MISCELLANEOUS FOUNDATIONS MACADAM. Where crushed stone is especially plentiful or cement is unusually expensive, a thick course of macadam may be more economical than a thin layer of concrete Concrete dis tributes the concentrated load of the wheel over a considerable area of the subgrade by virtue of both its thickness and its transverse strength; while macadam distributes the pressure only by virtue of its thickness. Therefore if the cement is omitted the thickness of the broken stone should be increased. The efficiency of a layer of broken stone in distributing the concentrated load on a wheel is proved by the fact that under very favorable circumstances 4 inches of macadam has successfully carried a considerable traffic (§ 321), while 6 inches of macadam are quite common (§ 320). If 4 or 6 inches of macadam without any other pavement will carry the traffic, the same thickness will certainly make a good foundation for a pavement of brick, or wood, or stone blocks.
A macadam foundation should be laid substantially as the lower course of a macadam road (§ 339). The rolling should be continued until the stones are firm under foot as one walks over the layer. if the stone is soft (there is no objection to a moderately soft stone for this purpose), the rolling will crush the top of the layer to such an extent that the surface will be nearly impervious. If the wear ing surface of the pavement is not too impervious, or if the drainage of the subsoil is not good, or if for any other reason it is desired to make the foundation impervious, it may be done by spreading stone dust upon the rolled macadam and continuing the rolling with or without sprinkling.