Tunnels are underground passages made for the purpose of continuing
roads, railways, or canals through hills or elevated parts of the
ground. In slight elevations “cuttings” are generally made, and it
becomes a question of expediency for the engineer as to which of
the two shall be chosen, as there will be a much greater quantity of
earth to remove from a deep cutting than from a tunnel; in fact, the
question is often at once determined by the demand for this material,
for if it should so happen that the level of the road is such, that
many or large embankments are required, the earth taken from the
cuttings will be wanted to construct them. As a general rule, tunnels
are only made where a hill has to be penetrated which is too high to
be cut through; another object sought by tunnelling is to cause the
road to pass beneath a canal, river, buildings, or roads, where the
level of the road or railway is far below them. The Thames Tunnel is
the most remarkable example of this kind of work.
Tunnels were constructed by the ancient Greeks and Romans, chiefly
for the purpose of forming aqueducts, and for draining lakes. A few
years back, tunnels were not very frequently undertaken, those that
were made being chiefly on the lines of canals, but they are now far
more numerous, on account of the great number of railways, on which
they frequently occur. Before a tunnel is undertaken, the nature of
the soil should be thoroughly examined by means of “borings,” sunk
down to the intended level, some soils being much less favourable
than others for this purpose. Quicksand, or sand percolated by
water, is one of the greatest impediments to tunnelling, while rock
(contrary to what would at first be thought) is one of the least, as
in this case there is no occasion to line the tunnel with brickwork,
which more than saves the extra expense of excavation, as that can
be rapidly done by blasting, where it is too hard to dig, while
in the former case the difficulty of draining off the water, and
the cost of brickwork of sufficient solidity to support such loose
earth, often prevent the undertaking. Tunnels of great length are
sometimes begun at several different parts by means of shafts sunk
down to the required level, and each made to unite into one, but
this depends upon where the earth is to be deposited; on railways it
is common that the “level” is so chosen that the earth from tunnels
and cuttings is always wanted to fill up hollows in their immediate
vicinity (see “Railways”), and when this is the case a series of
“trucks” drawn by an engine on temporary tramways remove the earth
from the tunnel as it progresses, to the next embankment, both works
proceeding at the same time.
When a small portion of the tunnel is excavated, the casing of
brickwork is begun from the mouth of it, and continued a small
piece at a time, following closely the excavation. The brickwork
is generally begun at each side, and carried up to the height at
which the arch springs; an inverted arch is then constructed from
one side to the other to form the floor, and “centreings” resting
upon uprights are placed, to “turn” the arch upon. These centreings
are only about two feet deep, and span the arch from side to side;
the men work with their backs to the part being excavated, and are
able to reach across the narrow centreing to lay the bricks. When
completed all round, and the spaces between the brickwork of the arch
and the surface of the excavation filled up with earth, the centreing
is shifted further in and a fresh ring of brickwork commenced,
the excavation going on at the same time. In many tunnels, large
perpendicular shafts are sunk down for the purposes of ventilation
and lighting.