120 CONSTRUCTION 



The amount of tar or asphalt necessary is about i to f 

 gallon per square yard. 



On top of this must be placed a thin layer of sand, 

 gravel or screenings, from 7 to 35 pounds per square yard. 

 In England the top dressing has been omitted, but in this 

 country it is generally specified, since by its use the road 

 may be much sooner opened to traffic. Though rolling is 

 not absolutely necessary, it greatly improves the surface. 

 Fig. 38 shows a type of road roller used in the Boston 

 parks. 



For parks with narrow, winding, and steep drives or 

 paths a tandem roller is to be preferred. Fig. 16, page 49, 

 shows a special tandem roller built for this purpose which 

 combines the short wheel base and narrow tread of the 

 tandem roller with the high compression given by the three- 

 wheeled roller. 



7. Impregnating Earth Roads with Crude Oils. This 

 method originated in California, where a high-grade asphaltic 

 oil is plentiful. The process consists in loosening the soil 

 for a depth of 6 inches. Heated oil is then applied and 

 thoroughly kneaded into the soil by means of a " rolling 

 tamper." The work can be done only in warm dry weather. 

 Porous soils are better adapted to this treatment than heavy 

 ones. Three hundred barrels per mile to be applied once 

 each year are sufficient for a 12-foot roadway. 



8. Impregnating New Macadam Pavements with Bitumi- 

 nous Materials by the Penetration Method. There are several 

 methods employed, for which see Highway Engineering, 

 by Blanchard and Drowne, to which work the author is 

 indebted for much of the information here presented on 

 pavements. One of the best is as follows: 



The upper course is built of crusher run from IJ-inch to 

 ^-inch size. On this without further filling of the voids 



