Mr. Peter W. Barlow, civil engineer, has recently obtained permission from the City Commissioners of Sewers to construct a subway beneath the Thames, which is to be carried from Lower Thames Street to the opposite shore of the river. The dimensions of the subway are to be sufficient to allow a loaded omnibus to pass through it. If constructed economically, there is no reason why it should not answer commercially.
We perceive by recent accounts from America that it was originally intended to connect the railways concentrating on both banks of the Mississippi at St. Louis, by a tunnel under the bed of the river; but this plan has been abandoned, and instead of it a “sub-aqueous iron tubular bridge” is to be laid on the bed of the river, which is about half a mile wide in this vicinity.
The difficulties connected with crossing the upper Indus at Attock, a thousand miles from its mouth, and 940 feet above sea level, have long been felt. Colonel Robertson, of the Madras Staff Corps, therefore, proposed a scheme for going under instead of over it. In his report on the subject submitted to Government in 1859, he stated that, as the geological formation at Attock, is a compact slate rock, it is easily worked; and under the bed of the river it is apparently not broken by any great fissures which might possibly endanger the tunnel. Colonel Robertson fixed the upper level of the excavation for the tunnel at 60 feet under low water cold weather mark, or, at the water’s deepest point, 20 feet below the bed of the river; the lining of the tunnel to be 2 feet thick, and as its height is 20 feet, the foundation level would be 82 feet below the low water level. To guard against all risk of inundation through floods, which raise the level of the river from 50 to 92 feet (it was at the latter height in 1841), the two entrances of the tunnel are to be 100 feet above low water level. The width of the river at the point selected is 1,215 feet. This portion of the tunnel is to be nearly on the level; but the gradient of the approaches to it on each side, each 3,720 long, is to be 1 in 40, or at the rate of 132 feet in the mile; the total length of actual tunnel to be 7,215 feet, as some portion of each approach is to be in open cutting. There are ten shafts, each 600 feet apart, except at the actual river, where they are 1,580 feet apart. In 1860 the works were commenced, and a drift gallery had been nearly carried through, when all operations were suspended; but it is intended that they be resumed in prospect of the railway between Lahore and Pesshawer being constructed. The revived estimate makes the cost of the tunnel £105,000, if the gradient on each side be 1 in 20; but if it be flattened to 1 in 30, the estimate is £143,300.[133]
The greatest tunnels that we know of connected with mining (irrespective of galleries for working in mines) are the great drainage galleries at the mines of Clausthal, in the Hartz, 11,377 yards, or 6½ miles long, and in many parts 900 feet below the surface of the superjacent mountain. The second is the Great Adit, which drains several of the important mines in the parish of Gwennap, Cornwall. It is from 30 to 60 feet below the surface, and is 30 miles long. There is an adit level of 10,000 yards to the celebrated silver-mines of Norway.
There are two modes by which it is proposed to carry a subway between France and England,[134] the first is by means of iron tubes laid on the bed of the ocean, the other is by actual tunnel. At the present time no less than three competitors present themselves for the honour of constructing the former, and there is only one whose scheme is before the public for the latter. The three advocates for the tubular system are Mr. James Chalmers, Mr. B. Hilmer, and M. Thome de Gammond, of Paris. Each has his mode of laying down and connecting the tubes together, but the great and distinguishing feature of the plan of M. De Gammon, is that he proposes to construct a great oceanic station, which is to be a kind of half-way halting house between the two shores. Here also is to be a harbour and three ship’s basins, so that any one returning from a long voyage and being in a hurry to get either to London or to Paris, or to any other place—it signifies not where—in England or the Continent, might land and at once proceed upon the terra firma portion of his journey. In the centre of the harbour there is to be a huge shaft 330 yards in diameter, which would serve the double purpose of ventilating the tunnel, and of providing means of ingress and egress between the Islet de Varne station and the upper and outer world. All these great advantages—tunnel, shaft, and railway—are to obtained at the cost, as estimated by M. De Gammond, of £7,200,000!
The advocates for the tubes insist that theirs is the right system, in consequence of its having been publicly stated that Mr. Hawkshaw has satisfied himself by many borings that the bottom of the English channel between Dover and Cape Grinez, has too many and too deep “faults” to permit of tunnelling. Mr. George Remington, C.E., however, considers he gets over the difficulty by avoiding the line originally selected for the Anglo-French tunnel. He therefore proposes Dungenness as his English starting point. The depths of the tunnel is, says Mr. Remington, to be from 90 to 130 feet below the bed of the channel, and there are to be three main shafts, the first at the point of Dungenness, the second on the shoal in mid-channel, where there are only eleven feet at low water spring tide, and the third at Cape Grinez. These shafts are to be 100 feet diameter, and being carried up considerably above the sea are to act as lighthouses. It would be inconsistent with the character of this work to enter into an account of the technical details which Mr. Remington proposes to adopt. We shall, therefore, limit ourselves to saying that in addition to the three intended permanent shafts, it is likewise proposed to put down ten temporary shafts, the cost of each of which is not to exceed £20,000. With these thirteen shafts, says the editor of Engineering, “the tunnel may be carried on in twenty-six sections, and the distance from shore to shore being twenty-six miles, gives only one mile for each section, or two miles for a shaft, and assuming an advance of only one yard a day for each headway, the whole distance may be accomplished in about five and a-half years,” exclusive of the couple of years required for sinking the shafts.