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 clone 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 dumped 

 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 



