Читать книгу Remarks on the Present System of Road Making - John Loudon McAdam - Страница 13

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Having communicated to your Honourable Board, some observations on making and repairing roads, in February, 1819, I beg leave to add the following, which have arisen from increased experience on the subject, and also from a desire of calling your attention to the effects of the late severe winter on the roads of the country, and the confirmation afforded to the opinions I have endeavoured to introduce on the construction of roads.

During the late winter, and particularly in the month of January, 1820, when the frost was succeeded by a sudden thaw, accompanied by the melting of snow, the roads of the kingdom broke up in a very alarming manner, and to an extent that created great loss and inconvenience by the interruption of communication, and the delay of the mails, and also occasioned a very heavy extra expenditure by the Post-office.

The obvious cause of this defect of the roads, was the admission of water from the loose and unskilful method of their construction. Previous to the severe frost, the roads were filled with water, which had penetrated through the ill-prepared and unskilfully laid materials: this caused an immediate expansion of the whole mass during the frost, and upon a sudden thaw, the roads became quite loose, and the wheels of carriages penetrated to the original soil, which was also saturated with water, from the open state of the road. By this means, many roads became altogether impassable, while the whole were rendered deep and inconvenient to be travelled upon.

In particular, it was observed, that all the roads of which chalk was a component part, became, generally, impassable; and even, that the roads made over chalk soils gave way in most places. This evidently proceeded from the absorbent quality of chalk, which renders it so tenacious of water, that I consider its use to be one of the most dangerous errors in road making. I was induced on former occasions to recommend particular care in making roads over chalk soils, and to advise a discontinuance of the practice of mixing chalk, clay, or any other matter that holds water, with the materials of a road. The experience of last winter has confirmed this opinion, and has shewn the ruinous effects of the former method.

Of all the roads which have been thoroughly re-made, according to the directions which I had the honour to submit to your Honourable Board last spring, not one has given way, nor has any delay taken place through the severity of the late season.

As every winter has, in some degree, presented such inconveniences, and as it has been observed that very severe winters occur in England every six or seven years, it is of great consequence to consider of the means of constructing the roads of the kingdom in such a manner as shall prevent their being in future affected by any change of weather or season.

The roads can never be rendered thus perfectly secure, until the following principles be fully understood, admitted, and acted upon: namely, that it is the native soil which really supports the weight of traffic: that while it is preserved in a dry state, it will carry any weight without sinking, and that it does in fact carry the road and the carriages also; that this native soil must previously be made quite dry, and a covering impenetrable to rain, must then be placed over it, to preserve it in that dry state; that the thickness of a road should only be regulated by the quantity of material necessary to form such impervious covering, and never by any reference to its own power of carrying weight.

The erroneous opinion so long acted upon, and so tenaciously adhered to, that by placing a large quantity of stone under the roads, a remedy will be found for the sinking into wet clay, or other soft soils, or in other words, that a road may be made sufficiently strong, artificially, to carry heavy carriages, though the sub-soil be in a wet state, and by such means to avert the inconveniences of the natural soil receiving water from rain, or other causes, has produced most of the defects of the roads of Great Britain.

At one time I had formed the opinion that this practice was only a useless expence, but experience has convinced me that it is likewise positively injurious.

It is well known to every skilful and observant road-maker, that if strata of stone of various sizes be placed as a road, the largest stones will constantly work up by the shaking and pressure of the traffic, and that the only mode of keeping the stones of a road from motion, is to use materials of a uniform size from the bottom. In roads made upon large stones as a foundation, the perpetual motion, or change of the position of the materials, keeps open many apertures through which the water passes.

It has also been found, that roads placed upon a hard bottom, wear away more quickly than those which are placed upon a soft soil. This has been apparent upon roads where motives of economy, or other causes, have prevented the road being lifted to the bottom at once; the wear has always been found to diminish, as soon as it was possible to remove the hard foundation. It is a known fact, that a road lasts much longer over a morass than when made over rock. The evidence produced before the Committee of the House of Commons, shewed the comparison on the road between Bristol and Bridgwater, to be as five to seven in favour of the wearing on the morass, where the road is laid on the naked surface of the soil, against a part of the same road made over rocky ground.

The practice common in England, and universal in Scotland, on the formation of a new road, is, to dig a trench below the surface of the ground adjoining, and in this trench to deposit a quantity of large stones; after this, a second quantity of stone, broken smaller, generally to about seven or eight pounds weight; these previous beds of stone are called the bottoming of the road, and are of various thickness, according to the caprice of the maker, and generally in proportion to the sum of money placed at his disposal. On some new roads, made in Scotland, in the summer of 1819, the thickness exceeded three feet.

That which is properly called the road, is then placed on the bottoming, by putting large quantities of broken stone or gravel, generally a foot or eighteen inches thick, at once upon it.

Were the materials of which the road itself is composed, properly selected, prepared, and laid, some of the inconveniences of this system might be avoided; but in the careless way in which this service is generally performed, the road is as open as a sieve to receive water; which penetrates through the whole mass, is received and retained in the trench, whence the road is liable to give way in all changes of weather.

A road formed on such principles has never effectually answered the purpose which the road-maker should constantly have in view; namely, to make a secure, level flooring, over which carriages may pass with safety, and equal expedition, at all seasons of the year.

If it be admitted, as I believe it is now very generally, that in this kingdom an artificial road is only required to obviate the inconvenience of a very unsettled climate; and that water with alternate frost and thaw, are the evils to be guarded against, it must be obvious that nothing can be more erroneous than providing a reservoir for water under the road and giving facility to the water to pass through the road into this trench, where it is acted upon by frost to the destruction of the road.

As no artificial road can ever be made so good, and so useful as the natural soil in a dry state, it is only necessary to procure, and preserve this dry state of so much ground as is intended to be occupied by a road.

The first operation in making a road should be the reverse of digging a trench. The road should not be sunk below, but rather raised above, the ordinary level of the adjacent ground, care should at any rate be taken, that there be a sufficient fall to take off the water, so that it should always be some inches below the level of the ground upon which the road is intended to be placed: this must be done, either by making drains to lower ground, or if that be not practicable, from the nature of the country, then the soil upon which the road is proposed to be laid, must be raised by addition, so as to be some inches above the level of the water.

Having secured the soil from under water, the road-maker is next to secure it from rain water, by a solid road, made of clean, dry stone, or flint, so selected, prepared, and laid, as to be perfectly impervious to water: and this cannot be effected, unless the greatest care be taken, that no earth, clay, chalk, or other matter, that will hold or conduct water, be mixed with the broken stone; which must be so prepared and laid, as to unite by its own angles into a firm, compact, impenetrable body.

The thickness of such road is immaterial, as to its strength for carrying weight; this object is already obtained by providing a dry surface, over which the road is to be placed as a covering, or roof, to preserve it in that state: experience having shewn, that if water passes through a road, and fill the native soil, the road, whatever may be its thickness, loses its support, and goes to pieces.

In consequence of an alteration in the line of the turnpike road, near Rownham Ferry, in the parish of Ashton, near Bristol, it has been necessary to remove the old road. This road was lifted and relaid very skilfully in 1816; since which time it has been in contemplation to change the line, and consequently, it has been suffered to wear very thin. At present it is not above three inches thick in most places, and in none more than four: yet on removing the road it was found, that no water had penetrated, nor had the frost affected it during all the late winter; and the natural earth beneath the road was found perfectly dry.

Several new roads have been constructed an this principle within the last three years. Part of the great north road from London by Hoddesdon in Hertfordshire—two pieces of road on Durdham Down, and at Rownham Ferry, near Bristol—with several private roads, in the eastern part of Sussex.

None of those roads exceed six inches in thickness, and although that on the great north road is subjected to a very heavy traffic, (being only fifteen miles distant from London) it has not given way, nor was it affected by the late severe winter; when the roads between that and London became impassable, by breaking up to the bottom, and the mails and other coaches were obliged to reach London by circuitous routes. It is worthy of observation, that these bad roads cost more money per mile for their annual repair, than the original making of this useful new road.

Improvement of roads, upon the principle I have endeavoured to explain, has been rapidly extended during the last four years. It has been carried into effect, on various roads, and with every variety of material, in seventeen different counties. These roads being so constructed as to exclude water, consequently none of them broke up during the late severe winter; there was no interruption to travelling, nor any additional expense by the Post-office in conveying the mails over them, to the extent of upwards of one thousand miles of road.

Many new roads, and to a considerable extent, are projected for the ensuing season. Some of them are to be assisted by grants or loans from government, and it will be a great saving of property, and enable government to extend their assistance more effectually, if these roads be made in the most approved and economical manner.

The unnecessary expense attending the making of new roads in the manner hitherto practised, is one great cause of the present heavy debt upon the road trusts of the kingdom. The principal part of the large sums originally borrowed, have been sunk in the useless, and in my opinion, mischievous preparation, of a foundation. This debt presses heavily on the funds of all the roads in England, and, in many cases, absorbs almost their whole revenue in payment of interest. In Scotland this pressure is still more heavily felt: indeed it is not of uncommon occurrence in that country, for creditors to lose both principal and interest of their loans to roads.

This causes not only a great and unnecessary loss in the first instance, and a deficiency of means for ordinary repair, and maintenance of the roads, but it also discourages the formation of new roads. Were a better and more economical system generally adopted and acted upon, many great additions and improvements of the communications of the country would take place, from which, at present, the landholders are deterred, by fear of the extent of the expense, and the difficulty of obtaining loans of money.

The measure of substituting pavements, for convenient and useful roads, is a kind of desperate remedy, to which ignorance has had recourse. The badness, or scarcity of materials, cannot be considered a reasonable excuse; because the same quantity of stone required for paving, is fully sufficient to make an excellent road any where: and it must be evident, that road materials of the best quality may be procured at less cost than paving stone.

The very bad quality of the gravel round London, combined with want of skill and exertion, either to obviate its defects, or to procure a better material, has induced several of the small trusts, leading from that city, to have recourse to the plan of paving their roads, as far as their means will admit. Instead of applying their ample funds to obtain good materials for the roads, they have imported stone from Scotland, and have paved their roads, at an expense ten times greater than that of the excellent roads lately made on some of the adjoining trusts. Very few of these pavements have been so laid as to keep in good order for any length of time; so that a very heavy expense has been incurred without any beneficial result, and it is to be lamented that this wasteful and ineffectual mode is upon the increase in the neighbourhood of London.

This practice has also been adopted in places where the same motive cannot be adduced: in Lancashire, almost all the roads are paved at an enormous cost, and are, in consequence, proverbially bad. At Edinburgh, where they have the best and cheapest materials in the kingdom, the want of science to construct good roads, has led the trustees to adopt the expedient of pavements, to a considerable extent; and at an expense hardly credible, when compared with what would have been the cost of roads on the best principles.

The advantages of good roads, when compared with pavements, are universally acknowledged; the extension of pavement is therefore to be deprecated as an actual evil, besides the greatness of the expence. Pavements are particularly inconvenient and dangerous on steep ascents, such as the ascent to bridges, &c. A very striking example of this may be observed on the London end of Blackfriars Bridge, where heavy loads are drawn up with great difficulty, and where more horses fall and receive injury, than in any other place in the kingdom. The pavement in such places should be lifted, and converted into a good road; which may be done with the same stone, at an expense not exceeding ten-pence per square yard. This road would be more lasting than the pavement, and, when out of order, may be repaired at less than one-tenth of the expense which relaying the pavement would require.

This measure has been adopted with great success, and considerable saving of expense, in the suburbs of Bristol, where the pavements were taken up, and converted into good roads, about three years ago.

The advantages of the system recommended is so obvious to common observation in the repair of old roads, and has been practised to an extent so considerable, during the last four years, that the minds of most people have become reconciled to it; and objections, founded on old prejudice and suspicion, have given way to experience, but the application of the same principles to the construction of new roads, has necessarily been much more limited. It will, therefore, require more liberality and confidence on the part of country gentlemen, and also more patient investigation of the principles on which the system is founded, before they will allow of its adoption on new lines of road. It is to be hoped, however, that the importance of the subject will recommend it to general consideration.