The heavy motor traffic of to-day demands highways of a type far different from the paved roads built by Roman engineers, or the coach roads built by men such as Telford and McAdam
CUTTING A LEVEL ROAD through a hill. Scrapers are being used as transport trucks to remove the spoil cut away by an excavator. A scraper is illustrated on below. When the cutting is finished the scrapers are formed in line abreast to clear away all the loose material so that a smooth road bed is left for the next operation.
ONE of the earliest activities of the engineer was the making of roads, that is, the deliberate creation of a hard surface to stand the wear and tear of traffic. From prehistoric times man, by following a definite route between certain points, had formed beaten pathways which might, in some sense, be called roads. One of these roads, dating from 3000 B.C., was used for the transport of lapis lazuli across Afghanistan, Persia and Arabia. Then there were the tracks from eastern Denmark to Italy, which came into being about 2000 B.C. and served the growing trade in amber. A new route, after the discovery in East Prussia of richer deposits of this highly prized substance, was established about 1000 B.C. In England neolithic and bronze age men were treading the tracks known as Ridgeways and Harrow Ways still traceable on the Downs.
The made road, too, dates back to 3000 B.C., for it was about that time that Khufu (Cheops) laid down the paved road to the Great Pyramid, referred to in the chapter “Monumental Engineering”. It appears that in the fifth century B.C. Carthage was the centre from which a number of roads radiated, though these were not paved with stone slabs but were made by mixing sand with stones and flints.
The practice was later taken up by the Romans, and in 312 B.C., the laying of their first great road, the Via Appia, or Appian Way, was begun. The remains of this road, to the credit of its builders, are still to be seen. It had two main characteristics, its straightness and narrowness. Its width was 15 feet, with a trench for drainage purposes on either side, thus providing room for only two vehicles abreast. Edward Gibbon, the historian, states that it was easy to travel on the Via Appia at the rate of 100 miles a day, and it is recorded that a rate of 200 miles a day was attained by the Emperor Tiberius travelling by chariot. This speed was never beaten until within two centuries ago. Remains of Roman roads are also to be found in Great Britain. Their construction is not merely a matter of antiquarian interest, but has had considerable influence on modern practice. The road was first marked out and trenches were dug to delimit it. The site was then excavated down to the subsoil, on which a layer of fine earth was pressed down, to be covered by square stone slabs set in mortar. Then came a thicker mass of rubble mixed with lime and above it a mixture of chalk, broken tiles, gravel and lime. This was rammed hard and received the final surface which, whenever it was possible, consisted of squared stones accurately fitted together. The total depth was about 3 feet and eight distinct operations were required. The result was a perfect and almost everlasting road.
This last sentence seems to conflict with the statement that only remains of Roman roads exist, not complete roads; but, with the decline of the Roman Empire, interest in the roads vanished and, where not too remote from towns and villages, they became quarries for the medieval builder who, in the matter of roads, was content with an earthen surface.
The general state of the roads in most European countries was, until the end of the eighteenth century, almost incredibly bad, though there were sporadic attempts at improvement locally. Thus, in 1346 tolls were imposed for road improvement. Three roads, all short and within the area of what is now central London, were kept up in this way. Nothing more appears to have been done with the toll system until about 1663, when it was put into operation on a part of the Great North Road. Then came the day of the turnpike system, which was often denounced as a means of enriching rapacious landlords, whereas it was the only means available at the time of raising money for road upkeep.
However this may be, the roads were still defective. Contemporary literature has much to say about them. Arthur Young, writing about 1767, exclaims: “Of all the cursed roads that ever disgraced this kingdom in the very ages of barbarism none ever equalled that from Billericay to the King’s Head at Tilbury”. He could not find words to describe the road between Preston and Wigan, in Lancashire, and he records having measured ruts in it four feet deep and filled with mud. He goes on to say that “the only mending it receives is tumbling in some loose stones, which serve no other purpose than jolting a carriage in the most intolerable manner”. The cause of all these troubles was that the first principles of making roads, that is, of making a surface that could withstand traffic, were not considered, much less understood, though the Romans had solved the problem 2,000 years before.
Roman Method Modified
The latter half of the eighteenth century, however, saw the birth of two men who were to put road construction on a scientific basis. These men were John Loudon McAdam, born at Carsphairn, Kirkcudbrightshire, on September 21, 1756, and Thomas Telford, born in Eskdale, Dumfriesshire, on August 9, 1757. The methods adopted by each differed fundamentally, and each was preceded by men who had almost anticipated them, McAdam by Robert Phillips, a London man, and Telford by the remarkable John Metcalf, a blind Yorkshireman.
Telford adopted a modification of the Roman method. A typical example of his system is that of his famous road to Holyhead. The foundation consisted of stone blocks about 7 in. deep and 4 in. wide at the top. These were set as closely as possible, any interstices being packed tight with stone chips. On this foundation was a layer of hard stone rammed down and having a thickness of 6 in. Then came the carpet — a term still used in roadmaking — of about 2½ in. of gravel or small stones. This road was 30 feet wide and was cambered, the centre being about 4 in. higher than the sides. In the Highlands alone Telford built some 920 miles of new road; but, although his system was a vast improvement on previous practice, it was expensive and its surface was apt to disintegrate, the unyielding bed below that surface imposing resistance to the weight of the vehicles on it. The Romans, in addition to a stone foundation, had a stone surface.
SCRAPER AT WORK removing the top soil, an operation which forms the first stage of road construction. A scraper is a knife-edged scoop which is lowered to take a cut and is towed by a tractor. The soil removed piles up in a hopper, as here shown. The scoop is lifted when the hopper is full and the load is carried away to be dumped.
McAdam, on the other hand, held that the road bed should “give” under the weight so that the surface would not be ground as it were between wheels and unyielding stone. McAdam was not a practising engineer, but what would be called nowadays a business man. After having left. school, he spent thirteen years in New York, returning in 1783. As Deputy Lieutenant of Ayrshire he saw the need for improving the roads of the county and when, in 1798, he went to Falmouth (Cornwall) as a naval victualling agent, his hobby was studying the roads of south-west England. In 1814 he stated that he had spent some £2,000 on these investigations during the preceding sixteen years and had travelled 30,000 miles. Later, appointed surveyor to the Bristol turnpike trust, he became eventually a recognized authority on roadmaking.
The Roman road was a sort of sandwich consisting of layers of more or less yielding material between the stone surface and the stone foundation. Telford had used what may almost be called the bottom half of this road, McAdam may be said to have used the top half. He dispensed with a hard foundation and placed his material directly on the stripped subsoil or even on the top soil itself. This material was stone broken to uniformly-sized pieces with sharp edges and was spread in a layer 10 in. or 12 in. thick, occasionally only 6 in. thick. The broken stone was soft enough for some of it to be ground away into a fine powder and under traffic the whole consolidated into a smooth coherent mass.
The binding action that takes place is not in any way similar to the chemical changes that occur when cement sets. It is due to a physical condition in which hygroscopic water, that is, water that cannot be evaporated, causes cohesion by means of its surface tension. This is the real explanation, though McAdam himself only grasped the fact that the binding occurred and did not know why it did so. At any rate, his system of roadmaking virtually superseded all others and his name is applied to all roads made of broken stone.
The Dust Menace
At first the ordinary traffic of the day and the action of rain were relied upon to consolidate the road metal, but about 1830 the road roller and the water cart were introduced to hasten the process. The roller, up to quite recently steam-driven, became heavier, and the watering arrangements more effective. For many years the waterbound macadamized road proved perfectly satisfactory for all kinds of traffic. It is true that after a prolonged drought it became dusty, but this did not affect its durability and a shower quickly laid the dust. McAdam made use of a cambered surface, as the Romans and Telford did.
The macadamized road, although it was so satisfactory, is being largely superseded for main highways, because of the introduction of motor traffic. It may at first appear that a rubber tyre cannot be so “hard” on a road surface as the iron tyres of the horse-drawn vehicle of the past, but the wheels of the latter are merely rolled over the surface, the tractive effort being exerted by the horse. The action of the wheel is, therefore, purely one of consolidation.
On the other hand, the driving wheels of a motor vehicle themselves transmit the tractive effort and thus there is a tearing action between them and the road surface. This action is aggravated by the greater area presented by the spreading of the pneumatic tyres. On a wet day these rubber surfaces have a sucking action and flat stones are torn out of their bed in the road to be left lying loose on the surface or to be flung off the road altogether. On a dry day the turbulent eddies raised by the passage of a swift car disperse the stone dust which in the old days settled down again and acted as a binder after the first shower.
MAKING A CONCRETE ROAD BY HAND. The two structures resembling a pair of broad-gauge railway lines are road forms. They are temporary walls of steel between which the concrete is poured. The concrete is transported in barrows from the mixer on the right. The surface is levelled by a screed board curved to the desired camber and with its ends resting on the top of the forms.
By 1906 the dust menace had become so formidable that in that year a Royal Commission was set up to see what could be done. Steps were taken and investigations were made in Great Britain and abroad. All sorts of chemical watering compounds were tried, but continual wetting, although it kept down dust, was found to be fatal to long life in a road. The use of a dressing of tar solved the difficulty, and even now most roads are of water-bound macadam and are satisfactory and easy to maintain, provided the traffic over them is not too heavy.
The use of coal tar, spread on the carpet of a macadam road and dusted over with fine road material before it had set hard, had been proved effective as far back as 1880 in France, and it had been used in Australia in 1886. The tar was originally painted on by hand with brushes. Later it was sprayed on to the surface through fine nozzles by air pressure. The modern tar sprayer or surface dresser consists of a large tank to hold the tar, which is kept fluid by a furnace and is pumped to the nozzles.
A few inches behind the nozzles are a number of brushes and a leather flap which assist in spreading the tar evenly. The tank is mounted on wheels so that it can be traversed. Sometimes, however, the machine is propelled under its own power. As soon as the tar has been put down a layer of gravel or stone chips is spread on it and the whole is rolled well down by a heavy roller. Nowadays, more often than not, the roller is driven by an oil engine, chiefly because this type of roller can be stopped and started up again whenever required, whereas the steam roller consumes fuel even when it is standing and takes a long time to get ready for work in the morning.
The tar-macadam road developed from the tar-sprayed road. The two are often confused, but are essentially different. Tar dressing closes the surface, makes it waterproof and stops the dust nuisance. Tar macadam provides a road in which the whole carpet is bound together by tar and is not therefore readily disturbed.
Road Waves
The tar-macadam type of road is generally made by laying waterbound macadam on the subsoil to a thickness dependent on the nature of the subsoil itself. On top of this come two or three layers of road metal which has been immersed in tar, each layer being consolidated by a roller of about 10 tons. Bitumen is sometimes used instead of tar for coating the metal. The tars first used were not of good quality and they became sticky in hot weather. Early bitumen was defective also, but both types are now much improved.
The asphalt road does not have a macadamized carpet. In it the heavy-coated stone is replaced by stone chips, gravel and so forth. These are mixed with the asphalt in a somewhat porridge-like mass, which is laid and rolled. The plant used for this type of road includes a rather complicated mixing machine with revolving drums, bucket elevators, weighing machines, and boilers for the asphalt or bitumen, or a combination of both.
The mix is generally spread by hand, mostly in two layers, and is afterwards rolled. The spreading machine now does the work more quickly and better than man. The asphalt road, as well as the newer types of road made with cold proprietary binding solutions, needs a firm foundation to support the carpet. Thus, in a way, road engineers are reverting to the practice of Telford.
This does not prove that McAdam was wrong. His principle of an elastic foundation was sound for the traffic of his day and holds still on roads, of which there are many, where the traffic has not greatly increased in weight. But the coming of the motor brought in a set of conditions that did not previously exist. The weight of a six-wheeled motor lorry, for instance, is much greater than the old two-wheeled horse-drawn cart, and the heavy motor bus had no parallel in the old days. Further, heavy motor traffic sets up a series of waves in a road. Apart from questions of wear, then, the modern main road has to be strong, and to attain this end that valuable engineering material, concrete, is widely used.
VIBRATORY CONCRETE FINISHING MACHINE at work on a road at Alperton, Middlesex. The machine runs on road forms and carries a vibrating screed which causes the concrete to set more densely than if left to settle naturally. This enables less cement to be used in the mix to obtain the necessary strength. The machine is driven by an internal combustion engine. A travelling concrete mixer is seen in the background at the right.
Concrete may form a strong foundation for the asphalt road, and is always thus used with a wood block surface. The wholly concrete road is rapidly coming into favour. Concrete was first used for roads in Michigan, U.S.A., in 1909. There are now nearly 100,000 miles of concrete roads in the United States. There are many ways of making a concrete road. A typical method consists of laying a base of heavy material on the subsoil, on top of which a layer of concrete is deposited, the concrete being a well-mixed mass of broken stone, sand, cement and water. Then comes a layer of steel reinforcement, that is, wire network varying in pattern with the type of road. The primary function of the reinforcement is not to strengthen the concrete, but to prevent it from cracking. The carpet is poured on the reinforcement and consists of a finer concrete, generally only cement, sand and water, though sometimes a fine gravel may be added.
The rotating concrete mixer is a familiar sight. The concrete base is tamped by a rammer, either hand-operated or a portable power machine, to fill up any voids. The concrete road, however, is not finished with its laying: it has to be “cured”. In other words, the rate of setting has to be controlled, as this rate affects its later hardness. The finished road is covered with canvas or matting which is constantly kept wet, and is allowed to remain for a week or more. It is important, too, with some types of base to prevent the water in the concrete from draining off too rapidly, so that in such circumstances a layer of coarse paper is generally laid over the foundation before the concrete is poured.
“Carpet” Made in Sections
The surface of a concrete road cannot be made continuous as can other types, for it contracts and expands to such an extent with changes of temperature that serious cracking would occur if some provision were not made to counter this movement. The carpet, then, is made in sections which are divided by narrow strips of wood, cork or other elastic but durable substance. The cross-section of the concrete road may vary according to its particular purpose. A typical road 30 feet wide might be 7 in. thick for a width of 10 feet in the centre, this thickness being uniform. The two side portions of 10 feet each might, on the contrary, be increased in thickness from 7 in. at their inner to 10 in. at their outer edges. There are many other types of road in existence. Roads paved with the stone setts, for instance, are common in the north of England. Roads paved with hard brick are used much in Holland, and cobbles are found in other parts of the Continent. Modern roads may be paved with rubber blocks or cast-iron slabs.
The engineer has not discovered a universal road formula, for that is not possible. In industrial districts, for instance, where heavy loads of machinery have to be transported, the roads need to be much stronger and harder than is necessary in other areas. Again, in some places the minimizing of noise from traffic is of paramount importance and this calls for a soft surface. In cities the network of underground sewers, pipes and cables demands a stiff structure. The conditions vary enormously, and the road engineer has a vastly more arduous task nowadays than had his predecessors.
The work of the road engineer is, further, not confined to the structure of the road bed itself. He has, for example, to lay out his highway, to make cuttings, to form embankments, to provide adequate drainage, to guard against blind corners, and to see that lighting and signalling are ample for the volume of present-day high-speed traffic.
POURING A CONCRETE ROAD BY MACHINE. The concrete mixer at the left is provided with a swinging Doom whicn carries a Ducket with a hinged bottom. The material is shovelled into the loading hopper, which is then raised to deliver the material to the mixer drum. Then water is added and the drum rotated. Next the mixed concrete is discharged into the bucket, which is run out and swivelled to deposit where required The concrete is here being poured on to paper, which prevents the water from draining away too rapidly.
