[1] There are two important secondary techniques for opening
subterranean and subaqueous ways, neither a method truly of
tunneling. One of these, of ancient origin, used mainly in the
construction of shallow subways and utility ways, is the "cut and
cover" system, whereby an open trench is excavated and then roofed
over. The result is, in effect, a tunnel. The concept of the other
method was propounded in the early 19th century but only used
practically in recent years. This is the "trench" method, a sort
of subaqueous equivalent of cut and cover. A trench is dredged in
the bed of a body of water, into which prefabricated sections of
large diameter tube are lowered, in a continuous line. The joints
are then sealed by divers, the trench is backfilled over the tube,
the ends are brought up to dryland portals, the water is pumped
out, and a subterranean passage results. The Chesapeake Bay Bridge
Tunnel (1960-1964) is a recent major work of this character.
[2] In 1952 a successful machine was developed on this plan, with
hardened rollers on a revolving cutting head for disintegrating
the rock. The idea is basically sound, possessing advantages in
certain situations over conventional drilling and blasting
systems.
[3] In 1807 the noted Cornish engineer Trevithick commenced a small
timbered drift beneath the Thames, 5 feet by 3 feet, as an
exploratory passage for a larger vehicular tunnel. Due to the
small frontal area, he was able to successfully probe about 1000
feet, but the river then broke in and halted the work. Mine
tunnels had also reached beneath the Irish Sea and various rivers
in the coal regions of Newcastle, but these were so far below the
surface as to be in perfectly solid ground and can hardly be
considered subaqueous workings.
[4] Unlike the Brunel tunnel, this was driven from both ends
simultaneously, the total overall progress thus being 3 feet per
shift rather than 18 inches. A top speed of 9 feet per day could
be advanced by each shield under ideal conditions.
[5] Ideally, the pressure of air within the work area of a
pneumatically driven tunnel should just balance the hydrostatic
head of the water without, which is a function of its total height
above the opening. If the air pressure is not high enough, water
will, of course, enter, and if very low, there is danger of
complete collapse of the unsupported ground areas. If too high,
the air pressure will overcome that due to the water and the air
will force its way out through the ground, through increasingly
larger openings, until it all rushes out suddenly in a "blowout."
The pressurized atmosphere gone, the water then is able to pour
in through the same opening, flooding the workings.
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