« PreviousContinue »
road ascends three rather steep and long hills, while the new road avoids, almost entirely, two of these hills, at the same time that it is shorter by 638 yards.
In tracing a road across a deep valley between two hills, it should be carried in a direction opposite to the fall of the valley, as by so carrying it, that is, by crossing the valley at the highest practicable point, the descent and ascent are diminished.
Thus, in going from A to B, across a valley, if it be found by levelling, that in a straight line the valley is too deep to make an embankment at a reasonable expense, the surveyor should try a line, A C B, higher up the valley, rather than in the direction A D B, where he would get into a lower level, (see Plate I. Fig 2.) Although this is the general principle, instances may occur, where a valley may be crossed with more advantage down stream; as, for instance, if the sides of a valley contract considerably, it may require much less embankment to raise the road to the same height, than if it were carried higher up the valley; see Plate I. Fig. 3., by which it appears that it would be more advisable to take the line A D B, than either the straight line A B, or the line A C B, higher up the valley.
Another instance where a valley may be crossed with more advantage down stream, is where detached or insulated hills are situated in the valley below the straight line of direction, as represented in Plate I. Fig. 4. Here it would be proper to pass the valley lower down, to take advantage of the intervening high ground, as will be seen by the section, in which it is evident that much less embankment will be required in the line A D B, than in either the direct line Ab B, or the line A c B, higher up the valley. Lately, when it was proposed by the Parliamentary Commissioners of the Holyhead Road, to improve the valley of the Geese Bridge, between Towcester and Daventry, on the road from London to Birmingham, six different surveys and plans of doing so were made. The report on these surveys is given in note A, more fully to explain the rules for crossing valleys. In many situations, particularly in mountainous countries, it will be found necessary to pass valleys or deep ravines by means of high arches of masonry, as in some parts of Scotland, where Mr. Telford has erected several great works of this description; of these, the most remarkable are the bridges over the Mouse Water, at Cartland Craigs, on the Lanark Road, represented in Plate I. Fig. 5. The bridge over Birkwood Burn, near Lesmahago, on the Glasgow
Road, represented in Plate I. Fig. 6., and the Fidlor Burn Bridge, on the Lanark Road, represented in Plate I. Fig. 7.*
The suspension bridge over the Menai Straits, in North Wales, is of a similar character, for, besides its use in passing these straits, it has improved the road by its being no longer necessary to descend to the level of the water. See Plate I. Fig. 8.
In most cases, however, valleys may be crossed by high embankments of earth, such as the chalk hill embankment near Dunstable, and that near Chirk, in North Wales.
In some situations it may be advisable to pass through a hill by means of a tunnel, instead of by deep cutting.
There are three works of this kind on the Simplon Road. One of them, "la grande galerie de Gondo," is 240 yards in length, 8 in breadth, and the same in height. † There is a similar
*To this list may be added the Dean Bridge over the Water of Leith at Edinburgh, which is above 100 feet high, and consists of four arches of 90 feet span; and a bridge at Pathhead, on the Coldstream road of five arches of 80 feet span.
+ It would appear from the following extract from a Memoir of M. Ceard, on the Simplon Road, that some great errors were made in laying out the line of this famous road.
work at Puzzuoli, near Naples, which is nearly half a league long; it is fifteen feet broad and as many high.
M. Ceard was the chief engineer, and claims the merit of being the author of the plan.
The plan of the road, once adopted, it became the duty of the head engineers, and more especially of the younger officers under their command, strictly to follow what had been decided upon. M. C, and the engineer with whom he acted, entertained, however, a different opinion, and exercised their discretion. After much labour was uselessly expended by them in attempts which they were obliged to abandon, the first ascent, which should have been an uniform inclined plane, rising regularly six inches in seventy-two, on the side of the mountain of Brandevald, was constructed in an irregular manner, the incline rising sometimes seven inches in seventy-two, sometimes eight, sometimes three, or less, and sometimes nothing. This error was committed in my absence, and when I discovered it, too much labour and money were expended, to abandon what had been executed, without considerable loss; besides which, the government would have been exposed to the public ridicule, which would be excited by the exposure of the blunders of those employed by it. Such was the first consequence of M. C's mismanagement in this ascent.
The second division of the road was arranged to proceed from the first summit to the bottom of the valley of Ganther. This was to have been effected by an inclined plane, at the rate of two inches in seventy-two; but the course followed by these engineers doubled this rate; and if I had not interrupted their departure from the original plan, the road would
The peculiar circumstances of a river may render it necessary to deviate from a direct line in laying out a road.
have been carried across three torrents, over which bridges must have been thrown, in situations almost inaccessible, and exposed to destruction by avalanches, instead of a single bridge, which alone was necessary by the original plan, situated also in a position free from those objections.
The consequences of this second error in arranging the inclinations of the road were, that it was necessary to elevate the line of the road, (in order to avoid the inconvenience of having to make a descent and then an ascent,) and to give double the necessary height to the bridge of Ganther, the position of which admitted of no change. It was necessary also to increase the height of the stone buttresses of the bridge, to the extent of at least twenty metres (sixty-five feet), to supply the place of the support which would otherwise have been derived from natural rocks; and finally, to construct considerable causeways, and to make the carpentry of the bridge of that height which the torrent required. Such were the consequences of M. C's proceedings.
I shall continue to investigate the labours of this young engineer.
Having to superintend, besides the Simplon road, fifty leagues of road in Italy, in the Valais, in Savoy, and upon Mount Jura, I could not be always present at the Simplon to watch the proceedings of these gentlemen; but I trusted that the superintending engineer, C-—, satisfied with the consequences of his first departure from the plan laid down. by the council, would follow, for the future, implicitly the