FOUNDATIONS FOR PAVEMENTS.
In the construction of a road-bed to support a pavement, the same principles are involved as in the earthwork of a common road, which has been discussed in Art. 26, and the same methods may be employed in handling the earth. In grading, the surface should be left high enough to allow for the compression pro duced in rolling. The amount of settling to be ex pected under the roller will vary with the character of the material and the weight of the roller. With a heavy steam roller, the compression may vary from inch for stiff soil in dry condition to about 3 inches for light porous soil. The allowance to be made can only be judged from experience with the soil in question.
The road-bed, after being brought to the proper grade, should be thoroughly compacted by rolling before placing the pavement. Sometimes in the use of a heavy roller, when the material is of a light nature, it is shoved forward in, a wave before the roller and re fuses to become compacted, in which case a thin layer of gravel or small stone placed upon the surface of earth before rolling may have the effect of consolidat ing the road-bed under the roller to a hard surface.
The roller should pass several times over the road bed. When low places are developed, which roll down below grade, they should be filled and rolled again until brought to proper grade. Passing the roller trans versely over recently filled trenches will always produce depressions which require refilling. Where such trenches exist, the rolling should be very carefully done.
In rolling, soft spots are sometimes discovered, which cannot be compacted by rolling. In such cases the soft material should be removed and replaced with better material to a sufficient depth to admit of roll ing the road-bed to a compact surface.
In some instances, repeated rolling of light material with a heavy roller may have the effect of working the material loose so that it moves in a wave before the roller, although the first rolling leaves the road-bed compact. In such cases it is desirable to avoid too much rolling.
Where much grading is to be done, it is usually de sirable to do the rough work before setting the curb upon the street, if a new curb is to be ,placed. It is, however, much easier to finish the grade after the curb is set, as a line across the street at the top of the curb is a convenient means of getting the elevation of points on the subgrade.