129 



ROAD. 



ROAD. 



130 



materials, and, when well executed, of distributing the pressure o: 

 carriages over a larger base ; while the size of the paving-stones them 

 selves prevents their Hinting into the earth, as small stones are liable 

 to do. The pavement also acts as a drain to the surface materials. 

 In addition to these, the plan has, in many situations, the advantage ol 

 economy, as the cost of a pavement is considerably less than that 

 of an equal depth of well-broken stone. In most, if not all, of the 

 cases in which the paving system has failed, the want of success may 

 be attributed to very imperfect execution ; as, if the stones are very 

 irregular in size or badly set, or the thickness of metal is insufficient 

 to protect the pavement from the shake of passing carriages, the stones 

 become deranged, and the subsoil, working up among them, quickly 

 -l .'.-;. : . 



The case of the Highgate Archway road, which has been before 

 alluded to, is a remarkable illustration of the absolute necessity of a 

 firm bottoming under some circumstances. This road is over a subsoil 

 of sand, clay, and gravel, and, being partly in a deep cutting (originally 

 intended for a tunnel), is much exposed to the influx of water. The 

 road, which w rather more than a mile and a half long, was originally 

 made of a quantity of gravel and sand laid on the natural soil, and 

 covered with broken flints and gravel ; but this plan not succeeding, 

 the road was taken up, and pieces of waste tin were laid on the sub- 

 soil, over which were spread gravel, flints, and broken stone. This 

 expedient did not produce the desired effect, and at length, in 1829, 

 the road was placed under the management of the Holyhead-road 

 Commissioners, iU proprietors having failed, notwithstanding an enor 

 mous outlay, and the application of 1 200 cubic yards of gravel annually, 

 to bring it into a satisfactory state. A thick coat of broken granite 

 Was spread on a portion of the road ; but, owing to toe unsoundness of 

 the foundation, it never consolidated, the stones wearing into smooth 

 pebbles by their attrition against one another, even down to the bottom 

 of the mass. The commissioners therefore determined, as paving-stones 

 could not bn procured without great expense, to lay a coating of Roman 

 cement and gravel as a bed for the road-metal, an experiment which 

 was attended with complete success. The work was executed by 

 Macneill, and consists of a composition of Roman cement with eight 

 times its quantity of washed gravel and sand, which, after being mixed 

 in a box, was laid on the bed of the road to a thickness of six inches 

 and a width of about eighteen feet. A few minutes after being laid, 

 tli- upper surface was indented, by means of a triangular piece of wood 

 sheeted with iron, with numerous channels or grooves, sloping about 

 three inches from the centre to the sides, these channels serving for 

 the stones to lie and fasten in, and for conducting any water that 

 might percolate through them into the side drains. This measure, 

 combined with an extraordinary extent of drainage, amounting in the 

 whole to a length of 12,303 yards, proved so complete a remedy that, 

 in the first winter after the cement was laid, coaches were able to go 

 up with four horses at a trot with the heaviest loads, though before 

 ment six horses had mounted with difficulty at a walking 

 pace. The effect of the alteration on the wear of the road was equally 

 satisfactory, four inches of quart* being worn away on the old bottom 

 while only half an inch of the same stone was worn where laid on the 

 cement foundation. The expenses of laying the cement composition, 

 including the formation of the bed of the road, was about ten shillings 

 per lineal yard, part of the gravel used being old. Macneill estimate I 

 the cost at from twelve to fifteen shillings per yard if new gravel were 



j ;:::..-. i 



The e8Vct of a |>ared or concrete foundation in diminishing the 

 draught appears, from the subjoined statement, founded on experi- 

 ment* with Mr. Macneill's road indicator, to be very great; but a more 

 extensive series of trials is desirable for a comparison of different sys- 

 tem* under various circumstances. The draught of a waggon weighing 

 21 cut. was found to be as. follows : 



On s well-mull- pavement SI lb. 



On s ro.il with iU Inches of hard broken itono on a rough 



ptvement 4.6 



On a timiUr road, with s foundation of Roman cement and 



travel In lien of parrmrnt . . . t . . . 4S 



Oa s rad with s thick coal inn of broken (Urns on earth . . 03 



OB s road with s thick coating of gravel on tsrth . . . 147 



the formation of the pavement of a metalled road, almost any 

 hard stone that may be easily dressed with the hammer may be used. 

 The stones should be tolerably regular in size, and laid in rows with 

 their broadest face downwards, the interstices being carefully filled up 

 with stone-chippings, so as to pin the whole pavement together, and 

 effectually prevent the earth from working up through the joints. In 

 one of Telford's specifications for the Holyhead road, the dimensions 

 of the stone* for a pavement 30 feet wide are given as 7 inches deep 

 in the middle of the road, 5 inches at 9 feet from the centre, 4 inches at 

 12 feet, and 3 inches at 15 fifteen feet, the stones to be laid lengthwise 

 across the road, and the upper edge in no case to exceed 4 inches wide. 

 All irregularities are to be broken off by the hammer, and the stone- 

 ised in packing the joints are directed to be wedged in by hand 

 or with light hammers. No ramming is necessary, and it is desirable 

 nt carts which are used in the conveyance of the road materials 

 from being drawn upon the pavement before it is covered with broken 

 stooe. 



ABTB AD SCI DIV. VOL. VII. 



Some road-makers use a pavement even on a substratum of rock, 

 where it is uneven, but in many cases it is unnecessary ; although, if 

 the surface be smooth, it should be picked to a degree of roughness 

 similar to that of a pavement, in order that the road materials may not 

 slide upon it. Where paving the whole width of a road might be too 

 expensive, the pavement is sometimes limited to a width of 16 or 18 

 feet in the centre. In situations where coarse stone of suitable 

 quality can be easily procured, it is found to be cheaper to make a 

 road with 6 inches of broken stone and a pavement, than with 10 inches 

 depth of broken stone without paving. Mr. Wingrove, surveyor of 

 the Bath roads, mentions the use of freestone brash, chalk, &c , for 

 forming the foundation pavement of metalled roads. Of late years 

 burnt clay ballast has been substituted for rough stone pavements in 

 the brick earth or blue clay districts around London, with considerable 

 success. 



The quality and right application of the road-metal, or broken stone, 

 which forms the surface of the road, is of great importance. As a 

 general rule, the hardest stone is to be preferred ; but this rule admits 

 of some qualification, some very hard stones being found to wear much 

 more rapidly than others of a softer but tougher quality. According to 

 Parnell, whose experience rendered him a good authority, the best 

 descriptions of road materials " consist of basalt, granite, quartz, 

 syenite, and porphyry rocks." The whinstones found in different parts 

 of the United Kingdom, Guernsey granite, Mountsorrel and Hartshill 

 stone of Leicestershire, and the pebbles of Shropshire, Staffordshire, 

 and Warwickshire, are among the best of the stones now commonly in 

 use. The schistus stones will make smooth roads, being of a slaty and 

 argillaceous structure, but they are rapidly destroyed by wet, by the 

 pressure of wheels, and they occasion great expense in scraping and 

 constantly laying on new coatings. Limestone is defective in the same 

 respect. It wears away rapidly when wet, and therefore, when the traffic 

 is very great, it is an expensive material. Sandstone is much too weak 

 for the surface of a road ; it will never make a hard one, but it is very 

 well adapted to the purpose of a foundation pavement. Flints vary 

 very much in quality as a road material. The hardest of them are 

 nearly as good as the best limestone, but the softer kinds are quickly 

 crushed by the wheels of carriages, and make heavy and dirty roads. 

 Gravel, when it consists of the pebbles of the hard sorts of stones, is a 

 good material, particularly when the pebbles are so large as to admit 

 of their being broken ; but when it consists of limestone, or sandstone, 

 it is often a very bad one ; for it wears so rapidly that the crust of a 

 road made with it always consists of a large portion of the earthy 

 matter to which it is reduced. This prevents the gravel from be- 

 coming consolidated, and renders a road made with it extremely 

 defective with respect to that perfect hardness which it ought to have." 

 Mr. Stevenson, in the article ' Road,' in the ' Edinburgh Encyclopaedia,' 

 states the distribution of road materials in the British islands to be 

 partial and irregular. " Throughout Scotland, and even as far south 

 as the approaching sources of the rivers Tees and Ribble, good road- 

 metal is generally to be met with, containing the numerous varieties 

 of granite, greenstone, basalt, porphyry, and limestone. South of this 

 boundary, as far as the Trent and the Deo in Cheshire, the formation 

 is chiefly carboniferous sandstone, and the softer varieties of limestone. 

 In the southern counties chalk and gravel soils chiefly occur, affording 

 flint and gravel, both of which, under proper management, make ex- 

 cellent roads. In North and South Wales we have all the varieties of 

 road-metal which are common to Scotland. In Ireland they have 

 excellent road materials, as granite and limestone are pretty generally 

 distributed." 



An interesting experiment has been tried on a part of the Holyhead 

 road between London and Birmingham, as to the eflect of iron amongst 

 the road-metal in diminishing the wear of the road. The iron is cast 

 in the form of cubes, about an inch square, and when the road was 

 consolidated, holes large enough to receive them were picked in its 

 surface. A single cube was then placed in each hole, so as to be level 

 with the road 1 , and the small stone-chips were beat down about the 

 iron with a mallet. One of these iron cubes is placed in every four 

 inches of surface. They very soon become firmly imbedded, so as not 

 to be disturbed by the rolling of carriages or the feet of the horses ; 

 and to assist their consolidation, it is recommended to water the road 

 freely, if the cubes should be inserted in dry weather. The iron was 

 applied in March, 1835, since which time the portion of road in 

 which they are used has continued in excellent repair, and the 

 wear is materially diminished. Mr. Macnoill, the patentee of this 

 method of road-making, considered it particularly applicable to streets, 

 on account of its durability, and believed that the expense would be 

 trifling, as iron of the worst quality might be used. It may be 

 observed that the draught on the piece of road on which the ex- 

 periment is made is very easy, and that horses do not show any 

 tendency to slip upon it. 



In the choice of materials, the expense of conveying them to tho 

 road must bo taken into consideration, but it is often better economy 

 to fetch good stone from a great distance than to use that which is less 

 durable, though readily procured ; a, in addition to the expense of 

 Frequent repairs to a road formed with weak materials, great additional 

 labour is imposed upon the horses, which have to wear down repeated 

 coats of fresh stones. This is one of the points in which the inex- 

 [>eriencc and ignorance of road surveyors have often been displayed, 



