withstanding constant vehicular traffic they should be designed so as to 

 discourage the practice of driving into them. 



4. The grade of gutters should be made as uniform as practicable in order 

 to prevent them from becoming partially filled by sedimentation, and should 

 not be less than 1 in 200. 



Brief directions for constructing the most common types of gut- 

 ters follow : 



COBBLE GUTTERS. 



Cobble gutters are ones constructed of rounded water-worn stones, 

 ranging in size from 4 to 8 inches in their longest dimension. The 

 stones should be sound, durable, and fairly uniform in shape. The 

 construction of the gutter should proceed about as follows : 



First, excavate a trench to receive the gutter. The trench should have a 

 width equal to the outside dimensions of the gutter and should be excavated to 

 a depth of about 12 or 14 inches below the proposed grade of the gutter. 



Second, spread sufficient broken stone or gravel in the bottom of the trench 

 to give a depth of about 6 inches when thoroughly compacted. The broken 

 stone or gravel should be of the kind ordinarily specified for macadam or 

 gravel roads and should be compacted thoroughly with hand rammers. 



Thiixl, spread a layer of bedding sand over the foundation to such a depth 

 that when the cobblestones are bedded into the sand and rammed thoroughly, 

 they will form a gutter true to grade, line, and cross section. For cobblestones 

 of fairly uniform size a depth of about 2 inches usually is sufficient for the 

 sand bedding. 



Fourth, place the cobblestones. In doing this the larger and more nearly 

 perfect stones should be selected and placed true to line and gi'ade in the edges 

 and in the row which forms the lowest part of the gutter. All stones should be 

 laid with their long diameter lengthwise of the gutter and should fit snugly 

 one against the other. Each stone should be rammed until it no longer yields 

 under the rammer, and any irregularities which may develop should be cor- 

 rected by resetting the affected portions. AYhile the ramming is in progress, 

 sufficient coarse sand should be spread over the surface to keep the joints 

 between the stones well filled. AYlien the ramming is complete, the surface of 

 the glitter should be ui^iformly true to line and grade. Wherever changes in 

 the dimensions of the gutter are necessary, they should be made gradually, 

 and not by abrupt off-sets. 



Fifth, where the grade of a cobble gutter exceeds about 5 per cent, and the 

 volume of water it must carry is at all considerable, the interstices between 

 the stones should be filled with Portland cement grout instead of sand, as 

 above described. A suitable grout for this purpose may be made by mixing 



