Despite the ultimate success of Brunel's Thames Tunnel in 1843, the
shield in that case afforded only moderately reliable protection
because of the fluidity of the soil driven through, and its tendency
to enter the works through the smallest opening in the shield's
defense. An English doctor who had made physiological studies of the
effects on workmen of the high air pressure within diving bells is
said to have recommended to Brunel in 1828 that he introduce an
atmosphere of compressed air into the tunnel to exclude the water
and support the work face.
This plan was first formally described by Sir Thomas Cochrane
(1775-1860) in a British patent of 1830. Conscious of Brunel's
problems, he proposed a system of shaft sinking, mining, and tunneling
in water-bearing materials by filling the excavated area with air
sufficiently above atmospheric pressure to prevent the water from
entering and to support the earth. In this, and his description of air
locks for passage of men and materials between the atmosphere and the
pressurized area, Cochrane fully outlined the essential features of
pneumatic excavation as developed since.
[Illustration: Figure 37.--THE GIANT ROOTS LOBE-TYPE BLOWER used for
propelling the car.]
In 1839, a French engineer first used the system in sinking a mine
shaft through a watery stratum. From then on, the sinking of shafts,
and somewhat later the construction of bridge pier foundations, by the
pneumatic method became almost commonplace engineering practice in
Europe and America. Not until 1879 however, was the system tried in
tunneling work, and then, as with the shield ten years earlier, almost
simultaneously here and abroad. The first application was in a small
river tunnel in Antwerp, only 5 feet in height. This project was
successfully completed relying on compressed air alone to support the
earth, no shield being used. The importance of the work cannot be
considered great due to its lack of scope.
[Illustration: Figure 38.--TESTING ALIGNMENT of the Broadway Subway at
night by driving a jointed rod up to street level. (_Scientific
American_, March 5, 1870.)]
In 1871 Dewitt C. Haskin (1822-1900), a west coast mine and railroad
builder, became interested in the pneumatic caissons then being used
to found the river piers of Eads' Mississippi River bridge at St.
Louis. In apparent total ignorance of the Cochrane patent, he evolved
a similar system for tunneling water-bearing media, and in 1873
proposed construction of a tunnel through the silt beneath the Hudson
to provide rail connection between New Jersey and New York City.
[Illustration: Figure 39.--HASKIN'S pneumatically driven tunnel
under the Hudson River, 1880. In the engine room at top left was the
machinery for hoisting, generating electricity for lighting, and air
compressing. The air lock is seen in the wall of the brick shaft.
MHT model--0.3" scale. (Smithsonian photo 49260.)]
[Illustration: Figure 40.--ARTIST'S CONCEPTION OF MINERS escaping
into the air lock during the blowout in Haskin's tunnel.]
It would be difficult to imagine a site more in need of such
communication. All lines from the south terminated along the west
shore of the river and the immense traffic--cars, freight and
passengers--was carried across to Manhattan Island by ferry and barge
with staggering inconvenience and at enormous cost. A bridge would
have been, and still is, almost out of the question due not only to
the width of the crossing, but to the flatness of both banks. To
provide sufficient navigational clearance (without a drawspan),
impracticably long approaches would have been necessary to obtain
a permissibly gentle grade.
Haskin formed a tunneling company and began work with the sinking of
a shaft in Hoboken on the New Jersey side. In a month it was halted
because of an injunction by, curiously, the D L & W Railroad, who
feared for their vast investment in terminal and marine facilities.
Not until November of 1879 was the injunction lifted and work again
commenced. The shaft was completed and an air lock located in one wall
from which the tunnel proper was to be carried forward. It was
Haskin's plan to use no shield, relying solely on the pressure of
compressed air to maintain the work faces and prevent the entry of
water. The air was admitted in late December, and the first
large-scale pneumatic tunneling operation launched. A single 26-foot,
double-track bore was at first undertaken, but a work face of such
diameter proved unmanageable and two oval tubes 18 feet high by 16
feet wide were substituted, each to carry a single track. Work went
forward with reasonable facility, considering the lack of precedent.
A temporary entrance was formed of sheet-iron rings from the air lock
down to the tunnel grade, at which point the permanent work of the
north tube was started. Immediately behind the excavation at the face,
a lining of thin wrought-iron plates was built up, to provide form for
the 2-foot, permanent brick lining that followed. The three stages are
shown in the model in about their proper relationship of progress. The
work is shown passing beneath an old timber-crib bulkhead, used for
stabilizing the shoreline.
The silt of the riverbed was about the consistency of putty and under
good conditions formed a secure barrier between the excavation and the
river above. It was easily excavated, and for removal was mixed with
water and blown out through a pipe into the shaft by the higher
pressure in the tunnel. About half was left in the bore for removal
later. The basic scheme was workable, but in operation an extreme
precision was required in regulating the air pressure in the work
area.[5] It was soon found that there existed an 11-psi difference
between the pressure of water on the top and the bottom of the working
face, due to the 22-foot height of the unlined opening. Thus, it was
impossible to maintain perfect pneumatic balance of the external
pressure over the entire face. It was necessary to strike an average
with the result that some water entered at the bottom of the face
where the water pressure was greatest, and some air leaked out at the
top where the water pressure was below the air pressure. Constant
attention was essential: several men did nothing but watch the
behavior of the leaks and adjusted the pressure as the ground density
changed with advance. Air was supplied by several steam-driven
compressors at the surface.
The air lock permitted passage back and forth of men and supplies
between the atmosphere and the work area, without disturbing the
pressure differential. This principle is demonstrated by an animated
model set into the main model, to the left of the shaft (fig. 39). The
variation of pressure within the lock chamber to match the atmosphere
or the pressurized area, depending on the direction of passage, is
clearly shown by simplified valves and gauges, and by the use of light
in varying color density. In the Haskin tunnel, 5 to 10 minutes were
taken to pass the miners through the lock so as to avoid too abrupt a
physiological change.
Despite caution, a blowout occurred in July 1880 due to air leakage
not at the face, but around the temporary entrance. One door of the
air lock jammed and twenty men drowned, resulting in an inquiry which
brought forth much of the distrust with which Haskin was regarded by
the engineering profession. His ability and qualifications were
subjected to the bitterest attack in and by the technical press. There
is some indication that, although the project began with a staff of
competent engineers, they were alienated by Haskin in the course of
work and at least one withdrew. Haskin's remarks in his own defense
indicate that some of the denunciation was undoubtedly justified. And
yet, despite this reaction, the fundamental merit of the pneumatic
tunneling method had been demonstrated by Haskin and was immediately
recognized and freely acknowledged. It was apparent at the same time,
however, that air by itself did not provide a sufficiently reliable
support for large-area tunnel works in unstable ground, and this
remains the only major subaqueous tunnel work driven with air alone.
[Illustration: Figure 41.--LOCATION OF HUDSON RIVER TUNNEL. (_Leslie's
Weekly_, 1879.)]
After the accident, work continued under Haskin until 1882 when funds
ran out. About 1600 feet of the north tube and 600 feet of the south
tube had been completed. Greathead resumed operations with a shield
for a British company in 1889, but exhaustion of funds again caused
stoppage in 1891. The tunnel was finally completed in 1904, and is
now in use as part of the Hudson and Manhattan rapid-transit system,
never providing the sought-after rail link. A splendid document of the
Haskin portion of the work is S. D. V. Burr's _Tunneling Under the
Hudson River_ published in 1885. It is based entirely upon firsthand
material and contains drawings of most of the work, including the
auxiliary apparatus. It is interesting to note that electric
illumination (arc, not incandescent, lights) and telephones were used,
unquestionably the first employment of either in tunnel work.
[Illustration: Figure 42.--ST. CLAIR TUNNEL. View of front of shield
showing method of excavation in firm strata. Incandescent electric
illumination was used. 1889-90. MHT model--1" scale. (Smithsonian
photo 49260-D.)]