Embankments.—Great care is necessary in making high embankments. No person should be entrusted with these works who has not bad considerable experience. The base should be formed, at first, to its fiull breadth ; the earth laid on in regular courses or layers, if not more than 4 feet in thickness, of a concave form, and no fresh course should be deposited until the preceding one is firm and consolidated. The slopes at which cuttings and embankments can be safely made entirely depend on the nature of the soil. In the London and plastic clay formations, it will not be safe to make the slopes of embankments or cuttings, that exceed 4 feet high, with a steeper slope than three to one. In chalk or marl, the slopes will stand at 1 to 1. In solid sandstone, at to 1, or nearly vertical. Before quitting this subject, it is proper to remark, that in every instance of deep cutting, the greatest pains should be bestowed in examinmg the character of the material to be removed: much difficulty will be avoided by proceeding in this way ; but on the whole, the best general rule to follow, is always to lay out a line of road, so as to avoid, as much as possible, deep cuttings and high embankments ; they are always attended with great expense, and are unavoidably liable to many objections."—Sir H. Parnell.
ifaterits/i, 4e.—The breadth of roads should vary according to circum stances. In the vicinity of large towns, where the traffic is considerable, the road should, in our opinion, be not less than sixty feet between the fences. Where there is less traffic, fifty feet will be sufficient. The whole breadth should, in these cases, be metalled, or laid with broken stones. Near London, and the capitals of Edinburgh and Dublin, perhaps 70 feet is not too great a width, and a footpath should be provided on each side. " The road," says Mr. Telford, in a specification for the Holyhead road, "is to be 30 feet wide, exclusive of footpaths, with a fall of 6 inches from the centre to the side channels." The bed of the new road being prepared for the reception of the materials, should, if of a wet or spongy nature, be well 'rammed' with chips of stone; in some situations it is advisable to lay a stratum of hand-laid stones, of from 5 to 7 inches in depth, with their broadest ends placed downwards, and the whole built compactly together. On this is to be laid the metal,' or broken stones, to the depth of at least 8 inches, broken of a uniform size, so as to form a solid and compact body. To insure uniformity in the size of the broken stones, various tests have been suggested; perhaps the most simple is, that every piece shall pass through a ring of 2i inches diameter. On this body of metal, no binding or gravel should be used ; the angular sides of the metal soon lock into each other, and form a smooth surface. In the selection of road-metal, we prefer the several varieties of green-stone. The best kinds of these are less friable than granite, when broken into small pieces. It is, however, often
necessary, for want of better materials, to use sandstone, common limestone, and chalk, even in districts where there is a great deal of traffic ; in some instances, where coal is abundant, sandstone is reduced to a vitreous mass kilns erected by the road side • but all such road-metal is now used very sparingly in the formation of modern roads, and confined chiefly to the bridle tracks.
The distribution of road-material is very irregular in the British Islands. Throughout Scotland, we meet with the numerous varieties of granite, green stone, basalt, porphyry, and limestone. In Cheshire, the formation is chiefly coal, sandstone, and the softer varieties of limestone. In the southern counties, chalk and gravel soils chiefly occur, both of which, under proper management, make excellent 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."— Edinburgh Encyclopedia.
The reports of the Commissioners of the London and Holyhead road contain a mass of useful and interesting information. The appointment of Mr. Telford was the commencement of a new era in the management of roads ; under his judicious superintendence were planned most of the improvements which have since been carried into effect ; and the correct principles on which they were conducted, is proved by the present state of this great road. Mr. Macneill succeeded Mr. Telford as the Resident-Engineer to the Commissioners, and has shewn himself not unworthy of his While assistant-engineer under Mr. Telford, be suggested an experiment for the purpose of rendering solid and dry the piece of road between Holloway and the Wellington Inn at Highgate.
In the year 1829 the Commissioners took the Highgate Archway road under their management, to put in complete repair. In order to accomplish this, several experiments were tried, by draining the surface and sub-sod, and by laying on a thick coating of broken granite ; but from tile wet and elastic nature of the sub-soil, the hardest stones were rapidly worn away by the wheels of carriages, but much more by the friction of the stones themselves against each other ; for, in a very short time, they were found to become as round and as smooth as gravel pebbles, even at the bottom of the whole mass of road materials. It was therefore evident, that to form a perfect road, which might be kept in repair at a moderate expense, it was necessary to establish a dry and solid foundation for the surface ; but as no stones could be obtained for making a foundation of pavement, but at a very great expense, a composition of Roman cement and gravel was suggested by Mr. Macneill, and this, on trial, was found to answer effectually.