The final model of the soft-ground series reflects, as did the Hoosac
Tunnel model for hard-rock tunneling, final emergence into the modern
period. Although the St. Clair Tunnel was completed over 70 years ago,
it typifies in its method of construction, the basic procedures of
subaqueous work in the present day. The Thames Tunnel of Brunel, and
Haskin's efforts beneath the Hudson, had clearly shown that by
themselves, both the shield and pneumatic systems of driving through
fluid ground were defective in practice for tunnels of large area.
Note that the earliest successful works by each method had been of
very small area, so that the influence of adverse conditions was
greatly diminished.
The first man to perceive and seize upon the benefits to be gained by
combining the two systems was, most fittingly, Greathead. Although he
had projected the technique earlier, in driving the underground City
and South London Railway in 1886, he brought together for the first
time the three fundamental elements essential for the practical
tunneling of soft, water-bearing ground: compressed-air support of the
work during construction, the movable shield, and cast-iron, permanent
lining. The marriage was a happy one indeed; the limitations of each
system were almost perfectly overcome by the qualities of the others.
The conditions prevailing in 1882 at the Sarnia, Ontario, terminal of
the Grand Trunk Railway, both operational and physical, were almost
precisely the same as those which inspired the undertaking of the
Hudson River Tunnel. The heavy traffic at this vital U.S.--Canada rail
interchange was ferried inconveniently across the wide St. Clair
River, and the bank and river conditions precluded construction of a
bridge. A tunnel was projected by the railway in that year, the time
when Haskin's tribulations were at their height. Perhaps because of
this lack of precedent for a work of such size, nothing was done
immediately. In 1884 the railway organized a tunnel company; in 1886
test borings were made in the riverbed and small exploratory drifts
were started across from both banks by normal methods of mine
timbering. The natural gas, quicksand, and water encountered soon
stopped the work.
[Illustration: Figure 43.--REAR VIEW OF ST. CLAIR SHIELD showing the
erector arm placing a cast-iron lining segment. The three motions of
the arm--axial, radial, and rotational, were manually powered.
(Smithsonian photo 49260-C.)]
It was at this time that the railway's president visited Greathead's
City and South London workings. The obvious answer to the St. Clair
problem lay in the successful conduct of this subway. Joseph Hobson,
chief engineer of the Grand Trunk and of the tunnel project, in
designing a shield, is said to have searched for drawings of the
shields used in the Broadway and Tower Subways of 1868-9, but unable
to locate any, he relied to a limited extent on the small drawings of
those in Drinker's volume. There is no explanation as to why he did
not have drawings of the City and South London shield at that moment
in use, unless one considers the rather unlikely possibility that
Greathead maintained its design in secrecy.
[Illustration: Figure 44.--OPENING OF THE ST. CLAIR TUNNEL, 1891.
(_Photo courtesy of Detroit Library, Burton Historical Collection._)]
The Hobson shield followed Greathead's as closely as any other, in
having a diaphragm with closable doors, but a modification of Beach's
sharpened horizontal shelves was also used. However, these functioned
more as working platforms than supports for the earth. The machine was
21-1/2 feet in diameter, an unprecedented size and almost twice that
of Greathead's current one. It was driven by 24 hydraulic rams.
Throughout the entire preliminary consideration of the project there
was a marked sense of caution that amounted to what seems an almost
total lack of confidence in success. Commencement of the work from
vertical shafts was planned so that if the tunnel itself failed, no
expenditure would have been made for approach work. In April 1888,
the shafts were started near both riverbanks, but before reaching
proper depth the almost fluid clay and silt flowed up faster than it
could be excavated and this plan was abandoned. After this second
inauspicious start, long open approach cuts were made and the work
finally began. The portals were established in the cuts, several
thousand feet back from each bank and there the tunneling itself
began. The portions under the shore were driven without air. When the
banks were reached, brick bulkheads containing air locks were built
across the opening and the section beneath the river, about 3,710 feet
long, driven under air pressure of 10 to 28 pounds above atmosphere.
For most of the way, the clay was firm and there was little air
leakage. It was found that horses could not survive in the compressed
air, and so mules were used under the river.
In the firm clay, excavation was carried on several feet in front of
the shield, as shown in the model (fig. 42). About twelve miners
worked at the face. However, in certain strata the clay encountered
was so fluid that the shield could be simply driven forward by the
rams, causing the muck to flow in at the door openings without
excavation. After each advance, the rams were retracted and a ring of
iron lining segments built up, as in the Tower Subway. Here, for the
first time, an "erector arm" was used for placing the segments, which
weighed about half a ton. In all respects, the work advanced with
wonderful facility and lack of operational difficulty. Considering
the large area, no subaqueous tunnel had ever been driven with such
speed. The average monthly progress for the American and Canadian
headings totaled 455 feet, and at top efficiency 10 rings or a length
of 15.3 feet could be set in a 24-hour day in each heading. The 6,000
feet of tunnel was driven in just a year; the two shields met
vis-a-vis in August of 1890.
The transition was complete. The work had been closely followed by the
technical journals and the reports of its successful accomplishment
thus were brought to the attention of the entire civil engineering
profession. As the first major subaqueous tunnel completed in America
and the first in the world of a size able to accommodate full-scale
rail traffic, the St. Clair Tunnel served to dispel the doubts
surrounding such work, and established the pattern for a mode of
tunneling which has since changed only in matters of detail.
Of the eight models, only this one was built under the positive
guidance of original documents. In the possession of the Canadian
National Railways are drawings not only of all elements of the shield
and lining, but of much of the auxiliary apparatus used in
construction. Such materials rarely survive, and do so in this case
only because of the foresight of the railway which, to avoid paying a
high profit margin to a private contractor as compensation for the
risk and uncertainty involved, carried the contract itself and,
therefore, preserved all original drawing records.
While the engineering of tunnels has been comprehensively treated in
this paper from the historical standpoint, it is well to still reflect
that the advances made in tunneling have not perceptibly removed the
elements of uncertainty but have only provided more positive and
effective means of countering their forces. Still to be faced are the
surprises of hidden streams, geologic faults, shifts of strata,
unstable materials, and areas of extreme pressure and temperature.