Fresh quantities of clay and bags of clay were now employed to fill up the second hole; and the enlarged dimensions of the former, occasioned by a settling or movement of the artificial ground, was also filled up to a level with the natural bed of the river. The clay was covered with a stratum of gravel, and this by a large and very thick tarpauling, which was kept down by cast-iron kintledge; another layer was thrown over the whole, to keep it as closely together as possible. This plan proved an effectual remedy against the further irruption of the water, but there were many persons who imagined that the plan we have just described would have been preferable to prevent the danger of a further influx of the water. Vessels of all descriptions were required to avoid anchoring over that part of the river where the works of the tunnel were going on.
The lamentable accident which we described was also productive of an excellent plan from an eminent member of the London Mechanics' Institution, which consists in introducing, a few yards behind the workmen, floodgates, so constructed, that the lower parts of the gates would be first shut by the water issuing in at the place where the work is carried on; and when the waters rise nearly halfway up, then to shut the middle parts of the gates; and when they rise near the top, to shut the top parts of the gates. This arrangement would have afforded all the workmen time, who could reach so far as the flood-gates, to get safe out, and prevent the tunnel from being filled with water. This plan would not only have tended to obviate much of the danger to be apprehended by the workmen, but greatly to diminish the enormous expense consequent upon such an accident. The small space between the shield and the flood-gates would soon become filled with mud and sand, and the bed of the river might then have been soon made good from above, as then there would have been no liability of the materials put down for that purpose being loosened and removed by the periodical ingress and egress of the water during the rise and fall of the tide.
The cut on the next page exhibits a transverse section of the tunnel, with the proposed gates, etc.
Fig. 1 represents a front view of the gates, with those on the right hand or eastern arch entirely closed, those in the other arch having been kept open for taking through the clay and building materials, as the excavation proceeded. In order to make the plan better understood, the water is represented as coming in, which, having just closed the lower pair of gates, is in the act of shutting the middle pair, while the upper pair is represented as standing open.
The lower pair of gates are bevelled off to an acute angle, which terminates at the outside of the upper edge; and to correspond with this, the lower edge of the middle gates are bevelled off in a contrary way, to lap over the others, as exhibited in the drawing. From this arrangement, it will be perceived that the middle gates would be partly in the water before it would run over the lower gates; and hence, the second gates would be shut as soon as the water began to run over the first. The same arrangement was made with respect to the middle and upper gates, except that the upper edges of the latter were bevelled to an angle on the inside, to fit a contrary bevel on the top of the gateway. It may be observed here, that none of the floodgates were made to open so far back as to become parallel with the side of the tunnel, consequently they are always in a situation to be acted upon by the water; and that the whole of the gates, as well as the framework in each arch, met in the middle of the arch, as flood-gates on canals do, at such an angle as to afford the greatest resistance to the pressure of the fluid.
To prevent the outward lateral pressure of the sides of the framing against the brickwork, and the injury it might thereby sustain, the opposite sides of the framing in each arch were to be connected by tie-beams, similar to those used in roofing.
It would not be necessary to make the whole area of the arch to open, as a. comparatively small opening would be sufficient for conducting the operation of the miners. The opening gates are therefore represented as occupying only a small portion in the middle of the strong framing which fills up the arch.
It will of course be of the greatest importance that the threshold, or cells, against which the bottoms of the first gates shut, be secured; and for that purpose the portion of the roadway which passes through the gates, is so balanced and supported, that a very small portion of water accumulated under it will disengage its supports, and project part of the roadway or platform outside of the gates. The threshold may be further secured, if necessary, by a covering of canvass so attached to the gates as to be rolled off by them in the act of shutting.
The joinings between the frame and brickwork, as well as the joinings round the gates, are to be made air and water tight, by triangular packings of leather or other soft material, which could be drawn into the crevices by a series of screw bolts passing through to the outside of the gates, where the workmen could at their leisure screw the packing up after all the gates were shut.
The method of moving the gates forward, and of securing them in their places, is shown in Fig. 2, where t represents a vertical section of a set of floodgates, supported in its place by three pair of strong beams, represented at s, fastened together at r; the other ends of these beams are attached to the floodgates, three on each side, at a small distance from the edge. The piece r rests upon a friction-roller or small wheel, and against a powerful screw-jack o, which is supported by the abutment p, fixed into the bottom of the tunnel, and kept in its place by the vertical beam q.
When the gates are to be moved forward, the triangular packing round the edges of the frame must be released, and moved back, by unscrewing the bolts which keep it in its place, andthen the gates are forced forward on the smooth supports on which they rest, by the screw o; and when they have been moved to their assigned place, the screw is returned into its box, and the abutments are brought up, and the whole apparatus again properly secured.
The box represented to the left of the eastern arch would be sufficiently capacious to hold two or three men; it is provided with two doors, one of which opens into the box, and the other into that part of the tunnel which would be full of water when the flood-gates are all closed. The use of this box is for a man, harnessed in James's diving apparatus, to enter the part filled with water, for the purpose of exploring and examining the works, and bringing out any thing of importance, requiring to be removed from the water. The man, having provided in his diving apparatus a sufficient supply of air for the time he intends to remain in the water, enters the box, and closes the door; he then, by means of a stop-cock, admits the water into the box, when the door between the box and the interior can be easily opened to admit him. In coming out, he has only to re-enter the box, shut the communication between the box and the interior, and then, by a stop-cock, let the water contained in the box issue into the open part of the tunnel.
This might be repeated as often as occasion may require, with very little escape of water, and with perfect safety to the diver; for, theingress of the water being entirely prevented by the flood-gates, it would be perfectly quiescent within them, and no danger need be apprehended from a change or derangement of any part of the works taking place during the miner's inspection. The small space between the shield and flood-gates would soon be filled, when all would become stationary; and consequently the principal cause of damage to the works, the rushing of a large quantity of water with great violence, would be removed. If it should be objected, that the time occupied in filling the space between the gates and the shield would be too short to allow the workmen to escape, it may be answered, that very little time would be required for all the workmen to get outside, where they would be perfectly safe, and might leisurely view the progress of the water in filling the space and closing the gates: besides, an irruption of the water, under such circumstances, would be of so little consequence, that there would be no occasion for detaining the men in attempting to stop the torrent, till their lives were in danger.
The water within being perfectly still, the bed of the river might be made good and the water pumped out.
The following statement is an account of the progress of the work. In 1836, 117 feet were completed; in 1837, only 28 feet; in 1838, 80 feet; in 1839,194 feet; and by March 1840, 76 feet. It was then 60 feet from the Wapping shore. This distance was completed early in 1843, and the public were enabled to pass from one side of the river to the other on the 25th of March in the same year.