EOAD MODELS. 13 



Section B shows the first course of gravel spread and rolled to a 

 thickness of about 4 inches at the center and 3 inches at the sides. 



Section C represents the finished road after the second course of 

 gravel has been spread and compacted. The second course when 

 compacted is 2 inches in thickness at the center and 1| inches at the 

 sides. 



Many gravels as found in nature are not suitable for use on the 

 road, unless modified by the addition or elimination of certain ma- 

 terials. The products of some pits are deficient in bonding materials, 

 such as sand or clay, and at the same time may have an excess 

 of large pebbles. In general, the gravel should be screened and the 

 graded material laid in courses according to the manner of macadam 

 road construction. 



MACADAM ROADS. 

 WATER-BOUND MACADAM ROADS. 



Plate VII, figure 1, illustrates the construction of a water-bound 

 macadam road 16 feet wide, 6 inches thick at the center, and 4 inches 

 thick at the sides when rolled. The crown of the finished road as 

 shown is one-half inch to the foot. The road is graded for a width of 

 33 feet. 



Section A represents the prepared subgrade properly crowned and 

 rolled; section B, the first course of broken stone, 4 inches thick 

 compacted; section C, the second course, 2 inches thick compacted; 

 and section D, the finished road. 



In preparing for the construction of a road of this type, the pro- 

 cedure given on page 7, for foundation, or subgrade, and shoulders, 

 should be followed. After the subgrade has been prepared, a layer 

 of broken stone of approved size and quality for the first or bottom 

 course should be spread evenly over it to such depth that it shall 

 have, when rolled, the required thickness. The depth of the .loose 

 stone may be gauged by laying upon the subgrade cubical blocks of 

 wood of the proper size, and spreading the stone evenly to conform 

 to them. 



The roller should be run along the edge of the stone backward and 

 forward several times on each side of the road before rolling the 

 center. If a filler is desired for the bottom course it should be clean, 

 coarse sand, or stone screenings supplemented by the product of the 

 crusher not otherwise used in top or bottom courses. It should be 

 spread uniformly over the surface and then swept in and rolled dry. 

 This process must be continued until no more will go in dry, when 

 the surface should be sprinkled to more effectually fill the voids. Any 

 irregularities or depressions may be made good with broken stone of 

 the size used in the bottom course, and screenings or filler should not 

 be used for this purpose. 



