[CHAPTER XXX.]
TUNNELS, VIADUCTS, EMBANKMENTS, AND WEIRS.
In the laying out of a canal there generally comes a juncture at which the engineer has to choose between a tunnel, a flight of locks, a lift, or series of lifts, and, finally, an embankment. There are also cases, although these are comparatively rare, in which a valley has been crossed, and the level of the water maintained, by aqueducts.
Tunnelling is always a costly operation, and it seldom happens that it gives a considerable advantage, if any, over locks in the matter of speed. Nevertheless, many of the early canal engineers were partial to tunnels, and hence there are many examples of such structures on the canal system of Great Britain. Of some of these we may suitably furnish particulars before proceeding to refer to more recent works of the same character.
One of the earliest canal tunnels of which we have any record was constructed by Brindley on the Bridgwater canal. This tunnel gave the Duke access from his canal into the coal works at Worsley, and after it had proceeded for some way straight into the hill, came at a great depth to be under a small brook or constant stream of water, by the side of which a large water-shaft was sunk, and a drum and a large brake-wheel erected over it, of such size that a man who stood before the lever had his two hands at liberty to pull the lines which connected the valves, and give signals to those below, while by lunging or stepping forwards, with his breast against the lever, he could in an instant stop the machinery in any part of its motion, or regulate the same at pleasure. There were two water-tubs, which were very large, and had a valve and pin to empty themselves quickly when they arrived at the bottom. They were suspended by large ropes or cables from the drum, while other large ropes descended therefrom through another or coal-shaft, by the side of the middle or principal tunnel, into and over the navigable tunnel, which is there at some 60 yards lower level. On this level, canal boats were used, similar in their dimensions to those above, and containing boxes, which being filled with coals at the several terminations of the canal, in the seams of coals, were pushed along by means of rings fixed along the roof of the tunnel at the proper height for a man, who walked on the top of the coals, to lay hold of, and shove the boat along by. The boat having arrived under the coal shaft, and one of the water-tubs being at the top of its shaft, the coal rope answering thereto was hooked on to the box of coals, and the descent of the water-tub, immediately on the ringing of a bell, drew up the same to the level of the principal canal, where, being drawn aside over an empty boat, it was lowered into the same by a slight reversion of the motion of the machine, when the interval of emptying the tub at the bottom by its valve gave time for hooking another box to the other rope which was at the bottom, when the other water-tub was filled, and the machine was suffered to move by the man who leant against the brake.
This arrangement was contrived and erected by James Brindley, and it was so constructed that when coals were not drawing, the alternate descent of the water-tubs worked some very large pumps, which were sufficient to lift all the mine water of the lower level into the middle canal, and to keep the lower canal always at the proper height for navigation.
The same tunnel of the Bridgwater canal was continued a considerable way farther into Worsley Hill, until, under Walkden Moor, another subterraneous canal or tunnel begins, at 35½ yards higher level, being nearly 60 yards from the surface. From the surface two shafts were sunk, one terminating in and over the upper tunnel or canal, and the other in and over the middle or principal canal. There is another canal still lower, and after passing close by the canal above. Between these shafts a large drum was erected on the surface, with a brake-wheel and a pair of strong ropes. An old account of the working of this tunnel states, that “two boats being arrived at the shafts on the upper canal, one of them loaded with boxes of limestone that was wanted at the furnaces, and another with boxes of coals intended to be transferred into an empty boat in the middle canal, the ends of the two ropes were fastened to a box of coals and a box of limestone, when the superior size and weight of the coal boxes drew the limestone to the surface, to be there landed and deposited, at the same time as the box of coals was deposited in the lower boat, ready to proceed on the canal to Manchester or other places.”
This method was, in 1797, superseded by an inclined plane for letting down the boats laden with coals from the higher to the middle level, and returning the empty boats and boxes.
At Brierley Hill, near Coalbrook Dale, the extremity of a branch of the Shropshire canal, great quantities of coal and iron, in crates made of iron, were let down one of two shafts, which connected with the termination of the canal above, and the ends of a railway in a tunnel below, from which limestone in similar crates was drawn up the other shaft to be placed in the boat. A barrel and brake-wheel were fixed between the tops of the shafts, and cranes with jibs, by which the crates could be raised and moved from the boat over the shaft or the reverse. These shafts, which were 120 feet deep, were not found to answer, in point of expense, so well as inclined planes, and Mr. Telford informs us (‘Plymley’s Report,’ p. 296-307) that “inclined planes have been substituted, on which crates of coal or iron pigs, or goods descend, and draw up other crates containing limestone for the use of the ironworks above, by means of ropes, a drum, and brake-wheel, with a much less portion of manual labour, and more expedition, than was done by the shafts above mentioned.”[282]
Marsden tunnel, on the Huddersfield Canal, is 5280 yards in length; Sapperton, on the Thames and Severn, 4300 yards; Penfax, on the Leominster and Kington, 3850 yards; Laplat, on the Dudley Canal, 3776 yards; Blisworth, on the Grand Junction Canal, 3080 yards; Ripley, on the Cromford, 3000 yards; Dudley, on the Dudley Canal, 2926 yards; Harecastle, on the Trent and Mersey canal, 2888 yards; Norwood, on the Chesterfield Canal, 2850 yards; Westheath, on the Worcester and Birmingham Canal, 2700 yards; Morwelham, on the Tavistock Canal, 2500 yards; Oxenhall, on the Hereford and Gloucester Canal, 2192 yards; and Braunston, on the Grand Junction, 2045 yards.
The longest tunnels that have been proposed, besides those stated above, were one of 5 miles on the once proposed extension of the Manchester, Bolton, and Bury Canal to the Calder river; and one of 4¼ miles on the Portsmouth and Croydon Canal, through the chalk hills to the south of the latter place.