CHAPTER X.

WOOD PAVEMENTS.

446. Carriageway pavements constructed of wood were introduced into London and laid on Stead's system in front of the Old Bailey in 1839. It is stated, however, that wooden blocks were used in Russia for some hundred years previous to that date, while in America wooden street pavements were tried experimentally in New York during 1835-6.

Most of the earlier attempts to construct a pavement with blocks of soft wood resulted in failure, owing to the inferior quality of the material, combined with the careless and improper methods adopted in laying the blocks without a foundation of cement concrete.

From the time when wood pavements were first laid in the metropolis many different forms were patented, but as most of these are now only known by name, few need description here.

Of late years the introduction of Australian hard woods, mostly from trees belonging to the eucalyptus family, and the improvement effected in cutting and laying the blocks, have resulted in this class of pavement being extensively used in many of the cities and provincial towns in the kingdom.

447. Compared with the other pavements of recognized merit, wood is practically noiseless, and it is no doubt owing to its possessing this quality that it has been so extensively employed in recent years. It also gives the minimum of traction of any pavement, asphalte excepted, and affords a good foothold for horses under varying weather conditions when the surface is kept in a thoroughly clean state.

The following are some of the wooden pavements referred to above. 448. Stead's Wood Pavement. This pavement was laid on a bed of gravel 3 inches thick on ground previously levelled and well beaten, the object of this layer being to drain the surface water percolating through the joints of the pavement. The blocks of wood consisted of Scotch or Norway fir, and were hexagonal in shape, 6 to 8 inches across and from 3 to 6 inches deep. The upper edges of the blocks were chamfered and grooves were formed across the wearing surface in some cases. In several instances there

was a concrete foundation 3 inches thick, composed of small broken stones flushed with Ardwick lime or Roman cement. Pavements were also constructed by Stead, composed of round blocks of wood placed vertically and laid close together, the interspaces being filled with sifted gravel or sharp sand. 449. De Lisle's Wood Pavement. This consisted of blocks the upper and under surfaces of which were cut diagonally to the grain, forming parallelopipeds, placed alternately in reversed positions; two holes were cut on each side of the blocks, and dowels or trenails inserted in these locked the blocks together.

450. Carey's Pavement.-This was the first durable wood pavement laid in London, many of the streets having been paved with it in 1841 and subsequent years. It consisted of pine blocks 6 to 7 inches wide, 13 to 15 inches long, and from 8 to 9 inches deep laid on a foundation of ballast; the cost of this paving was 14s. 2d. per square yard. The size of the blocks was reduced, in 1866, to 4 inches in width and from 5 to 6 inches in depth, the length being 9 inches. They were formed with double bevelled surfaces, which when laid, came together, the intention being to prevent the blocks shifting and also to distribute the pressure on one block over the contiguous ones. The joints, inch in width, were grouted with lime and sand mortar. It is worthy of note that the life of this pavement, taken on an average, extended to nine years on streets having a heavy traffic, and to eleven years on lighttrafficked thoroughfares. The total cost of construction and maintenance during the lifetime of this pavement amounted to 2s. 8d. per square yard in the former case and 2s. 5d. per square yard in the latter annually.

451. Improved Wood Pavement. This pavement, introduced from the United States, was originally laid with blocks, hexagonal in shape, 6 inches across and 8 inches deep. The first piece was laid in London in 1871; but the earlier method of forming this pavement was subsequently considerably modified. After excavating to the profile and section of the street, a layer of sand, fine mortar stuff or dry earth was spread to a depth of 4 inches. On this bed two layers of 1-inch deal boards, previously dipped in boiling tar, were laid close together transversely and longitudinally. The object of this flooring of planks was to form an elastic foundation and to distribute the weight applied on one block over a large surface. This method of forming the foundation was, for obvious reasons, abandoned, and a thin concrete one was substituted; one layer of the 1-inch boards, however, was retained, and was laid on the top of the concrete.

Upon this foundation, blocks, measuring 3 inches wide, 6 inches deep, and 9 inches long, dipped in tar, were laid. The longitudinal joints were made of an inch wide, and kept apart by a fillet nailed to the flooring board, the heading or end joints being laid close or butting. They were filled to a depth of 2 inches with a mixture of boiling tar and pitch, and

the space above was finished with fine dry ballast rammed into the joints by means of a caulking iron, and flushed with the tar and pitch composition. The work was completed by the whole surface being strewn with small gravel (to which was added boiling tar), and by sand then being thrown over the surface.

The object of sprinkling gravel over the surface was to indurate the wearing surface of the wood; while the elastic binding material, with which the joints were filled, was intended to produce an impervious pavement.

452. Ligno-Mineral Pavement.-This pavement, known as Trenaunay's system, was introduced from France. It appears to have been the first that was provided, as an original part of the system, with a hard concrete foundation moulded to the required curvature of the street. Upon the concrete foundation hard wood blocks of oak, elm, beech, or ash, 3 inches by 6 inches and 9 inches long, chamfered on the upper edges, were laid, the upper and lower surfaces of the blocks being cut at an angle of about 60 degrees to the grain of the wood, with a view to expose the fibre obliquely to the wearing surface, and also to distribute the weight of the traffic from one block to those adjacent in the line of thrust.

A groove cut horizontally along the sides of the blocks near the base was filled with asphaltic mastic; the joints were also partially grouted with the same composition, the remaining space being filled with lime and sand grout. The blocks, previous to being laid, were subjected to a process called 'mineralization' to render them tough and more durable. Besides the hard woods mentioned, fir was used, and was treated in a similar manner to preserve the wood; these blocks, however, were laid in an upright position with vertical fibres. This pavement was laid in various streets in London in the years 1872 and 1875.

453. Asphaltic Wood Pavement. This system of wood paving, originally patented by Copeland, was constructed in the following manner :—

The ground having been excavated as required, a foundation of blue lias lime concrete, consisting of one part of lime to five or six parts of ballast, was laid to a depth of 6 inches, finished to the proper contour of the road section. Upon this foundation a coat of mastic asphalte was laid from inch to inch in thickness as a cushion for bedding the pavement. The wood blocks, of Baltic fir, 3 inches wide, 6 inches deep, and 9 inches long, were laid in transverse courses with the grain of the wood upwards. Interspaces or joints about inch wide between each line of blocks were regulated by placing strips of wood against each course as they were laid. These were filled to a depth of 1 inch with melted asphalte, which partially re-melted and united with the coating spread on the foundation. The remainder of the joints was filled with a grout composed of hydraulic lime and sand, whereby the whole pavement was made into a solid mass.

The surface of the paving was finished with a top dressing of sharp sand or fine gravel strewn over it, which, when worked in by the wheel traffic, indurated the surface of the wood blocks.

In the original pavement, holes were pierced at the sides of the blocks and were filled with liquid asphalte in order to afford an additional key to the paving; this arrangement, however, was dispensed with in the later forms of this pavement.

This street covering was first laid in London in 1874, at the east end of Cannon Street. The foundation consisted of Portland cement concrete, 9 inches deep, over which a layer of mastic asphalte, inch thick, was spread. The blocks of soft wood steeped in tar were bedded in this mastic cushion and laid in courses across the street with ğ-inch joints. Each block had two holes about inch in diameter, and ğ inch deep bored in each side, and were filled with the liquid asphalte when the joints were being grouted. The upper part of the joints was rammed with screened gravel and filled with asphalte grout, and the pavement, as already mentioned, was covered with fine gravel and sand. The pavement was characterized as impervious to water, and the foundation when examined a considerable time after being laid, was found to be dry and in good condition.

454. Henson's Wood Pavement. The foundation for this system of wood paving was composed of 6 inches of blue lias lime concrete floated with a 2-inch layer of cement concrete of fine quality. Upon this was placed a layer of ordinary roofing felt previously saturated with a hot asphaltic composition of distilled tar and mineral pitch. This layer was intended to give elasticity to the road covering, which was composed of Swedish yellow deal blocks, 3 inches by 6 inches, and 9 inches long, laid with the grain upwards. The joints were formed with strips of saturated felt, the blocks being driven together so that the space between the courses was reduced to less than inch in width. At intervals of every three or four rows a course of blocks, grooved along the middle, was laid, to aid in giving proper foothold for horses. The surface was finished by spreading a hot bituminous compound, and then a coating of sand.

455. Mowlem & Co.'s Method. This description of wood pavement is laid on a foundation of concrete varying in thickness according to the nature of the subsoil and the traffic. The blocks of yellow deal, 3 inches by 6 inches or 7 inches deep, are laid direct on the foundation, the joints, varying in width from 3 inch to inch, being filled in with lias. lime and sand grouting, the surface of the blocks being afterwards indurated by strewing it with shingle which was run in by the vehicular traffic.

456. Duffy's Patent Wood Pavement.-It is claimed for this patent wood paving that the depth of the blocks can be materially reduced by the introduction of dowels or binding keys inserted into recesses or holes on either side of each block, effectually fastening them together, and distri

buting the weight of the traffic over a considerable area. By being so bound together, the blocks do not become loose or rise; it is an inexpensive pavement, and is easily laid. This system of paving was laid on the roadway of the Tower Bridge in 1895. It has also been recently laid in front of the Marylebone terminus of the Great Central Railway Com. pany.

Of the other patented systems of wood paving adopted and carried out, many of which are similar to those already described, with probably some slight alterations in detail, the following may be mentioned:-Harrison's, Newton's, Stone's, Gabriel's, Wilson's, Lloyd's, Trosser's, and Elli wood paving.

457. The experience acquired in laying the wood pavements described in the preceding paragraphs clearly points to simplicity of construction. The many patents for elastic foundations, interlocking the blocks, and forming grooves on the surface, have all been found to be extremely complicated and deficient in many important respects.

458. Gradients.-The steepest gradient which could be successfully laid with wood paving was, until recently, considered to be 1 in 40. Streets having steeper gradients are now paved with wood, as for instance Savoy Street and Arundel Street in London, which are situated on gradients of 1 in 12 and 1 in 14 respectively. It is, however, considered in modern practice that streets should not be paved with wood where the gradient exceeds 1 in 27. Under certain atmospheric conditions the paving becomes slippery, and requires sand or fine gravel sprinkled over the surface to ensure a good foothold. The frequency with which this has to be done increases as the gradient becomes steeper, and the expenditure under this head, in giving an artificial foothold, amounts to a considerable sum annually.

459. Contour. The convexity adopted for a wood pavement is generally to of the width of the carriageway, and may be made in the form of a segment of a circle, or according to the ordinates shown in fig. 121.

Methods of Constructing Wood Pavements.-The methods adopted in laying a wood pavement carriageway and practised at the present time will now be described. The wood may be either soft or hard.

460. Foundations.-Portland cement concrete is invariably adopted for forming the foundation of wood pavements of varying strength and thickness according to the traffic, nature of the subsoil, and position of the street. In other words, the foundations for a pitched and a wood pavement are similar; but in the case of the latter paving, an additional covering of fine concrete or cement rendering is applied on the top of the concrete foundation so as to form a true and even surface on which the blocks are directly laid without the intervention of any cushion layer.

In most cases the old foundation of concrete, which has done service for