The ordinary oiled road consists of a light covering of oiled soil upon the road surface. In some instances oiled surfaces are made 4 to 6 inches deep, by loosen ing the soil, saturating with oil, and then compacting by rolling to a firm surface. Difficulty has been found in compacting so deep a layer of oiled earth, and this has led to the invention of the petrolithic rolling tamper for this purpose. The rolling tamper is shown in Fig. 20. It consists of a heavy roller with a large number of tampers, or feet, projecting from its surface. These feet pack the material, beginning at the bottom, and thus compress the whole layer to uniform density. An oiled road constructed with this machine is called a petrolithic pavement. The following specifications used in Los Angeles for this class of streets illustrate the method of construction employed in such "After the street has been brought to the required grade and cross-section as above specified, the sur face shall be rolled with a roller weighing not less than 250 pounds to the inch width of tire until the surface is unyielding. Depressions made by the rolling shall
be leveled up with good earth and again rolled. Such portions of the street as cannot be reached by the roller, and all places excavated below grade and refilled, and all pipe trenches and other places that cannot be properly compacted by the roller, shall be tamped solid, and in case of wet weather or soft or muddy ground, making use of the roller unsafe or impracticable, the rolling shall not be undertaken until the ground has become sufficiently dry.
"The street shall then be tested for grade and cross section, and no further work shall be done upon it until a certificate shall have been issued stating that it is acceptable in these respects. It shall then be plowed to a depth of not less than six inches and thoroughly pulverized by cultivating and harrowing.