|The Construction and Works of the Highland
Railway by Joseph Mitchell, F.R.S.E., F.R.S., C.E.
Page 2 of 4
|The principal difficulties that arose in
laying out the line were in passing through the narrow defile at Dunkeld,
the beautiful demesne of the Duke of Athole, and again in penetrating through
the picturesque Pass of Killiecrankie, where the mountains, as it were,
close in upon each other for a great height; likewise in passing along the
narrow, precipitous, and rocky valley of the Garry, close to a large and
rapid mountain-stream; also the Park at Castle Grant, and the defile at
Huntley's Cave near Grantown. These points in particular required much study,
with repeated trial and contour levels, so as to obtain a knowledge of the
precise formation of the ground, and to choose the best direction at the
lowest possible cost.
At the Pass of Killicrankie the banks were so precipitous and steep at that point that the line had to be supported by breast of retaining walls to the extent of 690 lineal yards, and to the average height of 26 feet, the extreme height of one being 55 feet; and in order to carry the railway at the narrowest point in the Pass where the precipices close in, as it were, on either side, and afford scarcely any additional space beyond that occupied by the channel of the river, instead of supporting the line by breastwalls, it was deemed prudent to construct a viaduct of 10 arches, 60 feet above the river, which with a tunnel at the north end carried it successively through the Pass. At two other points in the line, in running up the sides of the Garry, breastwalls had to be formed, respectively 94 and 35 yards in length, and 15 feet in average height. All these breastwalls, extending to 1650 lineal yards, are built with lime, and set on a solid foundation of dry gravel or rock, at right angles to the face of the wall, which batters at the rate of 1½ inch to the foot. The writer prefers the curves to the straight batter, as it gives more effectual resistance if well built; but breastwalls are to be avoided wherever earth embankments can be substituted, as, in his experience, there are subtle influences in the Scottish Climate of alternate frost and wet in winter, which operate imperceptibly to their destruction, and they require careful and constant inspection.
In running through so large an extent of mountainous country the line, as might be expected, had to pass over some lengths of soft ground and morass. The principal of these were for two miles near the town of Nairn, also for about two miles near Keith, one mile near Dava Moor, and about a mile in crossing through a hollow at Drumochter on the summit of the Grampians. In all places where the ground was particularly soft, a uniform mode of treatment was adopted. Two parallel drains were first cut outside the fences, about 50 feet apart, from4 to 6 feet deep, and with slopes of 1 to 1. This drained off the surface-water; and, after making up the holes and other irregularities of the surface with turf, the space for the railway to a breadth of about 15 feet was covered with two or three layers of swarded or heather turf, having the sward side of the lower layer undermost, and that of the top layer up, the joints breaking band. In this way a good sustaining surface has uniformly been obtained.* On this bed of turf the ballast was laid for 2 or 3 feet in depth. This was quite sufficient to support the traffic, but as in some cases the bed of moss was from 20 to 30 feet in depth, the railway merely floated on the surface, and was in the first instance undulating, and yielded in some parts from 3 to 4 inches under the weight of the engines passing over. To obviate this undulation longitudinal beams of timber were tried at one place, 20 to 40 feet long, below the sleepers, but his was found objectionable, as rendering it more difficult to raise or repair this surface of the road; and an additional sleeper (making the sleepers 2 foot 6 inches from centre to centre, instead of 3 feet) was found preferable. There was nothing for it, at the worst, but to lift the road every other week as it sunk, until it had acquired a solid bearing. In many places we had to lay on 4, 5, or 6 feet in depth of additional gravel, and in one place no les than 27 feet, before the road became solid. In the course of two or three years, however, with the attention, the tails being fished, the lines through these mosses were all that could be desired for solidity and performance.
As the writer has said, in crossing so many mountain-rivers, bridges of magnitude had to be constructed, involving considerable varieties of execution. The principal of these bridges may now be described, and any peculiarity will be noticed which may have arisen during the progress of the works. It will be observed that the beds of the rivers in the north of Scotland differ in many respects from what is common in England, consisting frequently of depths of 10 or 12 feet of gravel and boulders, the solid and compact debris of successive floods, below which, if the country is of rocky formation, there is usually hard clay and then rock, or, as in the case of the mouth of the River Ness, after penetrating 12 feet of shingle and boulders, a sort of admixture of whitish clay and sand was obtained. In some cases we had to deal with soft clay and mud of great depth, but these were exceptions. Nor was it possible in general to ascertain, by boring, the precise nature of the foundations, because many of the boulders in the gravel were of large size, and were often mistaken for rock. The only way in which an approximate knowledge of the foundations could be obtained was by driving iron rods at various places, and when the bed of the river admitted of it, wooden piles. Still we worked very much in the dark; but the writer's long experience of these rivers, and of the nature of their floods, was of great advantage in enabling him to fix the depth of the foundations and the precise description of works to secure the necessary stability of construction. In only two of three cases was there any fear of sinking. What had chiefly to be guarded against was sudden and impetuous floods, sometimes accompanied with floating ice and trees, undermining the foundations and damaging the piers; it was therefore important to provide ample waterway. The construction of these bridges [lasted?]** over twelve years, and during that time there has been considerable changes in bridge building, by the adoption of iron cylinders for piers, and lattice girders in spanning the waterways, so that, as the works progressed, these improvements were adopted where found suitable.
*Author's note: "Had this plan, which the writer has found to answer to well both for roads and railway, been adopted in the clayey ground at Balaklava in the Crimea, a good road might have been formed, 1867".
**Transcriber's note: this word is illegible