EARTH, SAND-CLAY, AND GRAVEL ROADS. 55 



succeeding courses, because, in general, a thicker layer of gravel 

 may be compacted on the subgrade than when spread over a layer of 

 gravel already compacted. It usually is impracticable to have the 

 compacted thickness of any course greater than about 5 inches, and 

 quite frequently 3 or 4 inches is as much as can be compacted satis- 

 factorily at one time. 



(3) After each course of gravel is spread it should be harrowed 

 with a tooth harrow until the various sizes of particles and the binder 

 or cementing material are distributed thoroughly through the mass. 

 Then it should be compacted by rolling with a power roller weighing 

 about 10 tons, or by means of traffic. Where a roller is employed the 

 rolling should be continued until the particles of gravel are all well 

 bonded together and the surface presents a smooth, uniform appear- 

 ance. When completed, the surface of each course should be so firm 

 and unyielding that it will not be disturbed in any way by subsequent 

 traffic. 



(4) Where it is necessary to add sand or clay to the gravel in 

 order to fill the voids it should be done after each course is spread 

 and before it is harrowed, except that with some kinds of gravel it 

 may be permissible to add a limited amount of fine material to the 

 surface of the top course after the harrowing is completed and the 

 rolling is in progress. 



(5) Where the binder consists of some material other than clay, it 

 may be desirable to sprinkle each course with water while it is being 

 rolled, and even where clay is used as a binder a small amount of 

 sprinkling may be necessary in dry weather in order to secure a satis- 

 factory bond. The sprinkling should be done uniformly and in 

 such quantities as not to wash the fine material out from the gravel 

 or to soften the subgrade. 



(6) When the road surface is complete it should be uniform 

 in grade and cross section. If depressions occur under the roller 

 they should be corrected by adding gravel and continuing the rolling, 

 and this should be kept up until no depressions or appreciable waves 

 are produced by the roller in moving back and forth over the surface. 



(7) After the road is opened to traffic, it should be watched very 

 carefully for several months and all defects which develop should be 

 corrected immediately. The work of maintaining the road until the 

 surface no longer " picks up " or ravels under traffic should be con- 

 sidered an essential feature of the construction.. When traffic is 

 depended upon to compact the gravel, much dragging is necessary 

 in order to secure a smooth, well-bonded surface. In fact, the cost 

 of dragging, under such conditions, frequently may exceed the cost 

 of securing a well-bonded surface by means of rolling. 



Plate V, figures 1, 2, and 3, illustrates the different kinds of work 

 involved in gravel-road construction. 



