This novel undertaking appears to excite continually increasing interest.—A tunnel for carriages beneath such a river as the Thames being regarded as one, at least, of the various wonders for which England has become such an object of admiration.
The tunnel will consist of two archways, and, in order that there may be no obstruction to carriages, those going from the north to the south shore will pass through one, while those from the south to the north will occupy the other archway. The passages will be Macadamized, and there will be distinct paths for foot passengers. In the centre, between the two archways and dividing the two roads, will be a line of numerous arches of communication, spacious enough to admit of persons passing from one road to the other ; and in each of these arches will be fixed a strong gas light. The approaches to the entrance of the tunnel will be by circular descents of easy declivity, not exceeding four feet in every hundred. One smaller one for pedestrians; another larger one for carriages; and so easy will the descent be, that the heaviest loaded waggon will never need to have the wheel locked. The bed of the river has been rigidly examined by frequent and deep borings, in older to ascertain the nature of the strata, and the results have been, we are informed, highly encouraging ; the deep parts being uniformly of a tenacious, hard, blue clay. The machinery, by which this excavation is to be made, is the invention of Mr. Brunel. The iron shield, for the protection of the workmen, and for the security of the work as it proceeds, is of an ingenious combination, and of itself it is said to weigh ninety tons ; and this weight is literally to be pushed through the earth, entering at the shaft on the north side of the river. For this invention Mr. Brunel has obtained a patent. As this large shield, or three-storied iron scaffolding, is daily moved on to fill up the excavation made by the workingmen, it is to be followed closely by the bricklayers who will make all good; so that the incumbent earth will always be supposed by the iron shield, or by compact and well cemented brick work. The size of the tunnel will be 35 feet wide, by 20 feet in height ; each archway will have about 14 feet clear width (including foot path;) and the whole length of the tunnel will be about 1300 feet. Should nothing unforeseen prevent it, this novel communication is expected to be opened in less than 3 years. No bridge could cross the Thames where the tunnel is designed to be made, unless it were of a height so great that ships could sail beneath it. Such a bridge has been projected, but the design was soon abandoned. The tunnel may be regarded as a bridge reversed, and as a channel of intercourse between the two shores, at once commodious, and suited to its locality. There will be in this tunnel a thickness of 15 feet of earth between the crown of the tunnel and bed of the river. The following is a description of the mode adopted in carrying the work into effect, as set forth in Mr. Brunel’s plan :—
In the case of the drift-way made under the Thames at Rotherhithe, in 1809, the water presented no obstacle for nine hundred and thirty feet, 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 eruption, 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 operations that were carried on, under the most disadvantageous circumstances, to an extent of one thousand and eleven feet, and within one hundred and thirty feet of the opposite shore.
The second eruption could be stopped merely by throwing from above a quantity of 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 eruption increased, and the filling over the second very much sunk, the undertaking was abandoned.
The peculiar 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 thereby the surrounding ground in its natural state of density and solidity.
In so to effect an excavation of thirty-five feet in breadth by twenty five feet in height, Mr. Brunel proposes to have the body of the tunnel preceded by a strong framing of corresponding dimensions. The object of this framing is to support the ground, not only in front of the tunnel, but to protect at the same time the works of excavation on all sides. The body of the tunnel, which is to be constructed of brick, is intended to be fitted close to the ground; 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 forward all in one body, on account of the friction of its external sides against the surrounding earth, it is composed of twelve 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 brickwork when they are stationary. It is to be observed, that six alternate frames are stationary, while the six 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. Thus the progressive movement of the frame can readily be effected.
In order that a sufficient number of hands may be employed together with effect and security, each frame is divided into three small distinct apartments, which may properly be denominated cells. By this disposition the twelve frames form thirty-six cells, in which the men are to operate for excavating the ground. It is to be observed, that the ground in front of each cell is to be kept from falling in by means of a powerful shield, consisting of small boards, strongly secured by the frames. It is from within the cells that each workman is to cut out the ground, just in the way he would do if he were to cut out a recess in a wall for the purpose of fitting a closet into it; but with this precaution, that he is to remove each board, one after the other, and replace each in succession as he cuts the portion of the ground corresponding with it. When he has thus gained from three to six inches over the whole surface (an operation which it is expected may be made in all the cells nearly in the same time,) the frames are moved forward, and so much of the brick work added to the body of the tunnel. Thus in entrenched and secure cells, thirty-three men may be made to carry on an excavation which is six hundred and thirty feet superficial area, in regular order and uniform quantities, with as much facility and safety as if one drift only of nineteen feet square was to be opened by one man.
The draft carried on under the Thames, in 1809, which was about the size of one of these cells, proceeded at the rate of four to ten feet per day. In the plan now proposed, it was not intended the progress should exceed three feet per day, because the work should proceed with mechanical uniformity in all the points together.
The estimated expense of this undertaking, 160,000 l. is so much under that of a bridge, as to have occasioned doubts whether it has not been under-rated. But to remove these, it is sufficient to state, that it will be constructed almost entirely of brick, which is the cheapest of all building materials, not merely as a material, but also in the labor.
The Massachusetts Spy, and Worcester County Advertiser, September 07, 1825