CHAPTER VIII.

THE CONSTRUCTION OF NEW AND THE MAINTENANCE OF

EXISTING ROADS.

342. In a preceding chapter full details have been given, especially of the methods adopted in selecting the most suitable line in laying out a new road and the manner in which the construction is carried out, embracing the earthworks, drainage, and bridges.

The methods employed in quarrying the material, reducing the rock to road metal, and its subsequent consolidation by steam rolling, have also been described in detail, the cost of each operation being taken from the results of practical experience.

343. The method of constructing a new road, by which is meant the part forming the bottoming and top metalling, and the maintenance of roads generally, as practised at the present time, will now be described. To make and maintain these in an efficient manner requires careful investigation and consideration.

Modern exigencies are such that machinery is employed almost exclusively in the construction and repairing of roads, and has to a considerable extent superseded the principles and methods adopted in road making by the two great pioneers, Telford and Macadam.

344. In the method followed by Telford (who was called upon to lay out new roads and improve the line and gradients of existing ones in different parts of the country) the type of foundation adopted depended on the local supply of suitable material. Telford's name is associated with the system of hand-set stones as a pavement foundation on which the top metal or wearing surface is placed. In the construction of many new roads, and the alteration of existing ones, where suitable material could not be obtained for a paved foundation, Telford used what could be procured locally, such as gravel or layers of chalk and gravel.

345. Telford's Specification of Roads.-The following specification of a 30-foot pitched foundation roadway was that adopted by Telford, and a similar form is occasionally employed at the present time when making new roads:

"Upon the level bed prepared for the road materials a bottom course or layer of stones is to be set by hand in form of a close, firm pavement; the stones set in the middle of the road are to be 7 inches in depth; at 9 feet from the centre 5 inches; at 12 feet from the centre 4 inches; and at 15 feet 3 inches.

"They are to be set on their broadest edges lengthwise across the road, and the breadth of the upper part of the said pavement is to be broken off by the hammer, and all the interstices to be filled with stone chips firmly wedged or packed by hand with a light hammer, so that when the whole pavement is finished there shall be a convexity of 4 inches in the breadth of 15 feet from the centre.

"The middle 18 feet of pavement is to be coated with hard stones to a depth of 6 inches. Four of these 6 inches to be first put on and worked in by carriages and horses, care being taken to rake in the ruts until the surface becomes firm and consolidated, after which the remaining 2 inches are to be put on.

"The whole of the stone is to be broken into pieces as nearly cubical as possible, so that the largest piece in its longest dimensions may pass through a ring 2 inches inside diameter.

"The paved spaces on each side of the 18 middle feet are to be coated with broken stones or well-cleaned stony gravel up to the footpath or other boundary of the road so as to make the whole convexity of the road 6 inches from the centre to the sides of it, and the whole of the materials are to be covered with a binding of 11⁄2 inch of good gravel free from clay or earth.” * A cross-section of Telford's roads as specified above is shown in fig. 116. It has been already observed that Telford did not always make use of a pitched foundation, but employed such material as gravel, which was applied

on parts of the road between London and Shrewsbury, while this class of bottoming was utilized in many of the new roads constructed by him.

It was the complete separation of the road metalling from the subsoil that Telford aimed at in adopting a pitched foundation, in order to prevent the useful wearing material coming into contact with the natural soil. He also recommended, when the drainage was difficult to effect, that gravel,

* A Treatise on Roads, by Sir Henry Parnell.

sand, vegetable soil, and chalk should be used, but he generally advised the adoption of the former method when suitable material could be conveniently procured.

346. Macadam's System.-Macadam's system differed materially from that of Telford, but both Macadam and Telford insisted on the necessity of thorough drainage of the formation or bed of the road and subsoil, which had prior to that time been entirely neglected in road making.

Macadam was satisfied with laying the metalling directly on the surface of the ground, after the irregularities had been levelled and side ditches formed; or, in other words, he preferred to build up the road above the surface of the ground.

The method of construction (common previous to Macadam's time, and practised in many instances during recent years), that of cutting a trench or 'box' sufficiently deep to retain the road material—was condemned by Macadam as detrimental to efficient drainage.

He contended that the use of metal broken to a small cubical size, which would unite by their own angles into a compact impervious covering, ranging in thickness from 5 to 9 inches, combined with perfect subsoil drainage, was sufficient for any class of traffic.

To Macadam is due the credit of having been the first to direct public attention to the great necessity of properly breaking the material to a uniform size and cubical form, ranging from 1 to 2 inches, and weighing less than 6 oz. He was instrumental also in bringing the management of the highways into a regular system under qualified surveyors.

The services of Macadam and his staff of assistants were in great demand over a considerable part of Britain at the beginning of the present century. 347. Advantages and Defects of Macadam's and Telford's Methods of Construction. Of the two systems much can be said, especially when they are severally applied to the varying conditions and nature of the different soils met with in practice.

A paved foundation, to be of any advantage, requires to be very carefully executed with the best possible class of materials; even then there may be a large number of unfilled spaces (inseparable from this method of construction), which sometimes act as drains, so as to allow the water to reach and to soften the natural soil; the repetition of this process moves the foundation and destroys the cohesion of the wearing part of the road. Defective drainage, even of a temporary nature, will cause this; the subsoil gradually working upwards is mixed with the metalling, while the latter is pressed down into the bottoming of the road. A road formed entirely of broken stones and consolidated by wheel traffic does not present any voids, but as the interstices are composed to a large extent of soft material, more especially at the lower part resting on the subsoil, the crust of the road is liable to suffer if situated on a damp subsoil of a retentive nature.

The bottoming of a road is sometimes carried out in an indiscriminate manner, and the so-called 'pitched pavement' is often made with no better material than that obtained from old buildings in the form of freestone, bricks, and lime rubbish tipped on the formation level of the road in a promiscuous manner, and roughly graded to the required contour.

A system of construction combining the two methods mentioned may be adopted in general practice with great advantage. A bottoming, of stones about one-third of the size adopted for a pitched foundation, and consequently two to three times larger than ordinary macadam, spread to a sufficient thickness, then covered with a coating of ordinary road metal consolidated separately by steam rolling, has been attended with beneficial results. By adopting this method of making roads the cost of construction is reduced compared with the expensive methods inseparable from the different systems carried out by Telford and Macadam.

348. The great advance in the use of mechanical appliances in road construction and maintenance has made possible what could not hitherto have been accomplished, not only from a practical point of view, but by reason of the greater efficiency and economy attained.

Previous to entering into the details of the construction of roads it will be necessary to consider the nature of the soil or bed on which the proposed road will rest.

The preliminary work of grading and forming the cuttings and embankments described in the previous chapter which treated of the work necessary to be accomplished in general. The nature of the ground on which the road is to be placed demands attention, however, and apart from the under drainage therein treated, the several kinds of soil composing the strata necessarily require different treatment.

349. The Effects of Sub-soils on the Methods of Construction adopted. In constructing a new road over sand or gravel (the simplest case met with in practice), it is quite unnecessary to adopt a pitched foundation. On the other hand, if the road bed is a retentive clay, the spreading of metalling directly on the soil, according to Macadam's principle, would be a failure.

In the first case the natural drainage cannot be improved upon, and consequently an expensive pitched foundation can be dispensed with, while the system advocated by Macadam would be applicable.

On a road bed, the soil of which is of an argillaceous nature, subsoil drainage can only partially relieve the general surface on which the road materials are to be placed. Under these circumstances it would be inadvisable to set a pitched foundation directly on such a surface, as in the event of the subsoil drainage becoming temporarily inoperative, or should the material composing the crust of the road be disintegrated by the alternate action of frost and thaw resulting in the metalling being made permeable,

the road bed would be liable to saturation with water in rainy weather. The clay would then gradually rise between the paved foundation stones by reason of the pressure exerted by the wheel traffic pressing the metalling downwards. By such a process considerable injury to the macadam of the road would result, while the incorporation of an undue amount of soft material in the road coating must necessarily, sooner or later, deteriorate and possibly destroy it.

A road constructed on Macadam's principle would likewise suffer, probably to a greater extent, by being placed on a clay bed under similar conditions. In such circumstances it is advisable to apply a suitable material to entirely separate the soil from the road material. This is readily accomplished by spreading a layer of furnace ashes, sand, or other material of a similar nature from 3 to 6 inches thick over the surface. This at once forms a cushion to receive and thoroughly separate the road material from the soil, while at the same time this intermediate layer admits of perfect under drainage. It may be an expensive means of efficiently carrying out the work when suitable material cannot be procured locally, but economical maintenance can only be looked for by adopting approved methods of construction.

Too many roads, unfortunately, have been constructed without such precautions being taken, with results which are not only apparent from the relatively high cost of maintenance, but also from the surface requiring almost constant attention and repairs, while the necessity for scraping and the removing of mud during wet weather is greatly increased.

It is advisable when the road is formed in rock cuttings to spread a layer of sand or other material of a light nature, so as to fill up the irregularities of the surface, and that will also form a cushion for the road materials to rest on. Should the bottoming be placed on a hard surface, as in the case of rock, the material is apt to be pounded and destroyed during the process of consolidation.

350. The practice of thoroughly consolidating the ground when forming new roads by means of a light roller previous to commencing the actual structure, has much to recommend it. This operation should be carried out in dry weather, and any depressions on the surface that may be caused by the passage of the roller should be made good with the same class of material as the surrounding soil. The process of rolling must be repeated until a uniform and solid bed is obtained which will enable the surface of the ground to sustain the superstructure, and the weights brought upon it.

351. Method of Construction adopted at the present time. - A system of bottoming roads combining the methods practised by Telford and Macadam, has long been adopted by engineers and surveyors. It consists in spreading the metalling on a layer of 3- or 4-inch cube stones from 6 to 12 inches thick. The advent of the steam roller has materially aided this form