of construction, the practical and economical benefits of which show that this method may be looked upon as almost perfect for macadamized roads.

The first points to be considered are the width of the proposed road and the amount of contour or convexity required for adequately draining its surface.

352. Width of Roads. -The width of a new road will be determined by the prospective wheel traffic.

It is a common practice to make a road sufficiently wide to allow of two or more vehicles passing each other easily when travelling fast. For ordinary road traffic 12 feet of metalling will be sufficient, but this may be increased to 20, 25, 30, and 35 feet according to circumstances; but in any case it must be adapted to the probable amount of wheel traffic.

An unnecessarily wide road will cost more in proportion to maintain than one which is of the proper width, while the first cost of construction is, so far, practically thrown away. If a wide road is not kept in good order it assumes a very untidy appearance. A narrow road, on the other hand, is generally very difficult and expensive to maintain.

In most cases roads are made narrow, because the amount of money available will not suffice for making them wide; but there is no economy in making a road either narrower or wider than is required by the exigencies of the locality. When the requirements necessary to accommodate the traffic of future industries which may arise in a district cannot be determined with any degree of certainty, it is advisable to make provision for it in forming the earthworks. The road proper may be made to suit the existing requirements of the traffic, so that should necessity demand it the roadway can be widened at any future period.

The width of existing roads necessarily varies according to the geographical position which they occupy. An ordinary country road may, in view of its importance, be metalled to a width of from 10 to 15 feet, while a main road may vary from 15 to 25 feet.

Roads situated between large towns, and those in suburban districts, are usually 25 to 30 feet wide, while city roads generally have a width of from 30 to 45 feet. In many instances, however, 50 to 60 feet has been adopted in laying out new streets in the larger cities, while a width of 100 feet and over is proposed for some of the principal thoroughfares in the metropolis to relieve the congestion of vehicular traffic, which has been expanding to an enormous extent for many years past.

The widths stated do not include footpaths, which are usually provided. on one side of the road, and in many instances on both sides, as in villages and populous places.

The footpaths generally occupy a width of 6 to 7 feet, so that on county main roads, exclusive of side verges, the total width of roadway may be

from 21 to 31 feet, while in the case of suburban roads, with footpaths on either side, the entire width is generally from 37 to 42 feet.

353. Contour of Roads and Minimum Gradients.-The cross-section which should be given to a road of certain width is a matter of great importance. It is usual to construct a road which is practically level longitudinally with more convexity or a greater contour, than one which has a minimum gradient of 1 in 80 to 1 in 100. The latter inclination will, even should the convexity of the road be inconsiderable, enable the surface water to be efficiently drained to the side channels.

The best form of cross-section to give, like many other details in connection with road work, is a matter about which a great diversity of opinion exists. Macadam considered "that a road should be as flat as possible with regard to allowing the water to run off it at all, because a carriage ought to stand upright in travelling as much as possible." The rise or versed sine given by Macadam was 3 inches on a width of 18 feet, his view being that, if the surface be well made and smooth, the water would run off freely with such a convexity.

Telford recommended the form to be that of a flat ellipse, the rise being 6 inches on a road 30 feet wide.

Many authorities have advocated a cross-section composed of straight lines falling towards the channels at an inclination of 1 in 24, and joined by a short curve at the crown of the road. By adopting this form of crosssection it is obvious that the contour will soon be destroyed by the inclined flat sides wearing into hollows between the crown of the road and the haunches. When the central portion of a road is worn flat, or there is a want of convexity, the surface rapidly becomes destroyed by the wearing effects of the traffic and defective drainage combined. The practice of giving an excessive contour to roads, or, as it is sometimes termed, making it 'hogbacked' or 'barrelled,' should be avoided, as it is with difficulty that vehicles can travel over them with safety except at the centre. A road having an excessive rise, besides being dangerous, either causes the wheel traffic to be confined to one track at the crown of the road, or when travelling on the sides the tendency of the wheels to slip or travel towards the channels creates a considerable amount of wear on the road surface.

It is evident, under these circumstances, that the labour of horses is greatly increased, while the wear to the wheels of vehicles is aggravated. The excessive contour which a curve at the crown of a road joined by two straight lines produces is shown in fig. 117, which also shows a flat ellipse. for a similar width of road.

The best convexity for all practical purposes is a rise of from to of the width of the metalling. The form should be that of a flat elliptical curve, which, besides admitting of the greatest amount of wear to be

* Parliamentary Report on the Highways of the Kingdom, 1819, p. 22.

obtained from the metal coating, is sufficient to pass off the surface water freely.

The former rise gives a transverse gradient of 1 in 33 nearly, for threefourths of the width of the roadway, and this convexity is made use of on suburban roads and those maintained on the patching system. The latter rise, namely, of the width of the metalled portion of the roadway, is

FIG. 117.-Elliptical and straight line and curve contour.

that invariably adopted when the metal coating is spread the full width and steam-rolled; it is a gradient nearly equivalent to 1 in 25 for a considerable breadth of the central portion of the road. This additional rise is necessary, because after a few years' wear, the general contour approaches that of the former, or 1 in 33, and that before the surface has to be refaced. 354. Details of Construction.-When the road bed has been prepared, a layer of stones varying in size from 3- to 4-inch cubes, and weighing from 1 to 11 lbs., is spread over the surface to the intended width, the thickness being regulated according to the class of road and the amount of traffic likely to pass over it. The bottoming material may be prepared in the quarries and subsequently conveyed to the works, or (as is generally the method. adopted) the rough material is carted direct to the road under construction and deposited on the formation bed, when it is hand-broken and formed to the required width and thickness. Where the material for bottoming is broken in this manner the cost will be from 6d. to 8d. per ton (including spreading, but excluding the cost of quarrying and haulage).

A depth of from 6 to 9 inches for roads in country districts may be considered sufficient, the thickness being increased to 12 inches for suburban and town roads. The formation bed of county roads is generally made level transversely, the requisite contour being attained by increasing the thickness of the bottoming and metalling in the centre of the road. This, of course, presents a road the sides of which are not equal in strength to the central portion, but as from 50 to 75 per cent. of the wheel traffic on this class of roads passes along the middle part, the wear is there greatest and the flanks or bermes are relatively strong in proportion.

For suburban and town roads, or for exceedingly wide county roads, where the wheel traffic spreads entirely over the surface, and is therefore equally distributed upon the road, it is desirable to form the foundation of a uniform thickness and of equal strength. This is accomplished by making the formation bed of the road conform transversely to the contour which the

finished surface will assume when finished. Figs. 118 and 119 show a cross-section of a country road and that of a suburban road respectively.

The bottoming, whether for suburban or country roads, should in every case be steam-rolled, the interstices being filled with sand or gravel brushed into the joints during the final stages of consolidation. Artificial watering should not be permitted until the rolling is completed, when the work may be tested by sprinkling water over the surface to ascertain if the voids

Fall 9 of width. Metalling 2 to 24 gange

FIG. 118.-Cross-section of a country road.

between the stones forming the bottoming are thoroughly filled. Great care is necessary, however, when applying water under these circumstances, as should an undue amount reach the bed of the road and soften the soil, depressions will be formed by subsequent rolling, which may possibly result in a permanently weak place being formed.

In the event of depressions being formed, resulting from the unequal capacity of the soil for bearing the material composing the bottoming, or by reason of water finding its way to the road bed or formation, it becomes

necessary to add more material of the same nature and quality, in small quantities, until the whole surface is brought to a regular and uniform contour.

On this foundation or pavement a cushion layer of sharp clean sand or fine gravel should be spread, from 1 to 2 inches in thickness, to receive the top metalling. If this precaution is not taken, the metal coating, especially if it be of moderate thickness, will be crushed by the action of the roller wheels pressing the stones on the hard consolidated bottoming

R

The top metalling may now be proceeded with and spread to a thickness, according to the requirements considered necessary, but generally varying from 3 to 6 inches. If the latter thickness is necessary, it will be advisable to carry out the work by spreading the material in two layers and consolidating the first before applying the second coating.

355. In either case the rolling should be commenced at the water tables or outside edge of the metalling and be gradually extended towards the crown of the road. After repeated passages of the roller, and when the macadam has been properly set or interlocked, binding of a sandy description may be applied by shovels, care being taken to spread it evenly over the surface of the metal coating.

The rolling is continued meanwhile with the addition of an artificial supply of water and brushing, until all the interstices between the stones are filled up.

Great care must be exercised to prevent any unnecessary watering, which, besides doing no good to the metalling, might penetrate and do damage to the foundation by softening the natural soil. The surface is finished by applying a coating of screenings from the stone-breaker one-half inch thick, the roller passing backwards and forwards until a thoroughly solid and smooth surface is obtained, any surplus binding being swept off during the final stages of rolling.

356. Cost of Rolling Road Bottoming.-The cost of steam-rolling from 6 to 12 inches of bottoming and filling the voids with sand, assuming the latter has been obtained within a reasonable distance of the work, may be put down at 3d. or 4d. per ton of material consolidated. The consolidation of the top metalling in forming new roads requires a considerably longer time than does that of a coating of metal spread on an existing road. This may, in certain circumstances, amount to 1s. per ton of metalling consolidated, or an average of 8d. for the combined operation of rolling the bottoming and top metal when these quantities are of equal depth.

Maintenance of Macadamized Roads.—To maintain an existing road in good order it is necessary to perform the work either by annual repairs or by applying a wide and continuous coat of metalling and then steam-rolling it. Roads repaired by the latter method, depending on the quantity of metalling applied, may wear for a considerable number of years with but little attention.

357. Wear on Roads.-The wear of the macadam composing the crust of a road is due to two causes, namely, the traffic and the weather. The amount of the former, and the wearing effect due to wet weather and wheel traffic determine to a great extent the quantity of material necessary for the efficient maintenance of the surface of a road. Frost has the effect of expanding the moisture in the crust of a road in proportion to the amount of soft or muddy material which it contains. Alternating frost and thaw,