TUNNELS
THEIR USE IN REDUCING GRADES--THE HOOSAC TUNNEL--THE USE OF SHAFTS--
TUNNELLING UNDER WATER--THE DETROIT RIVER TUNNEL.
Sometimes the construction engineer of the railroad brings his new line
face to face with a mountain too steep to be easily mounted. Then he may
prepare to pierce it. Tunnels are not pleasant things through which to
ride. They are, moreover, expensive to construct, and when once
constructed are an unending care, necessitating expensive and constant
inspection. But--and that "but" in this case is a very large one--they
reduce grades and distances in a wholesale fashion; and when you reduce
grades you are pretty sure to be reducing operating expenses. A railroad
man will think twice in his opposition to a smoky bore of a tunnel that
will cost some three to five million dollars, when his expert advisers
tell him that that same smoky bore will save him a hundred thousand tons
of coal in the course of a year.
From almost its very beginnings the American railroad has been dependent
upon tunnels, and thus has closely followed European precedent. The
Alleghany Portage Railroad, to which reference has already been made,
passed through what is said to have been the first railroad tunnel in the
United States. It pierced a spur in the Alleghany Mountains, and it was
901 feet in length, 20 feet wide, and 19 feet high within the arch, 150
feet at each end being arched with cut stone. The old tunnel, built in
1832, which has not echoed with the panting of the locomotive for more
than half a century, is still to be found not far from Johnstown, Pa. It
simply serves the purpose to-day of calling attention to the durable
fashion in which the earliest of our railroad-builders worked.
Of the building of the Baltimore & Ohio, tunnel-construction formed an
early part, several paths being found across the steep profiles of the
Alleghanies. The Kingwood Tunnel, which B. H. Latrobe drove, was nearly a
mile long and the chief of these bores. But when the Hoosac Tunnel was
first proposed--piercing the rocky heart of one of the greatest of the
Berkshires--the country stood aghast. Four miles and a half of tunnel!
That seemed ridiculous away back in 1854, when the plan was first broached
and folk were not slow to say what they thought of such an absurd plan.
For twenty years it looked as though these scoffers were in the right--the
work of digging that monumental tunnel was a fearful drain on the treasury
of the commonwealth of Massachusetts, which was lending its aid to the
project. But the tunnel-diggers finally conquered--they almost always
do--and the Hoosac remains to-day the greatest of all mountain tunnels in
America. The system of continuous tunnels, by which the Pennsylvania
Railroad recently reached its terminal in New York, stretches from Bergen
Hill in New Jersey to Sunnyside, Long Island, a distance of some ten
miles. In fact the largest feature of recent tunnel-work in this country
has been in connection with terminal and rapid-transit development in the
larger cities. For a good many years New York and Baltimore, in
particular, have been pierced with these sub-surface railroads; it is a
construction feature that increases as our great cities themselves
increase. No river is to-day too formidable to be conquered by these
underground traffic routes. A river such as the Hudson or the Detroit may
sometimes halt the bridge-builders; it has but slight terror for the
tunnel engineers.
The tunnel-work is apt to be a separate part of the work of building a
railroad. It calls for its own talent, and that of an exceedingly expert
sort. If the tunnel is more than a half or three-quarters of a mile long
it will probably be dug from a shaft or shafts as well as from its
portals. In this way the work will not only be greatly hastened but the
shafts will continue in use after the work is completed as vents for the
discharge of engine smoke and gases from the tube. The work must be under
the constant and close supervision of resident engineers. The survey lines
must be corrected daily, for the tunnel must not go astray. It must drive
a true course from heading to heading. In the shafts plumb lines, with
heavy bobs, to lessen vibration, will be hung. Sometimes these bobs are
immersed in water or in molasses.
From the portals and from the bottoms of the shafts the headings are
driven. If the tunnel is to accommodate no more than a single track it
will be built from 15 to 16-1/2 feet wide, and from 21 to 22 feet high,
inside of its lining; so the general method is first to drive a top
heading of about 10 feet in height up under the roof of the bore. The rest
of the material is taken out in its own good season on two following
benches or levels.
Piercing a granite mountain is no rapid work. When the Pennsylvania
Railroad built its second Gallitzin Tunnel in 1903, 13 men, working 4
drills in the top heading, were able to drill 16 holes, each 10 feet deep,
in a single day. The engineers there figured that each blast removed
twenty-three cubic yards of the rock. At night, when the "hard-rock men"
were sleeping and their drills silent, a gang of fourteen "muckers"
removed the loosened material.
Slow work that. The Northern Pacific finding its way through the crest of
the Cascade Mountains by means of the great Stampede Tunnel, nearly two
miles in length, demanded that the contractor work under pressure and make
13-1/2 feet of tunnel a day. The contractor, working under the bonus plan,
did better. With his army of 350 "hard-rock men," "muckers," and their
helpers, and his tireless battery of 36 drills he sometimes made as high
as eighteen feet a day from the two headings. On a three-year job he beat
his contract time by seven days. The Northern Pacific paid the price, $118
for each lineal foot of tunnel. That was a high price, occasioned largely
by the fact that the work was carried forward in what was then an almost
unbroken wilderness. The Wabash finding its way through the great and
forbidding hills of Western Pennsylvania to Pittsburgh a dozen years later
was able to dig its succession of tunnels at an average cost of $4,509 for
100 feet. Of that amount $2,527 went for labor; and $260 was the price of
a ton of dynamite.
When the tunnel engineer finds that his bore is not to pierce hard-rock,
of whose solidity he is more than reasonably assured, he prepares to use
cutting-shields. These shields, proceeding simultaneously from the portals
and from the footings of the shafts, are steel rings of a circumference
only slightly greater than that of the finished tunnel. With pick and with
drill and dynamite, they constantly clear a path for it, whereupon it is
pressed forward in that path. Dummy tracks follow the cutting-shield; and
dummy locomotives--more likely electric than steam in these days--are used
in removing the material. Electricity has been a boon to latter-day
tunnel-workers. Its use for light and power keeps the tunnel quite clear
of all gases during the work of boring.
In rare cases, the rock through which the shield has been forced is strong
enough to support itself; in most works the engineers prefer to line the
bore, with brick and concrete, as a rule. This lining is set in the path
of the cutting-shield before its protection is entirely withdrawn; and so
the heavy roof-timbering which was formerly a trade-mark of the successful
tunnel engineer is no longer used.
Tunnel-boring becomes doubly difficult when the railroad is to be carried
under a river or some broad arm of the sea. Men work in an unnatural
environment when they work below the surface of great waters, and the
record of such work is a record of many tragedies. At any instant firm
rock may cease, silt or sand or an underground stream may make its
appearance and the helpless workmen find a ready grave. In work where
there is even the slightest expectation of such a contingency the
air-lock, with its artificial pressure to hold back the soft earth and
moisture is brought into use. In another chapter we shall see how the
caisson is operated. Suffice it to say now that the necessity of "working
under the air," brings no comfort to any one. It vastly hinders and
complicates the work of construction, and adds greatly to the expense.
Moreover, it has its own record of tragedies. Still it remains, to the
infinite credit of a national persistence, that there is no record in the
annals of American engineering where the workers have finally given up a
tunnel job. Lives have been sacrificed, good-sized fortunes swept away,
but in the end the resistless railroad has always found its underground
path.
The tunnel-workers can tell you of the accident when the subway was being
driven under the East River from Manhattan to Brooklyn, three years ago.
The cutting-shield, which was advancing from the Brooklyn side, suddenly
slipped out from the rock into the unprotected soft mud of the river
bottom. The heavily compressed air shot a geyser straight up to the
surface of the river some fifty feet above. A workman shot through the
geyser, pirouetted gayly for a fraction of a second above the river, then
dropped, to be picked up by the crew of a passing ferryboat. In a week he
was back at work again inside the cutting-shield. His fortune was the
opposite of that which generally awaits a man caught in a tunnel accident.
"It ain't as bad as it used to be," one of them informs you. "When I first
got into this profession, they didn't have the electricity for lights or
moving the cars or nothing. We used to try and get along with safety
lamps an' near choke to death. It was more like hell then than it is now."
[Illustration: "SOMETIMES THE CONSTRUCTION ENGINEER ... BRINGS HIS LINE
FACE TO FACE WITH A MOUNTAIN"]
[Illustration: FINISHING THE LINING OF A TUNNEL]
[Illustration: THE BUSIEST TUNNEL POINT IN THE WORLD--AT THE WEST PORTALS
OF THE BERGEN TUNNELS, SIX ERIE TRACKS BELOW, FOUR LACKAWANNA ABOVE]
[Illustration: THE HACKENSACK PORTALS OF THE PENNSYLVANIA'S GREAT TUNNELS
UNDER NEW YORK CITY]
But your interest in the man who was blown from the tunnel to the surface
of the river and escaped with his life is not entirely satiated, and you
ask more questions. What do they do when they strike soft mud like that?
"We get down and pray," he of the experience in this weird form of
construction engineering tells you. "We try to get the boys safely back
through the air-lock, and then we quit boring till we can fix things up
from outside. If it's a real bad case we've got to make land to bore
through. It's generally done by dumping rock and bags of sand from floats
just over where she blows out. It's a pretty rough way of doctoring her
up, but it has to go, and generally it does. All we want is to get it to
hold until we can set the rings of the tunnel.
"That ain't always the worst. I've been driving a bore under water this
way, when we struck stiff rock overhead and soft mud underneath the edge.
That's something that makes the engineers hump. You can't rest a cast-iron
tunnel like this on mud and you get a wondering if you've got to quit
after all this work under the durned old river, and let the boss lose his
money.
"The last time we struck a snag of that sort, the boss didn't give up. He
wasn't that kind. He had a chief engineer that was brass tacks from
beginning to end. What do you suppose that fellow did? He bored holes in
the bottom of the lining and drove steel legs right down to the next ledge
of solid rock below. There's that tunnel to-day, carrying 32,000 people
between five and six o'clock every night perched down there seventy feet
underground like a big caterpillar sprawled under the wickedest ledge o'
rock you ever see."
It takes a real genius of an engineer for this sort of work. He who drives
his bore into the unknown must be on guard for the unexpected. Emergencies
arise upon the minute, and the tunnel engineer must be ready with his
wits and ingenuity to meet them. Finally the day does come when the bores
from either shore are hard upon one another. If there has been blasting
under the bed of the river it is reduced to a minimum. The drills work at
half-speed, the fever of expectancy hangs over the men. Those who are
close at the heading catch faint sounds of the workmen on the other side
of the thin barrier--the last barrier of the river that was supposed to
acknowledge no conqueror.
The first tiny aperture between the two bores is greeted with wild cheers.
On the surface far above, the whistles of the shaft-houses carry forth the
news to the outer world; it is echoed and reëchoed by the noisy river
craft. The aperture grows larger. It is large enough to permit the passage
of a man's body; and a man, enjoying fame for this one moment in his life,
crawls through it. The men knock off work and have a rough spread in the
tunnel. At night the engineers and contractors banquet in a hotel. "Not so
bad," the chief engineer says quietly. "We were 3/8 of an inch out, in
8,000 feet." It was not so bad. It spoke wonders for his profession. To
carry forth two giant bores from the opposite sides of a broad river, and
have them meet within 3/8 of an inch of perfect alignment, was an
achievement well worth attention.
After that, the last traces of the rough rock and silt are removed, the
iron rings of the tunnel made fast together, the air pressure released,
the cutting-shields, that formed so essential a feature of the
construction, removed. Then there remains only the work of installing
conduits and wiring and laying the tracks before the tunnel is ready for
the traffic of the railroad.
* * * * *
The Michigan Central has recently finished a tunnel under the busy Detroit
River, at Detroit, which eliminates the use of a car-ferry at that point.
The tunnel was built in a manner entirely new to engineers. The river at
Detroit is about three-quarters of a mile wide, and its bed is of soft
blue clay, making it difficult to bore a tunnel safely and economically.
To meet this obstacle a new fashion of tunnel-building was created.
The tunnel itself consists of two tubes, each made from steel 3/8 of an
inch in thickness and reinforced every twelve feet by outer "fins." The
channel was dredged and a foundation bed of concrete laid. The sections of
the tunnel, each 250 feet long, were then put in position one at a time.
The section-ends were closed at a shore plant with water-tight wooden
bulkheads. They were then lashed to four floating cylinders of compressed
air and towed out to position. After that it was merely a matter of detail
to drop the sections into place, pour in more concrete and make the new
section fast. The wooden bulkheads next the completed tube were then
removed and the structure was ready for the track-layers. The sub-aqueous
portion of the new Detroit Tunnel is 2,600 feet long; it joins on the
Detroit side with a land tunnel 2,100 feet long, and on the Canadian side
with a land tunnel of 3,192 feet.
It takes more than a river, carrying through its narrow throat the vast
and growing traffic of the Great Lakes--a traffic that is comparable with
that of the Atlantic itself--to halt the progress of the railroad.