From the earliest periods of rock working, the quest for minerals
and metals was the primary force that drove men underground. It was
the technology of mining, the product of slow evolution over the
centuries, that became the technology of the early tunnel, with no
significant modification except in size of workings.
Every aspect of 16th century mining is definitively detailed in
Georgius Agricola's remarkable _De re Metallica_, first published in
Basel in 1556. During its time of active influence, which extended
for two centuries, it served as the authoritative work on the subject.
It remains today an unparalleled early record of an entire branch
of technology. The superb woodcuts of mine workings and tools in
themselves constitute a precise description of the techniques of the
period, and provided an ideal source of information upon which to base
the first model in the soft-ground series.
[Illustration: Figure 15.--CENTERING FOR PLACEMENT OF FINISHED
STONEWORK at west portal, 1874. At top-right are the sheds where
the lining brick was produced. (_Photo courtesy of State Library,
Commonwealth of Massachusetts._)]
The model, representing a typical European mine, demonstrates the
early use of timber frames or "sets" to support the soft material of
the walls and roof. In areas of only moderate instability, the sets
alone were sufficient to counteract the earth pressure, and were
spaced according to the degree of support required. In more extreme
conditions, a solid lagging of small poles or boards was set outside
the frames, as shown in the model, to provide absolute support of the
ground. Details of the framing, the windlass, and all tools and
appliances were supplied by Agricola, with no need for interpretation
or interpolation.
The basic framing pattern of sill, side posts and cap piece, all
morticed together, with lagging used where needed, was translated
unaltered into tunneling practice, particularly in small exploratory
drifts. It remained in this application until well into the 20th
century.
The pressure exerted upon tunnels of large area was countered during
construction by timbering systems of greater elaboration, evolved
from the basic one. By the time that tunnels of section large
enough to accommodate canals and railways were being undertaken as
matter-of-course civil engineering works, a series of nationally
distinguishable systems had emerged, each possessing characteristic
points of favor and fault. As might be suspected, the English system
of tunnel timbering, for instance, was rarely applied on the
Continent, nor were the German, Austrian or Belgian systems normally
seen in Great Britain. All were used at one time or another in this
country, until the American system was introduced in about 1855.
While the timbering commonly remained in place in mines, it would be
followed up by permanent masonry arching and lining in tunnel work.
Overhead in the museum Hall of Civil Engineering are frames
representing the English, Austrian and American systems. Nearby, a
series of small relief models (fig. 19) is used to show the sequence
of enlargement in a soft-ground railroad tunnel of about 1855, using
the Austrian system. Temporary timber support of tunnels fell from use
gradually after the advent of shield tunneling in conjunction with
cast-iron lining. This formed a perfect support immediately behind the
shield, as well as the permanent lining of the tunnel.