ROADS AND BRIDGES 



305 



as lime hardens and cements together the particles of 

 sand in mortar. Some of this cement is needed in 

 loose sands or gravels to bind them together in 

 making a firm road surface, since they are too loosely 

 knit together. 



Properties of surfacing materials. That part of the road- 

 way which receives the weight of the passing vehicles 

 and teams, to best serve its purpose, must have a number 

 of qualities of which solidity is the first requisite. It must 

 be so solid that the heavy loads will not break it down 

 and thus crumble the crust which is designed to lie 

 intact upon the substructure of the roadway. Thus, a 

 thin hard surface might not 

 have under it a solid basis 

 and it might be crushed into 

 the soft earth beneath it. 



Tn T^icmrA rSn ic cVinAxrn VinAsr Figure 179. Cross-section of macadam 



rigut 100 is snown LOW road A lower i ayer of 6 or more inches 

 mixed sand and gravel over M &S?^W ; ii*Siif^ 

 a peaty soil, unless a foot under x 



thick, will be shoved into the soft peat below. A road 

 surface made of macadam or telford, if sufficiently thick, 

 will not be crushed into the substructure, even if soft 

 clay underlies, though with a solid substructure a thinner 

 layer will suffice. 



Resistance to traction. A surface is desired which is 



hard and smooth at the top, 

 that the wheels of vehicles 

 may not sink in and be 

 constantly required to climb 

 over or displace an obstruc- 

 tion as if climbing up a hill. 

 As we require the best steel in the edge of an ax, so we 

 require the hardest, toughest material to endure the wear 

 at the surface of the road. (See Figure 177.) 



Durability. Not only is it expensive to make per- 

 manent grades, but road surfaces are costly, especially 





