DKAIlSrAGE METHODS FOR COUNTY ROADS, 55 



in some cases 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 thor- 

 oughly with a tooth harrow until the different sizes of particles and 

 the cementing material are distributed thoroughly through the mass. 

 Then it should be rolled repeatedly with a power roller weighing 

 about 10 tons until the different particles are all well bonded together 

 and the surface presents a smooth, uniform appearance. When com- 

 pleted, the surface of each course should appear firm and unyielding 

 and should be such that no particles of gravel will be disturbed by 

 men walking upon it. , ' 



4. Where it is necessary to add sand or clay to the gravel, in order 

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

 and before it is harrowed. But with some kinds of gravel and where 

 certain types of road crust are to be used, it is permissible to add a 

 limited amount of sand after the harrowing is all done and while the 

 rolling is in progTess, so as to secure a denser surface. This usually 

 applies to the top course of the foundation when it is desirable to 

 prevent bitumen, Portland-cement grout, or other similar material 

 used in the surface crust from being wasted by penetrating into the 

 foundation. 



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 limited amount of 

 sprinkling may be necessary in dry weather. The sprinkling always 

 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 foundation 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 the foundation is able to support the roller, 

 moving back and forth over it, without appreciable waves being 

 formed in the surface. When the rolling is complete the foundation 

 is ready to receive the surface crast immediately. 



COST OF GRAVEL FOUNDATIONS. 



In estimating the cost of a gravel foundation it is necessary to con- 

 sider : 



1. The purchase price of the gravel it is proposed to use ; 



2. The cost of delivering the gravel on the work ; and 



3. The cost of spreading and compacting. The first two of these 

 factors are evidently dependent to a very great extent on local condi- 

 tions, and their influence on the cost of a given foundation can not be 

 determined except by a study of the given conditions. The following 



