TUNNELS THROUGH HARD ROCK (Continued).
EXCAVATION BY DRIFTS: THE SIMPLON AND MURRAY HILL TUNNELS.
[Illustration: FIG. 55.--Diagram Showing Sequence of Excavations in
Drift Method of Tunneling Rock.]
=General Description.=--The method of tunneling through hard rock by
drifts is preferred by European engineers. All the great Alpine tunnels,
from the Mont Cenis tunnel to the Simplon, are examples of tunneling by
drifts. In this method the sequence of excavation is shown
diagrammatically by Fig. 55. The work begins by excavating a drift close
to the floor of the proposed tunnel (as shown in the center of the
figure) and far in advance of the excavation of any other part. The
section marked 2 is next removed and still later the portions marked 3.
Then with the removal of the parts marked 4 the whole section of the
tunnel will be open.
The drift is usually strutted by means of side posts carrying a
cap-piece placed at intervals, and having a ceiling of longitudinal
planks resting on the successive caps. In hard rock the roof of the
section does not, as a rule, require regular strutting, occasional
supports being placed at intervals to prevent the fall of isolated
fragments: When the rock is disintegrated or full of seams, a regular
strutting may be necessary, and this may be either longitudinal or
polygonal in type. When longitudinal strutting is employed, a sill is
laid across the roof of the drift, and upon this are set up two struts
converging toward the top and supporting a cap-piece close to the roof.
On this cap-piece are placed the first longitudinal crown bars carrying
transverse poling-boards. Additional props standing on the sill and
radiating outward are inserted as parts No. 3 are excavated. These
radial props carry longitudinal bars which in turn support transverse
poling-boards. When polygonal strutting is used, it may take the form of
three or five segment arches of heavy timbers.
In hard rock tunnels, as a rule, there is no danger of caving in because
of heavy pressures, and the whole section is left open for some time
before it is lined. The lining may be of concrete masonry, but in many
long tunnels, excavated through hard rock, the side walls are lined with
rubble masonry and the arch with brick, and, in some instances, even the
arch has been lined with rubble masonry. With skilful laborers at hand
the rubble masonry lining has proved most efficient and economical,
because the rock is utilized as it is excavated without any further
operation. Concrete, however, is more extensively employed for lining
tunnels than any other material.
Tunnels excavated by drifts enable simple means of hauling to be
employed, and this is one of the reasons why the method finds so much
favor with European engineers. The tracks are laid along the floor of
the drift, and carry all the spoil from parts Nos. 2, 3, and 4, as well
as from the front of the drift itself. As fast as the full section is
completed, this single track in the drift is replaced by two tracks
running close to the sides of the tunnel, or by a broad-gauge track with
a third rail.
THE SIMPLON TUNNEL.[8]
Before entering upon a description of the constructive details of this,
the longest railway tunnel in the world, it may be well to give a
general idea of the undertaking. Many schemes for the connection of
Italy and Switzerland by a railway near the Simplon Road Pass have been
devised, including one involving no great length of underground work,
the line mounting by steep gradients and sharp curves. The present
scheme, put forward in 1881 by the Jura-Simplon Ry. Co., consists
broadly of piercing the Alps between Brigue, the present railway
terminus in the Rhone Valley, and Iselle, in the gorge of the Diveria,
on the Italian side, from which village the railway will descend to the
existing southern terminus at Domo d’Ossola, a distance of about 11
miles.
[8] Abstract from a paper read before the Institution of Civil
Engineers by Charles B. Fox, Jan. 26, 1900.
In conjunction with this scheme a second tunnel is proposed, to pierce
the Bernese Alps under the Lötschen Pass from Mittholz to a point near
Turtman in the Rhone Valley; and thus, instead of the long détour by
Lausanne and the Lake of Geneva, there will be an almost direct line
from Berne to Milan _via_ Thun, Brigue, and Domo d’Ossola.
Starting from Brigue, the new line, running gently up the valley for
1¹⁄₄ miles, will, on account of the proximity of the Rhone, which has
already been slightly diverted, enter the tunnels on a curve to the
right of 1050 ft. radius. At a distance of 153 yards from the entrance,
the straight portion of the tunnel commences, and extends for 12 miles.
The line then curves to the left with a radius of 1311 ft. before
emerging on the left bank of the Diveria. Commencing at the northern
entrance, a gradient of 1 in 500 (the minimum for efficient drainage)
rises for a length of 5¹⁄₂ miles to a level length of 550 yards in the
center, and then a gradient of 1 in 143 descends to the Italian side. On
the way to Domo d’Ossola one helical tunnel will be necessary, as has
been carried out on the St. Gothard. There will be eventually two
parallel tunnels having their centers 56 ft. apart, each carrying one
line of way; but at the present time only one heading, that known as No.
1, is being excavated to full size, No. 2 being left, masonry lined
where necessary, for future developments. By means of cross headings
every 220 yds. the problems of transport and ventilation are greatly
facilitated, as will be seen later. As both entrances are on curves, a
small “gallery of direction” is necessary, to allow corrections of
alinement to be made direct from the two observatories on the axis of
the tunnel.
The outside installations are as nearly in duplicate as circumstances
will allow, and consist of the necessary offices, workshops,
engine-sheds, power-houses, smithies, and the numerous buildings
entailed by an important engineering scheme. Great care is taken that
the miners and men working in the tunnel shall not suffer from the
sudden change from the warm headings to the cold Alpine air outside; and
for this purpose a large building is in course of erection, where they
will be able to take off their damp working clothes, have a hot and cold
douche, put on a warm dry suit, and obtain refreshments at a moderate
cost before returning to their homes. Instead of each man having a
locker in which to stow his clothes, a perfect forest of cords hangs
down from the wooden ceiling, 25 ft. above floor-level, each cord
passing over its own pulleys and down the wall to a numbered
belaying-pin. Each cord supports three hooks and a soap-dish, which,
when loaded with their owner’s property, are hauled up to the ceiling
out of the way. There are 2000 of these cords, spaced 1 ft. 6 ins.
apart, one to each man. The engineers and foremen are more privileged,
being provided with dressing-rooms and baths, partitioned off from the
two main halls. An extensive clothes washing and drying plant has been
laid down, and also a large restaurant and canteen. At Iselle, a
magazine holding 2200 lbs. of dynamite is surrounded and divided into
two separate parts by earth-banks, 16 ft. high. The two wooden houses,
in which the explosive is stored, are warmed by hot-water pipes to a
temperature between 61° F. and 77° F., and are watched by a military
patrol; but at Brigue a dynamite manufactory, started by an enterprising
company at the time of the commencement of the works, supplies this
commodity at frequent intervals, thereby avoiding the necessity of
storing in such large quantities. This dynamite factory has been
largely increased, and supplies dynamite to nearly all the mining and
tunneling enterprises in Switzerland.
=Geological Conditions.=--Before the Simplon tunnel was authorized,
expert evidence was taken as to the feasibility of the project. The
forecasts of the three engineers chosen, in reference to the rock to be
encountered and its probable temperature, have, as far as the galleries
have gone (an aggregate distance of nearly 2¹⁄₂ miles), generally been
found correct. At the north end, a dark argillaceous schist veined with
quartz was met with, and from time to time beds of gypsum and dolomite
have been traversed, the dip of the strata being on the whole favorable
to progress, though timbering is resorted to at dangerous places. Water
was plentiful at the commencement; in fact, one inrush has not been
stopped, and is still flowing down the heading. The total quantity of
water flowing from the tunnel mouth is 16 gallons per second, of which 2
gallons per second are accounted for by the drilling machines. At
Iselle, however, a very hard antigorio gneiss obtains, and is likely to
extend for 4 miles. Very dry and very compact, it requires no timbering,
and represents no great difficulty to the powerful Brandt rock-drills,
which work under a head of 3280 ft. of water.
The temperature of the rock depends not only on the depth from the
surface, but largely upon the general form of that surface combined with
the conductivity of the rock. Taking these points into consideration
with the experience gained from the construction of the St. Gothard
tunnel, 95° F. was estimated as the probable maximum temperature, owing
to the height of Monte Leone (11,660 ft.), which lies almost directly
over the tunnel axis.
=Survey.=--After having determined upon the general position of the
tunnels, taking into consideration the necessary gradients, the
temperature of the rock, and a large bed of troublesome gypsum on the
north side, two fixed points on the proposed center line were taken,
one at each entrance of tunnel No. 1, and the bearings of these two
points, with reference to a triangulation survey made in 1876, were
calculated sufficiently accurately to determine, for the time being, the
direction of the tunnel. In 1898, a new triangulation survey was made,
taking in eleven summits, Monte Leone holding the central position. This
survey was tied into that of the Wasenhorn and Faulhorn, made by the
Swiss Government, and the accuracy was such that the probable error in
the meeting of the two headings is only 6 cms. or 2¹⁄₂ ins.
On the top of each summit is placed a signal, consisting of a small
pillar of masonry founded on rock, and capped with a sharp pointed cone
of zinc, 1 ft. 6 ins. high. An observatory was built at each end of the
tunnel in such a position that three of the summits could be seen, a
condition very difficult to fulfill on the south side owing to the depth
of the gorge, the mountains on either side being over 7000 ft. high.
Having taken the angles to and from each visible signal, and therefrom
having calculated the direction of the tunnel, it was necessary to fix,
with extreme accuracy, sighting-points on the axis of the tunnel, in
order to avoid sighting on to the surrounding peaks for each subsequent
correction of the alinement of the galleries. To do this, a theodolite
24 ins. long and 2³⁄₈ ins. in diameter, with a magnifying power of 40
times, was set up in the observatory, and about 100 readings were taken
of the angles between the surrounding signals and the required
sighting-points. In this manner the error likely to occur was diminished
to less than 1′. Thus at the north end two points were found about 550
yds. before and behind the observatory, while on the south side, owing
to the narrowness of the gorge, the points could only be placed at 82
yds. and 126 yds. in front. One of these sighting-points consists of a
fine scratch ruled on a piece of glass fixed in an iron frame, behind
which is placed an acetylene lamp,--corrections of alinement are always
done by night,--the whole being rigidly fixed into a niche cut in the
rock and protected from climatic and other disturbing agencies by an
iron plate.
=Method of Checking Alinement.=--The direction of heading No. 1 is
checked by experts from the Government Survey Department at Lausanne
about three times a year, and for this purpose a transit instrument is
set up in the observatory. A number of three-legged iron tables are
placed at intervals of 1 mile or 2 miles along the axis of tunnel No. 1,
and upon each of these is placed a horizontal plane, movable by means of
an adjusting screw, in a direction at right angles to the axis, along a
graduated scale. On this plane are small sockets, into which the legs of
an acetylene lamp and screen, or of the transit instrument, can be
quickly and accurately placed. The screen has a vertical slit, 3 ins. in
height, and variable between ¹³⁄₁₆ in. and ³⁄₁₆ in. in breadth,
according to the state of the atmosphere, and at a distance shows a fine
thread of light. The instrument, having first been sighted on to the
illuminated scratch of the sighting-point, is directed up the tunnel,
where a thread of light is shown from the first table. With the aid of a
telephone this light is adjusted so that its image is exactly coincident
with the cross hairs, and the reading on the graduated scale is noted.
This is done four or five times, the average of these readings being
taken as correct, and the plane is clamped to that average. The
instrument is then taken to the first table and is placed quickly and
accurately over the point just found (by means of the sockets), and the
lamp is carried to the observatory. After first sighting back, a second
point is given on the second table, and so on. These points are marked
either temporarily in the roof of the heading by a short piece of cord
hanging down, or permanently by a brass point held by a small steel
cylinder, 8 ins. long and 3 ins. in diameter, embedded in concrete in
the rock floor, and protected by a circular casting, also sunk in cement
concrete, holding an iron cover resembling that of a small manhole. From
time to time the alinement is checked from these points by the
engineers, and after each blast the general direction is given by the
hand from the temporary points. To check the results of the
triangulation survey, astronomical observations have been taken
simultaneously at each end. With regard to the levels, those given on
the excellent Government surveys have been taken as correct, but they
have also been checked over the pass.
=Details of Tunnels.=--In cross-section, tunnel No. 1 is 13 ft. 7 ins.
wide at formation level, increasing to 16 ft. 5 ins., with a total
height of 18 ft. above rail-level, and a cross-sectional area of about
250 sq. ft. This large section will allow of small repairs being
executed in the roof without interruption of the traffic, and will also
allow of strengthening the walls by additional masonry on the inside.
The thickness of the lining, never wholly absent, and the material of
which it is composed, depend upon the pressure to be resisted, and only
in the worst case is an invert resorted to. The side drain, to which the
rock floor is made to slope, will be composed of half-pipes of 7 to 1
cement concrete. The roof is constructed of radial stones.
Tunnel No. 2, being left as a heading, is driven on that side nearest to
No. 1, to minimize the length of the cross-headings, and measures 10 ft.
2 ins. wide by 6 ft. 7 ins. high. Masonry is used only where necessary,
and in that case is so built as to form part of the lining of the tunnel
when eventually completed. Concrete is put in to form a foundation for
the side wall, and a water channel. The cross-headings, connecting the
two parallel headings, occur every 220 yds., and are placed at an angle
of 56° to the axis of the tunnel, to avoid sharp curves in the
contractors’ railway lines. They will eventually be used as much as
possible for refuges, chambers for storing the tools and equipment of
the platelayers, and signal-cabins. The refuges, 6 ft. 7 ins. wide by 6
ft. 7 ins. high and 3 ft. 3 ins. deep, occur every 110 yards, every
tenth being enlarged to 9 ft. 10 ins. wide by 9 ft. 10 ins. deep and 10
ft. 2 ins. high, still larger chambers being constructed at greater
intervals.
=Method of Excavation.=--The work at each end of the tunnel is carried
on quite independently, consequently, though similar in principle, the
methods vary in detail, apart from the fact that different geological
strata require different treatment. Broadly speaking, the two parallel
headings, each 59 sq. ft. in section, are first driven by means of
drilling-machines and the use of dynamite, this work being carried on
day and night, seven days in the week; No. 1 heading is then enlarged to
full size by hand-drilling and dynamite. On the Italian side, where the
rock is hard and compact, breakups are made at intervals of 50 yds., and
a top gallery is driven in both directions, but, for ventilation
reasons, is never allowed to get more than 4 yds. ahead of the break-up,
which is gradually lengthened and widened to the required section. No
timbering is required, except to facilitate the excavation and the
construction of the side walls. Steel centers are employed for the arch;
they entail fewer supports, give more room, and are capable of being
used over again more frequently without damage. They consist of two
I-beams bent to a template and riveted together at the crown, resting at
either side on scaffolding at intervals of 6 ft.; longitudinals 12 ft.
by 4 ins. by 4 ins. support the roof. Hand rock-drilling is carried out
in the ordinary way, one man holding the tool and a second striking;
measurements of excavation are taken every 2 or 3 yds., a plumb-line is
suspended from the center of the roof, and at every half-meter (20 ins.)
of height horizontal measurements are taken to each side.
At the Brigue end a softer rock is encountered, necessitating at times
heavy timbering in the heading, and especially in the final excavation
to full size, Fig. 56. The bottom heading, 6 ft. 6 in. high, is driven
in the center, and the heading is then widened to the full extent and
timbered; the concrete forming the water channel and the foundation for
one side wall is put in; the side walls are built to a height of 6 ft. 6
ins., and the tunnel is fully excavated to a further height of 6 ft. 6
ins. from the first staging. The side walls are then continued up for
the second 6 ft. 6 ins., and from the second floor a third height of 6
ft. 6 ins. is excavated and timbered. Finally the crown is cleared out,
heavy wooden centers are put in, the arch is turned and all timbers are
withdrawn except the top poling-boards, supporting the loose rock.
[Illustration: FIG. 56.--Sketches Showing Sequence of Work in Excavating
and Lining the Simplon Tunnel.
1
2
3
4
5
6
7
8]
The masonry for the side walls is obtained either from the tunnel itself
or from a neighboring quarry, and varies in character according to the
pressure; but the face of the arch is always of cut or artificial
stones, the latter being 7 to 1 cement concrete. Where the alinement
heading, or the “gallery of direction,” joins the curving portion of
tunnel No. 1, the section is very much greater, and necessitates special
timbering.
=Transport (Italian Side).=--A small line of railway, 2 ft. 7¹⁄₂ ins.
gauge, with 40-lb. rails, enters all three portals; but since the
construction of a wooden bridge over the Diveria, the route through the
“gallery of direction,” across heading No. 2, to tunnel No. 1, is used
exclusively; this railway leads to the face in both headings, and, where
convenient, from one heading to the other by the cross-galleries.
Different types of wagons are in use; but in general they are
four-wheeled, non-tipping box wagons, supplied with brakes and holding 2
cu. yds. of débris. A special type of locomotive is used, designed to
pass round curves of 50 ft. radius, and supplied with a specially large
boiler to avoid firing in the tunnel.
[Illustration: FIG. 57.--General Details of the Brandt Rotary Drills
Employed at the Simplon Tunnel.]
=Method of Working.=--The drilling-machines employed are of the Brandt
type, Fig. 57, and are mounted in the following manner: A small
four-wheeled carriage supports at its center a beam, the shorter arm of
which carries the boring mechanism and the longer a counterpoise; near
its center is the distributor. In the short arm is a clamp holding the
rack-bar or butting column, which is a wrought-iron cylinder with a
plunger constituting a ram, and is jammed by hydraulic pressure between
the walls of the heading, thus forming a rigid support for the
boring-machine, and an efficient abutment against the reaction of the
drill. This rack-bar can be rotated on its clamp in a plane parallel to
the axis of the beam. Three or four separate boring-machines can be
mounted on the rack-bar, and can be adjusted in any reasonable position.
The boring-machine performs the double function of continually pressing
the drill into the rock by means of a hollow ram (_I_) and of imparting
to the drill and ram a uniform rotary motion. This rotary motion is
given by a twin cylinder single-acting hydraulic motor (_E_), the two
pistons, of 2⁷⁄₈ ins. stroke, acting reciprocally as valves. The cranks
are fixed at an angle of 90° to each other on the shaft, which carries a
worm, gearing with a worm-wheel (_Q_) mounted upon the shell (_R_) of
the hollow ram (_I_), and this shell in turn engages the ram by a long
feather, leaving it free to slide axially to or from the face of the
rock. The average speed of the motor is 150 revolutions to 200
revolutions per minute, the maximum speed being 300 revolutions per
minute. The loss of power between the worm and worm-wheel is only 15% at
the most; the worm being of hardened steel and the wheel of gun-metal,
the two surfaces in contact acquire a high degree of polish, resulting
in little wearing or heating. Taking into consideration all other
sources of loss, 70% of the total power is utilized. The pressure on the
drill is exerted by a cylinder and hollow ram (_I_), which revolves
about the differential piston (_S_), which is fixed to the envelope
holding the shell (_R_). This envelope is rigidly connected to the
bed-plate of the motor, and, by means of the vertical hinge and pin
(_T_), is held by the clamp (_V_) embracing the rack-bar. When water is
admitted to the space in front of the differential piston the ram
carrying the drilling-tool is thrust forward, and when admitted to the
annular space behind the piston, the ram recedes, withdrawing the tool
from the blast-hole. The drill proper is a hollow tube of tough steel
2³⁄₄ ins. in external diameter, armed with three or four sharp and
hardened teeth, and makes from five to ten revolutions per minute,
according to the nature of the rock. When the ram has reached the end of
its stroke of 2 ft. 2¹⁄₂ ins., the tool is quickly withdrawn from the
hole and unscrewed from the ram; an extension rod is then screwed into
the tool and into the ram, and the boring is continued, additional
lengths being added as the tool grinds forward; each change of tool or
rod takes about 15 secs. to 25 secs. to perform. The extension rods are
forged steel tubes, fitted with four-threaded screws, and having the
same external diameter as the drill. They are made in standard lengths
of 2 ft. 8 ins., 1 ft. 10 ins., and 11³⁄₄ ins. The total weight of the
drilling-machine is 264 lbs., and that of the rack-bar when full of
water is 308 lbs. The exhaust water from the two motor cylinders escapes
through a tube in the center of the ram and along the bore of the
extension rods and drill, thereby scouring away the débris and keeping
the drill cool; any superfluous water finds an exit through a hose below
the motors and thence away down the heading. The distributor, already
mentioned, supplies each boring-machine and the rack-bar with hydraulic
pressure from the mains, with which connection is effected by means of
flexible or articulated pipe connections, allowing freedom in all
directions. The area of the piston for advancing the tool is 15¹⁄₂ sq.
ins., which, under a pressure of 1470 lbs. per sq. in., gives a pressure
of over 10 tons on the tool, while for withdrawing the tool 2¹⁄₂ tons is
available. In the rock found at Iselle, namely, antigorio gneiss, a hole
2³⁄₄ ins. in diameter and 3 ft. 3 ins. in length is drilled, normally,
in 12 mins. to 25 mins.; a daily rate of advance of 18 ft. to 19 ft. 6
ins. is made in a heading having a minimum cross-section of 59 sq. ft.;
the time taken to drill ten to twelve holes, 4 ft. 7 ins. deep, is 2¹⁄₂
hrs.
When the débris resulting from one operation has been sufficiently
cleared away, a steel flooring, which is provided near the face to
enable shoveling to be more easily done, and to give an even floor for
the wheels of the drilling-carriage, is laid bare at the head of the
line of rails, and the drilling-machines are brought up on their
carriage by eight or ten men. When advanced sufficiently close to the
face, the rack-bar is slewed round across the gallery and is wedged up
against the rock sides; connection is made between the distributor and
the hydraulic main, by means of the flexible pipe, and pressure is
supplied by a small copper tube to the rack-bar ram, thereby rigidly
holding the machine. Next, connections are made between the three
drilling-machines and the distributor, and in 20 mins. from the time the
machine was brought up all three drills are hard at work, water pouring
from the holes.
The noise of the motors and grinding-tools is sufficient to drown all
but shouts; and where the extension rods do not fit tightly, small jets
of water play in all directions, necessitating the wearing of tarpaulins
by the men directing the tools. Lighting is done wholly by small
oil-lamps, provided with a hook to facilitate fixing in any crack in the
rock; electricity will probably be used to light that portion of the
tunnel which is completed.
Two men are allotted to each drill, one to drive the motor, the other to
direct and replenish the tool, one foreman and two men in reserve
completing the gang. A small hammer is freely used to loosen the screw
joints of the extension rods and drill. A hole is usually commenced by a
two-edged flat-pointed tool, until a sufficient depth is reached to
prevent the circular tool from wandering over the face of the rock, but
in many instances the hole is commenced with a circular tool. The
exhaust water during this period flows away by the hose underneath the
motor. In the antigorio gneiss, ten to twelve holes are drilled for each
attack, three to four in the center to a depth of 3 ft. 3 ins., the
remainder, disposed round the outside of the face, having a depth of 4
ft. 7 ins. The average time taken to complete the holes is 1³⁄₄ hr. to
2¹⁄₂ hrs. Instead of pulverizing the rock, as do the diamond drills, it
is found that the rock is crushed, and that headway is gained somewhat
in the manner of a circular saw through wood. The core of rock inside
the tool breaks up into small pieces, and can be taken out if necessary
when the drill requires lengthening.
The lowest holes, inclined downwards, are full of water; consequently
two detonators and two fuses are inserted, but apart from this, water
has little effect on the charge. The fuses of the central holes are
brought together and cut off shorter than those of the outer holes, in
order that they may explode first to increase the effect of the outer
charges. All portable objects, such as drills, pipe connections, tools,
etc., have meanwhile been carried back; the steel flooring is covered
over with a layer of débris to prevent injury from falling rock, and to
the end of the hydraulic main is screwed a brass plug pierced by five
holes; and immediately the explosions occur a valve is opened in the
tunnel, and five jets of water play upon the rock, laying the dust and
clearing the air. The necessity for this was shown on one occasion when
this nozzle was broken by the explosion and the water had to be turned
off immediately to avoid useless waste; on reaching the face, the
atmosphere was found to be so highly charged with dust and smoke that it
was impossible to distinguish the stones at the feet, although a lamp
had been placed on the ground; and despite the fact that the air tube
was in full blast, the men experienced great difficulty in breathing. A
truck is now brought up, and four men clear a passage in front, through
the heap of débris, two with picks and two with shovels, while on either
side and behind are as many men as space will permit. The stone is
thrown either to the sides of the heading or into the wagon, shoveling
being greatly aided by the steel flooring, which, before the explosion,
had been laid over the rails for nearly 10 yds. down the tunnel to
receive the falling rock. These steel plates are taken up when cleared,
and the wagon is pushed forward until the drilling-machine can be
brought up again, leaving the remaining débris at the sides to be
handled at leisure during the next attack. The roof and side walls are,
of course, carefully examined with the pick, to discover and detach any
loose or hanging rock. The times taken for each portion of the attack in
this particular antigorio gneiss are as follows: Bringing up and
adjustment of drills, 20 mins.; drilling, between 1³⁄₄ hr. and 2¹⁄₂
hrs.; charging and firing, 15 mins.; clearing away débris, 2 hrs.; or
for one whole attack, between 4¹⁄₂ hrs. and 5¹⁄₂ hrs., resulting in an
advance of 3 ft. 9 in., or a daily advance of nearly 18 ft.
From this it appears that the time spent in clearing away the débris
equals that taken up in drilling, and it is in this clearing that a
saving of time is likely to be effected rather than in the process of
drilling. Many schemes have been tried, such as a mechanical plow for
making a passage; at Brigue, “marinage,” or clearing by means of
powerful high-pressure water-jets, directed down the tunnel, was tried,
but the idea is not yet sufficiently developed.
Another series of experiments has been tried at Brigue with regard to
the utilization of liquid air as an explosive agent instead of dynamite;
and for this purpose a plant has been laid down, consisting of one
ammonia-compressor, two air-compressors, and two refrigerators,
furnishing ¹⁄₁₀ gallon of liquid air per hour at an expenditure of 17 H.
P. The system used is that of Professor Linde, who himself directs the
experiments. The great difficulty experienced is that of shortening the
interval of time that must elapse between the manufacture of the
cartridge and its explosion. The liquid oxygen, with which the
cartridge, containing kieselguhr (silicious earth) and paraffin, is
saturated, evaporates very readily, losing power every moment; hence the
effect of each cartridge cannot be guaranteed, and though it is an
exceedingly powerful explosive when used immediately after manufacture,
no practical result has yet been obtained.
=Power Station.=--Water is abundant at either end, and therefore
hydraulic power is the motive force employed. On the Italian side, a dam
5 ft. high has been thrown across the Diveria at a point near the Swiss
frontier, about 3 miles above the site of the installations. A portion
of the water thus held back enters, through regulating doors and
gratings, a masonry channel leading to two parallel settling tanks, each
111 ft. by 16 ft., whence, after dropping all its sand and solid matter,
the now pure water passes into the water-house, and, after flowing over
a dam, through a grating and past the admission doors, enters a metallic
conduit of 3-ft. pipes. Each of the settling tanks and the approach
canal are provided with doors at the lower end leading direct to the
river, through which all the sand and solid matter deposited can be
scoured naturally by allowing the river-water to rush freely through.
For this purpose the floor of the basins is on an average gradient of 1
in 30. For a similar reason the river-bed just outside the entrance to
the approach canal is lined with wooden planks, from which the stones
collecting behind the dam can be scoured by allowing an iron flap,
hinged at the bottom, to change its position from the vertical to the
horizontal in a gap left purposely in the dam, so causing a rushing
torrent to sweep it clean.
The chief levels are:
Level of water at dam 794.00 meters above sea level.
„ in water-house 793.70 „ „ „ „
„ at turbines 618.50 „ „ „ „
giving a total fall of 175.20 ms. or 570 ft., and a pressure of 17.52
atmospheres.
The quantity of water capable of being taken from the Diveria in winter,
when the rivers which are dependent upon the mountain snows for their
supply are at their lowest, is calculated to be 352 gallons per second.
Thus, taking the fall to be diminished by friction, etc., to 440 ft.,
and the useful effect at 70%, there is obtained 2000 H. P. on the
turbine shaft.
The metallic conduit varies in material according to the pressure; thus
cast-iron pipes 3 ft. in diameter and ¹³⁄₁₆ in. thick are used up to a
pressure of 2 atmospheres, from which point they are of wrought-iron.
The cast-iron portion has of late caused a good deal of trouble, owing
to settlement of the piers causing occasional bursts, consequently a
masonry pier has been placed under each joint of this portion. The
following table gives the thicknesses and diameters, varying with the
pressure:
+---------+-------------+-------------+------+
| WATER | THICKNESS. | DIAMETER. |WEIGHT|
|PRESSURE.| | | PER |
| | | | YARD.|
+---------+-------+-----+-----+-------+------+
| Head | Milli-|Inch.|Feet.|Inches.| Lbs. |
| in Feet.|meters.| | | | |
+---------+-------+-----+-----+-------+------+
| 246 | 6 | ¹⁄₄ | 3 | 0 | 326 |
| 311 | 7 | ... | 3 | 0 | 383 |
| 360 | 8 | ... | 3 | 0 | 431 |
| 393 | 9 | ... | 3 | 0 | 483 |
| 426 | 10 | ... | 3 | 0 | 556 |
| 476 | 12 | ... | 3 | 0 | 651 |
| 590 | 16 | ⁵⁄₈ | 3 | 3¹⁄₃ | 977 |
+---------+-------+-----+-----+-------+------+
This pipe is supported every 30 ft. on small masonry piers, on the top
of which is placed a block of wood hollowed out to receive the pipe,
thus allowing any movement due to the contraction and expansion of the
conduit. However, to prevent this movement becoming excessive, the pipe
is passed at intervals of 300 yds. to 500 yds. through a cubical block
of masonry of 13 ft. side, strengthened by longitudinal tie-bars. Five
bands of angle-bar riveted round the pipe, with their flanges embedded
in the masonry, constitute a rigid fixed point. Straw mats are thrown
over the pipe where it is exposed to the sun. The temperature of the
conduit is not, however, found to vary greatly, since the pipe is kept
full of water. To supply the rock-drills with water at a maximum
pressure of 100 atmospheres, or 1470 lbs. per sq. in., a plant of four
pairs of high-pressure pumps has been laid down, and a still larger
addition is in course of erection. At present, two Pelton turbines of
250 H.P. each, running at 170 revolutions per minute, drive the pumps,
by means of toothed gearing, at 63 revolutions per minute. These pumps
are of very simple but strong construction, single suction and double
delivery, entailing one suction and one delivery-valve, both heavy and
both of small lift. The larger portion of the plunger has exactly double
the cross-sectional area of the smaller portion, so that in the forward
stroke half of the water taken in at the last admission is pumped into
the high-pressure mains, and at the same time a fresh supply of water is
sucked in. During the backward stroke half of this new supply is pumped
into the mains, and the remainder enters the second chamber, to be
pumped during the next forward stroke. Thus the work done in the two
strokes is practically the same. The pumps are in pairs, and are set at
an angle of 90°, to insure uniform pressure and uniform delivery in the
mains. Their size varies; but at Iselle there are three pairs, with a
stroke of 2 ft. 2¹⁄₂ ins., and the plungers of 2¹¹⁄₁₆ in. and 1⁷⁄₈ ins.
(approximately) in diameter, supplying 1.32 gallons per second.
To avoid injury to the valves, the water to be pumped is taken from a
stream up the mountain side, and is passed through filter screens. The
high-pressure water, after passing an accumulator, enters the tunnel in
solid drawn wrought-iron tubes, 3¹⁄₈ ins. in internal diameter, ³⁄₁₆ in.
thick, and in lengths of 26 ft. The diameter of these mains varies with
their length, so as to avoid loss of pressure. With the 1250 yds. of
tunnel now driven 10 atmospheres are lost.
At Brigue the installations are, as far as possible, identical. The
Rhone water, however, before reaching the water-house, is carried from
the filter basins, a distance of 2 miles, in an armored canal built upon
the Hennebique system,[9] the walls and supporting beams, of cement
concrete, being strengthened by internal tie-bars of steel. The concrete
struts, resembling balks of timber at a distance, are occasionally 35
ft. high and 1 ft. 7¹⁄₂ ins. square. The metallic conduit is 5 ft. in
diameter, with a minimum flow of 176 cu. ft. per second and a total fall
of 185 ft. In case water-power should be unavailable, three
semi-portable steam engines, two of 80 H.P. and one of 60 H.P., are
always kept in readiness at each end of the tunnel, and are geared by
belts to the turbine shaft.
[9] Network of steel rods embedded in concrete.
=Ventilation.=--In tunneling, one of the most important problems to be
solved is that of ventilation, and it is for this reason that the
Simplon tunnel consists of two parallel headings with cross cuts at
intervals of 220 yds. At Brigue, a shaft 164 ft. deep was sunk through
the overlying rock until the “gallery of direction” was encountered. Up
this chimney the foul air is drawn by wood fires, the fresh air--a
volume of 19,000,000 cu. ft. per day, or 13,200 cu. ft. per
minute--entering by heading No. 2, penetrating up to the last cross
gallery, and returning by tunnel No. 1. The entrances of No. 1 and the
“gallery of direction,” besides those of all the intermediate cross
galleries, are closed by doors. By this arrangement, however, fresh air
does not reach the working faces; therefore a pipe, 8 ins. in diameter,
is led from the fresh air in No. 2 to within 15 yds. of the face of each
heading, and up this pipe a draft of air is induced by means of a jet of
water, the volume to each face being 800 cu. ft. per minute. One single
jet of water from the high-pressure mains, with a diameter of ¹⁄₁₆ in.,
is capable of supplying over 1000 cu. ft. of air per minute at the end
of 160 yds. of pipe, and during the attack the men at the drills are in
a constant breeze with the thermometer standing at 70° F. At Iselle, air
is blown into the entrance of heading No. 2 at the rate of 14,100 cu.
ft. per minute by two fans driven from the turbine shaft. This air
travels from the fans along a pipe 18 ins. in diameter, till a point 15
yds. up the tunnel is reached, where beyond a door the pipe narrows to
form a nozzle 10 ins. in diameter. This door is kept open to allow the
outside air to be induced up the tunnel, as the headings are at present
only 2500 yds. long, giving a resistance of not quite sufficient power
to cause the air to return. The fresh air then travels up No. 2,
crossing over the top of the “gallery of direction,” from which it is
shut off by doors, to the last cross gallery, returning by No. 1, and
finally leaving either by the “gallery of direction” or by No. 1. A
system of cooling the air and driving it on by means of a large number
of water-jets will be installed in No. 2 where that heading crosses over
the “gallery of direction,” but at present there is no need for it.
The average temperature at the face is 73° F. during the drilling
operation, 76° F. after firing the charges, and a maximum of 80° F.,
lately attaining to 86° F. on the south side, with 80° F. and 85° F.
before and after firing. The temperature of the rock is taken at every
110 yds. in holes 5 ft. deep, and shows a gradual increase according to
the depth of over-laying rock, to the conductivity of the rock, and to
the form of the mountain surface. The maximum hitherto reached on the
north side is 68° F., while on the south side, although a smaller
distance has been traversed, it attains to 79° F., due to the more rapid
increase in depth. Moreover, the temperature of the rock is observed at
the permanent stations, 550 yds. from the entrances, in its relation to
that of the tunnel and outside air, and though on the north side that of
the rock varies almost as quickly as that of the tunnel air, on the
south it is influenced very much less.
A few statistics may be of interest with regard to the progress of the
last three months (taken from the trimestrial report of January, 1900).
At Brigue, where there are three drilling-machines in No. 1 and two in
the parallel heading, the total length excavated was 995 yds. or 6409
cu. yds. in 89 working days, the average cross-sectional area being 57
sq. ft. This required 507 attacks and 3066 holes, which had a total
depth of 26,600 ft. and 14,700 re-sharpenings of the drilling-tool, with
44,000 lbs. of dynamite.
The average time occupied in drilling was 2 hrs. 45 mins., while
charging, firing, and clearing away the débris took 6 hrs., 35 mins. At
Brigue 648 men and 29 horses were employed at one time in the tunnel. At
Iselle the numbers were 496 men and 16 horses, working in shifts of 8
hrs. Outside the tunnel, in the shops, forges, etc., the men work 8 hrs.
to 11 hrs. per day, the total being 541 men at Brigue and 346 men at
Iselle. On the Italian side, where the rock is very much harder, there
were three drilling-machines in each heading; the total length
excavated, with a cross-sectional area of 62 sq. ft., was 960 yds. or
6700 cu. yds. in 91 working days. This required 61,293 re-sharpened
tools, 758 attacks, 7940 holes with a total depth of 33,000 ft., and
56,000 lbs. of dynamite. The average time spent in drilling was 2 hrs.
55 mins., and in charging and clearing 2 hrs. 36 mins. Thus, in the hard
gneiss, to excavate 1 cu. yd. of rock required 8¹⁄₂ lbs. of dynamite,
and each tool pierced 6¹⁄₂ ins. of rock before it required
re-sharpening.
THE MURRAY HILL TUNNEL.
The drift method of excavating tunnels was followed in Section IV of the
New York Subway, under Park Avenue between 33rd and 41st Streets. At
this point the four tracks of the subway pass under a rocky elevation,
known as Murray Hill, in two double track parallel tunnels, 43 ft.
apart, center to center. Here already existed a double track tunnel
which was built many years ago by the New York Central and Hudson River
R.R., and is now used by the Madison Avenue surface cars. The two subway
tunnels were driven close below the existing tunnel and also very near
the foundations of expensive residences along Park Avenue, particularly
on Murray Hill, one of the best residential sections of the city.
=Material Penetrated.=--The material penetrated by the excavation
consisted chiefly of a surface outcrop of the mica-schist rock which
underlies Manhattan Island. The rock was for the most part in compact
strata, dipping at about 45° from East to West, but at intervals an
unstable stratum was encountered which when free slid on the underlying
stratum. Troubles from such slides were experienced during the
construction of the tunnel.
=Cross-Section.=--The cross-section selected for the tunnels had
vertical side walls and a three-centered roof arch with the flattest
curve at the crown. The interior dimensions were 25 ft. wide and 16 ft.
high. The selected cross-section was not the best suited for a tunnel to
be driven through rock, where the sharpest curve should be at the top,
but in this case the flattened curve was chosen because of local
conditions; chiefly, the presence of the existing tunnel and the
consequent necessity of leaving a certain thickness of rock between it
and the new tunnel, without depressing very much the grade of the
subway.
[Illustration: FIG. 58.--Sequence of Excavation in the Murray Hill
Tunnel.]
=Excavation.=--The two parallel tunnels were driven exclusively from the
ends reached by shafts; thus the tunnels were attacked at four parts. It
was in these tunnels that a comparative test was made of the different
methods of driving tunnels through rock. The contractor applied the
heading and drift method at the southern ends of the tunnels, the
eastern tunnel being driven by means of a drift while in the western
tunnel the usual heading method was followed. This latter method is
illustrated in the chapter following and the eastern tunnel at 33rd
Street, excavated by means of a drift, is here considered.
Fig. 58 shows the sequence of cuts adopted for this tunnel. It was begun
by a bottom drift, about 10 ft. high, 8 ft. wide and 7 ft. deep, which
was located at one side of the axis of the tunnel, as indicated in the
figure. This drift was immediately widened by removing the portions
marked 2. About 50 ft. in the rear the part marked 3 was taken away,
thus clearing the entire lower portion of the tunnel. Section 4, about
50 ft. to the rear of section 3, was then broken down and removed.
The methods of drilling and blasting were as follows: In taking out the
original drift, a wedge-shaped center cut was made and then enlarged to
the full size of the drift by drilling parallel holes. The succeeding
sections, 2 and 3, were removed by driving parallel holes, while the top
section, 4, was taken away by a center cut and parallel holes. The
drills were mounted on columns, two drills to a column, and the holes
were usually drilled about 7 ft. deep, starting with a diameter of 2³⁄₄
in. and ending with a diameter of 1³⁄₄ in. They were blasted with 40%
dynamite in light charges, only a few holes being fired at a time,
usually not more than three or four.
[Illustration: FIG. 59.--Traveling Platform for the Excavation of the
Upper Side of the Murray Hill Tunnel.]
To remove section 4, a traveling platform 10¹⁄₂ ft. long and 25 ft. wide
was used. This platform, as shown in Fig. 59, consisted of two
longitudinal beams mounted on four double flanged wheels which were
running on tracks laid 23 ft. apart. Resting on top of these beams were
four 12 in. × 12 in. uprights braced in every direction against the
framework of the platform. This frame was built of 12 in. × 12 in. beams
laid longitudinally, the transverse beams being 12 in. × 14 ins. The
platform proper was made of 3 in. planks, and was set 9 ft. above the
tunnel floor. The columns supporting the drills for the excavation of
the upper section 4, were set up above the platform which was then
reinforced by other vertical props, as indicated by the dotted lines in
the figure. These props, however, were placed so as to leave a clearance
beneath the platform for the cars to carry away the débris from the
front. During the blasting the platform was moved back so that the
blasted rock fell to the floor of the tunnel, whence it was loaded into
boxes on the cars.
=Strutting.=--When the rock was seamy and full of fissures, running in
every direction, it was necessary to support the roof of the excavation.
This was done in the following manner: After part 4 was removed the
timbers supporting the roof of the excavation were set up. In this case,
the polygonal strutting was used. This consisted of heavy timber frames
placed transversely to the axis of the tunnel and supporting the planks
or poling-boards which ran longitudinally against the roof of the
excavation. The seven-segment arch frame was used in the Murray Hill
tunnel. At the bottom of part 4 were placed longitudinally 12 × 16 in.
beams and upon them rested the inclined segments which, with a
horizontal one, formed the arch frame as shown in Fig. 60. When the
pressures were too heavy the crown segment was reinforced by a 6 × 12
in. beam, kept in place by two 12 × 12 in. inclined props which rested
on the templates. As the tunnel was lined with concrete, the timbering
was left in place and it was built outside the line of the extrados of
the concrete lining. Timbering was only used for a short distance but it
necessitated a larger amount of rock excavation when it was required.
[Illustration: FIG. 60.--Timbering Used in the Murray Hill Tunnel.]
=Hauling.=--Great efficiency was shown in the method of hauling away the
excavated materials. Three narrow-gauge parallel tracks were laid on the
floor of the tunnel and extended to the faces of the advance drifts.
Small flat cars were run on these tracks. They carried steel boxes, 5
ft. square and 15 ins. deep, fitted with three lifting rings and chains.
When filled, the cars were run to the bottom of the shaft, the boxes
were hoisted by a stiff-legged derrick placed at the shaft head, and the
débris was dumped into storage bins of 300 cu. yds. capacity. These bins
were elevated 8 ft. above the street so that the wagons could be driven
under it to take loads of spoil by means of chutes. The broken rock was
loaded into the boxes by hand.
=Concrete Lining.=--The tunnel was lined with concrete which was
manufactured by a quite elaborate plant. A stone crushing plant,
consisting of bins for raw and crushed stone, was erected at the shaft
head and a mixing plant was suspended from the shaft. On the platform of
the shaft head were two bins side by side, one for crushed stone, the
other for sand; both of which communicated, by means of trap doors, with
a hopper chute. The materials from the hopper were delivered into a
measuring box where cement was laid on top of the other ingredients by
hand. They were then conveyed through a canvas chute into a cubical
mixer operated by an engine. The mixer discharged its contents into
skips set on cars at the bottom of the shaft and the concrete was hauled
inside the tunnel ready for use.
The construction of the lining was accomplished by means of traveling
platforms. The footing courses were laid first. Because these projected
inward about 18 ins. from the faces of the finished sidewalks it was
possible to lay a track rail on their top inner edges on each side of
the tunnel. These track rails carried the traveling platforms. There
were three of these platforms; the forward one was used for building the
side walls; the center one, for carrying a derrick; the last one, for
building the roof arch. The side wall platform was mounted on six
wheels. On each side there was mounted an adjustable lagging which was
curved to conform to the inside profile of the side wall. In operation
this platform was run to the point where the side walls were to be
constructed and the lagging was adjusted to position and fastened. Skips
of concrete were then hoisted on its top, their contents were shoveled
into the space between the lagging and the wall of the excavation and
were there rammed into place until the finished concrete had reached the
top of the lagging. When the concrete had set, the wedges holding the
lagging in place were loosened and the platform was moved ahead and
adjusted for building a new section of wall. The derrick platform was
23¹⁄₂ ft. wide and 18 ft. long. Transversely, it had three bays, two of
which were floored over and one was left without flooring to allow
passage for the concrete skips to and from the cars, on the tunnel floor
beneath. At the center of the floored area was mounted a derrick to
handle the skips. In operation, the derrick platform came between the
side wall platform ahead and the roof platform behind. The construction
of the roof platform was practically the same as the side wall platform
with the addition of roof arch centers at each bent on which lagging
could be placed. The mode of procedure was to erect the form for a small
space between the side walls already built and the haunches of the
center, to shovel concrete from the skips and to run it into place. Then
the roof lagging, a part at a time, was placed upward from the haunches
and the concrete was filled and rammed behind it. The lining was built
from the haunches upward until the two sides approached within a
distance of about 5 ft. from each other at the crown. This 5 ft. crown
strip or key was built by working from the rear toward the front end of
the platform.
=Plant.=--The plant used by the contractors for Section IV. of the
subway comprised a central power plant located about 4000 ft. from the
work. This was on 42nd Street near the East River and furnished power
for the work on both Sections IV. and V. The buildings consisted of an
engine room 63 × 30 ft. and a boiler room, 42 × 28 ft. In the former
room was located one Rand-Corliss air compressor, 22 × 40 × 48 ins.,
having a capacity of 5000 cu. ft. of free air per minute; in the latter
room there were two 200 H.P. water tube boilers. There were also the
necessary equipment of feed water pump, air condenser pump, etc. The
compressors discharged into a 20 × 5¹⁄₂ ft. receiver of riveted steel
through a 7 in. pipe. The air from the receiver was carried by a 10 in.
pipe 3.277 ft. to the corner of Park Avenue and 41st Street, and was
thence run south along Park Avenue in an 8 in. pipe, from which 3 in.
branches led to the four headings of the work.
=Ventilation.=--The ventilation of the tunnel caused very little
trouble. In cool weather the natural draft of the shafts and the air
discharged from the drills served to keep the atmosphere wholesome. In
warm weather, artificial means were necessary to clear the workings of
foul air, particularly after blasting. They comprised at each end a 4
ft. American exhaust fan drawing air from a 12 in. riveted galvanized
iron pipe, which extended to the working faces.
=Illumination.=--The tunnel was lighted by electric lamps which extended
even to the working face. During the blasting, however, all the lamps
and wires within 100 ft. from the front were removed and gasoline
torches were used; they were also employed before the electric lamps and
wires could be replaced, to light the tunnel during the operation of
clearing the débris.