ROADS AND BRIDGES 



325 



factory driveway. Where ashes can be procured, they 

 make a most useful substance for hardening the surface 

 of the road and to use as the lower portion of the surface 

 in bicycle paths. 



Mixing surface materials. The mixing of materials in 

 making up the surface of the roadway until recently has 

 been but little studied from a scientific standpoint. As 

 a general proposition, however, under the ordinary 

 climatic conditions of the United States, it may be said 

 that a mixture of about equal parts of gravel, sand and 



Figurt 210. Dump wagon, with lever and chain gear to open and close drop bottom. 



clay forms a good compact road surface, when the sub- 

 soil is not miry. The road builder must constantly use 

 his judgment in mixing the best materials secured from 

 cuts, from the roadside or from adjacent fields in mak- 

 ing up the dirt, gravel or sand-clay surface of the road- 

 bed of the common road. Where the top of the 

 substructure is made up of mixed sand and clay, and pos- 

 sibly some gravel, the problem is how to add gravel or 

 other coarse material which will make the road carry a 

 heavier load without cutting, will be smooth and hard 

 on the surface, and will endure the constant wear of 



