117 







unc*}mllr attend a deep cxcvUoii owing to the accumulation 

 water ; the remedy for which w described hereafter. Where em- 

 lrl'- are re>iuird. strong frucing U especially uecery to 



,m the occurrence of accident-. Some of the road* I 

 Tdford are conducted era* deep valley* by bridge* or viaduct* of 

 grml magnitude, in order to maintain the desired level without the 

 inonoreoteoce ao<l expeue of Urge earth embankment*. 



la old road* the bridge, erected for the pauage of mere are 

 freuuriitly made much amaller than is advisable, BO that the level of 

 Ute road u made too low, awl the water U impeded by the contracted 

 arena* to *uch a degree a* to occasion much damage during floods. 

 Modern lllniri by adopting bolder dimensions for the bridge*, and 

 forming rair.l apiiroache*, avoid thea inoooTenience*, and noore their 

 roach from the riak of obstruction by flood*. The railing of the road 

 wherever it paam through marahy or low land U a very necessary 

 ineamire. Many old road* still in u*s are sunk several feet below the 

 urfaoe of the ground, because they have originally been exposed to 

 the deetructire action of water, and the materials thus nftened have 

 been ground into mud and cleared away, until, by the repetition of 

 thM operation*, the road* hare been converted into deep trenches, 

 which are frequently flooded in winter. Of the extent to which Una 

 isjDOMi has been occasionally Buffered to go on, an idea may be formed 

 from the itatummt of Edgeworth, that " the stag, the hounds, ami the 

 horsemen hare been known to leap over a loaded waggon, iu a hollow 

 tray, without any obstruction from the vehicle." 



nducting a road through a mountainous district, in addition t > 

 srou* bridge* for the purpose of crossing ravines (for which 

 i suspension bridges have been occasionally applied, as in the 

 of the Menai strait), embankment* between retaining walls of 



tome, and wall* to upi>ort the road along the face of a precipice, are 

 frequently necessary. Some works of the latter character have excited 

 much admiration. If the slope of a precipice be only six inches 

 horizontal to a foot vertical, such a road may be formed by building a 

 will thirty feet high, based on steps cut into the rock, and cutting into 

 the rock to the depth of ten feet on a level with the top of the wall, 

 tbejpace between which and the face of the precipice is rilled in with 

 orth or (tone. By this means a platform twenty-five feet wide i* 

 obtained. Many work* of this character have been executed by 

 TVlford and other engineer*, in various parts of Scotland, iu the High- 

 laud roads, and those forming the communication between Edinburgh 

 and London : and others, the boldness of which commands universal 

 admiration, occur in the great mountain-passes of the Simplon and 

 Mont Ceois, which form imperishable monuments of the talent and 

 euetgj of the engineer* of Napoleon, by whom they were executed. 



n the work* are completed to the proper level for receiving the 

 Ji*nl material* that form the surface of the road, the earth should be 

 turned into the intended width and a nearly level surface, the foot- 

 path or path* being elevated a few inches above the bed of the 

 carriage-way. Thirty feet is the ordinary width of the carriage-way, 

 excluaire of footpaths, of the Holyhead road ; but the propriety of 

 reducing the width, in most places to twenty-four or twenty-five feet, 

 lus been suggested. This width may be more or less exceeded in 

 the vicinity of large towns, according to the amount of tratlie, but 

 cliould be exactly adhered to in other situations, as uniformity in 

 this particular greatly improves the appearance of a road, and also 

 contributes U economy, both as to the laud and materials, and 

 the cost of maintenance. Some engineers recommend that the bed 

 irtuMiH be made convex, in the same degree as the finished surface of 

 the road; but it is quite flat in the Holyhead road, by which means 

 greater depth of materials is allowed in the centre than at the sides 

 of the road. Much has been said on the subject of the best form 

 for the transverse section of a road. Formerly it was common to 

 make it very convex, often to a degree that was highly dangerous, with 

 the idea of throwing off water; but this notion is very fallacious, 

 because if a road be allowed to wear into ruts, no degree of convexity 

 that can be given is sufficient to keep it dry ; while, if the surface be 

 good, a very moderate slope is sufficient to carry off water, and a steep 

 inclination will cause it to run with such velocity as to wear away the 

 road materials. Another disadvantage of too great nn inclination is, 

 that, by throwing the weight of a carriage on one side, the vehicle 

 itself i* injured, and the overloaded wheels cut up the road more than 

 necessary. Some have gone so far in opposition to this practice as to 

 advocate perfectly flat or even concave roads, in favour of both of 

 which much may be said ; but the general practice of modern road- 

 makers i* to make the surface slightly convex. In Telford's roads the 

 convexity is elliptical, the fall being half an inch at four feet from the 

 centre, two inch at nine feet, and six inches at fifteen feet. It has 

 been recommended to form the cross section into three flat pl.mcs 

 that in the centre being horizontal, and the others slightly inclined 

 from it. Very narrow road* are often sloped in one direction only, 

 like one-half of a convex road ; and roads on the face of a steep hill 

 are occasionally treated in this manner, the surface water being con- 

 ducted toward* the hill, and carried off by drains under the road. 

 This plan ha* the advantage of checking any tendency in carriages to 

 roll or turn over toward* the least protected side of the road. 



Wherever the substratum of a road is wet and soft, great care is 

 necessary to make the bed solid. If the ground be boggy or marshy, 

 it is desirable to form an embankment of sufficient thickness to com- 



UCU1A 11:$ 



press the elastic foundation ; such embankment being some- 

 supported by faggots. Telford and most engineers recommend that 

 ramming with stone-chips should be resorted to where the lied is wet 

 und it|xiiiK>- ; and that where soft clay occurs, a stratum of earth 

 MIL! should be laid between it and the road materials, a precaution 

 which tends to diminish the injurious effect of frost on a road with a 

 clay bottom. Though great care is usually considered necessary in 

 order to obtain a firm foundation, McAdam and some others have not 

 only contended against its importance, but actually preferred, in cer- 

 tain cases, a yielding substratum to one of rock, ou the supposition 

 that the wear of the road is diminished by elasticity. Careful ol> 

 tions on the repairs of a road in Somersetshire, of which about 

 miles are supported by a morass, and five or six by limestone rock, 

 d a difference of expense in repair of about five to seven in 

 favour of the morass, though it was so soft that the vibration caused 

 by a carriage passing was sufficient to break the young ice in th 

 ditches ; but extended experience seems to confirm the more general 

 opinion in favour of a hard unyielding foundation. 



1 1. -ep ditches should be cut for the efficient drainage of the road, 

 which is of paramount importance ; and these should be on the field 

 side of the fences. They should extend to a depth of from 2 feet ti 

 inches to 4 feet below the bed of the road, according to the nature of 

 the ground. The earth thrown out from them is commonly used in 

 forming liauks for the hedges ; but in wet soils, where the ditch, 

 made hrgerand deeper than usual, the additional earth excavai 

 applied to raising the bed of the road. Where brick or stone-Co 

 drains are substituted for open channels, it is usual to build them with 

 open joints, to allow the passage of water through the sides. < 

 drains of masonry are introduced at intervals to connect the f\< It- 

 channels, and numerous minor drains filled with rubble stones 01 

 gravel arc formed iu the bed of the road. The latter are frequently of 

 the kind called mitre-drains, which are made V shaped in plan, diverg- 

 ing from the centre of the bed, and extending diagonally to the 

 their angle being regulated by the longitudiual slope of the road, K.I 

 that their inclination may not exceed 1 in 100. These may lie j 

 about sixty yards apart, or closer in wet soils, and they receive the 

 water that filters through the surface materials. In cuttings or exca- 

 vations it is advisable to make drains to catch the water descending 

 from the sides, and prevent its reaching the surface of the road. 



In treating of the choice and application of the hard mai 

 which compose the surface of the road, the formation of j,t- 

 roads, or those made of broken stone and similar materials, will lie 

 first considered; and afterwards that of the principal varieties . .1' 

 pavement. 



Metalled or Broken-Stone llxuLs. In the formation of metalled roads 

 the system adopted in the great works of Telford and his followers, is 

 the one most generally adopted by English engineers. The distin- 

 guishing characteristic of this system is the use of a rough pavement 

 of hand-laid stones on the bed of the road, to support the small broken 

 stone of which the surface is composed. In the very imperfect mode 

 of road-making formerly practised (which scarcely deserves the name 

 of a system), it was very common to cover a bad road with a 

 quantity of stones, often unbroken, and generally of very in 

 dimensions. These stones, owing to their rounded form and the soft- 

 ness of the substratum, never consolidated into a hard surface, and in 

 course of time sunk into the soft earth beneath, which worked up 

 among the stones in the form of mud. Thus enormous quanti 

 stone were used without producing a good road, the stone Kinking inio 

 the earth to a surprising extent. This evil is greatly diminished by 

 good drainage, and by the use of stones of uniform size broken into 

 angular pieces, which have a tendency to lock together into a hard and 

 compact mass, a fact of the highest importance iu the scie 

 road-making, and which appears to have been first prominently 1 : 

 forward by the late Mr. McAdam. A great extent of excellent road 

 has been made on the plan advocated by McAdam, who considered 

 paving unnecessary, and laid the broken stone immediately on the 

 surface of the earth, depending on its forming a hard crust imp. 

 to water, so that the earth, being always kept dry, may have no ten- 

 dency to work up among the metal or broken stone. McAdar, 

 no stones exceeding six ounces in weight, and gyre the preference to 

 those of about one ounce, or an inch diameter, which he spread over 

 the road in thin layers, each being worked over by carriages till in 

 some degree consolidated; and ho objected to the use of chalk or earth 

 mixed 'with the stone for the purpose of binding it together. He 

 considered a thickness of ten inches of broken stone, well < 

 to be sufficient for bearing any load, even where tile foundatio 

 morass, in which c,'e he considered no intermediate substance 

 sary. Near Bristol, a road in which the metalling had worn down to a 

 thickness of only four inches, was found to have kept the substratum 

 of earth perfectly dry. But, satisfactory as this plan of road-making 

 has proved in some cases, there are others iu which it ha 

 some in which a very large quantity of stone has been applied before a 

 firm road could be obtained. A road from Lewes to Eastbourne. 

 on Mr. McAdam's principle, is said to have required three i 

 materials in many parts before it was consolidated, though i 

 ultimately brought into a good state. 



The system of pitching or paving the bottom of a road has the 

 advantage of preventing the subsoil from working up among the road 



