With few exceptions, civil engineering is a field in which the ultimate
goal is the assemblage of materials into a useful structural form
according to a scientifically derived plan which is based on various
natural and man-imposed conditions. This is true whether the result be,
for example, a dam, a building, a bridge, or even the fixed plant of a
railroad. However, one principal branch of the field is based upon an
entirely different concept. In the engineering of tunnels the utility of
the "structure" is derived not from the bringing together of elements
but from the separation of one portion of naturally existing material
from another to permit passage through a former barrier.
In tunneling hard, firm rock, this is practically the entire compass
of the work: breaking away the rock from the mother mass, and,
coincidently, removing it from the workings. The opposite extreme in
conditions is met in the soft-ground tunnel, driven through material
incapable of supporting itself above the tunnel opening. Here, the
excavation of the tunneled substance is of relatively small concern,
eclipsed by the problem of preventing the surrounding material from
collapsing into the bore.
[Illustration: Figure 2.--HOOSAC TUNNEL. METHOD OF WORKING EARLY
SECTIONS of the project; blast holes drilled by hand jacking.
MHT model--1/2" scale. (Smithsonian photo 49260-L.)]
In one other principal respect does tunnel engineering differ widely
from its collateral branches of civil engineering. Few other physical
undertakings are approached with anything like the uncertainty
attending a tunnel work. This is even more true in mountain tunnels,
for which test borings frequently cannot be made to determine the
nature of the material and the geologic conditions which will be
encountered.
The course of tunnel work is not subject to an overall preliminary
survey; the engineer is faced with not only the inability to
anticipate general contingencies common to all engineering work, but
with the peculiar and often overwhelming unpredictability of the very
basis of his work.
Subaqueous and soft-ground work on the other hand, while still subject
to many indeterminates, is now far more predictable than during its
early history, simply because the nature of the adverse condition
prevailing eventually was understood to be quite predictable. The
steady pressures of earth and water to refill the excavated area are
today overcome with relative ease and consistency by the tunneler.
In tunneling as in no other branch of civil engineering did empiricism
so long resist the advance of scientific theory; in no other did the
"practical engineer" remain to such an extent the key figure in
establishing the success or failure of a project. The Hoosac Tunnel,
after 25 years of legislative, financial, and technical difficulties,
in 1875 was finally driven to successful completion only by the
efforts of a group who, while in the majority were trained civil
engineers, were to an even greater extent men of vast practical
ability, more at home in field than office.
DeWitt C. Haskin (see p. 234), during the inquest that followed the
death of a number of men in a blowout of his pneumatically driven
Hudson River Tunnel in 1880, stated in his own defense: "I am not a
scientific engineer, but a practical one ... I know nothing of
mathematics; in my experience I have grasped such matters as a whole;
I believe that the study of mathematics in that kind of work
[tunneling] has a tendency to dwarf the mind rather than enlighten
it...." An extreme attitude perhaps, and one which by no means adds to
Haskin's stature, but a not unusual one in tunnel work at the time. It
would not of course be fair to imply that such men as Herman Haupt,
Brunel the elder, and Greathead were not accomplished theoretical
engineers. But it was their innate ability to evaluate and control the
overlying physical conditions of the site and work that made possible
their significant contributions to the development of tunnel
engineering.
Tunneling remained largely independent of the realm of mathematical
analysis long after the time when all but the most insignificant
engineering works were designed by that means. Thus, as structural
engineering has advanced as the result of a flow of new theoretical
concepts, new, improved, and strengthened materials, and new methods
of fastening, the progress of tunnel engineering has been due more to
the continual refinement of constructional techniques.