[Illustration: Transverse Section of the Thames Tunnel.]
[Illustration: Section of Drift-way.]
As far back as the year 1802, a project was set on foot by some
enterprising gentlemen, with a view of opening an archway under the
Thames, between Rotherhithe and Limehouse, not far from the line of the
present tunnel. The engineer selected for this enterprise was
particularly qualified for such an undertaking, being an experienced
Cornish miner. Having made some borings at the Horse-ferry and on the
opposite side of the river, he reported that “he was firmly persuaded
the undertaking would not cost so much as had been conceived.” A
subscription was, in consequence, raised; and a company was formed,
under the denomination of the “Thames Archway Company.” Surveys, plans,
and estimates were made, and an Act of Parliament being obtained, the
work was begun. The engineer commenced operations by sinking a shaft of
11 feet diameter, at 330 feet from the line of the wharf on the
Rotherhithe side. But the obstacles which he encountered from the nature
of the ground increased to such a degree, as he proceeded, that at the
depth of 42 feet he was obliged to desist. A subsequent report of
borings, however, having proved very favourable, an enterprising
proprietor engaged to complete the shaft (reduced to 8 feet diameter) to
76 feet, at which depth it was discovered that it would be dangerous to
go deeper. At this stage of the proceedings, viz. in August, 1807, a
second engineer was engaged by the Company, a gentleman whose name had
been coupled with very great enterprises in the mining department.
Before opening the drift-way both engineers agreed to reduce its breadth
to 2 feet 6 inches at the top, and 3 feet at the bottom. At the depth of
76 feet they found the ground to consist of a _firm dry sand_; and there
they opened the drift, which they carried forward in a gentle ascent. In
November, 1807, when 394 feet of the drift had been completed, the
services of the first engineer were dispensed with after four years and
a half of hard labour. The Directors then agreed to give the second
engineer £1000, by way of premium, if he succeeded in reaching the
opposite shore. The drift was further extended to 814 feet, through
equally firm dry ground, with the precaution, which had been employed
from the beginning, of a substantial planking all the way. One hundred
and thirty-eight feet more were cut through a bed of calcareous rock
eight feet thick. But on the 21st of December, the head of the drift had
hardly entered two feet into the stratum, which lay immediately over the
rock, when the roof broke down in a loose state, leaving above head a
cavity large enough for a man to stand in it. It is to be observed that
there was no less than thirty feet of intervening ground between the
drift and the river at the time this accident happened. The engineer
succeeded in filling and securing the cavity; but, such was the nature
of the whole ground above the rock, that, under the influence of an
extraordinary high tide (on the 26th of January 1808), the ground again
made its way fast in a loose state into the drift, and the river soon
broke through 25 feet of ground. This same tide caused the destruction
of the Deptford and Lewisham bridges. The engineer having succeeded in
filling and closing this hole, the miners re-entered the drift, which
was reduced to three feet in height, for the purpose of clearing the
dangerous place. The miners had, therefore, to work on their knees:
however, notwithstanding every effort to attain the opposite shore, they
were driven away by the frequent bursts of sand and water. The engineer
having afterwards sounded the ground from above, reported that he had no
doubt the two fractures communicated underneath; and therefore admitted
that it was quite impracticable to go further except by means of a
cofferdam or caissons. On the 30th of March, 1809, the Directors offered
a reward for the most approved plan of completing the archway.
Fifty-four plans having been obtained by this announcement, they were
referred to the opinion of scientific men. These gentlemen reported that
they were unanimously of opinion, that an archway, of any useful size,
was impracticable under the Thames by an underground excavation on any
of the plans that had come before them; observing, at the same time,
that they did not pretend to assign limits to the ingenuity of other
men. A further trial was made by a third engineer, who operated from
above the river, but it proved equally fruitless. Thus ended in 1809 all
the exertions and the efforts made during nearly seven years, for the
purpose of accomplishing an archway under the bed of the Thames; at the
end of which period not so much as a drain had been completed, nor had
the miners succeeded in working in any of those strata wherein the
excavation for the archway must eventually have been effected.
Several years afterwards, Mr. Brunel was prevailed upon by one of the
most active promoters of the archway enterprise (Mr. J. Wyatt) to turn
his attention to the subject; and, being furnished with the documents
connected with the first attempt, he devised his plan with the
impression that both the excavation and the structure might be made on a
full scale at once.
Before proceeding to an exposition of the plan adopted by Mr. Brunel,
and of the means by which he has carried it into execution, we have to
state that the structure of the Thames Tunnel, as represented in the
annexed view, is 38 feet in width and 22 feet 6 inches in height
externally and that a length of 600 feet, in the style of a double
arcade, has been made, though one arch only is open to public
inspection. The excavation therefore made under the Thames for this
structure presents a sectional surface of 850 feet, which is equal to
sixty times the area of the drift; but for a more comprehensive
demonstration of this aggregate, the reader is to understand that the
excavation which has been made for the Thames Tunnel is larger than the
House of Commons, the dimensions of which are 32 feet in width and 25
feet in height, presenting therefore only 800 feet of sectional surface.
At high water, the head of the river is about 75 feet above the foot of
the excavation, and consequently three times the height of that room.
These circumstances, independently of the nature of the ground, are
sufficient to place the work of the Thames Tunnel among the boldest
enterprises in the art of engineering.
Notwithstanding that the first attempt had contributed to discourage all
idea of success, there were still sufficient evidences to indicate that
by beginning in the stratum of dry firm sand, and keeping close under
the stratum of clay forming the bottom of the river, there was space
enough to effect the object, although the nature of the intervening
ground had been ascertained to be very loose in many places. All the
information obtained from the miner’s report concurred with the opinions
of geologists in pointing out that the most eligible line for the Tunnel
was to keep as near to the bottom of the river as the security of the
work would permit. The first idea of the plan which appeared to the
engineer best calculated for making an excavation fit for the object
under so overwhelming a head of water, was suggested by the sight of a
piece of a keel of a ship which had been eroded by the operation of the
worm called the terido. From this he conceived it practicable, as his
specification describes it, to make a circular opening of sufficient
capacity at once. However, of the two modes which he described, he gave
the preference to that of proceeding by forming simultaneously several
contiguous excavations by means of an apparatus which has been
denominated the shield. This shield upon the whole partakes of the
character of a powerful cofferdam, applied in a horizontal instead of
the vertical direction. It consists of twelve parallel frames lying
close to each other like so many volumes in a bookcase. Each frame,
being nearly 22 feet in height, is divided into three stories: the whole
presents therefore thirty-six openings or cells. It is from these cells
that the miners, operating by small quantities at a time, like so many
teridos, are able to erode the ground in front, while others at the back
bring up a substantial incrustation, namely the brick structure. For
locomotive action each frame is provided with two substantial legs
resting on equally substantial shoes, (not unlike snow-shoes); these
legs are provided with articulations that fit the frames for a pacing
movement. The shield has perambulated 600 feet of its assigned career;
and has left behind a substantial structure in the form of a double
arcade, which is now as long or nearly as the Burlington Arcade.
With regard to the external form of the structure, and the mode adopted
for its execution, it must be obvious to persons acquainted with such
matters, that the most substantial form, and the best calculated at the
same time to prevent, as far as practicable, any derangement in alluvial
strata of various degrees of density, is the square form, as
corresponding with that mode of building which is technically called
underjoining and underlaying. Thus, in fact, the bed of the river, with
its contents, has been underlayed, just as the Customhouse has been, to
receive the superstructure.
An indispensable requisite in a work of this nature was, that it should
be made proof against the greatest disasters that were to be
apprehended, notwithstanding every precaution that could be taken. Mr.
Brunel’s plan was considered by his Grace the Duke of Wellington, by Dr.
Wollaston, and by those engineers and scientific men who had the
opportunity of examining the designs, and of hearing the description
given by the engineer, as being well calculated to accomplish the
contemplated object, although some apprehensions were raised at the time
as to what might result from so formidable an occurrence as an irruption
of the river, considering the extent of the devastation it might cause
in the ground and among the works. The engineer afforded such
explanations as allayed, in some degree, those apprehensions which, it
must be admitted, he has since completely dispelled by undeniable facts.
It was under these auspices that the plan was brought before the public
in 1823, and that in the month of February, 1824, subscriptions were
obtained to a large amount to carry it into effect, notwithstanding the
novelty of the scheme, and its risks.
[To be continued.]
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