A _tunnel_, in engineering, is a subterranean passage cut through a
hill, or under a river, for the purpose of carrying a canal, road, or
railway, &c. One of the most remarkable works of this kind, ever
executed, is the tunnel under the river Thames, planned by Mr. Brunel,
and successfully executed under his direction. Two previous attempts had
been made to carry a tunnel under the river; one in 1799, and the other
in 1804; but both were unsuccessful. In 1824, however, an act of
parliament, authorizing operations on the plan of Mr. Brunel, was
obtained; and shortly after the work was commenced. A short account of
the progress of the work will probably be the best mode of conveying a
notion of the nature and difficulty of tunneling in general.
Mr. Brunel began his operations by making preparations for a shaft fifty
feet in diameter, which he commenced one hundred and fifty feet from the
river on the Surrey side; this he effected by constructing on the
surface of the ground a substantial brick cylinder of that diameter,
forty-two feet in hight and three feet in thickness. Over this he set up
a steam-engine, necessary for pumping out the water, and for raising the
earth to be taken from within the cylinder, and then proceeded to sink
it bodily into the earth. By this means he succeeded in passing through
a bed of sand and gravel twenty-six feet deep, constituting, in part, a
quicksand, and in which the drift-makers of the former undertaking had
been compelled to suspend their work. The cylinder having been sunk to
the depth of sixty-five feet, the horizontal excavation was commenced at
the depth of sixty-three feet; and in order to have sufficient thickness
of ground to pass safely under the deep part of the river, the
excavation was made to descend two feet and three inches in every
hundred feet. This excavation is thirty feet wide, and twenty-two and a
half feet high, and the process of making it may briefly be described as
follows.
It was accomplished by means of a powerful apparatus of iron, called a
_shield_, and which consisted of twelve large frames, standing close to
each other, like so many volumes on the shelf of a book-case, these
frames being twenty-two feet in hight, and about three feet in width.
They were divided into three stages or stories, thus presenting
thirty-six cells or chambers for the miners. The front of each one of
these cells was protected by narrow boards, technically called
polling-boards, each of which was separately held in its place by an
apparatus constructed for the purpose. The miner commenced by removing
the upper polling-board in his division of the shield, thus exposing a
small portion of earth; into this earth he made an excavation of six
inches in depth, throwing the earth behind him, from whence it was
removed to the mouth of the tunnel, and from thence raised by steam to
the surface of the ground. He then replaced the polling-board, causing
it to press against the face of the newly excavated earth, and thus
advancing it six inches beyond the other polling-boards of his division.
Then successively taking down the remaining boards, excavating the earth
six inches behind them, and replacing the boards six inches further in
than before, he very soon had advanced that distance over the whole
length of his division. All the other miners in the thirty-six cells
having done the same, the framework was moved forward, and six inches
more of earth removed. It was in this way, by these slow degrees, that
the work was finally completed. As the frame-work advanced, it was
closely followed by a solid mass of brick-work, inclosing two arched
passages. These two passages were separated by a solid wall, three and a
half feet at the top and four at the bottom. Other arches, however, were
formed in this wall, for the purpose of opening a communication between
one tunnel and the other. The whole of the brick-work is laid in Roman
cement, and each archway is finished with a lining of cement, a
carriage-road, and a narrow foot-path adjoining the central wall.
This immense enterprise was not finally completed without serious delay
and apparently insurmountable obstacles. The works were thrice
interrupted: in 1826, by the breaking off of the clay, leaving the
shield exposed to the influx of the land-water for six weeks; also in
May, 1827, and in January, 1828, when the river broke in and filled the
tunnel. This was quickly remedied, however, by filling the holes or
chasms with strong bags of clay; the structure, on clearing the tunnel
of the water, being found in a most satisfactory state. Some time later,
the works were suspended for seven years, owing to the want of funds.
Parliament, however, after repeated applications, granted an advance for
their completion, and the works were resumed and continued, till they
were brought to a successful termination. The cost of the tunnel, with
the approaches on both sides of the river, was about three million and a
half dollars; much less than the cost of the modern metropolitan bridges
which span the Thames between Surrey and Middlesex.