I do not propose in this work to speak of any of the engineering operations necessary to lay out or construct long lines of connecting roadways, as that is a duty which seldom falls to a town surveyor to perform, and there are a great number of treatises and books upon the subject already published. The object of this chapter will be to give some information and hints upon the construction and maintenance of what are known as macadamised roads, suitable for urban and suburban traffic.

There can be little doubt that roadways of this description are expensive luxuries where the cost of their maintenance, owing to excessive traffic or other causes, exceeds 2s. per square yard per annum, but they are often necessary luxuries when the requirements of the locality are considered, a point to which I drew attention in the preceding chapter upon “Traffic.” For purposes of what may be styled “pleasure traffic,” macadamised roadways are unequalled when well constructed and maintained, but there are many objections to them which will be considered in their place in this chapter.

The word macadamised is, as is well known, derived from one John Loudon Macadam, who in the year 1816 first took up the question of putting broken metal upon a road instead of the boulders previously used.[12] His name, being rather a peculiar one, has been attached to this description of road ever since.

As a matter of fact, the “macadamised” roadways of the present day are constructed after a method introduced by Thomas Telford as an improvement upon Macadam’s principles, and a perusal of the two following specifications will, I think, show that there is not very much difference between the method introduced by Telford and that followed at the present time.

Specification of a Roadway as designed by Thomas Telford more than fifty years ago.[13]

“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 seven inches in depth; at nine feet from the centre five inches; at twelve feet from the centre four inches; and at fifteen feet three inches. They are to be set on their broadest edges lengthwise across the road, and the breadth of the upper edge is not to exceed four inches in any case. All the irregularities of the upper part of the said pavement are 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 four inches in the breadth of fifteen feet from the centre.[14]

“The middle eighteen feet of pavement is to be coated with hard stones to the depth of six inches. Four of these six 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 two inches are to be put on.

“The whole of this 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 of two and a half inches inside diameter.

“The paved spaces on each side of the eighteen middle feet are to be coated with broken stones or well-cleaned stony gravel up to the foot path or other boundary of the road, so as to make the whole convexity of the road six inches from the centre to the sides of it, and the whole of the materials are to be covered with a binding of an inch and a half of good gravel free from clay or earth.”

If the above specification, written more than fifty years ago, is compared with one of the present date, it will be seen that there is a strong resemblance between them.

Specification of a Roadway as now executed.

The cross section of the roadway when finished is to be an arc of a circle, with a rise of 1 in 27 from kerb to the centre of the roadway each way.[15] The roadway, when consolidated and finished, to be 12 inches in depth at the gutters and 15 inches at the centre, diminishing gradually from this point right and left to the depth named. The gutters to be 2 feet in width, formed of stone setts 6 inches by 6 inches, and laid in sand, on a firmly consolidated surface of small broken stone or gravel.

The earth road-bed on which the surface formation is to rest is to be excavated to the required depth, and when graded and shaped to its proper form, it is to be thoroughly and repeatedly rolled with a steam roller, and all depressions which then appear are to be filled with the same material as the road-bed, and rolled until the whole be uniformly compact and firm.

On the road-bed thus formed and compacted, a bottom layer of stone of a depth of 8 inches at the centre of the road, and gradually diminishing to 6 inches at the kerb, is to be set by hand, to form a close, firm pavement. The stones are to be laid, with their largest side down, in parallel lines across the street, breaking joint as much as practicable.[16] The width of the upper part of the stone not to be more than 8 inches, nor less than 6 inches. The stone not to exceed 15 inches in length. After being set closely together, the stones are to be firmly wedged by inserting a bar in all possible places, and placing between them stones as nearly as possible of the depth of the pavement, until the whole is bound in position. Projections of the upper part of this course are to be broken off, care being taken not to loosen the pavement; and no wedging is to be done within 20 feet of the face of the work being laid. The small interstices are to be filled in with stone chips firmly wedged with hammers. The whole is to be thoroughly rammed and settled to place, and all undue irregularities of surface broken off.

On the foundation course must be laid an intermediate layer of broken stones, varying in size from 3 inches in their greatest diameters to 1 inch in their smallest diameters. These irregular-sized stones may be either the “tailings” of the screened stones, or may be raked from the quarry, and placed on the roadway without being machine-broken; but they must nevertheless be so laid as to compact solidly, and must be clean broken stone, free from dust and dirt, and within the dimensions given above. This intermediate course must be 4 inches in depth at the centre of the roadway, gradually decreasing to 3 inches in depth at the gutters; it is to be thoroughly rolled with the steam roller until it be firm, compact, and solid. On its upper surface it must be identical in rise and form to the cross-section of the finished pavement, as specified above. In the laying of this course of stone a small quantity of binding material is to be used, sufficient only to fill up the crevices, and render this portion of the pavement solid. Preferably the binding is to be of fine screened gravel or sand, which is to be sufficiently watered during the process of rolling, so that the “licking up” of the road material, and its adherence to the rolling-wheels may be prevented.

On the intermediate course is to be laid the surface layer of broken stone.[17] It must be 2¹/2 inches in depth, and the stones must be practically uniform in quality, and as near an approach to a cube in form as possible. Each stone used in this layer must have passed through a 2¹/2-inch circular hole, and all stones that are wedge-shaped, and do not approach uniformity of measurement on their sides, are to be taken from the road with properly shaped rakes, and no stones allowed to remain which are not sound, strong, and equable in size and quality of material. The stones are to be raked into an even layer, and the steam roller passed over them twice or thrice. After this a quantity of fine screened gravel or sand is to be thrown on and sufficiently sprinkled to moisten the mass without “licking up.” The rolling is then to be continued (working the roller backwards and forwards, gradually from the gutter to the crown), with an occasional light watering of the pavement, until the cross-section be exact according to specification, the interstices filled in, the roadway firmly compacted and solid, and all excess of binding removed from the surface of the finished pavement.[18]

Telford’s object was the complete separation of the road metal from the subsoil by a firm and regular foundation, and this system has ever since held its ground. The advantages to be gained in constructing a roadway in this manner may be summed up as follows:—

(1.) Economy of construction, as a considerable quantity of metalling is saved; only 3 inches of properly broken stone and a little binding material being necessary, the foundation of the roadway (which really carries the traffic) may be made of a quality of stone unsuitable for road metal, or even of bricks or stones from old buildings that are being pulled down.

(2.) The prevention of the rising up or “spewing” of the clay or other soft material on which the roadway rests.

(3.) A solid foundation is secured which will successfully resist the weight and percussion of the traffic.

(4.) The increased facility for the drainage of the roadway water being ruinous to it.[19]

Instead of forming a paved or “pinned” foundation for macadamised roadways, sometimes what is called “hard core” is placed at the bottom of the road upon the surface formation.

This “hard core” is made of very heterogenous materials, often the waste products of the house refuse depÔt, and consists of ashes, old pots and pans, meat tins, old bottles, shells, and a variety of similar articles; sometimes the core is made of burnt ballast, but in no case does it make so good a foundation as stones set by hand.

Concrete has also been employed as a foundation with great success, but it is very expensive, and is seldom used except under streets paved with either granite, wood, or asphalte, of which I shall speak hereafter; for if the traffic was so great as to necessitate the use of concrete for a foundation it would surely be better to give the roadway a more durable surface than macadam.

The following tables, showing the thickness of the foundation and metalling of broken stone roads, is from a paper on roadways, read to the Association of Municipal and Sanitary Engineers, by Mr. James Hall, Borough Surveyor of Stockton, and may be of use to those who would like to know what proportions to use.

Chalk has sometimes been used for the bottom of a roadway, but where this is likely to be affected by frost it is the worst material that can be used, as it is likely to blow up the roadway.

With regard to the annual outlay upon macadamised roadways, the following comparative tables[20] prepared by Mr. Ellice Clark, the then Surveyor of Derby, may be of interest.

The following is a table of the cost of streets in Paris per square yard per annum.[21]

The cost of maintaining macadamised roadways as compared with that of granite setts has been said to be as high as 5 to 1 and that this cost if capitalised for 12 or 13 years will equal the first expense, interest on money, and the necessary repairs for a granite paved roadway.

The following table gives the cost per annum per square yard for the maintenance of macadamised roadways in different places, so far as I have been able to collect them:

In Birmingham the macadamised streets have worn down 6 inches in one year, with a traffic of 2484 vehicles passing in 10 hours.

With reference to the great cost of maintenance in Paris, the following particulars[22] may here be given;

“The surface of the street is picked by gangs of men, metal from 2¹/2 to 9 inches in thickness is then laid on, a coating of sand is then spread upon it, it is watered and rolled at per kilometre ton, that is, at per ton weight of roller per kilometre travelled, at a cost of about 15·33d. per ton mile for the first 250,000 ton miles, and at reduced rates for additional service. The materials used for the roads are flints costing 4s. 6¹/2d. per cubic yard for light traffic roads; for medium traffic, hard millstone at 11s. 4d.; and for the heaviest and greatest traffic, porphyry at 15s. 9d. The average total cost of maintenance of the streets is 1s. 8¹/2d. per square yard per annum for the first-class roads, and 1s. 1¹/2d. for the lighter traffic; the highest cost for maintenance is as high as 10s. 9d. per square yard, the lowest 9¹/4d. per annum.”

It may be well to mention that 73 per cent. of the streets in Paris are paved, 5 per cent. are coated with asphalte, and 22 per cent. are macadamised.

The contour, or best form of cross section that should be given to a roadway, has often exercised the minds of engineers, but for all practical purposes evenness of surface and regularity of section in a macadamised roadway are of more importance than the slight difference between straight lines and curves, which might only tend to confuse the workmen. Formerly it was the practice to employ a complicated gauge in the form of a straight-edge fitted with plummet or level and sliding bars, but a good eye, assisted by a long straight-edge and spirit-level and three boning rods, is generally found to be sufficient, and if the centre of the roadway is kept level with the heel of the footpath, a sightly cross-section is generally the result; or say 6 inches to 9 inches higher in centre of a roadway 30 feet in width between the kerbs, 3 inches to 4 inches where it is from 18 to 20 feet in width.

The following detailed section of a macadamised roadway is one which I am in the habit of specifying for suburban districts, as it is easily set out and constructed, and answers all purposes most admirably.

Large illustration (66kB)