TUNNEL, a name given to a subterraneous arch-way driven through a hill or beneath a river, for the purpose of making a canal or a road, or a subterraneous bridge. In our article NAvtoximax, we have already given an account of some of the most celebrated tunnels cut for the purposes of canals. We shall therefore at present limit our selves to a notice of the new plan of tunnelling in vented by that celebrated engineer M. Brunel, and practised in the excavation of the tunnel under the Thames. This notice was drawn up by the editor of this work for a scientific journal.
As the celebrated author of this plan has had the kindness to favour us with a description and draw ings of his new method of tunnelling, which, though printed, are, we believe,not intended for separate pub lication, we conceive that our readers will he highly gratified by an explanation of a method which, in point of ingenuity and utility, has not been sur passed by any of our modern improvements iu the useful arts.
The writer of this notice had the peculiar grati fication of examining, in 1818, in company with his much respected friend Professor Pictet, all the original drawings, on a large scale, at the house of Mr Brunel, and of having them explained by that distinguished engineer. Mr: Brunel then men tioned that the idea upon which his new plan of tunnelling is founded, was suggested to him by the operations of the Teredo, a tcstaceous worm, cov ered with a cylindrical shell, which eats its way through the hardest wood, and has, on this account, been called by Linimus calamitas navium. The same happy observation of the wisdom of nature, led our celebrated countryman Mr. Watt to deduce the construction of the Flexible from the mechanism of the lobster's tail.
The difficulties (says Mr. Brunel) which have opposed themselves to every attempt that has been hitherto made to execute a tunnel under the bed of a river, have been so many and so formidable, as to have prevented its successful termination in those instances where the attempts have been made.
To propose, therefore, the formation of a tunnel after the abandonment of these several attempts, may appear somewhat presumptuous. On inquir ing, however, into the causes of failure, it will be found that the chief difficulty to be overcome lies in the inefficiency of the means hitherto employed in forming the excavation upon a large scale.
In the case of the drift-way made under the Thames at Rotherhithe in 1809, the water pre sented no obstacle for 930 feet; and, when a great body of quicksand gave way and filled the drift, the miners soon overcame this obstruction, and were able to proceed until they were stopped by a second irruption, which in a few minutes filled it. Nothing
comes more satisfactorily in support of the system that is adopted here, than the result of the opera tions that were carried, under that circumstance, to an extent of 1011 feet, and within 130 feet from the opposite shore.
It is to be remarked that, at the second irrup tion, on examining the bed of the river, a hole was discovered four feet diameter, nine feet deep, with the sides perpendicular,—a proof that the body of quicksand was not extensive; but what is most re markable is, that this hole could be stopped merely by throwing from above clay, partly in bags, and other materials: and, after pumping the water out under a head of twenty-five feet of loose ground, and thirty feet of water, the miners resumed the work, and proceeded a little further; but finding the hole at the first irruption increased, and the filling over the second very much sunk, the under taking was abandoned.
The character of the plan before us, consists in the mode of effecting the excavation, by removing no more earth than is to be replaced by the body of the tunnel, retaining thereliy the surrounding ground in its natural state of density and solidity.
In order so to effect an excavation thirty-four feet in breadth by eighteen feet six inches in height, the author of this plan proposes to have the body of the tunnel preceded by a strong framing of cor responding dimensions, as represented in the ac companying drawings (Plate DXXVI.), and in"the model proposed to be submitted for inspection. The object of this framing is to support the ground, not only in front of the tunnel, but at the same time to protect the work of excavation in all directions. The body of the tunnel, which is to be constructed in brick, is intended to be fitted close to the ground, (See Fig. 1-3.); and, in proportion as the framing is moved forward, so the brick work is made to keep pace with it. But, as this framing could not be forced for•ard'all in one body, on account of the friction of its external sides against its surrounding earth, it is composed of eleven perpendicular frames, which admit of being moved singly and independently of each other, in proportion as the ground is worked away in front. These several frames are provided with such mechanism as may be necessary to move them forward, as well as to secure them against the brick-work, when they are stationary. It is to be observed that six alternate frames are stationary, while the five intermediate ones are left free, for the purpose of being moved forward when required; these, in their turn, are made stationary for relieving the six alternate ones, and so on.