Mr. Wm. H. Grant, Superintending Engineer of the New York Central Park, in his report upon the park roads, says:[46] “At the commencement of the macadam roads, the experiment was tried of rolling and compacting the stone by a strict adherence to Macadam’s theory, that of carefully excluding all dirt and foreign material from the stones, and trusting to the action of the roller and the travel of teams to accomplish the work of consolidation. The bottom layer of stone was sufficiently compacted in this way to form and retain, under the action of the rollers (after the compression had reached about its practical limit) an even and regular surface; but the top layer, with the use of the heavy roller loaded to its greatest capacity, it was found impracticable to solidify and reduce to such a surface as would prevent the stones from loosening and being displaced by the action of waggon-wheels and horses’ feet. No amount of rolling was sufficient to produce a thorough binding effect upon the stones or to cause such a mechanical union and adjustment of their sides and angles together, as to enable them mutually to assist each other in resisting displacement. The rolling was persisted in with the roller adjusted to different weights up to the maximum load (12 tons) until it was apparent that the opposite effect from that intended was being produced. The stones became rounded by the excessive attrition they were subjected to, their more angular parts wearing away, and the weaker and smaller ones being crushed.”
“The experiment was not pushed beyond this point. It was conclusively shown, that broken stones of the ordinary sizes, and of the very best quality for wear and durability, with the greatest care and attention to all the necessary conditions of rolling and compression, would not consolidate in the effectual manner required for the surface of a road while entirely isolated from and independent of other substances. The utmost efforts to compress and solidify them while in this condition after a certain limit had been reached, were unavailing.”
From the foregoing it is very evident that some description of binding material is essential in making a road under a roller. Where traffic is allowed to consolidate a road it is different, as then the stones are knocked about and are sufficiently abraded against each other to form a binding material for themselves. Too much binding material or too much water should not be used in forming a road with a steam roller. It is unfortunately frequently the case that a road is made quickly only to go to pieces with the traffic in a few weeks. The surface of a well-constructed macadamised roadway should after being rolled look almost like an encaustic pavement. If there is too much binding material in the joints of the stones, the first heavy rain washes it out and the surface of the roadway quickly goes to pieces.
The following description of the manner in which it is recommended that the roller should be applied is taken from an excellent little pamphlet on Steam Road Rolling, by Messrs. Aveling and Porter, the well-known makers of steam-rollers, and although local circumstances must guide the surveyor in all his works, the particulars may be of use:—
“In the best practice the roadway is excavated, graded, and properly formed to a depth of 14 inches from the level of the gutters, with a cross section conforming to the cross section of the road when finished; it is then thoroughly and repeatedly rolled with the steam roller, all depressions being carefully filled and rolled before the stone is put on. On the bed thus formed and consolidated a layer of stones 8 inches thick is set by hand, and rammed or settled to place by sledge hammers, all irregularities of surface being broken off and the interstices wedged with pieces of stone. The intermediate layer of broken stone, of a size not exceeding 3 inches in diameter, is then evenly spread to a depth of 4 inches and thoroughly rolled, and this is followed by rolling in half-an-inch of sand. The surface layer of stone, broken to a size not larger than 2 inches diameter, and to a form as nearly cubical as possible, is then put on to a depth of 3 inches, thoroughly rolled, and followed as before by sand, also rolled. Finally, a binding composed of clean, sharp sand is then applied, well watered and most thoroughly rolled with the steam roller, until the surface becomes firm, compact and smooth, the superfluous binding material being swept off and removed.”
And the following account of the method adopted in the United States at Hartford may also be of interest.[47]
“The surface of the road is excavated to a suitable depth—say, 18 inches; preparing the form for the pavement with the precautions as for a common pavement; 4 inches of gravel and proper drainage where required, provided blocks of stone of any irregular shape are selected for the pavement, of about 7 inches in thickness. The blocks are set by hand with great care, as closely in contact at their base as practicable. The surface between the blocks is filled with chippings of stone carefully laid in. A layer of broken stone, 4 inches thick, is laid over this pavement. The road-covering thus prepared should be rolled with the steam roller until the upper layer has become perfectly compact and consolidated. The second layer, about 3 inches in depth, is then laid on; a coating of clean coarse gravel, 1¹⁄₂ inch thick, termed ‘binding,’ is spread over the surface, and the whole well rolled as before, and you have the requisites of a good road—viz., clean, hard, and even at all seasons. No road should be considered made until it is completely rolled. A road made in the manner above described, and kept perfectly clean, hard, and even, with materials of a good tough quality, would show extremely little wear on the surface; indeed, it has been found in France to be less than ¹⁄₂ an inch in a year, on a road of great traffic.”
In the neighbourhood of New York the steam roller is used as follows:—
Two and a half inches of trap rock is laid and lightly rolled until the stones have become a little compacted, then coarse screenings are added, and it is again rolled; after this a layer of about 2 inches of stones are added and rolled with coarse screenings as before. Fine screenings or stone dust is then applied, and the roadway is then rolled until every interstice is filled up; it is then well watered and again rolled.
With reference to the employment of the steam roller in repairs of roads, the following description is given of the method adopted by the Surveyor to the Tottenham Local Board, near London.[48]