Such was the deep interest taken by ingenious and scientific men for the pro secution of this tunnel, that soon after the first irruption, Mr. Brunel received, (according to report,) no less than 260 written plans, which, together with ver bal communications, made altogether 400 proposed remedies for the disaster. Amongst these there were some which displayed considerable ingenuity ; and the best, according to our information, was the following, which we insert, as the application of the principle of its construction may hereafter prove of eminent utility in tunnelling under a body of water. The inventor was a Mr. Garvey, a modeller, and an active member of the London Mechanics' Institution, and who, we regret to add, fell an early victim to the cholera in 1832. Mr. Garvey's plan, as stated by himself; " consists in placing at the bottom of the river, directly over the part undergoing excavation, a large platform or raft, with ledges proceeding downwards to fix into the soil, to prevent the water from entering the excavation." The nature and operation of this will be understood by reference to the drawing on page 808, where S S represents a section of the tunnel ; It the mud, gravel, &c., constituting the bed of the river; A B, the square platform, about twice the width of the tunnel, consisting of two layers of planks, crossing each other at right angles, and made water and air-tight by a stratum of artificial leather, tarpauling, or other elastic waterproof material, between the layers; G G, and H H, represent sections of the ledges or rims, which may be made of iron, or wood pointed with iron ; the platform must be loaded sufficiently to sink in water. F is a pipe for the escape of the air while the platform is descending in the water ; and E is a pump to draw off the water from under it, when it reaches the bot tom; e o are sliding valves, to be opened or shut at pleasure, by the cords pass ing over the pulleys m m and a n ; the bent pipes I s are for the escape of the air or water, flrom the space between the ledges G and H. When the apparatus is put down to the bottom of the river, the water is to be removed from under neath by the pump E, which will produce a very great hydrostatic and pneu matic pressure on its surface, and cause the points of the ledges, G and H, to penetrate the bed of the river, and the whole to become firmly fixed in its place. The cavity M, which extends of conrse all round the raft, is made conical, for the purpose of compressing the soil between the rims as they are forced down, and thus preventing the entrance of the water at the edges.
When the apparatus is to be moved forward to a new station, the pump E is to be a condensing air-pump, by changing the valves ; and air is to be forced under the raft till it is disengaged from the bottom, when it can with facility be moved forward in the water, and sunk as before.
When the bed of the river is very irregular and gravelly, it may be necessary to dredge it, and put down clay in some parts before the platform is brought to its place.
Having described that which is stated to have been the best of the rejected plans, (as acknowledged by Mr. Brunel to the inventor of it, the late Mr. Garvey,) we shall proceed to notice that which was unfortunately adopted in preference. The concavity in the bed of the river, and the hole through which the water rushed into the tunnel on the 18th of May, was first filled with clay, bags of clay and gravel ; a large fiat wooden raft, (without ledges,) was then sunk over the new-made ground, to prevent any sudden displacement of it, and by that means afford a full protection to the workmen, when they might recommence excavating underneath. The water, however, found its way under
the raft, and the powerful engine and pumps were employed for a considerable period without lowering the level of it in the tunnel. The works were about half emptied of the water, when the force of the tide raised up one side of the raft, threw off the weights which had kept it down, when it floated up to the surface of the river. The ground in another part contiguous to the former bole now gave way, and the tunnel was again filled with water. Fresh quantities of clay and bags of clay were then itmlAoyed 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 imp 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. Although this plan has proved an effectual remedy as far as it has been applied, the repetition of such remedies, whenever quicksand may be met with, or irruptions formed in the future progress of the work, must be attended with a wasteful expense: we therefore submit that Mr. Garvey's plan deserves the preference, as it may be shifted from place to place, as the work proceeds.
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 fewyards behind the workmen, flood-gates, so constructed, that the lower parts of the gates will be first abut by the water issuing in at the place where the work is carried on ; and when the waters rise nearly half way up, then to shut the middle parts of the gates; and when it rises near to the top, to shut the top parts of the gates. This arrangement would afford 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 tend to obviate much of the danger to be apprehended by the workmen, but greatly diminish the enormous expense consequent from 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 be soon made good from above, as then there would be 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 gates, &c.
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 proceeds. In order to make the plan better understood, the water is repre sented 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 stand ing open.