TUNNEL. In engineering, a subter ranean passage cut through a hill or under a river, for the purpose of carry ing a canal, road, railway, &c.
In the construction of railways and canals, it is sometimes absolutely neces sary, and very frequently expedient, to have recourse to tunnelling, either to preserve the requisite level, or shorten the distance, or to lessen the expense of open cutting. The circumstances on which the question of expediency de pends, are often of a very complicated nature ; but, generally speaking, it must be decided by considerations for in the present state of engineering, a tunnel may be made of almost any length, and through materials of any descrip tion, from a granite rock to a quicksand. The nature of the ground can hardly be said to interpose any farther obstacle than what may be occasioned by the expense.
One of the greatest tunnels in the world, is that under the river Thames below London Bridge—a short notice of it is here inserted.
Some previous attempts had been made to carry a tunnel under the river below London Bridge. In 1799, one was projected at Gravesend; but the project was soon abandoned. In 1804, another was attempted from Rotherhithe to Lime house. A shaft of 11 feet in diameter was sunk to the depth of 42 feet, and continued at a reduced diameter of 8 feet to the depth of 76 feet, whence a drift was carried. 923 feet under the river, and to within 150 feet of the opposite shore, where difficulties of so formidable a na ture arose that the engineer reported farther progress to be impossible. The scheme, however, continued to be agi tated; and in 1823, Mr. Brunel proposed a plan which has at length been carried successfully into execution.
The act of parliament authorizing the operation was obtained in June, 1824, and shortly after the work was com menced. at Rotherhithe. The shaft at this place is 150 feet from the river. It was formed by building a cylinder of brickwork 50 feet in diameter, 42 feet in height, and 3 feet in thickness, on the top of which a steam engine was erected for raising the water and earth. The cylinder was let down bodily into the ground, forcing its way through a bed of gravel and sand 26 feet deep, and full of land water. The shaft was sunk to the depth of 65 feet, and from this level another smaller shaft, 25 feet in diameter, was sunk, destined to be a well or reser voir for the drainage of water. The ex cavation for the body of the tunnel was commenced at a depth, of 63 feet, and was carried on at a declivity of 2 feet and 3 inches per 100 feet, in order to have sufficient thickness of ground to pass safely under the river. The excavation is 88 feet in breadth, and 228 feet in height, presenting a sectional area of 850 feet ; and the base, at the deepest part of the river is 76 feet below high-water mark. The body of the tunnel is of brickwork in Roman cement.
In the neighborhood of the city of Buffalo, N.Y., is the tunnel of the Water Works Company in the rock under the Eric Canal and the Black Rock harbor to the Niagara river, about half a mile be yond the city line.
The perpendicular shaft or well is about 8 feet in diameter and 30 feet deep, nearly the whole being through rock. From the bottom of the well starts the tunnel, which is nearly circular, and about 68 feet in diameter, running nearly hori towards the bed of the river, which is distant about 360 feet. A slight slope upward, as the tunnel advances, allows the water which pours into it from springs or crevices in the rock, to run back into the well out of the way of the workmen who are engaged incessantly, day and night, in blasting the rock. They have now (1851) proceeded about 280 feet from the well, progressing at about 2 feet per day. Only four of the miners em are able to work at once, changing three times during the twenty-four hours. The work is all done by lamp-light. The Railroad Tunnel at New Hamburg, N. Y., is 830 feet long, and through so lid rock. At the south end is a cut 500 feet long, 80 feet wide, and 50 feet deep, all through the rock before reaching the tunnel ; through two shafts sunk to it, one 70 feet in depth, the other 56, a glimpse of daylight may be obtained. at the north end one other deep cut is found, nearly as formidable as that at the south, being 200 feet long and 70 deep, making the entire deep cut ting through the rock, all inclusive, no less than 1530 feet.
To carry on this work Messrs. Ward, Wells & Co., the contractors, employ 400 men, keep in steady operation nine black smiths' shops with two fires each, to re pair and temper tools, have 12,000 lbs. of cast steel in drills and tools in constant use, and have consumed 6000 kegs of powder, of 25 lbs. each, in fourteen months. The tunnel is 19 feet high and 24 feet wide, where finished, and will be so all the way through. It is not yet completed.
The Great Tunnel, of the Baltimore and Ohio Railroad, is one of the greatest works of civil engineering now going on in this country. It is a few miles from Morgantown, West Virginia, and is through a mountain (for a rail-track) a mile and a quarter wide. There are al ready sunk three shafts, some 20 by 19 feet, and from 175 to 185 deep.
The Huddersfield and Manchester Rail road Tunnel, in England, is more than three miles in length, being the largest in the world, at a depth of 625 feet below the ridge of the hill, which it pierces. The tunnel is so perfectly straight that on a clear day one can see through from either end.
In France, there is an extensive tunnel three miles in length, on the railroad be tween Marseilles and Avignon. Its height is 80 feet, and width 24 feet, and its depth below the surface of the ground 600 feet.
The cost of tunneling was, $2,040,000.