Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/610

586 ROADS ends inclining in opposite directions in alternate courses. The upper edges of the blocks were chamfered, and there was a cham- fered groove near the bottom. In a few years this form of block was abandoned for rectangular blocks, and mineralized fir was sub- stituted for beech. The blocks were bedded in Portland cement and laid with joints one-fourth of an inch wide, partly filled with asphalt and then grouted with mortar. The adoption of a bed of concrete as the weight-bearing foundation of the road marks a new departure, and in all the more recent systems of wood pave- ment a substantial foundation of concrete is an essential feature. In Norwich, however, a large quantity of wood pavement has been laid on the old street foundation, the blocks being bedded in gravel and sand and rammed, and the joints grouted with lime and sand. The experience of from four to seven years has proved the pavements to be successful, but the foundation is exceptionally dry and hard and the traffic not very heavy. With a concrete foundation there is no reason for complicated shapes and contrivances for locking the blocks together ; and wood pavements in their modern form consist of rectangular blocks (obtained by cutting off the end of a deal plank), bedded on the concrete with the fibres of the wood vertical, thus constituting a slightly elastic wearing surface on a rigid foundation, by which the weight of the traffic is borne. There is, however, considerable variation in the method of bedding and jointing the blocks. The Asphaltic Wood Pavement Company laid half an inch of asphalt upon the concrete, and formed the lower part of the joint of asphalt and the upper part of a grout of Port- laud cement and gravel, the advantage claimed being a slightly elastic bed for the blocks and water-tight joints. The blocks have been laid in unset cement over the concrete and rammed to an even surface ; but the ramming is liable to split the blocks, and the indentations formed in the cement surface of the foundation have to be removed when the time comes for renewing the blocks. It is now more usual to bed the blocks directly on the concrete, a smooth surface being formed either with the concrete itself or by a floating of cement, and to fill the joints with a grout of cement and gravel. A cement joint adheres to the blocks, resists wet, and does not wear down too much below the surface of the wood, snson and so form a receptacle for mud. In Hensoii's system, which ee- has been largely used, the blocks are bedded and jointed with int. ordinary roofing felt, a strip of which, cut to a width equal to the depth of the blocks, is placed between every two courses. The joint is made as close as possible by driving up the blocks as every eight or ten courses are laid with heavy mallets, a plank being laid along the face of the work. A perfectly close and slightly elastic joint is thus formed. A continuous layer of felt is likewise laid over the concrete foundation to give a slightly elastic bed to the blocks. A V-shaped groove along the centre of every fourth block was at first considered necessaiy for foothold, but its use has been dis- continued except on gradients steeper than 1 in 30. The surface of the pavement is dressed over with a hot bituminous compound, and covered with fine clean grit. This method of laying a wood pavement, although somewhat more expensive, is probably the best that has hitherto been devised for smoothness and durability. The blocks are laid in courses across the streets, any change in the direction of the latter being accommodated by shorter courses ending with wedge-shaped blocks. At street junctions the courses are laid diagonally, or meeting at right angles. Two or three courses are laid parallel with the kerb to form a water channel. The blocks may be laid close end to end across the street if some allowance be made for expansion by wet, without which the kerb- stones and footways will be displaced, or the courses will be bent in reversed curves. To afford relief the joints of the courses parallel to the kerb may be left open, or the course next the kerb may be left out until expansion has ceased, the space being temporarily filled in with sand. In the direction of the traffic, joints more or less wide are generally thought necessary for foothold. A wide joint allows the fibres of the wood to spread and give way at the upper corner of the blocks for want of lateral support, and it also forms a receptacle for mud and wet Experience has shown that the space of three -fourths of an inch or one inch, once thought necessary for foothold, may safely be reduced to one -fourth or three -eighths of an inch. For spacing the courses to form the joints strips of wood of the proper thickness may be laid in and removed before the joints are filled, or they may be nailed to the lower part of the blocks. Two fillets have been nailed on, or three cast-iron studs fixed in the sides of each block to keep them steady in place until the joints are filled and thoroughly set. The latter method secures more uniformity in the width of the joints, aterials. There is some difference of opinion as to the best material for a wood pavement. Pitch pine and the harder red and yellow deals are the most durable, but they are less elastic than the softer woods, and are apt to wear slippery. Soft white woods have been recom- mended for the sake of a more elastic surface ; but on the whole either Memel or Swedish yellow deal is generally considered the best material. Whatever wood is used, it should be sound, close-grained, even in quality, free from knots and sap, and from the blue tinge which is a sign of incipient decay. After the blocks are cut, all those that are unsound, knotty, or badly shaped should bo rnre- f'ully rejected, as defective blocks soon cause holes in the surface and must be replaced, or the adjoining blocks will suffer undue wear and the surface become irregular. The breadth of the blocks never now exceeds 4 inches, and it is generally 3, the length being determined by the breadth of the deal or batten from which they are cut. The depth is usually 5 or 6 inches ; 5 inches are con- sidered by many to be enough to give sufficient depth for as long as the pavement will retain a sufficiently good surface without renewing the wood, and blocks of that depth have been laid in many London streets. It is doubtful if any advantage is derived from creasoting or from dipping the blocks in creasote oil or coal tar. Dipping affords a cover for the use of defective or inferior Avood, and thorough creasoting, though it preserves the wood from decay, has little or no influence on the wear, which in almost all cases determines the life of the blocks. With a curved cross section like that already described a rise Cross from the mean level of the channels to the crown of the road equal sectio to one-sixtieth or one-seventieth of the width of the carriage-way is enough. The necessary profile must be accurately given to the concrete foundation when wet. Wooden moulds or templates are fixed across the street 10 or 12 feet apart, over which a straight batten is worked to give the concrete the required form and a smooth surface. The moulds are removed when the concrete is fartially set and the spaces are made good with cement mortar, n a level street provision should be made in the foundation for a Founda- fall in the side channels towards the gullies of not less than 1 in tion. 150, and the necessary modifications of cross section at the inter- section of streets must also be provided for. Every care should be taken to ensure a good homogeneous concrete for the foundation, as upon that the strength of the road depends. With a well-made Portland cement concrete a thickness of 6 inches is sufficient. It should be allowed to set thoroughly before the blocks are laid, and traffic should not be allowed to pass over it for a week. The finished pavement should be covered with a thin layer of sharp grit, which is forced into the wood by the traffic and forms a hard face. Several applications of grit are desirable at first, and from time to time afterwards, both as a protection to the wood and to prevent slipperiness. Systematic cleansing is required to prevent slipperiness and foul smells, and to preserve the pavement. Cleans- ing may be aided by washing, and when it is thoroughly carried out but little watering is required to keep down the dust. A wood pavement is the quietest for the residents, pleasant to travel over, and favourable to the wear of vein n les. Traction on it is easy and foothold good, so that it may be laid on gradients as steep as 1 in 20. The wear of wood pavements in London is stated by Mr Stayton to be from '065 inch per year in Sloane Street, with a traffic of 279 tons per yard of width per day, to '456 inch per year in Fleet Street, with a traffic of 1360 tons per yard of width per day. Reduced to a standard of traffic of 750 tons per yard per day, the comparative annual wear becomes '175 in the former and '251 in the latter street. In Parliament Street, Westminster, blocks removed after four years in places where patching was required had lost 1^ to 1 inches in thickness, equal to one-third of an inch per year under traffic stated to be 1106 tons per yard of width per day. From information afforded by Mr Haywood it appears that in the City of London under traffic of from 300 to 660 vehicles per yard of width per day of 12 hours the wear is from '2 to -3 inch per year, and that in King William Street, London Bridge, under a traffic of about 1200 vehicles per yard of width in 12 hours the wear was found to be 2| inches in 3| years in the middle of the road, or '81 inch per year. This is the heaviest traffic to which wood pavement has been sub- jected. The wear is generally considered to be as much due to the horses' feet as to the wheels, and the action of the former is more destructive on steep gradients. Towards the end of the life of the blocks the wear is more rapid than at first. Few wood pavements retain a sufficiently good surface after about six years' wear without extensive repairs, and it is probably not advantageous to lay blocks of a greater depth than will provide for a duration of seven years ; 5 inches are almost always sufficient for this. Wood pavements of plain blocks on a cement concrete bed are Cost, now (1885) laid at from 10s. 6d. to 12s. 6d. per square yard, a con- siderable reduction on the prices paid for patented systems a few years ago. Of the above prices 2s. 3d. to 3s. 9d. is the cost of the foundation, which does not require renewal like the blocks. As- suming the average life of the latter to be seven years, Mr Stayton estimates the annual cost of wood paving in Chelsea with a traffic of 500 to 750 tons per yard of width per day to be Is. 9d. per square yard, which includes the cost of original construction, repairs and renewals, and interest, spread over fifteen years. Cleansing and sanding are estimated to cost 5d. per square yard in addition. Asphalt Paving. Asphalt was first used for street paving in As: Paris in 1854. It was introduced in London in 1869, when Thread- PAVING needle Street was paved by the Val de Travers Asphalt Company,.. and since then it has been extensively used for paving both streets and footways. The material is a hard limestone impregnated with bitumen in the proportion of from 6 to 8 per cent, in the Seyssel