Roads And Streets
ROADS AND STREETS. These words embrace the two divisions into which the lines of communication made by man for vehicular and pedestrian traffic between different places may be roughly classified. In current usage " road " is applied as a general term for all broad made ways from place to place, whether with separate side-paths for foot-passengers or not, while " street " is confined to the roads through towns, villages and other inhabited places, more or less lined by houses and other buildings on either side. The present article is confined to the methods adopted in making roads, from the first great roadmakers, the Romans, down to modern times. The roadways of times anterior to the Romans, at least in Europe, were merely the tracks worn by the feet of pedestrians and animals, and the wheels of vehicular traffic.
Etymologically considered, " road " in its current usage is late in its appearance. The first quotation in the New English Dictionary is from Shakespeare (/ Henry IV. 2, i. 16). The true O.E. word was weg, way, common to Teut. languages, and probably allied to Lat. via. The O.E. rod meant the act of riding, and is formed from ridan, to ride, and is thus used of a journey on horseback, and in compounds of a track or course, cf . suianrad, the swan's track, a poetic word for the sea- or streamrod, course of a stream, hweolrad, wheel-track, etc. A special use of the word, occurring as early as the Anglo-Saxon Chron. c. 900, was for a hostile foray, an " inroad," a " raid," which is the N. Eng. doublet of " road," and has superseded it in general use. Another use, which still survives, and shows the origin, is that of a space of water where ships may " ride at anchor in security from stress of weather, a roadstead." " Street " (O.E. straet) represents the Lat. strata via, paved way (from slernere, to strew, pave). It is one of the few words adopted in O.E. from the Romans.
The earliest roads about which anything definite is known, so far as construction is concerned, are those of ancient Rome, one of the oldest of which and the most celebrated for the grandeur of its works the Appian Way was roads." commenced in 312 B.C. Roman roads are remarkable for preserving a straight course from point to point regardless of obstacles which might have been easily avoided. They appear to have been often laid out in a line with some prominent landmark, and their general straightness is perhaps due to convenience in setting them out. In solidity of construction they have never been excelled, and many of them still remain, often forming the foundation of a more modern road, and in some instances constituting the road surface now used. It is consequently possible, with the help of allusions of ancient writers, to follow the ideal mode of construction, though this was not always adopted. Two parallel trenches were first cut to mark the breadth of the road; loose earth was removed until a solid foundation was reached; and it was replaced by proper material consolidated by ramming, or other means were taken to form a solid foundation for the body of the road. This appears often to have been composed of four layers, generally of local materials, though sometimes they were brought from considerable distances. The lowest layer consisted of two or three courses of flat stones, or, when these were not obtainable, of other stones, generally laid in mortar; the second layer was composed of rubble masonry of smaller stones, or a coarse concrete; the third of a finer concrete, on which was laid a pavement of polygonal blocks of hard stone jointed with the greatest nicety. The four layers are found to be often 3 ft. or more in thickness, but the lower ones were dispensed with on rock, on which the paving stones were sometimes laid almost directly. The paved part of a great road appears to have been about 14 ft. wide, and on either side, and separated from it by raised stone edgings, were unpaved sideways, each of half the width of the paved road. Where, as on many roads, the surface was not paved, it was made of hard concrete, or pebbles or flints set in mortar. Sometimes clay and marl were used instead of mortar, and it would seem that where inferior materials were used the road was made higher above the ground and rounder in cross section. Streets were paved with large polygonal blocks laid as above described, and footways with rectangular slabs. Specimens are still to be seen in Rome and Pompeii, while in Britain many of the roads were of hard gravel or had a cobbled surface. There are no traces of Roman influence in the later roads in England, but in France the Roman method appears to have been followed to some extent when new roads were constructed about the beginning of the 18th century. A foundation of stones on the flat was laid, and over that two layers of considerable thickness, of larger and smaller stones, bordered by large stones on edge, which appeared on the surface of the road. In 1764 Tresaguet set the foundation-stones on edge and reduced the thickness of the upper layers, and his method was generally followed until the influence of John Loudon McAdam (1756-1836) began to be felt. A French chaussee with accotements still retains some ' resemblance to the old Roman roads.
The almost incredibly bad state of the roads in England towards the latter part of the zyth century appears from the English accounts cited by Macaulay (Hist. c. iii.). It was due roads, chiefly to the state of the law, which compelled each parish to maintain its own roads by statute labour, but the establishment of turnpike trusts and the maintenance of roads by tolls do not appear to have effected any great improvement. At the time of Arthur Young's six months' tour in 1770 the roads would seem to have been almost as bad as ever, and it is doubtful if there was much improvement up to the beginning of the igth century. The turnpike roads were generally managed by ignorant and incompetent men until Telford and McAdam brought scientific principles and regular system to their construction and repair. The name of Telford is associated with a pitched foundation, which he did not always use, but which closely resembled that which had been long in use in France, and the name of McAdam often characterizes roads on which all his precepts are disregarded. Both insisted on thorough drainage and on the use of carefully prepared materials, and adopted a uniform cross section of moderate curvature instead of the exaggerated roundness given before; but, while Telford paid particular attention to a foundation for the broken stone, McAdam disregarded it, contending that the subsoil, however bad, would carry any weight if made dry by drainage and kept dry by an impervious covering. McAdam was engaged more with the repair of old roads than with the construction of new ones, and, though it is not possible to agree with all his doctrines, the improvement which he effected in road management and maintenance was great and lasting.
Construction of Roods. A road should be as short as possible between two points to be connected, but straightness must often be sacrificed to avoid difficulties and expense and to secure good gradients. The latter should be as easy as practicable, having regard to the country to be traversed, and it is desirable that there should be a ruling gradient than which none should be steeper. On the level macadamized road in ordinary repair the force which the horse has to put forth to draw a load may be taken as one-thirtieth of the load. But in going uphill the horse has also to lift the load, and the additional force to be put forth on this account is very nearly equal to the load drawn, divided by the rate of gradient. Thus on a gradient of i in 30 the force spent in lifting is one-thirtieth of the load, and in ascending a horse has to exert twice the force required to draw the load on a level. In descending, on the other hand, on such a gradient, the vehicle, when once started, would just move of itself without pressing on the horse. A horse can without difficulty exert twice his usual force for a time, and can therefore ascend gradients of i in 30 on a macadamized surface without sensible diminution of speed, and can trot freely down them. These considerations have led to i in 30 being generally considered as the ruling gradient to be aimed at on first-class roads, though i in 40 has been advocated. Telford adopted i in 30 as the ruling gradient on the Holyhead road through North Wales, and there are only two gradients steeper, in places where they were unavoidable. All unnecessary rises and falls should be avoided, but a dead level is unfavourable for drainage, and on this account i in 100 to i in 150 is the flattest gradient that is desirable. Such slight rises and falls are probably rather favourable than otherwise to ease of draught by horses.
In transverse section, roads in the United Kingdom generally consist of a carriage-way, with spaces on each side, on one or both of which there may be a footpath, and fences and 'section ditches. The width of the carriage-way may be from 15 ft., which allows of the easy passage of two vehicles, to 30 or 50 ft. for roads of importance near towns. The side spaces may be from 4 or 5 to 8 or 10 ft. wide; wide sides give the Sun and air access to the road, and tend to keep it dry, and also afford space for the deposit of road materials and scrapings. In cuttings or on embankments the transverse section has of course to be modified. The road surface should have just enough convexity to throw the wet off freely, and a very moder- ate amount is sufficient when a good surface is maintained. On a too convex road the traffic keeps to the middle, and wears ruts which retain the water, so that the surface is not so dry as with a flatter section which allows the traffic to distribute itself over the whole width. Telford used a cross section differing slightly from an arc of a circle in being more convex in the middle than at the sides. J. Walker recommended two straight lines joined in the middle of the road by a curve, and inclined about i in 24 towards the sides, the objection to which is that the flat sides are liable to wear hollow. On the whole a curve of the form of a flat ellipse is the best; the rise in the curve from the sides to the centre need not exceed one-fortieth of the width, and one-sixtieth is generally enough on well-kept roads. It is generally best to obtain the requisite convexity by rounding the formation surface or seat of the road and giving a uniform thickness to the coating of stone, but often, especially in country roads where the traffic is not very heavy and keeps mainly to the centre, the formation is made level and the convexity is obtained by using more road material at the centre than the sides. When there is not a kerb there should be a " shouldering " of sods and earth on each side to keep the road materials in place, and to form with the finished surface the water tables or side channels in which the surface drainage is collected, to be conveyed by outlets at frequent intervals to the side ditches. The outlets are open cuts through the sides or drains beneath the footpaths. The side ditches should be deep enough thoroughly to drain the foundation of the road, and cross or mitre drains under the road communicating with the side ditches may be required in wet soil. A thorough drainage of the subsoil is of the greatest importance, and it is economical in the end to go to considerable expense to secure it. In a cutting, or where there are no side ditches, the surface water may be taken off by gratings and under drains beneath the side channels.
Macadam Roads. The thickness to be given to a road made altogether of broken stone will depend on the traffic it is intended for. On a good well-drained soil a thickness of 6 in. will make an excellent road for ordinary traffic, and McAdam's opinion that 10 in. of well-consolidated material was sufficient to carry the heaviest traffic on any substratum if properly drained has proved to be generally correct. In a new road the loss of thickness during consolidation must be allowed for, and the materials should be laid about one-half thicker than the coating is intended to be. When the materials are not rolled, a thickness of 3 to 6 in. should be laid first, and when that has partly consolidated under the traffic other coats may be added to make up the full thickness. There is great wear and waste of the materials in consolidating if they are laid too thickly at once. Inferior material is sometimes used in the lower part of the road coating, especially when the surface is to be of granite or other hard expensive stone. Thus flints or gravel may be used for the lower 5 or 6 in. of a road to be coated with 3 or 4 in. of granite. Telford covered the broken stone of new roads with 15 in. of gravel to act as a binding material. McAdam absolutely interdicted the use of any binding material, leaving the broken stone to work in and unite by its own angles under the traffic.
If the ideas of the inventor are strictly followed, macadam, when the fine network of joints is thinly masked with hardened mud worn from the stone, comes near to a perfect surface. But stones that will pass through a ring of a given size may be twice as much in length, and unless their form is about that of a cube not exceeding ij in. on its longest side, they cannot be rammed or rolled into the regular mosaic characteristic of the true macadam. The best modern roads are of handbroken stone dressed slightly on the surface with stone chips, while the mass of the road-metal is kept free from any kind of binding. Some roadmakers, however, have found the large irregularly shaped stones from the machine so difficult to consolidate that they have had to reconsider the question of binding. The engineer of Central Park, New York, found that, with the greatest care and attention to rolling, such stones would not consolidate properly without admixture; indeed they became more intractable the more they were abraded by rolling. G. F. Deacon of Liverpool advocated a binding composed of large chips of trap rock or else of siliceous gravel from the size of three-quarters of an inch down to that of a pin's head, together with about one-fourth part of macadam sweepings obtained in wet weather. This will enable the roller to consolidate the road-metal in a third of the time required for broken stone alone. The harder materials here suggested differ essentially from the sand and dirt formerly used for binding, since they fill up all the vacant spaces and cannot be washed down.
A new road is preferably finished by rolling, since in that way the materials are consolidated with less waste, and wear and tear of vehicles is saved. A 1 5-ton steam-roller, 7 ft. wide, giving upwards of 2 tons weight per foot can thoroughly consolidate 1000 to 2000 sq. yds. of newly laid materials per day.
A pitched foundation, as used by Telford, consists of flat stones set on edge in courses across the road with the broader edges downwards. All inequalities must be knocked off, and small stones and chips must be firmly pinned into the interstices with a hammer, so as to form a regular convex surface with every stone fixed firmly in place. A foundation of cem'ent concrete 6 in. thick was used by Sir J. Macneill on the Highgate Archway (London) road on a bad clay bottom, and common lime concrete was subsequently used elsewhere. A bed of lias lime concrete 1 2 in. thick was laid as a foundation in Southwark Street and on the Thames Embankment, but it is too expensive for a macadamized road under ordinary circumstances. Foundations of large and rough hard-core should be rolled down to a surface close enough to keep the finer pieces of road-metal from dropping down, so as to create hollows which, though they may escape the roller, will be detected by the laden wheel and by the pounding of the heavy hoof. But there is no foundation equal to sand, which has the property of spreading pressure over an enlarged area. A 12 -in. bed of sand rolled down to 8 in. has been recommended, but military engineers have found that a layer of so little as 3 or 4 in. is sufficient as a foundation for macadam in very bad ground that has been rolled, or on an embankment that has had time to settle.
Tar Macadam. Broken stone mixed with some bituminous composition has been found very suitable for suburban roads, and for towns where the nature of the traffic requires smooth roadways reasonably free from noise and dust. In its simplest form, tar macadam is made from a good hard limestone broken into the usual sizes, the fine chips being used for top-dressing. In a shed a large hearth is formed of stone flagging, under which the flues of a furnace are constructed, and upon the hearth the broken stone is spread in a layer just as thick as the heat may be able to penetrate, to dry off the moisture and make the stones distinctly hot. The load of an ordinary barrow is tipped on an iron plate and gas tar is poured over it (from 8 to 12 gals, per cubic yard), while a couple of men with shovels turn it over exactly as they would turn concrete. No more tar should be used than is required completely to blacken the whole surface of every stone; and when this has been done, the stone can be thrown upon the heap, where it may be kept for one or two months, under cover, to allow the volatile oils to evaporate. Fine sittings are treated in the same way. When it has been properly seasoned, the mass should assume a greenish lustre; and when cut into by a shovel, the particles will cling together and creep down slowly so that the heap is said to be " alive." In that state it may be used. The tar ought to be boiled, and if too thin, a little pitch may be added to it, though not enough to make the heap consolidate. A mixture of tar with pitch and creosote oil is used by more precise makers, one formula being 12 gals, tar, J cwt. pitch and 2 gals, creosote oil to a ton of stone. But these ingredients differ considerably in their chemical composition, and the proportions have to be varied according to experience. Moreover, as regards the tar and pitch used in the manufacture of pave- ments, the varieties that come directly from a vegetable source are liable to melt in hot and to become brittle in cold weather; coal tar is only moderately proof against these extremes.
Tar macadam must be put down in dry weather. If the material seems too dry, hot tar may be applied as before, but only as an expedient, and with great economy, so that the pavement may not soften in the Sun. Upon a well-rolled foundation of hard material a layer of the coarser macadam should be put and rolled, then a layer of the smaller grade. For a road of light traffic a coat of the fine siftings may be put down and heavily rolled to a finished surface. For a road of heavier traffic the second coat should be dressed before rolling with tarred stone of a gauge of three-quarters of an inch to an inch and a quarter, and rolled first with a roller of not more than 10 or 12 cwt., then with one of 30 cwt. After the traffic has been turned on the road for a few days it should again be rolled as heavily as may be necessary to restore any parts that have been disturbed. But such roads are often consolidated by steam-rollers of 10 or 15 tons. For refacing an old road the prongs attached to a steam-roller will easily lift the old layer. Small depressions may be well tarred and levelled up with fine stuff, and the whole surface may be dressed every three years with tar and a fresh coat of fine chips. If the surface of the road is irregular, water will hang upon it, and frost may cause it to become slippery. The lack of affinity between granite and bitumen prevents the use of tar macadam upon roads of heavy traffic.
Concrete Macadam. Rocks h'ke granite and syenite may be used in combination with Portland cement. The ingredients are mixed in about the proportion of four parts of broken stone that has first been well wetted, one and a quarter or two parts of clean sharp sand, and one of cement put on in two layers, the second being rolled by hand to the required shape and to a good surface. It should remain for two or three weeks to dry and set. Want of elasticity may be urged against concrete macadam, and it is productive of dust, but in some cases it has proved satisfactory.
Gravel Roads. Smooth rounded gravel is unsuitable for roads unless a large proportion of it is broken, and about an eighth part of ferruginous clay added for binding. Rough pit gravel that will consolidate under the roller may be applied in two or more layers, but each must be of similar composition, or the smaller stuff will work downwards. A gravel road should be always under inspection, and repairs should be done without delay. A track for equestrian exercise should be made of hoggin or fine gravel, that will remain soft when raked or harrowed and watered. It should be well drained. A foundation of rough hard core will let the hoggin pass down into it, so that the hard core will appear at the surface. The best material is rough chalk sufficiently rolled to stop the gravel while draining off the surface water.
Stone Pavements. Early pitched roadways consisted of pebbles or rounded boulders (" cobblestones ") bedded in the natural surface or in sand or gravel. The next step in advance was to employ roughly squared blocks; but the wide and irregular joints admitted the water to the subsoil, and the mud worked up and the stones sank irregularly under the traffic. Telford, who was called upon to report on the street pavements of the parish of Hanover Square in 1824, saw the necessity of cutting off all connexion between the subsoil and the paving stones. He recommended a bed of about 6 in. of clean river ballast, rendered compact by being travelled upon for some time before the paving was laid, but he subsequently considered that nothing short of 12 in. of broken stone, put on in layers 4 in. thick and completely consolidated by carriages passing over them, would answer the purpose. He recommended paving stones of considerable depth and of from 4! to 6 or 7! in. in breadth for the greatest thoroughfares, and he pointed out the importance of working the stones flat on the face and square on all sides, so as to joint close and preserve the bed or base as nearly as possible of the same size as the face, and of carefully placing together in the same course stones of equal breadth. Many pavements thus laid with stones of considerable breadth still remain, but experience proved that it was a mistake to suppose that broad stones having a larger base would support better the weight and shocks of heavy traffic; on the contrary, a wide stone has a tendency to rock on its bed, and also to wear round on the top and become slippery. To obtain an evener surface and a better foothold for the horses the stones were reduced in width, and in 1840 a granite pavement was laid by Walker on Blackfriars Bridge, which may be considered the first of modern set pavements. The stones were 3 in. broad and 9 deep; they were laid on a bed of concrete i ft. thick and were jointed with mortar. The reduction of breadth to about 3 in. was generally followed, but it was some time before a concrete foundation was employed to any great extent, the frequent breaking up to which streets are subject having prevented it. In London a foundation of broken stone has been continued in some thoroughfares, the sets being evenly bedded in gravel upon it and rammed with a heavy wooden rammer. Hard core a mixture of broken stone, clinker, brick rubbish and old building materials has also been largely used to form a foundation. In the northern towns of England cinders have been employed, and where the traffic is exceptionally heavy a pitched foundation of stones on edge has been laid when the sets were not paved upon an old macadamized surface. The concrete for a foundation to a paved street should be made with the 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 in. 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.
The best materials for pavement sets are the hard igneous and metamorphic rocks, though millstone grit and other hard sedimentary rocks of the same nature are used when the traffic is comparatively light. Excessively hard stone which wears smooth and slippery is objectionable in spite of its durability.
Joints simply filled in with gravel are of course pervious to water, and a grout of lime or cement does not make a permanently watertight 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, which is allowed to percolate and fill up the interstices, the pavement being 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 dry; and the pavement with bituminous grout of this kind 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.
A pavement consisting of broad, smooth, well-jointed blocks of granite for the wheel tracks, and pitching between for the horse 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 extensively used for the ordinary street traffic. The tractive force required is small, while the foothold on the horse track is good; but the tramstones are slippery for horses to pass over. The rigidity of the roadway renders it more suitable for slow heavy traffic than for light quick vehicles, and the improvement in other pavements has limited the application of this one in ordinary streets.
Brick Paving. Since about 1885 brick as a paving for carriage-ways has been adopted to a considerable extent, chiefly in the form of shale bricks, in American cities. The clay is a hydrated silicate of alumina, containing about 24% of alumina with 15% of iron, lime, soda, potash and magnesia. Lime is injurious, but alkalis to the extent of 3% are needed to ensure a slight degree of vitrification. Various tests arc used to determine their liability to absorb moisture and to be abraded. That for abrasion is made by rolling half-bricks in an iron barrel or rattler in company with pieces of castiron for a given time, and noting the effect on the surfaces, but particularly on the angles, which should be tough enough to resist chipping. Comparisons are also made with test pieces of granite that are mixed with the bricks. To guard against chipping, the best-made bricks are pressed over again, and the upper angles rounded to a radius of three-eighths of an inch. Upon a foundation of concrete or well-rolled ballast a cushion or bed of coarse sand from half an inch to 3 in. thick is laid, and on this the bricks are set. They are then rolled till level, or are heavily rammed, a plank being interposed between the bricks and the rammer. No channelcourses are used. Pitch is poured in at the joints, but by no means on the surface, as that would make them slippery. Brick roadways have stood well under hard wear for fourteen years. Although in the United Kingdom bricks are produced unequalled for hardness and finish, no serious attempt has been made to introduce a tough brick for roadways that will neither chip nor wear smoothly. In various experiments with bricks that seemed most suitable they stood hard traffic for about a year. Clay of absolutely uniform character, and kilns that will ensure perfect equality in firing, are requisite. Slag bricks, made to interlock in the form of a double hexagon, the surface being grooved to a small pattern, have stood good tests for wear and foothold on a perfectly level surface. Many attempts have been made to use compositions, into which asphalt or cement usually enters, for making blocks or slabs, square or hexagonal, that can be laid down on a concrete foundation. A mosaic of macadam set in an iron frame is fixed by running molten slag into the back of the block. Small square pieces of oak are formed into blocks, end-grain upwards. Staffordshire blue bricks, made with holes to hold wooden plugs, have been used with some success. Broad blocks not firmly fixed down usually become loose and tilt when subjected to traffic.
Asphalt Paving. Asphalt was first used for street paving in Paris in 1854. It was introduced in London in 1869, when Threadneedle 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% in the Seyssel rock, and from 10 to 12 in that from Val de Travers. Asplialts containing less than the former proportion have not sufficient coherence for street pavements, and those containing more than the latter proportion soften from heat in the summer. Asphalt is employed either as a mastic or compressed. The mastic is previously prepared in cakes and is melted for use in caldrons with a small quantity of bitumen, and for a street pavement is thoroughly mixed with sand or grit. It is spread in one thickness on a concrete foundation, covered with sand, and beaten to an even surface. This material has not proved so successful for street surfaces as compressed asphalt. To produce this, the rock asphalt, previously reduced to a fine powder by mechanical means, is heated in revolving ovens to from about 220 to 250 F., spread while still hot, and compressed into a solid mass by hot disk-shaped rammers, and afterwards smoothed with irons heated to a dull redness. The original rock is thus, as it were, reconstructed by taking advantage of the power of coherence of the molecules under pressure when hot. In heating the powder the moisture combined in the limestone must be driven off without reducing the proportion of the bitumen more than is unavoidable. The powder cools very slowly, and may be conveyed long distances from the ovens; it may even be kept till the next day before use. When laid it should still retain a temperature of from 150 to 200. It is spread evenly with a rake by skilled workmen for the whole width of the street to a thickness about two-fifths greater than the finished coating is intended to be. Ramming is commenced with light blows to ensure equality of compression throughout, and is continued with increased force until the whole is solidified. The ramming follows up the spreading, so that a joint is required only when the work is interrupted at the end of a day, or from some other cause. In a few hours after it has been laid an asphalt pavement may be used for traffic. When finished, its thickness may be from ij to 2j in., according to the traffic; a greater thickness than the latter cannot be evenly compressed with certainty. The asphalt loses thickness by compression under the traffic for a long time and to the extent, it is said, of onefifth or one-fourth, but the wear appears to be very small. The wear-resisting power of the asphalt is due to its elasticity; tracks are made by the wheels at first, but when thoroughly compressed by the traffic the surface retains little or no trace of the heaviest loads. Repairs are easily and quickly made by cutting out defective places and ramming in fresh heated powder, which can be done in the early morning without stopping the traffic. An unyielding foundation is indispensable ; it should be of the best Portland cement concrete, 6 in. in thickness, which must be well set and perfectly dry throughout before the asphalt is laid, or the steam generated on the application of the hot powder will prevent coherence and lead to cracks and holes in the asphalt, which quickly enlarge under the traffic. For the same reason the asphalt should be laid in dry weather. The concrete foundation must be carefully formed to the proper profile, with an inclination towards the sides of not more than i in 50, which is sufficient with so smooth a surface. About i in 50 is the steepest gradient at which an asphalt pavement can be safely laid. When either dry or wet it affords good foothold for horses, but when beginning to get wet, or drying, it is often extremely slippery. This is said to be due to dirt on the surface, and not to the nature of the material. Sand is strewed over the surface to remedy the slipperiness; it tends, however, to wear out the asphalt, and great cleanliness is the best preventive. An asphalt pavement can be kept cleaner than any other, is impervious to moisture, and dries quickly. While the road is kept clean, a very slight depression is made by the horseshoe, which for foothold is a great advantage. The noise made on asphalt by horse-traffic is about the same as that made on hard wood, and is not much more than is necessary for the safety of foot-passengers. In American cities asphalt has been adopted in a totally different form. All asphalt pavements are composed of a very large proportion, perhaps five parts in six, of a hard non-bituminous material. In America it is found cheaper to get the purer bitumen of the island of Trinidad, and to procure in the localities the bulky material required for admixture a coarse angular sand with a little pure carbonate of lime. An asphaltic cement is made from refined asphaltum. Of this, from 12 to 15% is used with 70 to 80% of sand and 5 to 15% of limestone dust. These materials are heated and stirred together into a stiff mastic paste to form the wearing surface of the road. Upon the concrete foundation is first spread a layer of fine bituminous concrete called '" binder, " i| in. thick, to unite the wearing surface to the concrete foundation. Upon the binder the asphalt is laid to a thickness of 2 in., being spread with iron rakes and brought to its finished surface by the steam roller. Obviously this is a process requiring great judgment and experience; but the system has become established in America, to the exclusion of European methods. Its great recommendation is the freedom from slipperiness that is said to result from the admixture of sharp sand, and this freedom is really the one quality in which asphalt pavement is seriously deficient. This system has been introduced into England.
Wood-Paving. Wood pavements were introduced in England in 1839. Hexagonal blocks of fir, 6 to 8 in. across and 4 to 6 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 pavements 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 " 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 in. 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 in. 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 provided 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, which is now generally employed. Australian hard woods have to a large extent supplanted the fir and pine which were at one time used as the materials for wood-paving. The softer woods, which afford reasonably good foothold and are comparatively noiseless, wear rapidly under heavy traffic, and are very liable to decay. Moreover, the wood actually used has been of mixed qualities, and when a block fails, those near it suffer; thus holes are formed, so that the pavement has to be renewed before its time. English oak and beech, which are perhaps too hard, have been used with varying results; but the Australian woods of the genus Eucalyptus have been most extensively tried, and with the most satisfactory results. Those which are best known are jarrah and kauri, but tallow wood, blackbutt, blue-gum, red-gum, and spotted-gum, with others, have been tried. Of these, one or two are too dense and hard to afford foothold, others are not easily procured, but jarrah and kauri are used extensively. When cut from the matured heart-wood they are uniform in quality, hard enough for durability, and rough enough to afford fairly good foothold. A very large quantity of wood has been used in London under the name of American red-gum. In substance it comes between the soft and hard woods above mentioned. Wood blocks for paving must be cut with the utmost precision as to the depth of 5 or 6 in. and the breadth of 3 in. The usual length of 8 or 9 in. should also be kept well enough for bond. A long block is liable to tilt. As to depth, although a slight depression may be of little account, the least projection in a block will be immediately noted as a jolt by the swift-moving wheel. The laying and jointing of wood blocks on concrete is still a matter of experiment. They may be set on a half-inch bed of sand, which is supposed to, though it is doubtful whether it actually does, make the pavement elastic to the tread. If the blocks are not accurately gauged, the sand enables the paviour to adjust them to a uniform surface. But the practice most approved is to pave directly upon the smoothly finished concrete, trusting for elasticity to the wood. On the revival of wood-paving it was thought necessary, for foothold, to leave wide joints filled with small gravel grouted with cement; but this is mischievous. The cement breaks up, and when the blocks shrink, the filling-in is driven downwards, and when they again get wet, they have less room to expand, the side kerbs are driven back, and the foot-pavements are displaced, so as to require relaying. To guard against this, a space of about 2 in. has been left between the pavement and the kerb, to be temporarily filled with clay or sand, which can be cleared out as the pavement expands. But cement has no affinity for wood, and its use, together with the wide joints that were thought necessary to give foothold, has been abandoned. They permitted the edge of the block to be beaten down below the centre, so as to produce a succession of ridges, having much of the character of a " corduroy " road. Asphalted felt placed in the joints has not succeeded. A method very successfully adopted is to leave the end joints slightly open, and to place strips or laths one-tenth of an inch thick between the courses, so that hot pitch can be poured down to fill the joint and cover the surface. The roadway is then strewn with fine sharp gravel. Hard-wood blocks so laid expand very slightly, so that a space of an inch and a quarter is sufficient between the kerb and the two courses of blocks that are usually laid parallel to it ; this, when filled with pitch, is more than enough to allow for expansion. Paving has been laid with close joints, small vessels of hot pitch being provided, into which each paviour dips the blocks more or less completely before laying them; but wood blocks are more commonly laid dry, a little pitch being brushed over the surface. The gradual abandonment of the wide joints once considered necessary for foothold will be noticed. Soft wood seems to wear under very heavy traffic about five times as fast as hard wood.
Plank Roads. In opening up a new country, roads, temporary or permanent, must be made with such materials as may happen to be at hand. The plank road often used in American forests makes an excellent track for all kinds of traffic. Upon that side of the space devoted to the road, which the heavy traffic leading to a town will use, two parallel rows of sills 15 to 20 ft. long, 12 in. wide and 4 deep are laid longitudinally flatwise 4 ft. from centre to centre, the earth being well packed and rammed to the level of their faces. The joints are not opposite; a short piece of sill is put either under or by the side of each joint. Cross-boards about 8 ft. 3 in. long and 3 in. thick are laid down loosely, so that groups of four boards together will project on alternate sides of the road 3 or 4 in., forming a shoulder to enable vehicles to get on to the track at any point. The remainder of the road space is formed as an earthen track, 12 ft. wide, for light vehicles. Its slope outwards may be i in 16, that of the plank road i in 32. If the soil is too bad for the earthen track, short lengths of plank road of double width are made at intervals to form passing places. The cross boards are spiked down on five sills, and are sprung so as to give a fall both ways.
Log Roads. The log road is formed across swamps by laying young trees of similar length close together. This is ridiculed as a " corduroy " road, but it is better than the swamp. Good temporary roads may be made by laying down half logs roughly squared upon the ground, close together or with spaces between of a couple of inches, into which earth is well rammed. They may be 8 or 9 ft. long, alternate logs being made to project a foot on each side for convenience of driving on and off the track.
Charcoal Roads. When fuel is available, good roads can be formed of burned materials. Clay is burned into ballast for foundations, or for a temporary track. In American forests charcoal roads have been largely used. Logs from 6 in. to 2 ft. in diameter are piled along the whole route, the stack being 9 ft. broad at the base, 6 ft. high and 2 ft. broad at the top. Dry materials for lighting are intermixed, and the stack is covered up with sods and earth from the side ditches. When burned, the charcoal is simply raked down so as to form a i s-ft. road of a well-rounded section. These roads are dry and hard, and otherwise satisfactory.
The mode of carrying a road across a bog upon a foundation of faggots or brushwood is well known. In India the native roads have been made equal to heavy traffic by laving branches of the mimosa across the track. And in the great plains, where the soil, when dry, would otherwise be made deep in dust, this is entirely prevented by laying across the track a coarse reed or grass like the pampas-grass, and covering it with 3 or 4 in. of loam.
Sand Dressing. In carrying traffic over a clay soil a covering of 3 or 4 in. of coarse sand will entirely prevent the formation of the ruts which would otherwise be cut by the wheels; and if the ground has already been deeply cut up, a dressing of sand will so alter the condition of the clay that the ridges will be reduced by the traffic, and the ruts filled in.
Noiseless Roads. A comparatively noiseless pavement may be formed with bricks made of cork granulated and mixed with fibre and asphalt; they are set in pitch, and seem to be suitable for rather steep gradients. For a perfectly noiseless pavement, such as is specially required where a carriage entrance under bedrooms is used by night, no substance is equal to indiarubber. For this purpose it is made in inch sheets about 3 ft. wide and as long as the width of the roadway; it is fixed over concrete and secured by iron clips. This arrangement carries the whole of the passenger traffic to St Pancras Station, London, and also a considerable amount of traffic passing under the Euston Square Station Hotel.
Dustless Roads. The necessity for making roads dustless has been rendered urgent by the advent <Sf the motor-car. The oldest and least efficacious method is to convert the dust into mud by the aid of the watering cart; at the best, however, the improvement is temporary, though attempts have been made to obtain more lasting results by using a solution of some hygroscopic salt such as calcium chloride. Various special preparations of petroleum and other oils have been introduced as palliatives, but the most promising treatment for existing macadam roads consists in distributing tar by hand or machine over the surface, care being taken to make the application in fine weather when the roads are dry. The radical solution of the problem, however, is to be sought in the adoption of improved methods and materials for construction, probably with a bituminous binding or matrix.
This same problem of the motor-car, which, by its rapidity of movement, rendered many of the old country roads in England (suitable, or at least tolerable, as they were for slowmoving traffic) positively dangerous for the new traffic by reason of their narrowness, sharp corners, etc., has been responsible for the passing by the legislature of a very important measure, the Development and Road Improvement Funds Act 1909. This act, in its second part, deals with the question of road improvement, and establishes a Road Board, making it a body corporate. The Board is given powers to make advances to county councils or other highway authorities for the construction of new roads or the improvement of existing roads, as well as itself to construct and maintain new roads. The expression " improvement of roads " is defined by the act as including the widening of a road, the cutting off corners, levelling, treating a road for mitigating dust nuisance, etc. Power is given to the Board to acquire land for the purposes of road improvements. The expenses of the Board are met out of a road improvement grant each year, the greater part of which it was proposed should be provided by diverting the tax on motor spirit and on motor vehicles levied under the Finance Act of 1909-10.
Watering. On macadamized roads in Great Britain watering is only good for the road itself when the materials are of a very silicious nature and in dry weather. With other materials the effect is to soften the road and increase wear. In and near towns watering is required for the comfort of the inhabitants, but it _ should not be more than enough to lay the dust without softening the road, and the amount required for this may be greatly reduced by keeping the surface free from mud, and by sweeping off the dust when slightly wetted. Pavements are watered to cleanse them as well as to lay the dust, but it must be remembered that both wood and asphalt are more slippery when wet, and that therefore watering should be obviated as far as possible by thorough cleansing. Hydrostatic vans, by improvements in the distributing pipes and regulating valves, water a wide track uniformly with an amount of water which can be regulated at pleasure. Where hydrants exist in connexion with a water supply at high pressure, street watering can be effected by a movable hose and jet, a method much more effective in cleansing the surface, but using a much larger quantity of water. Another method which has been tried, but not much used, is to lav perforated pipes, at the back of the kerb on each side of the road, from which jets are thrown upon the surface. The first cost is considerable, and the openings for the jets are liable to choke and get out of order. Deliquescent salts have been used for street watering, by which the surface is kept moist, but at the expense of the moisture in the air. Sea water has the same effect in a less degree.
Cleansing. The principal streets of a town are generally cleansed daily, either by hand-sweeping and hand-scraping or by machines. Sir Joseph Whitworth's machine consists of a series of revolving brooms on an endless chain, whereby the mud or dust is swept up an incline into the cart. A less costly and cumbersome machine consists of a revolving brush mounted obliquely, which sweeps a track 6 ft. wide and leaves the dust or mud on one side to be gathered up by hand. A horse scraping-machine which delivers the mud at the side is also used, the 'blades of the scrapers being mounted obliquely and covering a width of 6 ft. For general use, more especially in the country, scraping-machines, worked by a man from side to side of the road, and scraping a width of about 4 ft., are more convenient.
All street surfaces suffer from the constant breaking up and disturbance to which they are subjected for the purpose of laying and repairing gas and water pipes. Subways, either under the middle of the road or near the kerbs, in which the pipes may be laid and be always accessible, have often been advocated, and in a few instances have been constructed; but they have not hitherto found general favour.
Footways. Gravel is the most suitable material for country or suburban footways; it should be bottomed with a coarser material, well drained and should be laid with a roller. An inclination towards the kerb of about half an inch in a foot may be given, or the surface may be rounded, to throw off the wet. Where greater cleanliness is desirable and the traffic is not too great a coal-tar concrete similar to that already described, but of smaller materials, makes a good and economical footway. The coating should be 2 5 or 3 in. thick, composed of two or three layers each well rolled, the lower layer of materials of about ij in. gauge, and the upper of a half or a quarter of an inch gauge, with Derbyshire spar or fine granite chippings over all. Concrete footways require to be carefully made and must be allowed to set thoroughly before they are used. Concrete has a tendency to crack from contraction, especially when in a thin layer, and it is better to lay a footway in sections, with joints at intervals of about 2 yds. Concrete slabs, especially when silicated and constituting artificial stone, make an excellent footway. The material is composed of crushed granite, gravel or other suitable material, mixed with Portland cement and cast in moulds, and when set saturated with silicate of soda. This paving has proved more durable than York stone flagging, but it is more slippery, especially when made with granite. York stone makes a good and pleasant foot pavement, but is somewhat expensive considering its durability; it is apt to wear unevenly and to scale off when the stone is not of the best quality. It should not be laid of a less thickness than 2 in.; 2\ or 3 in. are more usual. The flags should be square jointed, not under-cut at the edges, and should be well bedded and jointed with mortar. Caithness flag is much more durable than York stone and wears more evenly; it is impervious to wet and dries quickly by evaporation. The edges are sawn, and the hardness of the stone renders it difficult to cut it to irregular shapes or to fit openings. Staffordshire blue bricks and bricks made of scoria from iron furnaces are both very durable, though somewhat brittle. Asphalt either laid as mastic or compressed is extensively used for footways; the former is considered inferior in durability to York stone and the latter superior to it. Asphalt should not be laid less than three-fourths of an inch thick on 4 in. of cement concrete, and I in. of asphalt is desirable where there is great traffic.
Footways in a street must be retained by a kerbing of granite, York stone, Purbeck or other stone sufficiently strong to stand the blows from wheels to which it is subjected. It should be at least 4 in. wide and 9 deep and in lengths of not less than 3 ft. A granite kerb is usually about 12 by 6 in., either placed on edge or laid on the flat. When set on edge a kerb is generally bedded on gravel with a mall; when laid on the flat a concrete bed is desirable. In a macadamized street pitched or paved water channels are required to prevent the wash of the surface water from undermining the kerb. The pitching consists of cubical blocks of hard stone about 4 in. deep, bedded on sand or mortar, or preferably on a bed of concrete. A paved channel consists of flat stones about I ft. wide inclining slightly towards the kerb. Moulded bricks and artificial stone are also used both for side channelling and for kerbing. Such an inclination must be given to the channel as will bring the surface water to gullies placed at proper intervals, and the level of the kerbing and consequently of the footway will depend to some extent on the surface drainage as well as on the levels of adjacent houses. To lay out a street satisfactorily the longitudinal and transverse sections must be considered in relation to these matters as well as to the levels of intersecting streets.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)