The interior surfaces of tunnels through ground merely unstable are
amenable to support by various systems of timbering and arching. This
becomes less true as the fluidity of the ground increases. The soft
material which normally comprises the beds of rivers can approach an
almost liquid condition resulting in a hydraulic head from the
overbearing water sufficient to prevent the driving of even the most
carefully worked drift, supported by simple timbering. The basic
defect of the timbering systems used in mining and tunneling was
that there was inevitably a certain amount of the face or ceiling
unsupported just previous to setting a frame, or placing over it the
necessary section of lagging. In mine work, runny soil could, and did,
break through such gaps, filling the working. For this reason, there
were no serious attempts made before 1825 to drive subaqueous tunnels.
In that year, work was started on a tunnel under the Thames between
the Rotherhithe and Wapping sections of London, under guidance of the
already famous engineer Marc Isambard Brunel (1769-1849), father of
I. K. Brunel. The undertaking is of great interest in that Brunel
employed an entirely novel apparatus of his own invention to provide
continuous and reliable support of the soft water-bearing clay which
formed the riverbed. By means of this "shield," Brunel was able to
drive the world's first subaqueous tunnel.[3]
[Illustration: Figure 16.--WEST PORTAL UPON COMPLETION, 1876.
(_Photo courtesy of New-York Historical Society._)]
The shield was of cast-iron, rectangular in elevation, and was
propelled forward by jackscrews. Shelves at top, bottom, and sides
supported the tunnel roof, floor, and walls until the permanent brick
lining was placed. The working face, the critical area, was supported
by a large number of small "breasting boards," held against the ground
by small individual screws bearing against the shield framework. The
shield itself was formed of 12 separate frames, each of which could be
advanced independently of the others. The height was 22 feet 3 inches:
the width 37 feet 6 inches.
The progress was piecemeal. In operation the miners would remove one
breasting board at a time, excavate in front of it, and then replace
it in the advanced position--about 6 inches forward. This was repeated
with the next board above or below, and the sequence continued until
the ground for the entire height of one of the 12 sections had been
removed. The board screws for that section were shifted to bear on the
adjacent frames, relieving the frame of longitudinal pressure. It
could then be screwed forward by the amount of advance, the screws
bearing to the rear on the completed masonry. Thus, step by step the
tunnel progressed slowly, the greatest weekly advance being 14 feet.
In the left-hand portion of the model is the shaft sunk to begin
operations; here also is shown the bucket hoist for removing the
spoil. The V-type steam engine powering the hoist was designed by
Brunel. At the right of the main model is an enlarged detail of the
shield, actually an improved version built in 1835.
The work continued despite setbacks of every sort. The financial ones
need no recounting here. Technically, although the shield principle
proved workable, the support afforded was not infallible. Four or five
times the river broke through the thin cover of silt and flooded
the workings, despite the utmost caution in excavating. When this
occurred, masses of clay, sandbags, and mats were dumped over the
opening in the riverbed to seal it, and the tunnel pumped out. I. K.
Brunel acted as superintendent and nearly lost his life on a number of
occasions. After several suspensions of work resulting from withdrawal
or exhaustion of support, one lasting seven years, the work was
completed in 1843.
Despite the fact that Brunel had, for the first time, demonstrated a
practical method for tunneling in firm and water-bearing ground, the
enormous cost of the work and the almost overwhelming problems
encountered had a discouraging effect rather than otherwise. Not for
another quarter of a century was a similar project undertaken.
The Thames Tunnel was used for foot and light highway traffic until
about 1870 when it was incorporated into the London Underground
railway system, which it continues to serve today. The roofed-over
top sections of the two shafts may still be seen from the river.
A number of contemporary popular accounts of the tunnel exist, but one
of the most thorough and interesting expositions on a single tunnel
work of any period is Henry Law's _A Memoir of the Thames Tunnel_,
published in 1845-1846 by John Weale. Law, an eminent civil engineer,
covers the work in incredible detail from its inception until the
major suspension in late 1828 when slightly more than half completed.
The most valuable aspect of his record is a series of plates of
engineering drawings of the shield and its components, which, so
far as is known, exist nowhere else. These formed the basis of the
enlarged section of the shield, shown to the right of the model of the
tunnel itself. A vertical section through the shield is reproduced
here from Law for comparison with the model (figs. 21 and 23).
[Illustration: Figure 17.--SOFT-GROUND TUNNELING. The support of
walls and roof of mine shaft by simple timbering; 16th century.
MHT model--3/4" scale. (Smithsonian photo 49260-J.)]
[Illustration: Figure 18.--SOFT-GROUND TUNNELING. The model of a 16th
century mine in the Museum of History and Technology was constructed
from illustrations in such works as G. E. von Löhneyss' _Bericht vom
Bergwerck_, 1690, as well as the better known ones from _De re
Metallica_.]
[Illustration: Figure 19.--THE SUCCESSIVE STAGES in the enlargement
of a mid-19th century railroad tunnel, using the Austrian system of
timbering. MHT model.]
[Illustration: Figure 20.--M. I. BRUNEL'S THAMES TUNNEL, 1825-1843,
the first driven beneath a body of water. MHT model--1/4" scale.
(Smithsonian photo 49260-F.)]