ROAD DRAINAGE. 1 93 



travelled road, which soon renders it unfit for use. To 

 prevent this the surface of a dirt road should be made 

 crowning as much as ten inches in a 20-foot roadway. 

 The side ditches will suffer much by erosion and irreg- 

 ular washing unless the flow is controlled by occa- 

 sional cutoffs by means of small cross-culverts, which 

 will divert and discharge the water at the lower side of 

 the road right-of-way without injury to either road or 

 adjoining land. These cross-drains should be located 

 at favorable points along the grade, and should consist 

 of good sewer-pipe not less than 10 inches in diameter 

 laid diagonally across the road track. With proper 

 selection of the location and the adoption of the plans 

 suited to the work, the top of the culvert may be placed 

 18 inches below the surface of the road and will con- 

 stitute a durable improvement worth many times its 

 cost. The joints of the pipe should be laid in good 

 cement mortar, and should, if possible, have a grade 

 of I foot in 20. These cross-drains being laid in solid, 

 not filled earth, are not open to the objection urged 

 against pipe culverts which are laid for waterways and 

 covered with loose embankment. 



Sewer-pipe^ Culverts ^ and Cross- drains. 



Experience with large earthen pipes for road cul- 

 verts has demonstrated that they frequently fail, not by 

 reason of lack of strength to resist the compression 

 that they may be called upon to bear, but because of 

 the jar or vibration which is communicated to them 

 through the material with which they are covered. 

 These failures are notable in locations where the pipes 

 are bedded in and covered with gravelly, rocky, or 



