TUNNELS THROUGH HARD ROCK; GENERAL DISCUSSION; REPRESENTATIVE MECHANICAL
INSTALLATIONS FOR TUNNEL WORK.
The present high development of labor-saving machinery for excavating
rock makes this material one of the safest and easiest to tunnel of any
with which the engineer ordinarily has to deal. To operate this
machinery requires, however, the development of a large amount of power,
its transmission to considerable distances, and, finally, its economical
application to the excavating tools. The standard rock excavating
machine is the power drill, which requires either air or hydraulic
pressure for its operation according to the special type employed. Under
present conditions, therefore, the engineer is limited either to air or
water under compression for the transmission of his power. Steam-power
may be employed directly to operate percussion rock drills; but owing to
the heat and humidity which it generates in the confined space where the
drills work, and because of other reasons, it is seldom employed
directly. Electric transmission, which offers so many advantages to the
tunnel builder, in most respects is largely excluded from use by the
failure which has so far followed all attempts to apply it to the
operation of rock drills. As matters stand, therefore, the tunnel
engineer is practically limited to steam and falling water for the
generation of power, and to compressed air and hydraulic pressure for
its transmission.
Whether the engineer should adopt water-power or steam to generate the
power required for his excavating machinery depends upon their relative
availability, cost, and suitability to the conditions of work in each
particular case. Where fuel is plentiful and cheap, and where
water-power is not available at a comparatively reasonable cost,
steam-power will nearly always prove the more economical; where,
however, the reverse conditions exist, which is usually the case in a
mountainous country far from the coal regions, and inadequately supplied
with transportation facilities, but rich in mountain torrents,
water-power will generally be the more economical. In a succeeding
chapter the power generating and transmission plants for a number of
rock tunnels are described, and here only a general consideration of the
subject will be presented.
=Steam-Power Plant.=--A steam-power plant for tunnel work should be much
the same as a similar plant elsewhere, except that in designing it the
temporary character of its work must be taken into consideration. This
circumstance of its temporary employment prompts the omission of all
construction except that necessary to the economical working of the
plant during the period when its operation is required. The power-house,
the foundations for the machinery, and the general construction and
arrangement, should be the least expensive which will satisfy the
requirements of economical and safe operation for the time required. It
will often be found more economical as a whole to operate the machinery
with some loss of economy during the short time that it is in use than
to go to much greater expense to secure better economy from the
machinery by design and construction, which will be of no further use
after the tunnel is completed. The longer the plant is to be required,
the nearer the construction may economically approach that of a
permanent plant. As regards the machinery itself, whose further
usefulness is not limited by the duration of any single piece of work,
true economy always dictates the purchase of the best quality. Speaking
in a general way, a steam-power plant for tunnel work comprises a boiler
plant, a plant of air compressors with their receivers, and an electric
light dynamo. When hydraulic transmission of power is employed, the air
compressors are replaced by high-pressure pumps; and when electric
hauling is employed, one or more dynamos may be required to generate
electricity for power purposes, as well as for lighting. In addition to
the power generating machines proper, there must be the necessary piping
and wiring for transmitting this power, and, of course, the equipment of
drills and other machines for doing the actual excavating, hauling, etc.
=Reservoirs.=--When water-power is employed, a reservoir has to be
formed by damming some near-by mountain stream at a point as high as
practicable above the tunnel. The provision of a reservoir, instead of
drawing the water directly from the stream, serves two important
purposes. It insures a continuous supply and constant head of water in
case of drought, and also permits the water to deposit its sediment
before it is delivered to the turbines. The construction of these
reservoirs may be of a temporary character, or they may be made
permanent structures, and utilized after construction is completed to
supply power for ventilation and other necessary purposes. In the first
case they are usually destroyed after construction is finished. In
either case, it is almost unnecessary to say, they should be built amply
safe and strong according to good engineering practice in such works,
for the duration of time which they are expected to exist.
=Canals and Pipe Lines.=--For conveying the water from the reservoirs to
the turbines, canals or pipe lines are employed. The latter form of
conduit is generally preferable, it being both less expensive and more
easily constructed than the former. It is advisable also to have
duplicate lines of pipe to reduce the possibility of delay by accident
or while necessary repairs are being made to one of the pipes. The pipe
lines terminate in a penstock leading into the turbine chamber, and
provided with the necessary valves for controlling the admission of
water to the turbines.
=Turbines.=--There are numerous forms of turbines on the market, but
they may all be classed either as impulse turbines or as reaction
turbines. Impulse turbines are those in which the whole available energy
of the water is converted into kinetic energy before the water acts on
the moving part of the turbine. Reaction turbines are those in which
only a part of the available energy of the water is converted into
kinetic energy before the water acts on the moving vanes. Impulse
turbines give efficient results with any head and quantity of water, but
they give better results when the quantity of water varies and the head
remains constant. Reaction turbines, on the contrary, give better
results when the quantity of water remains constant and the head varies.
These observations indicate in a general way the form of turbine which
will best meet the particular conditions in each case. The number of
turbines required, and their dimensions, will be determined in each case
by the number of horse-power required and the quantity of water
available. The power of the turbines is transmitted to the air
compressors or pumps by shafting and gearing.
=Air Compressors.=--An air compressor is a machine--usually driven by
steam, although any other power may be used--by which air is compressed
into a receiver from which it may be piped for use. For a detailed
description of the various forms of air compressors the reader should
consult the catalogues of the several makers and the various text-books
relating to air compression and compressed air. Air compressors, like
other machines, suffer a loss of power by friction. The greatest loss of
power, however, results from the heat of compression. When air is
compressed, it is heated, and its relative volume is increased.
Therefore, a cubic foot of hot air in the compressor cylinder, at say,
60 lbs. pressure, does not make a cubic foot of air at 60 lbs. pressure
after cooling in the receiver. In other words, assuming pressure to be
constant, a loss of volume results due to the extraction of the heat of
compression after the air leaves the compressor cylinder. To reduce the
amount of this loss, air compressors are designed with means to extract
the heat from the air before it leaves the compressor cylinder. Air
compressors may first be divided into two classes, according to the
means employed for cooling the air, as follows: (1) Wet compressors, and
(2) dry compressors. A wet compressor is one which introduces water
directly into the cylinder during compression, and a dry compressor is
one which admits no water to the air during compression. Wet compressors
may be subdivided into two classes: (1) Those which inject water in the
form of spray into the cylinder during compression, and (2) those which
use a water piston for forcing the air into confinement.
The following brief discussion of these various types of compressors is
based on the concise practical discussion of Mr. W. L. Saunders, M. Am.
Soc. C. E., in “Compressed Air Production.” The highest isothermal
results are obtained by the injection of water into the cylinders, since
it is plain that the injection of cold water, in the shape of a finely
divided spray, directly into the air during compression will lower the
temperature to a greater degree than simply to surround the cylinder and
parts by water jackets which is the means of cooling adopted with dry
compressors. A serious obstacle to water injection, and that which
condemns this type of compressor, is the influence of the injected water
upon the air cylinder and parts. Even when pure water is used, the
cylinders wear to such an extent as to produce leakage and to require
reboring. The limitation to the speed of a compressor is also an
important objection. The chief claim for the water piston compressor is
that its piston is also its cooling device, and that the heat of
compression is absorbed by the water. Water is so poor a conductor of
heat, however, that without the addition of sprays it is safe to say
that this compressor has scarcely any cooling advantages at all so far
as the cooling of the air during compression is concerned. The water
piston compressor operates at slow speed and is expensive. Its only
advantage is that it has no dead spaces. In the dry compressor a
sacrifice is made in the efficiency of the cooling device to obtain low
first cost, economy in space, light weight, higher speed, greater
durability, and greater general availability.
Air compressors are also distinguished as double acting and simple
acting. They are simple acting when the cylinder is arranged to take in
air at one stroke and force it out at the next, and they are double
acting when they take in and force out air at each stroke. In form
compressors may be simple or duplex. They are simple when they have but
one cylinder, and duplex when they have two cylinders. A straight line
or direct acting compressor is one in which the steam and air cylinders
are set tandem. An indirect acting compressor is one in which the power
is applied indirectly to the piston rod of the air cylinder through the
medium of a crank. Mr. W. L. Saunders writes in regard to direct and
indirect compression as follows:--
“The experience of American manufacturers, which has been more
extensive than that of others, has proved the value of direct
compression as distinguished from indirect. By direct compression is
meant the application of power to resistance through a single straight
rod. The steam and air cylinders are placed tandem. Such machines
naturally show a low friction loss because of the direct application
of power to resistance. This friction loss has been recorded as low as
5%, while the best practice is about 10% with the type which conveys
the power through the angle of a crank shaft to a cylinder connected
to the shaft through an additional rod.”
=Receivers.=--Compressed air is stored in receivers which are simply
iron tanks capable of withstanding a high internal pressure. The purpose
of these tanks is to provide a reservoir of compressed air, and also to
allow the air to deposit its moisture. From the receivers the air is
conveyed to the workings through iron pipes, which decrease gradually in
diameter from the receivers to the front.
=Rock Drills.=--The various forms of rock drills used in tunneling have
been described in Chapter III., and need not be considered in detail
here except to say that American engineers usually employ percussion
drills, while European engineers also use rotary drills extensively. A
comparison between these two types of drills was made in excavating the
Aarlberg tunnel in Austria, where the Brandt hydraulic rotary drill was
used at one end, and the Ferroux percussion drill was used at the other
end. The rock was a mica-schist. The average monthly progress was 412
ft., with a maximum of 646 ft., with the rotary drills, and an average
of 454 ft. with the percussion drill.
=Excavation.=--Since considerable time is required to get the power
plant established, the excavation of rock tunnels is often begun by
hand, but hand work is usually continued for no longer a period than is
necessary to get the power plant in operation. Generally speaking, the
greatest difficulty is encountered in excavating the advanced drift or
heading. Based on the mode of blasting employed, there are two methods
of driving the advanced gallery, known as the circular cut and the
center cut methods. In the first method a set of holes is first drilled
near the center of the front in such a manner that they inclose a cone
of rock; the holes, starting at the perimeter of the base of the cone,
converge toward a junction at its apex. Seldom more than four to six
holes are comprised in this first set. Around these first holes are
driven a ring of holes which inclose a cylinder of rock, and if
necessary succeeding rings of holes are driven outside of the first
ring. These holes are blasted in the order in which they are driven, the
first set taking out a cone of rock, the second set enlarging this cone
to a cylinder, and the other sets enlarging this cylinder to the
required dimensions of the heading. The number of holes, however, varies
with the quality of rock and they are seldom driven deeper than 4 or 5
ft. This method of excavating the heading, which is commonly followed by
European engineers, is illustrated in Figs. 50 to 52. In these figures
are indicated the number of holes in each round and the sequence of
rounds for the soft, medium and hard rock, as used in the Turchino
tunnel of the Genova Ovada Asti line of the Mediterranean Railway of
Italy. The heading was about 9 ft. square, and five sets of holes were
used in blasting, the depths being 3.91, 4.26 and 4.6 ft. for soft,
medium and hard rock, respectively, and the amount of dynamite consumed
was 2.38, 3.91 and 5.1 pounds per cubic yard for the three classes of
rock.
[Illustration: ~in Soft Rock~
~in Medium Rock~
~in Hard Rock~
FIGS. 50 to 52.--Arrangement of Drill Holes in the Heading of Turchino
Tunnel.]
[Illustration: FIGS. 53 and 54.--Arrangement of Drill Holes in the
Heading of the Fort George Tunnel.]
In the center-cut method, which is the one commonly employed in America,
the holes are arranged in vertical rows, and are driven from 8 to 10 ft.
deep. Fig. 53 shows the arrangement of the holes, and the method of
blasting them, as used in the excavation of the heading for the Fort
George tunnel of the New York rapid transit. The two center rows of
holes converge toward each other so as to take out a wedge of rock;
others are bored straight, or parallel, with the vertical plane of the
tunnel. Those bored around the perimeter are driven either outward or
upward, according as they are located, close to the sides or roof of the
tunnel. In this case, the holes of the center cut were driven 9 ft.
deep, while all the other holes were bored to a depth of 8 ft.
The width of the advanced gallery or heading depends upon the quality of
the rock. In hard rock American engineers give it the full width of the
tunnel section; but this cannot be done in loose or fissured rock, which
has to be supported, the headings here being usually made about 8 × 8
ft. The wider heading is always preferable, where it is possible, since
more room is available for removing the rock, and deeper holes can be
bored and blasted.
The important rôle played by the power plant and other mechanical
installations in constructing tunnels through rock has already been
mentioned. In some methods of soft-ground tunneling, and particularly in
soft-ground subaqueous tunneling, it is also often necessary to employ a
mechanical installation but slightly inferior in size and cost to those
used in tunneling rock. It is proposed to describe very briefly here a
few typical individual plants of this character, which will in some
respects give a better idea of this phase of tunnel work than the more
general descriptions.
=Rock Tunnels.=--The tunnels selected to illustrate the mechanical
installations employed in tunneling through rock are: The Mont Cenis,
Hoosac Tunnel, the Cascade Tunnel, the Niagara Falls Power Tunnel, the
Palisades Tunnel, the Croton Aqueduct Tunnel, the Strickler Tunnel in
America, and the Graveholz Tunnel and the Sonnstein Tunnel in Europe. In
addition there will be found in another chapter of this book a
description of the mechanical installations at the St. Gothard,
Pennsylvania and other tunnels.
_Mont Cenis Power Plant._--The mechanical installation consisted of the
Sommeilier air compressors built near the portals. The Sommeilier
compressors, Mr. W. L. Saunders says, were operated as a ram, utilizing
a natural head of water to force air at 80 lbs. pressure into a
receiver. The column of water contained in the long pipe on the side of
the hill was started and stopped automatically by valves controlled by
engines. The weight and momentum of the water forced a volume of air
with such a shock against the discharge valve that it was opened, and
the air was discharged into the tank; the valve was then closed, the
water checked; a portion of it was allowed to discharge, and the space
was filled with air, which was in turn forced into the tank. Only 73% of
the power of the water was available, 27% being lost by the friction of
the water in the pipes, valves, bends, etc. Of the 73% of net work, 49.4
was consumed in the perforators, and 23.6 in a dummy engine for working
the valves of the compressors and for special ventilation.
The compressed air was conveyed from each end through a cast-iron pipe
7⁵⁄₈ in. in diameter, up to the front of the excavation. The joints of
the pipes were made with turned faces, grooved to receive a ring of
oakum which was tightly screwed and compressed into the joint. To
ascertain the amount of leakage of the pipes, they and the tanks were
filled with air compressed to 6 atmospheres, and the machines stopped;
after 12 hours the pressure was reduced to 5.7 atmospheres, or to 95% of
the original pressure.
Sommeilier’s percussion drilling machines were used in the excavation of
this tunnel. They were provided with 8 or 10 drills acting at the same
time, and mounted on carriages running on tracks. These were withdrawn
to a safe place during the blasting, and advanced again after the broken
rock was removed from the front and the new tracks laid.
Machine shops were built at both ends of the tunnel for building and
repairing the drilling machines, bits, tools, etc. A gas factory was
built at each end for lighting purpose.
_Hoosac Tunnel._--The Hoosac tunnel on the Fitchburg R.R. in
Massachusetts is 25,000 ft. long, and the longest tunnel in America. The
material through which the tunnel was driven was chiefly hard granitic
gneiss, conglomerate, and mica-schist rock. The excavation was conducted
from the entrances and one shaft, the wide heading and single-bench
method being employed, with the center-cut system of blasting which was
here used for the first time. The tunnel was begun in 1854, and
continued by hand until 1866, when the mechanical plant was installed.
Most of the particular machines employed have now become obsolete, but
as they were the first machines used for rock tunneling in America they
deserve mention. The drills used were Burleigh percussion drills,
operated by compressed air. Six of these drills were mounted on a single
carriage, and two carriages were used at each front. The air to operate
these drills was supplied by air compressors operated by water-power at
the portals and steam-power at the shaft. The air compressors consisted
of four horizontal single-acting air cylinders with poppet valves and
water injection. The compressors were designed by Mr. Thomas Deane, the
chief engineer of the tunnel.
_Palisades Tunnel._--The Palisades tunnel was constructed to carry a
double track railway line through the ridge of rocks bordering the west
bank of the Hudson River and known as the Palisades. It was located
about opposite 116th St. in New York City. The material penetrated was a
hard trap rock very full of seams in places, which caused large
fragments to fall from the roof. The excavation was made by a single
wide heading and bench, employing the center-cut method of blasting with
eight center holes and 16 side holes for the 7 × 18 ft. heading.
Ingersoll-Sergeant 2¹⁄₂ in. drills were used, four in each heading and
six on each bench, and 30 ft. per 10 hours was considered good work for
one drill.
The power-plant was situated at the west portal of the tunnel, and the
power was transmitted by electricity and compressed air to the middle
shaft and east portal workings. The plant consisted of eight 100 H. P.
boilers, furnishing steam to four Rand duplex 18 × 22 in. air
compressors, and an engine running a 30 arc light dynamo. The compressed
air was carried over the ridge by pipes, varying from 10 ins. to 5 ins.
in diameter, to the shaft and to the east portal, and was used for
operating the hoisting engines as well as the drills at these workings.
Inside the tunnel, specially designed derrick cars were employed to
handle large stones, they being also operated by compressed air. This
car ran on a center track, while the mucking cars ran on side tracks,
and it was employed to lift the bodies of the cars from the trucks,
place them close to the front, being worked where large stone could be
rolled into them, and return them to the trucks for removal. In addition
to handling the car bodies the derrick was used to lift heavy stones.
The hauling was done first by horse-power, and later by dummy
locomotives.
_Croton Aqueduct Tunnel._--In the construction of the Croton Aqueduct
for the water supply of New York City, a tunnel 31 miles long was built,
running from the Croton Dam to the Gate House at 135th St. in New York
City. The section of the tunnel varies in form, but is generally either
a circular or a horseshoe section. In all cases the section was designed
to have a capacity for the flow of water equal to a cylinder 14 ft. in
diameter. To drive the tunnel, 40 shafts were employed. The material
penetrated was of almost every character, from quicksand to granitic
rock, but the bulk of the work was in rock of some character. The
excavation in rock was conducted by the wide heading and bench method,
employing the center-cut method of blasting. Four air drills, mounted on
two double-arm columns were employed in the heading. The drills for the
bench work were mounted on tripods. Steam-power was used exclusively for
operating the compressors, hoisting engines, ventilating fans and pumps;
but the size and kind of boilers used, as well as the kind and capacity
of the machines which they operated, varied greatly, since a separate
power-plant was employed for each shaft with a few exceptions. A
description of the plant at one of the shafts will give an indication of
the size and character of those at the other shafts, and for this
purpose the plant at shaft 10 has been selected.
At shaft 10 steam was provided by two Ingersoll boilers of 80 H. P.
each, and by a small upright boiler of 8 H. P. There were two 18 × 30
in. Ingersoll air compressors pumping into two 42 in. × 10 ft. and two
42 in. × 12 ft. Ingersoll receivers. In the excavation there were twelve
3¹⁄₂ in. and six 3¹⁄₈ in. Ingersoll drills, four drills mounted on two
double arm columns being used on each heading, and the remainder mounted
on tripods being used on the bench. Two Dickson cages operated by one
12 × 12 in. Dickson reversible double hoisting engine provided
transportation for material and supplies up and down the shaft. A
Thomson-Houston ten-light dynamo operated by a Lidgerwood engine
provided light. Drainage was effected by means of two No. 9 and one No.
6 Cameron pumps. At this particular shaft the air exhausted from the
drills gave ample ventilation, especially when after each blast the
smoke was cleared away by a jet of compressed air. In other workings,
however, where this means of ventilation was not sufficient, Baker
blowers were generally employed.
_Strickler Tunnel._--The Strickler tunnel for the water supply of
Colorado Springs, Col., is 6441 ft. long with a section of 4 ft. × 7 ft.
It penetrates the ridge connecting Pike’s Peak and the Big Horn
Mountains, at an elevation of 11,540 ft. above sea level. The material
penetrated is a coarse porphyritic granite and morainal débris, the
portion through the latter material being lined. The mechanical
installation consisted of a water-power electric plant operating air
compressors. The water from Buxton Creek having a fall of 2400 ft. was
utilized to operate a 36 in. 220 H. P. Pelton water-wheel, which
operated a 150 K. W. three-phase generator. From this generator a 3500
volt current was transmitted to the east portal of the tunnel, where a
step-down transformer reduced it to a 220 volt current to the motor. The
transmission line consisted of three No. 5 wires carried on cross-arm
poles and provided with lightning arresters at intervals. The plant at
the east portal of the tunnel consisted of a 75 H. P. electric motor,
driving a 75 H. P. air compressor, and of small motors to drive a
Sturtevant blower for ventilation, to run the blacksmith shop, and to
light the tunnel, shop, and yards. From the compressor air was piped
into the tunnel at the east end, and also over the mountain to the west
portal workings. Two drills were used at each end, and the air was also
used for operating derricks and other machinery. For removing the spoil
a trolley carrier system was employed. A longitudinal timber was
fastened to the tunnel roof, directly in the apex of the roof arch. This
timber carried by means of hangers a steel bar trolley rail on which the
carriages ran. Outside of the portal this rail formed a loop, so that
the carriage could pass around the loop and be taken back to the working
face. Each carriage carried a steel span of 1¹⁄₂ cu. ft. capacity, so
suspended that by means of a tripping device it was automatically dumped
when the proper point on the loop was reached.
_Niagara Falls Power Tunnel._--The tail-race tunnel built to carry away
the water discharged from the turbines of the Niagara Falls Power Co.,
has a horse-shoe section 19 × 21 ft. and a length of 6700 ft. It was
driven through rock from three shafts by the center-cut method of
blasting. In sinking shaft No. 0 very little water was encountered, but
at shafts Nos. 1 and 2 an inflow of 800 gallons and 600 gallons per
minute, respectively, was encountered. The principal plant was located
at shaft No. 2, and consisted of eight 100 H.P. boilers, three 18 × 30
in. Rand duplex air compressors, a Thomson-Houston electric-light plant,
and a sawmill with a capacity of 20,000 ft. B. M. per day. The shafts
were fitted with Otis automatic hoisting engines, with double cages at
shafts Nos. 1 and 2, and a single cage at shaft No. 0. The drills used
were 25 Rand drills and three Ingersoll-Sergeant drills. The pumping
plant at shaft No. 2 consisted of four No. 7 and one No. 9 Cameron
pumps, and that at shaft No. 2 consisted of two No. 7 and two No. 9
Cameron pumps and three Snow pumps. An auxiliary boiler plant consisting
of two 60 H. P. boilers was located at shaft No. 1, and another,
consisting of one 75 H. P. boiler, was located at shaft No. 0.
_Cascade Tunnel._--The Cascade tunnel was built in 1886-88 to carry the
double tracks of the Northern Pacific Ry. through the Cascade Mountains
in Washington. It is 9850 ft. long with a cross-section 16¹⁄₂ ft. wide
and 22 ft. high, and is lined with masonry. The material penetrated was
a basaltic rock, with a dip of the strata of about 5°. The rock was
excavated by a wide heading and one bench, using the center-cut system
of blasting. A strutting consisting of five-segment timber arches
carried on side posts, spaced from 2 ft. to 4 ft. apart, and having a
roof lagging of 4 × 6 in. timbers packed above with cord-wood. The
mechanical plant of the tunnel is of particular interest, because of the
fact that all the machinery and supplies had to be hauled from 82 to 87
miles by teams, over a road cut through the forests covering the
mountain slopes. This work required from Feb. 22 to July 15, 1886, to
perform. In many places the grades were so steep that the wagons had to
be hauled by block and tackle. The plant consisted of five engines, two
water-wheels, five air compressors, eight 70 H. P. steam-boilers, four
large exhaust fans, two complete electric arc-lighting plants, two fully
equipped machine-shop outfits, 36 air drills, two locomotives, 60 dump
cars, and two sawmill outfits, with the necessary accessories for these
various machines. This plant was divided about equally between the two
ends of the tunnel. The cost of the plant and of the work of getting it
into position was $125,000.
_Graveholz Tunnel._--The Graveholz tunnel on the Bergen Railway in
Norway is notable as being the longest tunnel in northern Europe, and
also as being built for a single-track narrow-gauge railway. This tunnel
is 17,400 feet long, and is located at an elevation of 2900 feet above
sea-level. Only about 3% of the length of the tunnel is lined. The
mechanical installation consists of a turbine plant operating the
various machines. There are two turbines of 100 H. P. and 120 H. P.
taking water from a reservoir on the mountain slope, and furnishing 220
H. P., which is distributed about as follows: Boring-machines, 60 H. P.;
ventilation, 30 to 40 H. P.; electric locomotives, 15 H. P.; machine
shop, 15 H. P.; electric-lighting dynamo, 25 H. P.; electric drills, the
surplus, or some 40 H. P. The boring-machines and electric drills will
be operated by the smaller 100 H. P. turbine.
_Sonnstein Tunnel._--The Sonnstein tunnel in Germany is particularly
interesting because of the exclusive use of Brandt rotary drills. The
tunnel was driven through dolomite and hard limestone by means of a
drift and two side galleries. The dimensions of the drift were 7¹⁄₂ ×
7¹⁄₂ ft. The power plant consisted of two steam pressure pumps, one
accumulator, and four drills. The steam-boiler plant, in addition to
operating the pumps, also supplied power for operating a rotary pump for
drainage and a blower for ventilation. The hydraulic pressure required
was 75 atmospheres in the dolomite, and from 85 to 100 atmospheres in
the limestone. The drift was excavated with five 3¹⁄₂ in. holes, one
being placed at the center and driven parallel to the axis of the
tunnel, and four being placed at the corners of a rectangle
corresponding to the sides of the drift, and driven at an angle
diverging from the center hole. The average depths of the holes were 4.3
ft., and the efficiency of the drills was 1 in. per minute. One drill
was employed at each front, and was operated by a machinist and two
helpers, who worked eight-hour shifts, with a blast between shifts at
first, and later twelve-hour shifts, with a blast between shifts. The 24
hours of the two shifts were divided as follows: boring the holes, 10.7
hours; charging the holes, 1.1 hours; removing the spoil, 11.7 hours;
changing shifts, 0.5 hour. The average progress per day for each machine
was 6.7 ft. The total cost of the plant was $17,450.
_St. Clair River Tunnel._--The submarine double-track railway tunnel
under the St. Clair River for the Grand Trunk Ry. is 8500 ft. long, and
was driven through clay by means of a shield, as described in the
succeeding chapter on the shield system of tunneling. The mechanical
plant installed for prosecuting the work was very complete. To furnish
steam to the air compressors, pumps, electric-light engines,
hoisting-engines, etc., a steam-plant was provided on each side of the
river, consisting of three 70 H. P. and four 80 H. P. Scotch portable
boilers. The air-compressor plant at each end consisted of two 20 × 24
in. Ingersoll air compressors. To furnish light to the workings, two 100
candle-power Edison dynamos were installed on the American side, and two
Ball dynamos of the same size were installed on the Canadian side. The
dynamos on both sides were driven by Armington & Sims engines. These
dynamos furnished light to the tunnel workings and to the machine-shops
and power-plant at each end. Root blowers of 10,000 cu. ft. per minute
capacity provided ventilation. The pumping plant consisted of one set of
pumps installed for permanent drainage, and another set installed for
drainage during construction, and also to remain in place as a part of
the permanent plant. The latter set consisted of two 500 gallon
Worthington duplex pumps set first outside of each air lock, closing the
ends of the river portion of the tunnel. For permanent drainage, a
drainage shaft was sunk on the Canadian side of the river, and connected
with a pump at the bottom of the open-cut approach. In this shaft were
placed a vertical, direct-acting, compound-condensing pumping engine
with two 19¹⁄₂ in. high-pressure and two 33³⁄₈ in. low-pressure
cylinders of 24 in. stroke, connected to double-acting pumps with a
capacity of 3000 gallons per minute, and also two duplex pumps of 500
gallons capacity per minute. For permanent drainage on the American
side, four Worthington pumps of 3000 gallons’ capacity were installed in
a pump-house set back into the slope of the open-cut approach. For the
permanent drainage of the tunnel proper two 400 gallon pumps were placed
at the lowest point of the tunnel grade. Spoil coming from the tunnel
proper was hoisted to the top of the open cut by derricks operated by
two 50 H. P. Lidgerwood hoisting-engines. The pressure pumping plant
for supplying water to the hydraulic shield-jacks at each end of the
tunnel consisted of duplex direct-acting engines with 12 in. steam
cylinders and 1 in. water cylinders, supplying water at a pressure of
2000 lbs. per sq. in.