54 BULLETIN T2i, U. S. DEPARTMENT OF AGRICULTURE. 



clay should be shaken down dry with a harrow and in no case 

 wetted and rolled until the whole mass is well mixed. If large quan- 

 tities of clay are to be added it is better to place it in a layer between 

 a first and second layer of the cleaner gravel. The foundation then 

 can be wetted and rolled without preliminary harrowing, and the 

 clay will not stick to the roller wheels. Wet clay in contact with the 

 roller can almost never be mixed into the gravel, and endless trouble 

 results from permitting it to occur. 



Gravel foundations vary in compacted thickness from about 4 

 inches up to about 10 inches, depending on the condition of the sub- 

 grade and the character of the road crust, as well as somewhat on 

 the quality of gravel used. The most usual thickness is from 5 to 

 T inches. Except that where the road crust is to be of some highly 

 improved type a thickness of about 8 inches generally is used. In 

 general, foimdations of more than about 5 inches in thickness must 

 be constructed in two or more courses, as otherwise, unless the per- 

 centage of clay is unwarrantably high, it is almost impossible to 

 produce a thorough bond between the particles by harrowing and 

 rolling. Figure 14 shows a typical cross section for a gravel foun- 

 dation upon which the limiting dimensions are indicated. This 

 cross section shows the subgrade and foundation crowned to con- 

 form with the surface of the finished road, and probably is more 

 frequently employed than any other form, but in some cases the sub- 

 grade is made flat, or even slightly " dished," and the whole crown 

 of the road made up either in the foundation or the road crust. 

 This latter arrangement provides for additional thickness of the 

 pavement near the center where it usually is desirable to have the 

 greatest strength. The principal precautions to observe in con- 

 structing gravel foundations may be described briefly and com- 

 mented upon as follows : 



1. The gravel should be delivered on the work in wagons or cars 

 especially adapted to spreading each load uniformly over that part 

 of the subgrade for which it is intended. Where loads are dumped 

 in one spot and spread later with shovels, as is done frequently, it is 

 almost impossible to secure uniform density of the foundation by 

 subsequent harrowing and rolling. The spots where the loads are 

 dumped will nearly always be more densely compacted than the areas 

 in between and, as a result, uneven settlement soon will develop. 



2. Where it is necessary to spread the gravel in more than one 

 course the thickness of the different courses should be approximately 

 the same, except that the first course may be made somewhat thicker 

 than the others because, in general, a thicker layer of gravel may be 

 compacted on the subgrade than when spread over another layer of 

 gravel, although compacted. It usually is impracticable to have 

 the finished thickness of any course greater than about 5 inches, and 



