In the Museum of History and Technology has recently been established
a Hall of Civil Engineering in which the engineering of tunnels is
comprehensively treated from the historical standpoint--something not
previously done in an American museum. The guiding precept of the
exhibit has not been to outline exhaustively the entire history of
tunneling, but rather to show the fundamental advances which have
occurred between primitive man's first systematic use of fire for
excavating rock in mining, and the use in combination of compressed
air, iron lining, and a movable shield in a subaqueous tunnel at the
end of the 19th century. This termination date was selected because it
was during the period from about 1830 to 1900 that the most
concentrated development took place, and during which tunneling became
a firmly established and important branch of civil engineering and
indeed, of modern civilization. The techniques of present-day
tunneling are so fully related in current writing that it was deemed
far more useful to devote the exhibit entirely to a segment of the
field's history which is less commonly treated.
[Illustration: Figure 3.--HOOSAC TUNNEL. WORKING OF LATER STAGES with
Burleigh pneumatic drills mounted on carriages. The bottom heading is
being drilled in preparation for blasting out with nitroglycerine.
MHT model--1/2" scale. (Smithsonian photo 49260-M.)]
The major advances, which have already been spoken of as being ones of
technique rather than theory, devolve quite naturally into two basic
classifications: the one of supporting a mass of loose, unstable,
pressure-exerting material--soft-ground tunneling; and the
diametrically opposite problem of separating rock from the basic mass
when it is so firm and solid that it can support its own overbearing
weight as an opening is forced through it--rock, or hard-ground
tunneling.
To exhibit the sequence in a thorough manner, inviting and capable of
easy and correct interpretation by the nonprofessional viewer, models
offered the only logical means of presentation. Six tunnels were
selected, all driven in the 19th century. Each represents either a
fundamental, new concept of tunneling technique, or an important,
early application of one. Models of these works form the basis of the
exhibit. No effort was made to restrict the work to projects on
American soil. This would, in fact, have been quite impossible if an
accurate picture of tunnel technology was to be drawn; for as in
virtually all other areas of technology, the overall development
in this field has been international. The art of mining was first
developed highly in the Middle Ages in the Germanic states; the tunnel
shield was invented by a Frenchman residing in England, and the use of
compressed air to exclude the water from subaqueous tunnels was first
introduced on a major work by an American. In addition, the two main
subdivisions, rock and soft-ground tunneling, are each introduced by
a model not of an actual working, but of one typifying early classical
methods which were in use for centuries until the comparatively recent
development of more efficient systems of earth support and rock
breaking. Particular attention is given to accuracy of detail
throughout the series of eight models; original sources of descriptive
and graphic information were used in their construction wherever
possible. In all cases except the introductory model in the
rock-tunneling series, representing copper mining by early
civilizations, these sources were contemporary accounts.
The plan to use a uniform scale of reduction throughout, in order
to facilitate the viewers' interpretation, unfortunately proved
impractical, due to the great difference in the amount of area to be
encompassed in different models, and the necessity that the cases
holding them be of uniform height. The related models of the Broadway
and Tower Subways represent short sections of tunnels only 8 feet or
so in diameter enabling a relatively large scale, 1-1/2 inches to the
foot, to be used. Conversely, in order that the model of Brunel's
Thames Tunnel be most effective, it was necessary to include one of
the vertical terminal shafts used in its construction. These were
about 60 feet in depth, and thus the much smaller scale of 1/4 inch
to the foot was used. This variation is not as confusing as might be
thought, for the human figures in each model provide an immediate and
positive sense of proportion and scale.
Careful thought was devoted to the internal lighting of the models, as
this was one of the critical factors in establishing, so far as is
possible in a model, an atmosphere convincingly representative of work
conducted solely by artificial light. Remarkable realism was achieved
by use of plastic rods to conduct light to the tiny sources of tunnel
illumination, such as the candles on the miners' hats in the Hoosac
Tunnel, and the gas lights in the Thames Tunnel. No overscaled
miniature bulbs, generally applied in such cases, were used. At
several points where the general lighting within the tunnel proper has
been kept at a low level to simulate the natural atmosphere of the
work, hidden lamps can be operated by push-button in order to bring
out detail which otherwise would be unseen.
The remainder of the material in the Museum's tunneling section
further extends the two major aspects of tunneling. Space limitations
did not permit treatment of the many interesting ancillary matters
vital to tunnel engineering, such as the unique problems of
subterranean surveying, and the extreme accuracy required in the
triangulation and subsequent guidance of the boring in long mountain
tunnels; nor the difficult problems of ventilating long workings, both
during driving and in service; nor the several major methods developed
through the years for driving or constructing tunnels in other than
the conventional manner.[1]