Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/609

Rh ROADS 585 best Portland cement, thoroughly mixed in proper proportions with the sand and gravel or other materials used, water being added as sparingly as possible. A thickness of 6 inches of well- made cement concrete is sufficient for the heaviest traffic, and it can be cut out in slabs for pipe-laying or repairs and can be relaid and cemented in its place. To obtain the best result a new foundation should not be paved upon for a week. A foundation of bituminous concrete is sometimes used where only a thin bed can be laid, in consequence of there being an old foundation which it is undesirable to disturb. It is made by pouring a composition of coal-tar, pitch, and creasote oil while hot over broken stone levelled and rolled to the proper form, and then spreading a thin layer of smaller broken stone over the surface and rolling it in. It has the advantage that it can be paved upon a few hours after it has been laid. aterials. The best materials for pavement sets are the hard igneous and metamorphic rocks, though millstone grit and other hard sedi- mentary rocks of the same nature are used when the traffic is com- paratively light. Excessively hard stone which wears smooth and slippery is objectionable in spite of its durability. Penmaen-Mawr stone, which is much used in many of the large Lancashire towns, is of this character, and its use was discontinued in London in consequence of its slipperiuess and noise. Guernsey granite (syenite) and Mount Sorrel granite (syenite) have the same nature in a less degree, and in London Aberdeen blue granite is preferred, as, though it wears faster, it keeps a rough surface. Walker's observations on the wear of tram stones showed that Aberdeen granite wore three and a quarter times as fast as Guernsey granite, and in the set pavement of Blaekfriars Bridge it was found that after thirteen and a half years' wear the Aberdeen stone had worn 1 J inches, while the Guernsey granite had only worn one-fourth of an inch (equal to '11 and '019 inch per year respectively), or that the former had worn six times as fast as the latter. Observations made by Mr Haywood showed the general rate of wear of Aberdeen granite under heavy traffic in the City of London pavements to have been from '14 to '23 inch per year. The rate of wear of Penmaen- Mawr and Carnarvonshire sets in Liverpool under the greatest traffic is stated to be seldom more than '02 inch per year. A certain proportion between the depth and the length and breadth of sets is required for stability. A shallow stone is more easily tilted tip by a heavy weight coming on one edge, and a nar- row stone has a tendency to turn over sideways. The length, measured across the street, must be sufficient to break joint pro- perly, as two or more joints nerrly in- a line lead to the formation of grooves. For the softer stones a breadth of 4 or 5 inches may be adopted, but for sets of granite or other hard material, with which the joints must be depended on for foothold, the breadth should not much exceed 3 inches. The depth should not be less than twice the breadth, and, as deeper sets weigh more and cost more than shallower ones and the loss by wear is but slight, there is some reason for not exceeding the minimum depth. Where, however, the speedy relaying of a street pavement is of more im- portance than a saving in first cost, deeper sets are used, and when they have become so worn as to be uneven the street is relaid with new sets and the old ones are removed to be redressed for use in other streets, the sets being used again and again in less important streets as their depth is reduced. In London sets 3 inches wide, 10 to 15 long, and originally 9 deep are used in this manner. In Liverpool sets 4 to 14 inches wide, to average with the joints, 5 to 7 inches long, and 6 to 7 deep according to the traffic are used. In Manchester the sets are 3 to 3^ inches wide, 5 to 7 long, and 5 to 6 deep, or 7 in exceptional situations. Sets should be well squared and not taper from the face downwards ; both joints and face should be free from irregular projections. On a concrete foundation sets are generally bedded on a thin layer of sand or fine gravel ; sometimes they are laid in a bed of fine cement concrete, enough of which is spread over the concrete foundation to be covered while fresh by the sets, which are put in place and smartly tapped, and the joints are grouted at once with cement grout. To allow the cement to become thoroughly set it is desirable that traffic over the pavement should not be allowed for a fortnight, if that can be arranged. The courses of sets are laid square across the street, no advantage arising from a slanting direction, which makes the wear more irregular. At junctions of streets the courses are laid meeting at an angle at the centre line of the narrower street, so that the courses may not run in the direction of the traffic. On steep inclines the sets are sometimes slightly tilted on their beds, forming a serrated surface to give foothold, "and slate has been in- serted in the joints for the same purpose. The water channels are _ formed by two or three courses of sets laid parallel to the kerb, lointing. Joints simply filled in with gravel are of course pervious to water, and a grout of lime or cement does not make a permanently water- tight joint, as it becomes disintegrated under the vibration of the traffic. Grouted joints, however, make a good pavement when there is a foundation of concrete or broken stone or hard core. Where there is not a regular foundation imperviousness in the joints is of great importance. In some of the Lancashire towns the joints have for many years past been made by first filling them with clean gravel, well shaken in by ramming, and then pouring in a composition of coal-tar, pitch, and creasote oil. The Manchester pavements are good examples of this system of trusting to impervious jointing to prevent unequal settlement. The founda- tion, where there is not already an old road surface, is a bed of cinders about 1 foot thick, over which are laid 3 inches of gravel, which are thoroughly consolidated by allowing the traffic to pass over them. The sets are evenly bedded and well rammed after the joints have been filled with clean gravel, ramming and gravel- ling being repeated till the joints are full of gravel. The mixture of coal-tar, pitch, and creasote oil, well boiled, is then poured over the surface and allowed to percolate and fill up all interstices in the joints, and the pavement is finished by covering it with small gravel. Joints so formed are impervious to wet and have a certain amount of elasticity ; the foundation is kept diy ; and the pavement keeps its form well for many years. The objection is made that in hot weather the composition runs from the joints and makes the streets unpleasant for foot-passengers. This sort of jointing is used in Liverpool and some other large towns, where the sets are laid on a concrete foundation. The elasticity diminishes vibration and noise, and pavements so jointed are said to wear better than others. A curve like that before described, flattening gradually towards Cross the sides, and having a rise equal to one-sixtieth of the width of section the carriage-way, is a common cross section for a paved street. Sometimes the rise is even less. A pavement consisting of broad, smooth, well -jointed blocks of Granit< granite for the wheel tracks, and pitching between for the horse tramwj track, was laid by Walker in Commercial Road (London) for the heavy traffic to the West India Docks in 1825, and similar pavements have been successfully used elsewhere, principally for heavy traffic, in streets only wide enough for one vehicle. In Milan, Turin, and other towns of northern Italy tramways of the same sort are ex- tensively used for the ordinary street traffic. The tractive force required is small, while the foothold on the horse track is good ; but the tram-stones are slippery for horses to pass over. The rigid- ity of the roadway renders it more suitable for slow heavy traffic than for light quick vehicles, and the improvement in other pave- ments has limited the application of this one in ordinary streets. Wood Paving. Wood pavements were introduced in England in WOOD 1839. HexagonaL blocks of fir, 6 to 8 inches across and 4 to 6 PAVING deep, were bedded in gravel laid on a foundation previously levelled and beaten. The blocks were either bevelled off at the edges or grooved across the face to afford foothold. Other wood pave- ments were tried in London about the same time, but they soon got out of order from unequal settlement of the blocks, and most of them lasted but a few years. The best of these was Carey's, which consisted of blocks 6^ to 7^ inches wide, 13 to 15 long, and 8 or 9 deep, the sides and ends having projecting and re-entering angles locking the blocks together with the view of preventing unequal settlement. Pavements on this system were laid in Minc- ing Lane in 1841 and in Gracechurch Street in 1842. In the latter street the blocks appear to have been relaid every three or four years and to have been entirely removed about every eleven years, until the pavement was removed in 1871, to be superseded by asphalt. Experience led to a reduction in the width of the blocks to 4 inches and in the depth to 5 or 6, and the salient and re-entering angles disappeared from the sides. With these modifications Carey's pavement remained in iise from 1841 until after the introduction of more modern systems in recent years. The " improved wood pavement " was first used in London in 1871. After the foundation was formed to the proper cross section a bed of sand 4 inches deep was laid, upon which came two layers of inch deal boards saturated with boiling tar, one layer across the other. The wooden blocks were 3 inches wide, 5 deep, and 9 long ; they were dipped in tar and laid on the boards with the ends close together, but transversely the courses were spaced by fillets of wood three-fourths of an inch wide nailed to the floor and to the blocks. The joints were filled up with clean pebbles rammed in and were run with a composition of pitch and tar, the surface being dressed with boiling tar and strewed with small sharp gravel and sand. In this pavement a 'somewhat elastic foundation was pro- vided in the boards, which were also intended to prevent unequal settlement of the blocks ; but the solidity of the pavement depended upon its water-tightness, for, when the surface water reached the sand, as it did sooner or later, settlement and dislocation of the blocks under the traffic arose. Pavements on this system were laid between 1872 and 1876, and were kept in repair and relaid from time to time, but about 1877 the plank foundation was abandoned for a foundation of cement concrete. A concrete foundation for a wood pavement appears to have been Founds first employed in a pavement laid in 1872 in Gracechurch Street by tion. the Ligno-Mineral Company. The concrete was of blue lias lime 4 inches thick formed to the curve of the road. The blocks were of beech, mineralized by a special process, 3 inches wide, 4J deep, and 7 long, with the ends cut to an angle of 60, so that each block might derive support from the next one. They were laid with the XX. 74