ROLLING THE STONE. Rolling is a very important part of the construction of a broken-stone road. The subgrade should be rolled to prevent the stone from being forced into the earth. The lower course of the stone should be rolled to compact it, so that the pieces will not move one upon the other under the traffic; and the top course should be rolled to pack or bind the pieces into place, to prevent their being knocked out by the horses' feet. Roll ing accompanied by sprinkling is necessary also to work the bind ing material into the interstices so as to make the surface water tight. Roads that have been consolidated by traffic are largely held together by mud, and after long use are fairly smooth and hard in dry weather, but soon become soft and muddy during a wet time.
The stone is put on in two or three layers,—according to the total thickness of the finished road,—and each course is thoroughly rolled before the next is added. The courses should not be more than 4 to 6 inches thick. When a telford foundation is used, broken stone is spread over the pavement to bring the top surface to the proper form and height, after which it is rolled.
The rolling should proceed gradually from both sides tow ard the center. If the weight of the roller can be varied, com mence with the unballasted roller, and increase the weight as the stone becomes consolidated. If the surface of the layer shows a wavy motion after being rolled three or four times, the subgrade is too wet, and time should be given it to dry out. Some coarse brittle granitic reeks begin to crawl and the sharp edges to break off after the roller has passed over them a few times; but a light sprinkling of sand or stone screenings will prevent this, and fa cilitate the consolidation of the layer. All irregularities of the surface developed by the rolling should be corrected by filling the depressions with stone of the size used in the layer.
The rolling should be continued until the stone ceases to creep in front of the roller, and until the macadam is firm under the foot as one walks over it. When the rolling is complete, one of the larger stones of the course can be crushed under the roller without indenting the surface of the layer.
When the first course has been consolidated, a second, usually a thinner one of smaller stones, is added, and then rolled the same as the first. Finally a third course consisting of about half an inch of sand or fine stone and stone dust is added. The roller is then passed over this layer, with the result that the bits are ground to powder. As the rolling of this course proceeds it is sprinkled, the aim of the sprinkling and rolling being to work the fine material into the cavities between the pieces of crushed stone, thus binding the whole into a solid mass. The proper binding of the road is the most important part of the construction, and will be more fully considered presently (see § 345).
In New York City, 5 inches of crushed gneiss on telford and 5 inches of trap on the gneiss, bound with trap screenings, was rolled with a 15-ton steam roller at the rate of 40.6 sq. yd. per hour, or 10 cu. yd. per hour. Although it is common to give the amount of rolling in terms of the time required, the statement is somewhat indefinite, since the work accomplished varies with the speed of the roller and also with the length of run, i. e., with the time lost in starting and stopping. The usual speed of steam rollers is 2 to 24 miles per hour. The above work is equivalent to 0.553 ton miles per sq. yd., or 2.246 ton-miles per cubic yard. The number of trips was 130.* In making repairs, a 6-inch course of 2-inch trap was rolled at the rate of 26.2 sq. yd. per hour, or 4.4 cu. yd. per hour. The work amounted to about 0.859 ton-miles per sq. yd., or 5.177 ton-miles per cu. yd. The number of trips over the surface was 201.t An area of 22,000 square yards of a 3-inch course of 2-inch trap upon an old broken-stone road, bound with trap-rock screenings and rolled with a 10-ton steam roller, was finished at an average rate of 47.15 sq. yd. per hour of rolling, the extremes being 38.4 and 61.1 sq. yd. per hour. This was an average of about 4.0 cu. yd. per hour.$ A 6-inch course of to 24-inch trap rock, bound with lime stone screenings, was rolled with a 124-ton steam roller at an aver age rate of 31.4 sq. yd. per hour, or 5.2 cu. yd. per hour.§ The Hudson County Boulevard (Jersey City, N. J.) consists of 8 inches of telford, 24 inches of 24-inch stone, 14 inches of 14 inch stone, and then 4 to 1 inch of coarse screenings—all trap rock. The macadam top was supposed to roll down to 4 inches, i. e., 411 to 5 inches of loose stone was supposed to roll to 4 inches. The rolling was distributed about as follows: On the telford, 10 to 12 passages; on the 24-inch course, 8 to 10 passages; on the 14-inch course, 10 to 12; and on the screenings, 80 to 90,—making a total of 100 to 120 passages of the roller over the road.
The above examples are representative of American prac tice in building the best crushed-stone roads, and represent consid erably more rolling than is customary in either England or France. In Paris the porphyry roadways with courses from 3 to 4 inches thick receive from 0.234 to 0.41 ton-miles per sq. yd., or 2.99 to 3.78 ton-miles per cu. yd.II The number of passages of the roller varies from 75 to 100. In Paris some streets were " thrown open to traffic when the rolling had reached about the point when in this country the application of screenings would commence." * It is believed that the American roads are enough better to pay for the greater amount of rolling they receive. As a rule, American crushed-stone roads are better constructed than those in Europe, notwithstanding the fact that the European roads are frequently cited as models for imitation in America. The superiority of American roads is partly due to the greater amount of rolling they receive, and partly to the greater quantity and better quality of binding material used. As a rule the broken-stone roads are better maintained in Europe than in America—probably because of the cheaper hand labor.