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The Thames Tunnel (continued)

The first operation connected with the works, was the construction of a shaft; and for this purpose land was bought on the Rotherhithe bank, about fifty yards from the river. On March 2, 1825, the ceremony of laying the first stone of the shaft was performed.

Mr. Smith, our chairman, attended by most of the members of the Court of Directors, and a very numerous cortége of friends invited on the occasion, proceeded from the Tunnel Wharf to the ground, where they were received among the cheers of a great concourse of people. Mr. Smith addressed the assembly in a very eloquent speech suitable to the occasion, and performed the ceremony of laying the first stone. From this day dates the beginning of the work. [1]

The mode in which Sir Isambard decided to construct the shaft was one not uncommonly adopted in the construction of wells; but to apply it to sinking a shaft fifty feet in diameter was a novel and bold undertaking. The brickwork intended to form the lining of the shaft was built on the surface of the ground, and the earth being excavated from within and underneath the structure, it sank gradually down to its final position.

The brickwork was 3 feet thick, bound together by iron and timber ties, and there were built into it 48 perpendicular iron rods, one inch in diameter, fastened to a wooden curb at the bottom, and to another curb at the top of the wall, by nuts and screws.

Drawing of a Thames Tunnel shaft
The shaft, from whence the Tunnel works are carried on, was built at Rotherhithe in the form of a tower, 50 feet in diameter, 42 feet in height, and 3 feet thick, at about 150 feet from the edge of the wharf, and it was sunk into its position by excavating the earth within. From An explanation of the works of the tunnel under the Thames from Rotherhithe to Wapping (1836).

When the shaft or tower of brickwork was completed up to the top, 42 feet in height, the next step was to remove the blockings on which it rested, and this being done the gravel was excavated and hoisted up, and the shaft descended by its own weight.

The Rotherhithe shaft was only sunk forty feet in this manner; the remaining twenty feet, in order to leave the opening for the Tunnel, was constructed by under-pinning, or underlaying, as it was then termed. The underlaying was commenced in the beginning of June. [2]

By July 4 they had got down to the level of the intended foundation of the shaft, having passed into a stratum of gravel, black pebbles embedded in greenish sand, with little or no water; from which circumstance Sir Isambard was of opinion that it was unconnected with the stratum of gravel above.

July 12.—Engaged on a general drawing for the great shield, and in preparing some instructions for moving the same (a very intricate operation!)

July 22.—Underlaying is a very laborious mode of proceeding. The sinking of a wall well bound as the first, would evidently be the best and cheapest mode for making another tower of 50 feet diameter.

On the 28th Sir Isambard enters in his journal the following additional observations upon the success of his plans for sinking the shaft:—

Considering the great labour necessary for securing the ground for the underlaying, the waste of planking, and of shores, and the time necessarily taken up in moving about, in securing and in baling out the water, and the many causes of interruption, and the imperfect way that things are done in underlaying, it is quite conclusive that the original plan of making a shaft, by sinking the structure, is the safest and the most economical. What is done is sound, and when once in place, may be secured with foundations in a very easy manner. The brickwork of the shaft is remarkably hard. Had it been made with brick facings and rubble stone, it would certainly be water-tight, and almost impenetrable by ordinary ways. The vertical ties and the circular wall bands are not to be dispensed with in a structure destined to be moved as the present has been.

On August 11 the underlaying was completed, and preparations were made for constructing a reservoir in the bottom of the shaft for receiving the permanent pumps. This was finished on October 11, with great difficulty, owing to the nature of the ground, which consisted of loose sand containing a large quantity of water.

August 19.—Engaged at home in revising my plans for the manner of carrying on the horizontal excavation, more particularly of penetrating through the shaft. This part of the operation requires indeed very great attention, as it presents great difficulties, arising from the wall to be broken through, and chiefly from the angular opening that is to be made at each extremity. Then another consideration is the uniting the brick arches to the brickwork of the shaft.

September 16.—Engaged in the early part of the day on revising my plans of future operations in the Tunnel work, and in adapting them to the nature of the ground as it is found at the various depths we have penetrated: namely, to about 73 feet. Went afterwards to Maudslay to request that the great shield may be completed.

October 14.—Engaged in the early part of the morning in making Great shield, some arrangements for the working of the great shield. Too much attention cannot be given to that subject at the early part; for, when once in its place, it would be extremely inconvenient to make any alteration.

October 15.—The dome of the reservoir will be covered to-day about noon; the bottom of the shaft will therefore be completed. They are now preparing to apply two frames of the shield. The ground now open in the front is remarkably hard; it consists of pebbles imbedded in a chalky substance, with hard loose stones of the nature of the Kentish rag. Everything is going on well. Devised with Isambard how to make our wells for the descent of the materials, &c. [marginal note: Preparing for the Frames]

Thus at last the shaft was completed, and Sir Isambard was able to commence the Tunnel itself, which he ultimately determined to construct in the form of a rectangular mass of brickwork, 37½ feet wide and 22 feet high, pierced by two parallel horseshoe archways, each 14 feet wide and 17 feet high.

[1] The paragraphs in small type, without any reference, are from Sir Isambard’s journals. The sentences inserted at the side are his marginal summary [editor’s note: in this copy they will be in square brackets following the paragraph, and noted as “marginal note:”]. Occasionally a few words are added (in square brackets) by way of explanation.

[2] The shaft subsequently made on the Wapping shore was sunk to its full depth without any under-pinning.

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