54 .BULLETIN 463, U. S. DEPARTMENT OF AGRICULTURE. 
an even, uniform appearance. Also, it should be as firm and unyield- 
ing as the conditions will permit. 
Some of the most important details requiring careful attention in 
preparing the subgrade, and which are perhaps most frequently over- 
looked, are (1) backfilling culvert trenches so as to prevent subsequent 
settlement, (2) exclusion of vegetable matter from fills, (3) provision 
for draining wet-weather springs which occur in the subgrade. and 
(4) the matter of thoroughly loosening and distributing the materials 
contained in old road crusts. These details are treated briefly in 
another part of this bulletin. 
After the roadbed has been graded and drained properly, and the 
details mentioned above have received proper attention, the work of 
preparing the subgrade consists simply in forming a trench, as shown 
in figure 22, to receive the gravel surface. The trench may be formed 
largely with a grading machine, the operation of which is described 
on page 21. but the final shaping should be effected by means of picks 
and shovels and rolling. It is customary to provide grade stakes at 
intervals of about 50 feet, to serve as a guide for the pick and shovel 
work, and where extreme accuracy is desired cords may be stretched 
between the stakes to insure that the subgrade conforms to the 
required grade throughout. The rolling is done ordinarily with a 
power roller weighing about 10 tons. 
In order that the subgrade may be well drained during the proc- 
ess of spreading and compacting the gravel it is frequently neces- 
sary to provide shoulder drains at comparatively short intervals. 
Such drains are constructed by opening small ditches through the 
shoulders and partially filling them with gravel. (See specification 
on page 66.) 
THE GRAVEL SURFACE. 
The principal precautions to observe in constructing a gravel road 
surface, after the subgrade is prepared, may be commented upon 
briefly as follows : 
(1) The gravel should be delivered on the work in wagons or cars 
especially adapted for spreading each load uniformly over that part 
of the subgrade for which it is intended. Where loads are diunped 
all in one spot and spread later with shovels, as is done frequently, it 
is very difficult to secure uniform density of the surface crust by 
subsequent harrowing and rolling. The spots where the loads are 
dumped nearly always will be more densely compacted than the areas 
between, and, as a result, uneven settlement will develop soon. 
(2) The gravel should be spread in two or more courses, and the 
thickness of the different courses should be approximately the same, 
except that the first course may be made somewhat thicker than the 
