Page:Historic highways of America (Volume 15).djvu/116

 permanent embankments. Where possible the longitudinal grade should be kept down to one foot in thirty feet, and should under no circumstances exceed one in twenty, while that from center to sides should be maintained at one foot in twenty feet.

Wherever the subgrade soil is found unsuitable it should be removed and replaced with good material rolled to a bearing, i.e., so as to be smooth and compact. The roadbed, having been brought to the required grade and crown, should be rolled several times to compact the surface. All inequalities discovered during the rolling should be leveled up and rerolled. On the prepared subgrade, the earth should be spread, harrowed if necessary, and then rolled to a bearing by passing the unballasted road roller a number of times over every portion of the surface of the section.

In level countries and with narrow roads, enough material may be excavated to raise the roadway above the subgrade in forming the side ditches by means of road machines. If not, the required earth should be obtained by widening the side excavations, or from cuttings on the line of the