TUNNEL

(Sax. tcenel.) 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 conslruction 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 con siderations of economy, for in the present state of engineer ing, 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 conveyer ally.ing a not i on of the extent and difficulty of such works genSome 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.In1804, another was attempted from Rother hi the to Limchouse. 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 liver, and to within 150 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, continued 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 feel in thickne , on the top of which a steam engine was erect ed 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 20 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 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 is38 feet in breadth, and 22.J 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 R: man cement. The means by which this extensive excavation wr.n effected, consisted of a powerful apparatus of iron, which hag been called a shield. It was formed of twelve great trames lying close to each other, and severally provided with the mechanism nece ary to move them forward, and to swum [s. 1292]