32 BULLETIN 463, U. S. DEPARTMENT OP AGRICULTURE. 



more than about 12 inches thick, can be compacted fairly well in this 

 way, provided that the grading teams are made to travel over the 

 entire surface of each layer, and that the material is spread uni- 

 formly. Where the road surface is to be of some highly improved 

 type, each layer of the embankment should be rolled thoroughly with 

 a power roller weighing not less than about 10 tons. 



Settlement of embankments, aside from that due to shrinkage, 

 may be caused by the gradual flattening of the slopes or by lack of 

 stability of the ground surface over which the embankment is con- 

 structed. The question of proper slopes for various materials has 

 been discussed. The question of ground surface stability usually does 

 not arise as a grading problem except where an embankment is being 

 constructed over very marshy land. If proper precautions are not 

 observed, the embankment material may be absorbed gradually by the 

 marsh until the entire roadbed has disappeared, an occurrence which 

 is not infrequent. 



Where drainage of the marsh is impracticable, the lower portion 

 of the embankment, which would come in contact with the marsh 

 water, should be formed of some nonslaking material that will 

 cement together and distribute the weight of the embankment over 

 the entire bottom area. Some varieties of gravelly clay are excellent 

 for this purpose. Where the marshy matter is very soft and deep, 

 it may be necessary to lay a wide foundation bed of logs, or fascines, 

 upon which to construct the embankment, but such a foundation bed 

 would not obviate the necessity for using a nonslaking material in 

 the lower portion of the embankment. 



Where embankments are constructed on very sloping ground, or 

 where old embankments are to be widened, there always is a tend- 

 ency for the new material to slip along the plane of separation. 

 In such cases the old surface should be roughened before the new 

 material is deposited. Where the old surface is earth, it usually can 

 be roughened sufficiently by plowing a series of rather deep furrows 

 about 2 feet apart. Old rock surfaces ordinarily are fairly rough 

 to begin with, but to prevent slipping it may be necessary^ in some 

 cases to blast steps into such surfaces before depositing embankment 

 material. 



Eetaining walls, slope walls, and cribbing also are frequently used 

 to prevent embankments from slipping. 



FINISHING THE SURFACE. 



No matter how the grading of an earth road may be accomplished 

 it usually is economical to bring the road surface to its final shape 

 by means of a grading machine. In making excavations it is not 

 generally considered practicable to form the crown and side ditches 



