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. 
