The following table[85]is also given as showing the comparative cost of wood paving with macadam and bituminous concrete paving in Liverpool.
In concluding this chapter upon wood paving, I will give a specimen specification for work of this description.
—Excavate the ground to a depth of — inches below the level of the proposed finished surface of the roadway.[86]The formation surface thus excavated must be well watered and rolled or punned if found necessary, and any soft or made earth removed to such a depth as may be found to be sufficient.[87]
—Upon the excavated formation surface a bed — inches thick of concrete is to be laid, composed of one part of good approved Portland cement to two of fine, sharp river sand, and three of clean river ballast or broken stone. The concrete to be finished off with an even and smooth top surface conforming with the contour line of proposed finished roadway.
—Upon the concrete thus laid, and after it has sufficiently set, wood blocks are to be laid.[88]These blocks must be of the best description of Baltic red timber[89](or such other timber as shall be specified), sound and thoroughly well seasoned, free from all sap, shakes, large and loose knots or other defects, and any that may be rejected by the surveyor as unfitted for the work shall be at once removed from the works or broken up. The blocks must not be less than 6 inches or more than 12 inches in length by 3 inches in width and 6 inches in depth, they are to be carefully laid with the fibre of the wood placed vertically, their endsmust butt with close joints to each other, and each course must be kept ³⁄₈ of an inch apart by means of wooden laths, which are afterwards removed.
—The joints are then to be carefully run with a grouting composed of one part of best approved Portland cement to two parts of fine, sharp, clean river sand. (In some cases a hot bituminous mixture or asphalte is run between the joints as a grouting.)
—The whole surface of the pavement is then to be spread with a coating, at least ¹⁄₂-inch in thickness, of fine sharp gravel or chippings.
The following heads of general conditions under such a contract may also be useful.
Alteration of gullies, sewer man-holes etc., will be done at the expense of the sanitary authority.
Contractor must make good at once any damage caused to gas or water mains or services—time penalty for delay.
Maintenance of work after completion for a specified time.
Power must be reserved to surveyor to suspend work during bad weather or from other causes.
Heavy time penalties for non-completion of contract by a certain date.
Payments to be made to contractor on surveyor’s certificate, up to 80 per cent. of whole contract, remaining 20 per cent. to be paid at end of (say) 2 years after completion.
With the above specimen specification I conclude the chapter on Wood Paving.
[67]The first wood pavement laid in London was in front of the Old Bailey, in 1839.[68]Vide‘Minutes of Proceedings of the Institution of Civil Engineers,’ vol. i. p. 131.[69]This specification is the company’s own, as advertised when they first began business; for many reasons the boards have since been discontinued, and other alterations introduced into the system.[70]I am unable to ascertain if this plan has ever been tried anywhere.[71]It is also affirmed by the Borough Surveyor of Sunderland that this process dispenses with watering.Vide‘Proceedings of the Association of Municipal and Sanitary Engineers and Surveyors,’ vol. iii. p. 72.[72]Vide‘A Practical Treatise on Roads, Streets, and Pavements,’ by Q. A. Gillmore, p. 166, which see also for a good account of wood pavements in the United States.[73]Vide‘Wood Pavements,’ by Henry Allnutt, 1880, p. 22.[74]The power of absorbing water by wood varies from 9·37 to 174·86 per cent. in dry wood. In its ordinary state the power varies from 4·36 to 150·64 per cent. The quantity of water contained in wood in its natural state varies from 4·61 to 13·56 per cent.Vide‘Minutes of Proceedings of the Institution of Civil Engineers,’ vol. lvi. p. 300.[75]Vide‘Roads and Roadways,’ by George Waller Wilcocks, 1879, p. 34.[76]Vide‘Minutes of Proceedings of the Institution of Civil Engineers,’ vol. lviii. p. 82.[77]Ibid, vol. lx. p. 293.[78]Ibid, vol. lviii. p. 45.[79]‘Report upon Asphalte and Wood Pavements,’ by William Haywood, (1874) p. 44.[80]See ‘Paper on Wood Pavements,’ by R. S. Rounthwaite, Boro’ Surveyor, Sunderland, ‘Proceedings of the Association of Municipal and Sanitary Engineers and Surveyors,’ vol. vii. p. 48.[81]The surveyor of the parish of St. George’s Hanover Square, London, says, “My experience of wood, and I have laid down 25,000 yards, is that it is perfectly free from smells, even on a cab rank.” Report of a Committee of the Paddington Vestry on Wood and other Pavements (1878) p. 30.[82]Mr. Allnutt says on this: “As to the swelling of the wood, it has been remarked that even brick walls have been forced out. We do not see what provision can be made for this; but leaving the channel by the kerb stone for the last work may relieve the lateral pressure, and perhaps it would be as well for the blocks not to be so dry when being laid down.”Vide‘Wood Pavement as carried out on Kensington High Road, Chelsea, etc.’ by Henry Allnutt (1880) p. 15.[83]Vide‘Report on Asphalte and Wood Pavements,’ by William Haywood, 1874, pp. 38 and 41.[84]The Ligno-Mineral Paving Company and the Improved Wood Paving Company offered to maintain their pavements, if laid, for terms of ten years and fourteen years respectively; their tenders were not accepted.[85]Vide‘Paper on Street Carriageway Pavements,’ by G. F. Deacon, ‘Minutes of Proceedings of the Institution of Civil Engineers,’ vol. lviii. p. 23.[86]If the road material thus excavated is macadam, it may be screened and used as concrete in the foundation, if approved by the surveyor. The granite pitching of crossings, channel gutters, etc., must remain the property of the sanitary authority, as well as the surplus macadam.[87]It is important to give sufficient notice to gas and water companies in order that they may attend to their mains and services before the foundations are put in.[88]Sometimes about half an inch of fine sand is spread upon the surface of the concrete upon which the wood blocks are bedded.[89]If the blocks are to be creosoted, the number of pounds of creosote that should be absorbed in a cubic foot of the wood should be specified; this is generally about 10 lb. of creosote to 1 cubic foot of wood.
[67]The first wood pavement laid in London was in front of the Old Bailey, in 1839.
[68]Vide‘Minutes of Proceedings of the Institution of Civil Engineers,’ vol. i. p. 131.
[69]This specification is the company’s own, as advertised when they first began business; for many reasons the boards have since been discontinued, and other alterations introduced into the system.
[70]I am unable to ascertain if this plan has ever been tried anywhere.
[71]It is also affirmed by the Borough Surveyor of Sunderland that this process dispenses with watering.Vide‘Proceedings of the Association of Municipal and Sanitary Engineers and Surveyors,’ vol. iii. p. 72.
[72]Vide‘A Practical Treatise on Roads, Streets, and Pavements,’ by Q. A. Gillmore, p. 166, which see also for a good account of wood pavements in the United States.
[73]Vide‘Wood Pavements,’ by Henry Allnutt, 1880, p. 22.
[74]The power of absorbing water by wood varies from 9·37 to 174·86 per cent. in dry wood. In its ordinary state the power varies from 4·36 to 150·64 per cent. The quantity of water contained in wood in its natural state varies from 4·61 to 13·56 per cent.Vide‘Minutes of Proceedings of the Institution of Civil Engineers,’ vol. lvi. p. 300.
[75]Vide‘Roads and Roadways,’ by George Waller Wilcocks, 1879, p. 34.
[76]Vide‘Minutes of Proceedings of the Institution of Civil Engineers,’ vol. lviii. p. 82.
[77]Ibid, vol. lx. p. 293.
[78]Ibid, vol. lviii. p. 45.
[79]‘Report upon Asphalte and Wood Pavements,’ by William Haywood, (1874) p. 44.
[80]See ‘Paper on Wood Pavements,’ by R. S. Rounthwaite, Boro’ Surveyor, Sunderland, ‘Proceedings of the Association of Municipal and Sanitary Engineers and Surveyors,’ vol. vii. p. 48.
[81]The surveyor of the parish of St. George’s Hanover Square, London, says, “My experience of wood, and I have laid down 25,000 yards, is that it is perfectly free from smells, even on a cab rank.” Report of a Committee of the Paddington Vestry on Wood and other Pavements (1878) p. 30.
[82]Mr. Allnutt says on this: “As to the swelling of the wood, it has been remarked that even brick walls have been forced out. We do not see what provision can be made for this; but leaving the channel by the kerb stone for the last work may relieve the lateral pressure, and perhaps it would be as well for the blocks not to be so dry when being laid down.”Vide‘Wood Pavement as carried out on Kensington High Road, Chelsea, etc.’ by Henry Allnutt (1880) p. 15.
[83]Vide‘Report on Asphalte and Wood Pavements,’ by William Haywood, 1874, pp. 38 and 41.
[84]The Ligno-Mineral Paving Company and the Improved Wood Paving Company offered to maintain their pavements, if laid, for terms of ten years and fourteen years respectively; their tenders were not accepted.
[85]Vide‘Paper on Street Carriageway Pavements,’ by G. F. Deacon, ‘Minutes of Proceedings of the Institution of Civil Engineers,’ vol. lviii. p. 23.
[86]If the road material thus excavated is macadam, it may be screened and used as concrete in the foundation, if approved by the surveyor. The granite pitching of crossings, channel gutters, etc., must remain the property of the sanitary authority, as well as the surplus macadam.
[87]It is important to give sufficient notice to gas and water companies in order that they may attend to their mains and services before the foundations are put in.
[88]Sometimes about half an inch of fine sand is spread upon the surface of the concrete upon which the wood blocks are bedded.
[89]If the blocks are to be creosoted, the number of pounds of creosote that should be absorbed in a cubic foot of the wood should be specified; this is generally about 10 lb. of creosote to 1 cubic foot of wood.
The word asphalte in its generally accepted sense implies a natural rock consisting of pure carbonate of lime, intimately combined and impregnated with mineral bitumen in very variable proportions; that used for roads or footpaths should not contain less than 7 or more than 12 per cent. of bitumen.
The rock when broken takes an irregular fracture without definite cleavage; it is principally derived from Val de Travers, Seyssel, Sicily, Chieti, Auvergne, Lobsann, and Limmer. Its grain should be regular and homogeneous, the finer the grain the better.[90]
When exposed to the atmosphere asphalte gradually assumes a grey tint, by reason of the bitumen evaporating from the surface leaving a thin film of limestone behind. The stone is usually taken from open quarries, but at Val de Travers shafts are sunk and the general treatment is similar to a coal mine.
Bitumen, it must be borne in mind, is itself a mineral product found in Trinidad and some other places; it is composed of carbon, hydrogen, and oxygen.
The weight of a cubic yard of natural asphalte is about 3874 lbs., its specific gravity is 2·114, but this of course varies with its percentage of bitumen.
The following is a test for asphalte given by Mr. Deland in a paper he read before the Institution of Civil Engineers in the year 1880.[91]
“A specimen of the rock freed from all extraneous matter, having been pulverised as finely as possible, should be dissolved in sulphurate of carbon, turpentine, ether or benzine, placed in a glass vessel and stirred with a glass rod. A dark solution will result, from which will be precipitated the pulverised limestone. The solution of bitumen should then be poured off. The dissolvent speedily evaporates, leaving the constituent parts of the asphalte, each of which should be weighed so as to determine the exact proportion. The bitumen should be heated in a lead bath and tested with a porcelain or Baumé thermometer to 428° Fahr. There will be little loss by evaporation if the bitumen is good, but if bituminous oil is present the loss will be considerable—gritted mastic should be heated to 450° Fahr. The limestone should next be examined. If the powder is white and soft to the touch it is a good component part of asphalte, but if rough and dirty on being tested with reagents it will be found to contain iron pyrites, silicates, clay, etc. Some asphaltes also are of a spongy or hygrometrical nature. Thus, as an analysis which merely gives so much bitumen and so much limestone may mislead, it is necessary to know the quality of the limestone and of the bitumen.
“For a good compressed roadway an asphalte composed of pure limestone and 9 to 10 per cent. of bitumen, non-evaporative at 428° Fahr., is the most suitable. Asphaltes containing much more than 10 per cent. of bitumen get soft in summer and wavy, those containing much less have not sufficient bind for heavy traffic, although asphalte containing 7 per cent. of bitumen properly heated does well for court yards, as it sets hard when cold.”
For roadways “compressed” asphalte should be used and not “mastic,” which is only fitted for footpaths, court-yards, etc. Compressed asphalte roadways are constructed as follows:
The asphalte rock is first crushed in a “Blake’s” or othersuitable crusher, then pulverised in what is known as a “Carr’s disintegrator,” until it is reduced to a powder; this powder is then heated up to between 212° and 250° Fahr. in revolving cylinders and is laid about 2¹⁄₂ inches in thickness upon a concrete foundation previously prepared for its reception, the powder is carefully raked to the required contour and then either rolled or punned with iron punners previously heated to prevent the adhesion of the powder to them.
A roadway thus prepared presents many advantages over macadam, granite setts or wood, the following passage amply describing one of them:[92]
“An indispensable feature of a weight-carrying pavement must be the absolute exclusion of water at the surface as nearly as it can be insured, and in this one respect it cannot be questioned that a surface like asphalte has no equal, the absorption being so gradual as to be inappreciable during any possible continuance of moisture.”
In addition to this indisputable fact the advantage of durability is claimed for asphalte, but this must vary considerably with the quality of the material and of the work. Mr. Hayward estimates the life of an asphaltic Val de Travers compressed roadway at 17 years, and it is claimed for it that it will wear until it becomes quite thin, very heavy traffic breaking it up when it is worn to about ³⁄₄ of an inch thick. Another advantage claimed for asphalte is cleanliness, and this is evidently indisputable, as, being impervious, none but imported mud or dust can be formed upon it.
In addition to the foregoing the following advantages are also claimed:
Pedestrians can walk on asphaltic roadways as well as on the footways.
It is comparatively noiseless under traffic, though in thiscase wood is better, as the clatter of the iron-shod horses’ feet upon asphalte is very apparent.
It is expeditiously laid, and when repairs are necessary they can easily be effected; no pavement shows less signs of openings being made in it for gas and water-pipe repairs than asphalte.
The rapid laying causes less inconvenience to traffic in the streets.
Ease of traction; but here steps in the one great objection to asphalte as a roadway paving, viz. danger to horses by slipping and falling, of which I shall say more hereafter.
Cellars and vaults under the streets are kept dry, by reason of its impermeability to moisture.
Easily cleansed, especially by mechanical sweeping, and snow is easily removed.
It is very pleasing to the eye, being so uniformly regular and of good colour.
There is no vibration or concussion in travelling over it, and apart from the question of safety it is delightful to drive over it.
It is a cool pavement at night; it does not absorb heat during the day, and consequently none radiates from it after the sun has gone down.[93]
The great objection to asphalte as a material for roadways arises from the fact that it is extremely slippery when damp,[94]irrespective of temperature, and this in the climate of England is frequently the case. The result of this slipperiness is, that not only do horses frequently fall upon it, but it is also difficultto stop a horse when drawing a load, thus causing more risk to foot-passengers of being run over, and straining the horse considerably in its efforts. Again, in thoroughfares crowded with vehicular traffic, constant stoppages occur, and in starting again it is painful to witness the struggles of the horses to keep their footing and overcome the inertia of their load. When a horse falls he has very great difficulty in rising, but on the other hand, although he may be strained, a horse never breaks his knees upon this class of pavement. How far this might be altered ifallthe streets of a town were paved with asphalte, is a fair matter for argument, as it is asserted that horses are very nervous on going from one pavement to another, and accidents frequently happen in consequence.
The strewing of sand upon asphalte renders it less slippery, but in addition to the interference of the traffic whilst this is being done, there are the further objections, of the possible injury of the sand cutting into the asphalte, the expense of labour and materials, and the mud caused thereby which has afterwards to be removed. Another plan is to frequently wash the asphalte with water, but this is expensive and only of temporary benefit.
Another objection to asphaltic roadways is that they cannot with safety be constructed of greater gradient than 1 in 60, and it must also be borne in mind that fine weather is necessary both for the construction and repairs of a roadway of this description.
Very little smell, and that not of an unpleasant character, arises from the work when compressed asphalte is being used, the mastic is however temporarily unpleasant to those who dislike the odour.
With reference to the question of the cost of compressed asphalte for roadways: it is of course a matter depending upon local circumstances as to the first cost, but it must be remembered that the compressed asphalte hitherto laid hasbeen nearly all that of the Val de Travers Company, who charge a fixed price per square yard for laying according to thickness required, the distance of the locality from London, and other local circumstances. With reference to maintenance, this is a question dependent mainly upon traffic, but here again the company will undertake to keep in repair at so much per square yard per annum for a certain number of years.
It would, however, perhaps be a better plan not to enter into such an agreement, but to arrange for repairs under a schedule of prices, but this must greatly depend upon the character of the work in the first place, and other local considerations.
Mr. Ellice Clarke gives the following as the cost of Val de Travers compressed asphalte.[95]
The cost is reduced to 100,000 tons per annum per yard of width.
Nothing is charged for renewal, as the annual sum for maintenance provides the asphalte in perpetuity.[96]
The following table[97]may here be of use:
Table showing the agreed Cost per Annum of certainAsphalte Carriageway Pavements in the City of London.
The cost of foundations is included in this table, but their thickness is not mentioned; the excavation was done for the contractors.
With the one serious objection of slipperiness, compressed asphalte seems a most suitable material for the surface of a roadway, but that objection is of considerable weight when we reflect that the great object of roadways is that of “traffic,” and it is for that purpose they are constructed; still, in cities where a heavybusinesstraffic is going on, this class of roadway has so many advantages that where cheap horses are driven it might be used; where, however, valuable horses are used for pleasure driving, as in the west end of London and the corresponding better parts of cities, some other description of roadway should be maintained.
Mastic asphalte will be described in the chapter onfootpaths.
—The excavation and concrete[98]foundation may be specified to be executed in a manner similar to that contained in the specimen specification for wood paving,[99]except of course that the excavation will be shallower in this case.
—The asphalte to be used shall be the pure unadulterated natural rock known as the Val de Travers, and be unmixed with any foreign or other matter whatever. The rock after being properly broken, shall be ground in a Carr’s disintegrator to a powder of such fineness, that not more than per cent. shall be left on a sieve containing meshes to the square inch and decrepitation by heat will not be accepted. This powder shall be heated to 240° F. or such othertemperature as shall be found desirable, so as to eliminate all moisture, and carefully transported to the street in covered iron carts, in order that not more than 20° F. of heat shall be lost in transit. The powder must be spread upon the concrete inches in thickness[100]and carefully raked so as to have regularity of depth and surface.
—The powder must then be rammed with iron punners of not less weight than 10 lb. heated so as to prevent the adhesion of the asphalte. The ramming must be done lightly at first, so as to ensure equality of thickness, and afterwards augmented to heavy blows. Where the rammers are not available a T tool must be employed.
To meet some of the objections to compressed asphalte as a material for roadways the “Imperishable Stone Paving Blocks” have been introduced in America; they consist of asphalte formed into rectangular blocks under pressure of about one ton to the square inch, these are laid close together without any grouting, and a pavement of this description is said to combine all the advantages of wood and asphalte, though sufficient time has not yet elapsed to prove this.
In Salford, Manchester, etc., I believe “Woodward’s Patent Molten Ironstone Blocks” are used with some success where there is not any very exceptionally heavy traffic.
One of the principal reasons of durability in asphalte pavement is its elasticity, and it should be remembered that compressed asphalte does not begin to “wear” until all compression has ceased; this is the case with no other system of pavement—stone and wood both begin “wearing” from the day the traffic commences. Under ordinarily heavy traffic it may be estimated that it will take two years to complete the compression of asphalte, and the weight of a square foot of this pavement will at the expiration of that time be nearly the same as on the day it was laid, though the thickness isreduced during the first two years as much as it will be in the following eight.
Much is said about the advisability ofgoodanddryconcrete, but it may be as well to explain the reasons that necessitate so much care in the foundation. First, it should be always borne in mind that asphalte pavement is nothing more than a tough “carpet,” and has no power of itself of offering resistance to heavy traffic; consequently, if the substratum or concrete is not thoroughly solid and resisting, the weight of traffic will crush it, and the asphalte will at once give way in all directions. The concrete should be made strong enough to resist the traffic, and the asphalte is a simple covering to protect the concrete from direct contact with the wear and friction caused by the traffic. So much for the strength, but the dryness is of even still greater importance; for the best asphalte, laid by skilled workmen, on thoroughly first-rate but damp concrete, will rapidly go to pieces—a phenomenon takes place, which, although quite natural, is little realised by most engineers. When the hot asphalte is laid, the water is immediately sucked up and turned into steam, which tries to escape through the heated powder, and the result is that although the surface of the asphalte is smooth, the mass is really disintegrated from underneath by its bitter enemy “water,” and as soon as the surface begins to wear, the fissures formed by the passing of the steam appear on the surface and the whole pavement falls to pieces: thus accounting for some of the failures this description of roadway has met with under unskilled treatment.
This completes the subject of roadways; I will turn to that of footpaths in thenext chapter.