HIGHWAY CONSTRUCTION r> 



This effect is due to the tendency of water which contains iron to 

 lay down that substance and to take lime in its place when the 

 opportunity for so doing occurs. Such gravels are termed ferru- 

 ginous. They are commonly found in a somewhat cemented state, 

 and when broken up and placed upon roads they again cement, even 

 more firmly than in the original state, often forming a roadway of 

 very good quality. 



When no gravel but that found in rivers or on the seashore can 

 be obtained, one-half of the stone should be broken and mixed with 

 the other half; to the stone so mixed a small quantity of clay or 

 loam, about one-eighth of the bulk of the gravel, must be added: 

 an excess is injurious. Sand is unsuitable. It prevents packing in 

 proportion to the amount added. 



Preparing the Gravel. Pit gravel usually contains too much 

 earth, and should be screened before being used. Two sieves should 

 be provided, one with meshes of one and one-half inches, so that all 

 pebbles above that size may be rejected, the other with meshes of 

 three quarters of an inch, and the material which passes through it 

 should be thrown away. The expense of screening will be more 

 than repaid by the superior condition of the road formed by the 

 cleaned material, and by the diminution of labor in keeping it in 

 order. The pebbles larger than one and a half inches may be 

 broken to that size and mixed with clean material. 



Laying the Gravel. On the roadbed properly prepared a layer 

 of the prepared gravel four inches thick is uniformly spread over the 

 whole width, then compacted with a roller weighing not less than 

 two tons, and having a length of not less than thirty inches. The 

 rolling must be continued until the pebbles cease to rise or creep in 

 front of the roller. The surface must be m6istened by sprinkling 

 in advance of the roller, but too much water must not be used. 

 Successive layers follow, each being treated in the above described 

 manner until the requisite depth and form has been attained. 



The gravel in the bottom layer must be no larger than that in 

 the top layer; it must be uniformly mixed, large and small together, 

 for if not, the vibration of the traffic and the action of frost will 

 cause the larger pebbles to rise to the surface and the smaller ones 

 to descend, and the road will never be smooth or firm. 



