Macadam road's as commonly constructed consist of two or more layers of broken stone, each layer being rolled to a firm bearing before placing the next. The broken stone is usually placed directly upon the earth road-bed.
In constructing a macadamized roadway, the road bed is .first brought to the proper grade in the usual manner, and rolled to a uniform surface. The surface of the road-bed is either flat or raised at the middle to the same section as is to be given the finished road surface. The inclined form is usually employed, and seems preferable on account of affording better drain age in case any water finds its way through the surface layer.
On village streets where curb and sidewalks are employed, this section of the road-bed may extend to the curbing (as shown in Fig. 3), but on country roads a bench of earth should be left at the side between the broken stone and the gutter in order to confine the broken stone while it is being compacted, and prevent the spread of the surface materials. The forth of the road-bed before placing the stone would then be as shown in Fig. 21, where the completed road is to be of the fora} given in Figs. 5 and 17. Where the road-bed is in embankment, it is common to construct the earth embankment to the height of the finished surface, and afterwards excavate the material necessary to admit of placing the surface layers. The embankment should be allowed to settle and become thoroughly compacted before the broken stone is placed upon it, and it is desirable with new embankments that they be used for a short time by the traffic upon the earth surface be fore finishing the road; where, however, the material is well compacted in construction and can be thor oughly rolled this is not necessary.
In constructing the road-bed its proper drainage must be considered, and where necessary to prevent its becoming wet under the broken stone some means should be adopted to artificially drain it.
Upon the completion of the road-bed, a layer of broken stone, usually from 3 to 5 inches in thickness, is placed upon it and thoroughly rolled. Upon this a second layer is placed and likewise rolled to a uniform surface. Sometimes a third layer is added, or in case of a very thin road it may consist of a single layer, the number of layers depending upon the thickness of the road. When no roller is used, the stone is. usually spread on the surface of the road-bed to the full thick ness desired for the road, and left to the action of the traffic.
The upper layer constitutes the wearing surface of the road, and upon this it is usually necessary to place a thin layer of finer material called binding material, which may consist of rock chips, sand, small gravel, or sometimes loam, and is washed and rolled into the inter stices of the rock, with the object of forming a com pact and impervious surface. Binding material is in like manner often added to the lower layers of the road, although this has not been common practice. The object should be to fill the voids in the rock as completely as possible, serving to make the road one solid mass, to bind the rock more firmly together, and to prevent the percolation of water through the surface.