thoroughly set and become hard, and till then the pedestrian traffic should be diverted or accommodated by placing a temporary platform of wood over the surface. In hot, dry weather the concrete, after being laid, should be protected by spreading sand over the surface, or by placing old cement bags, kept thoroughly wet, on the pavement.

581. One objection to this description of pavement for footways is that repairs are difficult to make should the covering be lifted for the purpose of laying or renewing pipes. The greatest objection is the prolonged disturbance of the pedestrian traffic during the time it is being laid and setting, a serious matter in a busy thoroughfare, where shops abut on the footway. This class of pavement should only be laid in good weather, heavy rain or the prevalence of frost being detrimental to its durability.

582. Concrete Slabs.-In consequence of these objections in connection with monolith pavements, concrete slabs, which are free from the defects mentioned, are now frequently used. Victoria stone and many other descriptions of concrete slabs are made in a similar manner; they are formed in wooden moulds lined with metal, to facilitate their being removed when set.

They are composed of broken stone, gravel, and sand mixed with Portland cement and water; in many cases a solution of silicate of soda is added. They are made in various sizes, the most useful dimensions for paving work being 30 inches in length and 24 inches in width, and 24 inches square by 2 inches thick. The process of making the slabs hard and dense is carried out by some firms by subjecting the coucrete to great pressure in the moulds.

The aggregate, consisting preferably of inch granite chippings, washed to remove the associated dust, is thoroughly mixed in a dry state with Portland cement, the proportion adopted being three of the former to 1 of the latter. After this has been accomplished, water is added in a careful manner, and the whole is mixed until thoroughly incorporated. The

compo' is then placed in moulds and worked at the corners and sides with a trowel to ensure clean, accurate arrisses all round the slab, and finished to a smooth surface. The moulds containing the concrete are by some processes subjected to a mechanical movement of a jolting nature, which motion gets rid of many of the air cavities.

The slabs are removed from the moulds at the expiration of two days, and stored so as to become air-dried for seven to nine days, and then, should the process be adopted, immersed in a bath containing a solution of silicate of soda. After being so treated for a week or fourteen days according to the capacity of absorption, the slabs are stacked in the open for some months to season, previous to being used. Fig. 126 shows a footway paved with slabs.

Many manufacturers of these concrete slabs use only one class of

granite chippings, which is produced principally at the quarries in Aber deenshire, Bonawe, Furness, Leicester, Guernsey, and Quenast (Belgium).

583. Adamant Paving Slabs.-This paving material is made of finely crushed Aberdeen granite and the finest Portland cement. The concrete, when properly mixed, is placed in moulds and then subjected to hydraulic pressure. This effectually disposes of both air cavities and moisture, the result being a dense, non-porous, and homogeneous stone or slab, which can be immediately handled. By this process the slabs can be manufactured and laid in the course of a few days; the ordinary sizes made are 3 feet × 2 feet, 2 feet x 2 feet, and 2 feet square by 2 inches in thickness.

These slabs for paving footways are largely used in Aberdeen, Ediuburgh, and many English cities and towns. Paving slabs are extensively made in certain towns from the clinker produced by refuse destructors, mixed with Portland cement. These are not, however, so durable as concrete paving made with granite chipping, but they form a cheap material which is quite suitable for footways in subsidiary streets. Pavements formed with concrete slabs are easily cleansed, and being non-absorbent, dry quickly after rain; they can be expeditiously laid and wear evenly.

584. Bricks or Tile Paving.-Bricks or tiles to be of any value as a footway paving require to be of hard vitrified stoneware; they do not form an ideal paving material, but are greatly used in many towns in the Midland counties of England.

Blue Staffordshire, and Buckley bricks measuring about 10 inches x 5 inches x 2 inches deep, with a chequered or diamond pattern surface, are those most largely used; they make a durable pavement when laid on a foundation of concrete.

Owing to the numerous joints, this paving is difficult to cleanse; and when worn becomes slippery and uneven. The appearance of brick paving is not in its favour; it does not make a pleasant surface to walk upon, but it is well suited for many footways in the streets of manufacturing towns. The bricks can be lifted and relaid at small cost, which is a point in its favour, especially where the footways are often disturbed in introducing or repairing service pipes and house connections. The life of this paving is estimated at various periods up to thirty years.

585. Brick paving is much used in the United States as a material for covering footways in the towns where it is extensively employed for carriageway pavements.

586. Cork Paving and Sanitary Tiles.-Compressed cork paving for footways are practically noiseless, are not slippery, and are non-absorbent; the blocks measure 9 inches x 4 inches x 1 inch; they are jointed with a bituminous grout laid on a concrete foundation 3 inches in thickness.

587. The sanitary tiles for footways are made in 8-inch squares or 5-inch hexagons, and 2 inches thick; they may be laid on a 3-inch concrete

foundation, or on a bed of gravel well punned and a layer of sand. The blocks are laid closely together, the joints being closed by compression under traffic. It affords a safe foothold for pedestrians, is smooth and even, and does not become slippery through wear. It is practically waterproof, dries quickly after rain, and can be quickly and easily repaired in all conditions of weather. It is stated to be durable and economical.

588. Footways situated in country districts are generally formed with from 4 inches to 6 inches of coarse gravel or freestone chips, and finished with fine gravel or the screenings from a stone-breaking machine. Furnace cinders are also used in districts where they can be easily obtained. This description of footways has been treated in Chapter VIII., and serve the purpose very well when properly kept in repair. For suburban districts, and in small provincial towns, tar paving may be advantageously employed where suitable materials can be obtained.

CHAPTER XIV.

SUBWAYS.

589. The construction of subways or conduits under the street level for the reception of gas and water mains, electric wires and other conductors, so as to obviate the perpetual breaking up of the street pavements in order to lay new pipes, discover and repair leakages, or for other purposes, is a subject of considerable public interest and an important element in the obstruction of vehicular traffic. It has received a large amount of attention from time to time, but little has been accomplished up to the present. Recently it has been proposed to construct high-level viaducts and tunnels or subways in addition to forming new and widening existing thoroughfares at important points, as a solution of the congestion of street traffic in London.

590. The ever increasing pedestrian traffic has likewise received attention; foot bridges over the carriageways, and subways with a flight of steps leading to the pavements at important street crossings have been proposed, the latter being the method adopted in connection with the Central London Railway scheme at its terminus in the open space in front of the Mansion House, which point has been aptly described as the "heart of the city."

591. The necessity for subways is evident when it is considered that any examination of the existing mains to ascertain their condition is impossible, especially where many are placed under the roadways, and it is only when undetected leakages, which may have existed for a long period, become apparent, that measures are taken to discover and remedy the evil. The only recourse under the circumstances is to break up the street pavement, an operation which may, in many instances, extend over a considerable area of the thoroughfare, and involve considerable expenditure and inconvenience.

In large cities and towns the annual cost incurred in tracing and making good the leakage of water and gas mains and connections of different public companies must be considerable, while the deterioration of the pipes which must necessarily take place when situated in a subsoil of a variable nature

is also a serious matter. The expenditure thus incurred must be enormous when taken collectively, and must not be overlooked; although when divided. amongst many companies it may not appear such a formidable figure.

592. It is stated that in the City of London many of these leakages cost about £20 to discover, exclusive of the actual repairs to the water or gas mains, or the other connections placed under the ground. In most instances the carriageway pavement is irretrievably damaged when disturbed for this purpose, its life being materially shortened, besides leading to possible accidents and causing the greatest inconvenience to the traffic and loss of trade to business people.

In addition to the large annual expenditure connected with the examination for leakages and the subsequent reinstatement of the pavement, the effect of the percolation of water and the escape of gas permeating the subsoil must necessarily create a very unsanitary condition of matters in the surroundings.

593. Early Subways.-One of the earliest attempts to avoid the disturbance of street pavements in connection with laying or repairing pipes under ground, so far as the author is aware, was carried out in Liverpool; it is described by Prof. Rankine.* "One method of obviating this is to provide 'side-trenches,' to contain those underground works, being narrow excavations lined at the sides with brick walls, and situated under the outer edge of the foot pavement, by the flags of which they are covered. The wall of the side trench next the roadway is strengthened against the pressure of the earth by means of transverse walls, with openings in them for the passage of sewers and pipes; and between these transverse walls the longitudinal wall is slightly arched horizontally. The other longitudinal wall of the side trench forms the back of a row of cellars under the foot pavement. The side walls of the cellars are in a line with the transverse walls of the side trench, and act as buttresses to give it stability. In the example given in Mr Newland's report for 1848 the side trench is 13 feet deep from surface of footway to foundation, 21 feet wide inside, and has cross walls at every 7 feet; the brickwork is one brick, or 9 inches thick. It contains an oval sewer pipe 27 inches x 18 inches, a 10-inch water-pipe and a 10-inch gaspipe. Sewers which are large enough to be traversed by men may be repaired by getting access to them through subterranean passages leading into them from trap-doors in the foot pavement."

The earliest subways of any consequence constructed in this country were carried out in 1862 in Southwark Road and Garrick Street, London, and in Victoria Street, Nottingham, in the year 1863. For many years past subways have been constructed in connection with most of the Metropolitan improvements, such as the Holborn Viaduct, Thames Embankment, Charing

* Civil Engineering, by Prof. Rankine, pp. 629 and 630, taken from Reports by the Borough Engineer of Liverpool (Mr James Newlands).