Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/608

Rh 584 ROADS coating of a well-maintained road the proportion of stones of various sizes varies, but generally from one-third to one-half is found to consist of detritus under three-eighths of an inch in diameter, and there is a very constant proportion of about one-fifth of mud and detritus under one-thirtieth of an inch in diameter. This appears to be the amount necessary to fill the voids between the fragments of stone when compacted together. In an ill-kept road, from which the mud is not removed, the proportion of detritus is much higher, and mud may constitute nearly one-half of the coating. In pro- portion as the detritus and mud are kept down to the minimum by constant removal from the surface, so will the road be able to resist the action of wet and frost and the wear of the traffic, rear. The wear of materials, resulting in their gradual reduction to detritus, is due to the joint action of the traffic and the weather, which cause surface wear, wear arising from cross breaking, and from rubbing of the stones together. When there is no movement in the body of the road, and the wear is confined to the crushing and grinding at the surface, it is the least possible ; but, when a road is weak from insufficient thickness or solidity on a yielding foundation, bending and cross breaking of the coating take place under passing loads in addition to the surface wear, and the effects are aggravated by the softening action of water finding its way into the road through cracks formed in the surface and by the disinte- grating action of frost. The wear and waste are thus far larger than on roads of sufficient strength, properly maintained. The destruc- tive effect of wheels is greater as the diameter is less, and to a much greater degree as the tire is narrower. On hard and strong roads no greater width of wheel than 4 inches is useful, as a wider tire does not bear evenly, but on yielding roads a greater width is of some advantage, though it does not prevent damage from bending and cross breaking of the whole coating under excessive loads. A good deal of attention has been given by French engineers to the measurement of traffic, wear, and the consumption of road materials. Without a knowledge of the amount of traffic accurate comparisons of wear are impossible, and an account of the traffic on the roads of France is taken periodically in "collars" or horses drawing loads, and in the weight drawn. Traffic as measured by weight drawn has of late been observed in some of the streets of London and Liverpool, and has been reduced for comparison to the weight per foot or yard of width of the carriage-way. Wear may be measured by loss of thickness in the coating; but the loss of stone in proportion to detritus must also be ascertained before all the effects of wear can be determined. The accurate measurement of wear as practised by the French engineers is a complicated process, and it must suffice here to state that measured by thickness the wear is seldom found to exceed half an inch, or on the most fre- quented roads of France one inch, of consolidated surface per year, and that about 100 cubic yards of good materials per mile per year are considered as the average consumption under 100 collars of traffic per day. Observations in the United Kingdom on roads well and systematically maintained have confirmed these results, fainte- The new materials may be added to the road either in thin coats ance. and small patches year by year or in a thick coat consolidated by rolling. The first method, by which the wear is replaced annually and the traffic is depended on to work the materials into the road, can be followed with excellent results, and at no great inconveni- ence to the public under proper management when the traffic is not excessive. Considerable care in the use of materials is required that none may be unnecessarily applied. The annual employment of one-fifth or one-sixth the quantity which it would take to cover the whole surface one stone in thickness is often sufficient to replace wear, and it will then take five or six years to coat every part of the road if it is covered regularly. It is therefore important to apply the new materials only where they are needed, and not to use them where the road is already sufficiently thick. The irregularity of wear and of thickness enables a good roadman to judge where new materials must be applied, and he will apply them in small quanti- ties wherever weak places appear. To facilitate this the materials should be placed in heaps by the roadside in the summer, and they should be carefully spread in the autumn and attended to after- wards to ensure consolidation without waste. By good manage- ment a large quantity of materials may be incorporated in a road before the middle of the winter without harassing the traffic, and the strength may not only be maintained but increased. On a hard strong road consolidation may be aided by loosening the sur- face with a pick ; generally only the margin of a patch need be picked up. But if the road is soft or weak it is better not to disturb the surface at all. Binding may sometimes be used to aid con- solidation, but it is seldom necessary if the materials are properly laid and attended to, as the coating already contains detritus enough. In the second method a coating of materials is laid on at once sufficient to endure the wear of several years with such slight repairs as may be necessary to keep a good surface, and, when the wear has gone as far as it can be safely allowed to go, the process is repeated. Unless the wear is very considerable there is no economy in this method, though the convenience to the public, especially in towns, is undeniable. Consolidation by rolling (after the manner already described) is essential, and it is generally desirable to loosen the old surface to ensure the incorporation of the new coating with it. Scraping and attention are required between one coating and another and also slight repairs to the surface, as, however well the materials may be laid and rolled, the wear of the ordinary traffic will search out places which have escaped the full pressure of the roller and produce inequalities. Besides a regular application of new materials to replace wear, there must be in road maintenance on proper principles a systematic removal of the detritus by scraping or sweeping, which must be regarded as keeping the whole coating in proper condition, and not as mere surface cleansing. The wear should also be reduced as far as possible by providing sufficient thickness to carry the traffic, by keeping an even surface on which water can never stand and soak, and by good drainage both of surface and subsoil. An adequate amount of skilled manual labour is necessary for economy of main- tenance, and this and the constant attention which is required to keep a road in good order are best secured by putting a man in permanent charge of a defined length. In the autumn and winter, when more labour is wanted, extra men should work under the directions of the permanent road labourer, whose knowledge of his length of road will enable him to employ them to the best ad vantage. Concrete macadam, formed by grouting with lime or cement Concret mortar a coat of broken stone laid over a bed of stone previously and tar well rolled, has been tried as an improvement on an ordinary mac- macadamized surface, but not hitherto with much success. When adara. cleanliness is of importance, and great durability is not required, tar macadam or bituminous concrete may be usefully employed. It is sometimes made by first spreading a coating of broken stone and consolidating it by a roller, and then pouring over it a mixture of coal-tar, pitch, and creasote oil, upon which a layer of smaller stone is spread and rolled in, and the surface finished with stone clappings rolled in. More usually the broken stone and bituminous mixture are well incorporated together before they are spread, the stone sometimes being previously heated. The lower layer, about 4 inches thick, may be of stone broken to 2^ inches gauge, and the next layer, about 2 inches thick, may be of smaller stone. Each layer must be well rolled, and when perfectly solid a thin coating of fine stone or granite drippings is spread over the surface and rolled in. Hard limestone is found to be more suitable than sili- cious or igneous rocks for this material. A road surface well made in this manner will last several years under light traffic without any repairs, and it can easily be patched when necessary. Stone Pavements. Early pitched roadways consisted of pebbles STONB or rounded boulders, bedded in the natural surface or in sand or PAVn- gravel. The next step in advance was to employ roughly- squared MESTB. blocks ; but the wide and irregular joints admitted the water to the subsoil, and the mud worked up and the stones sank irregu- larly under the traffic. Telford, who was called upon to report on the street pavements of the parish of Hanover Square in 1824, saw the necessity of cutting off all connexion between the subsoil and the paving stones. He recommended a bed of about 6 inches of Found clean river ballast, rendered compact by being travelled upon for tton. some time before the paving was laid, but he subsequently con- sidered that nothing short of 12 inches of broken stone, put on in layers 4 inches thick and completely consolidated by carriages passing over them, would answer the purpose. He recommended paving stones of considerable depth and of from 4 to 6 or 7^ inches in breadth for the greatest thoroughfares, and he pointed out the importance of working the stones flat on the face and square on all sides, so as to joint close and preserve the bed or base as nearly as possible of the same size as the face, and of carefully placing together in the same course stones of equal breadth. Many pavements thus laid with stones of considerable breadth still re- main, but experience proved that it was a mistake to suppose that broad stones having a larger base would support better the weight and shocks of heavy traffic ; on the contrary, a wide stone has a tendency to rock on its bed, and also to wear round on the top and become slippery. To obtain an evener surface and a better foot- hold for the horses the stones were reduced in width, and in 1840 a granite pavement was laid by Walker on Blackfriars Bridge, which may be considered the first of modern set pavements. The stones were 3 inches broad and 9 deep ; they were laid on a bed of concrete 1 foot thick and were jointed with mortar. The reduc- tion of breadth to about 3 inches was generally followed, but it is only of late years that a concrete foundation has been employed to any great extent, the frequent breaking up to which streets are subject having prevented it. In London a foundation of broken stone has been continued in the chief thoroughfares, the sets being evenly bedded in gravel upon it and rammed with a heavy wooden rammer. Hard core a mixture of broken stone, clinker, brick rubbish, and old building materials has also been largely used to form a foundation. In the northern towns of England cinders have been employed, and where the traffic is exceptionally heavy a pitched foundation of stones on edge has been laid when the sets were not paved upon an old macadamized surface. The con- crete for a foundation to a paved street should be made with the