TUNNEL

(Sax. |-oenel.) In Engineering, a subterranean pa age cut through a hill or under a river, for the purpose of carrying a canal, road, railway, . In the construction of railways and canals, it is sometimes absolutely nece ary, and very frequently expedient, to have recourse to tunnelling, either to preserve the requisite level, or shorten the distance, or to le en the expense of open cutting. The circumstances on which the question of expediency depends are often of a very complicated nature; but generally speaking, it must be decided by considerations of economy, for in the present state of eng i nee ering, a tunnel may be made of almost any length, and through materials of any description, 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. On account of the peculiar nature of the difficulties to be overcome, the tunnel which has recently been carried under the bed of the Thames, from Rother hi the to Wapping, may perhaps be regarded as the most remarkable work of the kind which has ever been executed. A short account of this operation will probably be the best mode of conveying a notion of the extent and difficulty of such works generally..Some previous attempts had been made to carry a tunnel under the river below London bridge. In 1799, one was projected at Grave send; but the project was soon abandoned. In 1804, another was attempted from Rother hi the 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, 1269 TUNNEL. whence a drift was carried 923 feet under the river, and to within 160 feet of the opposite shore, where difficulties of so formidable a nature arose that the engineer reported farther progre to be impo ible. The scheme, however, contmued to be agitated; and in 1823, Mr. Brunei proposed a plan which has at length been carried succe fully into execution. The act of parliament authorizing the operation was obtained in June, 1824, and shortly after the work was commenced at Rother hi the. 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 thickne , 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 laiid 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 reservoir for the drainage of water. The excavation 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 thickne of ground to pa safely under the river. The excavation is 38 feet in breadth, and 22^ feet in height, presenting a sectional area of 850 feet; and the base, at the deepest part of the river, is 76 feet below higii-water mark. The body of the tunnel is of brickwork in Roman cement. The means by which this extensive excavation was effected, consisted of a powerful apparatus of iron, which has been called a shield. It was formed of twelve great frames lying close to each other, and severally provided with the mechanism nece ary to move them forward, and to secure them, when stationary, against the brickwork. The frames were 22 feet in height, and about 3 feet in breadth, and each divided into three stages or stories; the whole thus presenting thirty-six compartments or cells for the workmen. 'I'hese compartments were open at the back j but in front small boards, which could be separately removed and replaced at pleasure, were pre ed firmly against the ground to be excavated by screws. Each workman in his cell operated upon the surface opposed to him, taking out one of the small boards at a time, cutting the ground away to the depth of a few inches, and then replacing the board before he proceeded to the next; and when from 3 to 6 inches were thus removed over the whole surface of the shield, the frame was moved forward, and the work secured by immediately adding so much more of brickwork to the body of the tunnel. By this means, the excavation and tho structure were carried forward simultaneously. The tunnel consists of a double and capacious archway, one side being appropriated to carriages pa ing in one direction, and the other to those pa ing in the contrary, with paths for foot pa engers by the sides of the carriage roads. The middle wall between the two archways was first built solid for greater strength, but openings were afterwards cut at short distances, so that each has a ready communication with the other. The two shafts at the extremities will contain easy flights of steps for foot pa engers; and the acce for carriages is intended to be by circular roadways 200 feet in diameter, the declivity of which will not exceed 4 feet in 100. The entire length of the tunnel is 1300 feet, and the expense is said to have been about 1200/. per yard. In the course of the work, two irruptions of the river took place, the first on the 18th of May, 1827, after the excavation had been advanced to the distance of about 4(;0 feet; and the second in January, 1828. These accidents were repaired by filling the chasms in the river with bags of clay; and on clearing the tunnel of water, the structure was found, on both occasions, to be in a perfectly sound state, and to have sustained, in fact, little or no injury. After the second irruption, however, the works, from want of funds, remained suspended for seven years; but a istance having at length been obtained from government, they Averc resumed in 1835, and proceeded without further interruption till the tunnel reached the termination on the Wapping side of the river,, early in the present year, 1842. This extraordinary work, which during its progre was visited by multitudes of persons from all parts of the world, is, doubtle , destined to remain for ages a proud monument of the genius and perseverance of the engineer by whom it was contrived and succe fully executed. The establishment of railway communication in this country has given rise to some stupendous undertakings in the way of tunnelling. One of the most remarkable, perhaps the most remarkable of all. Is the Box Tunnel, on the Great Western Railway, an account of which is given in the Companion to the British Almanac for 1842, and the Railway Times for July 10, 1841. This tunnel pierces through Box Hill, between Chippenham and Bath, part of which is 400 feet above the level of the railway. " It is 96S0 feet long, 39 feet high, and 35 wide to the outside of the brickwork. The shafts for making 4 M 3 [s. 1282]