398 ROADS AND PAVEMENTS 



thoroughly drained, and all disintegrated or worn-out material 

 and vegetable matter must be removed. The subgrade must 

 be brought to a uniform surface, free from hollows, and must 

 be thoroughly consolidated. The voids in the mass of the 

 broken stone must be eliminated by rolling and by adding 



FIG. 1 



fine dust; this dust should not be mixed with the stone, but 

 should be applied after the stones have received a slight com- 

 paction by rolling. The broken stones should not be left 

 loose to be compacted by the traffic, but should be consolidated 

 by rolling with a roller of suitable weight to bring each piece 

 of stone into close and firm contact with the adjacent pieces. 

 Two systems cf construction are employed: 



Macadam's system consists essentially in spreading and 

 compacting one or more uniform layers of suitable rock, broken 

 into pieces of nearly uniform size, directly on an earth foundation 

 that has been previously formed to the proper grade and cross- 

 section and thoroughly compacted by rolling. A cross-section 

 of a macadam road is shown in Fig. 1. 



Telford's system is much the same as Macadam's, except 

 that the layer of broken stone forming the wearing surface is 

 spread on a paved foundation. This paved foundation is 

 formed by blocks of stone from 3 to 8 in. in depth, set close 

 together on their broadest edges. The cross-section of a telford 

 roadway is shown in Fig. 2. The 

 blocks of stone are set on the earth 

 foundation, and their sizes are 

 graduated according to their posi- 

 FlG. 2 tion, as shown. 



Each of these systems has its 



place in the successful construction, of roads. The choice 

 depends entirely on the character and condition of the natural 

 soil. If this is composed of clay, not easily drained, a telford 

 foundation will be preferable; but, if the soil is easily drained, a 



