TUNNEL
(Fr.
tonnel
, later
tonneau
, a diminutive from Low Lat.
tonna
,
tunna
, a tun, cask), a more or less horizontal underground passage made without removing the top soil. In
former times any long tube-like passage, however constructed,
was called a tunnel. At the present day the word is sometimes
popularly applied to an underground passage constructed by
trenching down from the surface to build the arching and then
refilling with the top soil; but a passage so constructed, although
indistinguishable from a tunnel when completed, is more correctly termed a “covered way,” and the operations “cutting”
and “covering,” instead of tunnelling. Making a small tunnel,
afterwards to be converted into a larger one, is called “driving
a heading,” and in mining operations small tunnels are
termed “galleries,” “drift ways” and “adits.” If the underground
passage is vertical it is a shaft; if the shaft is begun
at the surface the operations are known as “sinking”; and
it is called a “rising” if worked upwards from a previously
constructed heading or gallery.
Tunnelling has been effected by natural forces to a far greater
extent than by man. In limestone districts innumerable
swallow-holes, or shafts, have been sunk by the rain water
following joints and dissolving the rock, and from the bottom
of these shafts tunnels have been excavated to the sides of
hills in a manner strictly analogous to the ordinary method of
executing a tunnel by sinking shafts at intervals and driving
headings therefrom. Many rivers find thus a course underground.
In Asia Minor one of the rivers on the route of the
Mersina railway extension pierces a hill by means of a natural
tunnel, whilst a little south at Seleucia another river flows
through a tunnel, 20 ft. wide and 23 ft. high, cut 1600 years
ago through rock so hard that the chisel marks are still discernible.
The Mammoth Cave of Kentucky and the Peak
caves of Derbyshire are examples of natural tunnelling.
Mineral springs bring up vast quantities of matter in solution.
It has been estimated that the Old Well Spring at Bath has
discharged since the beginning of the 19th century solids
equivalent to the excavation of a 6 ft. by 3 ft. heading 9 m.
long; and yet the water is perfectly clear and the daily flow
is only the 150th part of that pumped out of the great railway
tunnel under the Severn. Tunnelling is also carried on to an
enormous extent by the action of the sea. Where the Atlantic
rollers break on the west coast of Ireland, or on the seaboard
of the western Highlands of Scotland, numberless caves and
tunnels have been formed in the cliffs, beside which artificial
tunnelling operations appear insignificant. The most gigantic
sub aqueous demolition hitherto carried out by man was the
blowing up in 1885 of Flood Rock, a mass about 9 acres in
extent, near Long Island Sound, New York. To effect this
gigantic work by a single instantaneous blast a shaft was sunk
64 ft. below sea-level, from the bottom of which 4 m. of
tunnels or galleries were driven so as to completely honeycomb
the rock. The roof rock ranged from 10 ft. to 24 ft. in thickness,
and was supported by 467 pillars 15 ft. square; 13,286 holes,
averaging 9 ft. in length and 3 ins. in diameter, were drilled
in the pillars and roof. About 80,000 cub. yds. of rock were
excavated in the galleries and 275,000 remained to be blasted
away. The holes were charged with 110 tons of “rackarock,”
a more powerful explosive than gunpowder, which was fired
by electricity, when the sea was lifted 100 ft. over the whole
area of the rock. Where natural forces effect analogous results,
the holes are bored and the headings driven by the chemical
and mechanical action of the rain and sea, and the explosive
force is obtained by the expansive action of air locked up in the
fissures of the rock and compressed to many tons per square foot
by impact from the waves. Artificial breakwaters have often
been thus tunnelled into by the sea, the compressed air blowing
out the blocks and the waves carrying away the debris.
With so many examples of natural caves and tunnels in
existence it is not to be wondered at that tunnelling was one
of the earliest Works undertaken by man, first for dwellings
and tombs, then for quarrying and mining, and finally for
water-supply, drainage, and other requirements of civilization.
A Theban king on ascending the throne began at once to drive
the tunnel which was to form his final resting-place, and persevered
with the work until death. The tomb of Mineptah at
Thebes was driven at a slope for a distance of 350 ft. into the
hill, when a shaft was sunk and the tunnel projected a farther
length of about 300 ft., and enlarged into a chamber for the
sarcophagus. Tunnelling on a large scale was also carried on
at the rock temples of Nubia and of India, and the architectural
features of the entrances to some of these temples might be
studied with advantage by the designers of modern tunnel
fronts. Flinders Petrie has traced the method of underground
quarrying followed by the Egyptians opposite the Pyramids.
Parallel galleries about 20 ft. square were driven into the rock
and cross galleries cut, so that a hall 300 to 400 ft. wide was
formed, with a roof supported by rows of pillars 20 ft. square
and 20 ft. apart. Blocks of stone were removed by the workmen
cutting grooves all round them, and, where the stone was not
required for use, but merely had to be removed to form a
gallery, the grooves were wide enough for a man to stand up
in. Where granite, diorite and other hard stone had to be cut
the work was done by tube drills and by saws supplied with
corundum, or other hard gritty material, and water-the drills
leaving a core of rock exactly like that of the modern diamond
drill. As instances of ancient tunnels through soft ground
and requiring masonry arching, reference may be made to the
vaulted drain under the south-east palace of Nimrod and to the
brick arched tunnel, 12 ft. high and 15 ft. wide, under the
Euphrates. In Algeria, Switzerland, and wherever the Romans
Went, 'remains of tunnels for roads, drains and Water-supply
are found. Pliny refers to the tunnel constructed for the
drainage of Lake Fucino as the greatest public work of the time.
It was by far the longest tunnel in the world, being more than
m. in length, and was driven under Monte Salviano, which
necessitated shafts no less than 400 ft. in depth. Forty shafts
and a number of “cuniculi,” or inclined galleries, were sunk,
and the excavated material was drawn up in copper pails, of
about ten gallons capacity, by windlasses. The tunnel was
designed to be 10 ft. high by 6 ft. wide, but its actual cross section
varied. It is stated that 30,000 labourers were occupied
eleven years in its construction. With modern appliances
such a tunnel could be driven from the two ends without
intermediate shafts in eleven months.
No practical advance was made on the tunnelling methods of
the Romans until gunpowder came into use. Old engravings
of mining operations early in the 17th century show that
excavation was still accomplished by pickaxes or hammer and
chisel, and that wood fires were lighted at the ends of the
headings to split and soften the rock in advance (see fig. 1),
![](//upload.wikimedia.org/wikipedia/commons/thumb/2/24/EB1911_Tunnel%2C_1.jpg/300px-EB1911_Tunnel%2C_1.jpg)
(From Agricola's
De re metallica
, Basel, 1621.)
Fig
. 1.?Method of mining, 1621.
Crude methods of ventilation by shaking cloths in the headings and by placing inclined boards at the top of the shafts are also on record. In 1766 a tunnel 9 ft. wide, 12 ft. high and 2880 yds. long was begun on the Grand Trunk Canal, England, and completed eleven years later; and this was followed by many others. On the introduction of railways tunnelling became one of the ordinary incidents of a contractor's work; probably upwards of 4000 railway tunnels have been executed.
Tunnelling under Rivers and Harbours
.?In 1825 Marc Isambard Brunel began, and in 1843 completed, the 'Thames tunnel between Rotherhithe and Wapping now used by the East London railway. He employed a peculiar “shield,” made of timber, in several independent sections. Part of the ground penetrated was almost liquid mud, and the cost of the tunnel was about £1300 per lineal yard. In 1818 he took out a patent for a tunnelling process, which included a shield, and which mentioned cast iron as a surrounding wall. His shield foreshadowed the modern shield, which is substituted for the ordinary timber work of the tunnel, holds up the earth of excavation, affords space within its shelter for building the permanent walls, overlaps these Walls in telescope fashion, and is moved forward by pushing against their front ends. The advantages of cast-iron walls are that they have great strength
in small space as soon as the segments are bolted together, and they can be caulked water-tight.
In 1830 Lord Cochrane (afterwards 10th earl of Dundonald)
patented the use of compressed air for shaft-sinking and tunnelling
in water-bearing strata. Water under any pressure can
be kept out of a sub aqueous chamber or tunnel by sufficient
air of a greater pressure, and 'men can breathe and work
therein?for a time?up to a pressure exceeding four atmospheres.
The shield and cast-iron lining invented by Brunel,
and the compressed air of Cochrane, have with the aid of later
inventors largely removed the difficulties of sub aqueous tunnelling.
Cochrane's process was used for the foundation of bridge
piers, &c., comparatively early, but neither of these devices was
employed for tunnelling until half a century after their invention.
Two important sub aqueous tunnels in the construction
of which neither of these valuable aids was adopted are the
Severn and the Mersey tunnels.
The Severn tunnel (fig. 16),
4
+
1
⁄
3
m. in length for a double line of railway, begun in 1873 and finished in 1886, Hawkshaw, Son, Hayter & Richardson being the engineers and T. A. Walker the contractor, is made almost wholly in the Trias and Coal Measure formations, but for a short distance at its eastern end passes through gravel. At the lowest part the depth is 60 ft. at low water and 100 ft. at high water, and the thickness of sandstone over the brickwork is 45 ft. Under a depression in the bed of the river on the English side there is a cover of only 30 ft. of marl. Much water was met with throughout. In 1879 the works were flooded for months by a land spring on the Welsh side of the river, and on another occasion from a hole in the river bed at the Salmon (Pool. This hole was subsequently filled with clay and the works completed beneath. Two preliminary headings were driven across the river to test the ground. “Break-ups” were made at intervals of two to five chains and the arching was carried on at each of these points. All parts of the excavation were timbered, and the greatest amount excavated in any one week was 6000 cub. yds. The total amount of water raised at all the pumping station sis about 27,000,000 gallons in twenty-four hours.
The length of the Mersey tunnel (fig. 15) between Liverpool and Birkenhead between the pumping shafts on each side of the river is one mile. From each a drainage heading was driven through the sandstone with a rising gradient towards the centre of the river. This heading was partly red out by a Beaumont machine to a diameter of 7 ft. 4 in. and at a rate attaining occasionally 65 lineal yds. per week. All of the tunnel excavation, amounting to 320,000 cub. yds., was got out by hand labour, since heavy blasting would have shaken the rock. The minimum cover between the top of the arch and the bed of the river is 30 ft. Pumping machinery is provided for 27,000,000 gallons per day, which is more than double the usual quantity of water. Messrs Brunlees & Fox were the engineers, and Messrs Waddell the contractors for the works, which were opened in 1886, about six years after the beginning of operations.
In 1869 P. W. Barlow and J. H. Greathead built the Tower
foot-way under the Thames, using for the first time a cast-iron
lining and a shield which embodied the main features of Brunel's
design. Barlow had patented a shield in 1864, and A. E. Beach
one in 1868. The latter was used in a short masonry tunnel
under Broadway, New York City, at that time. In 1874
Greathead designed and built a shield, to be used in connexion
with compressed air, for a proposed Woolwich tunnel under
the Thames, but it was never used. Compressed air was first
used in tunnel work by Hersent, at Antwerp, in 1879, in a
small drift with a cast-iron lining.
In the same year compressed air was used for the first time
in any important tunnel by D. C. Haskin in the famous first
Hudson River tunnel, New York City. This was to be of
two tubes, each having internal dimensions of about 16 ft.
wide by 18 ft. high. The excavation as fast as made was lined
with thin steel plates, and inside of these with brick. In
June 1880 the northerly tube had reached 360 ft. from the
Hoboken shaft, but a portion near the latter, not of full size,
was being enlarged. Just after a change of shifts the compressed
air blew a hole through the soft silt in the roof at this spot,
and the water entering drowned the twenty men who were
working therein. From time to time money was raised and the
work advanced. Between 1888 and 1891 the northerly tunnel
was extended 2000 ft. to about three-fourths of the way across,
with British capital and largely under the direction of British.
engineers?Sir Benjamin Baker and E. W. Moir. Compressed
air and a shield were used, and the tunnel walls were made of
bolted segments of cast iron. The money being exhausted,
the tunnel was allowed to fill with water, and it so remained
for ten years; Both tubes were completed in 1908.
The use of compressed air in the Hudson tunnel, and of
annular shields and cast-iron lined tunnel in constructing
the City & South London railway (1886 to 1890) by Greathead,
became widely known and greatly influenced sub aqueous
and soft-ground tunnelling thereafter. The pair of tunnels
for this railway from near the Monument to Stockwell, from
10 ft. 2 in. to 10 ft. 6 in. interior diameter, were constructed
mostly in clay and without the use of compressed air, except
for a comparatively short distance through water-bearing
gravel. In this gravel a timber heading was made, through
which the shield was pushed. The reported total cost was
£840,000. Among the tunnels constructed after the City &
South London work was well advanced, lined with cast-iron
segments, and constructed by means of annular shields and
the use of compressed air, were the St Clair (Joseph Hobson,
engineer) from Sarnia to Port Huron, 1889?1890, through clay,
and for a short distance through water-bearing gravel, 6000 ft.,
18 ft. internal diameter; and the notable Blackwall tunnel
under the Thames (Sir Alexander Binnie, engineer, and S.
Pearson & Sons, contractors), through clay and 400 ft. of water saturated
gravel, 1892?1897, about 3116 ft. long, 24 ft. 3 in.
in internal diameter. The shield, 19 ft. 6 in. long, contained a
bulkhead with movable shutters, as foreshadowed in Baker's proposed
shield (fig. 2).
![](//upload.wikimedia.org/wikipedia/commons/thumb/0/00/EB1911_Tunnel%2C_2.jpg/240px-EB1911_Tunnel%2C_2.jpg)
Fig
. 2.?B. Baker's pneumatic shield.
Numerous tunnels of small diameter have been similarly constructed under the Thames and Clyde for electric and cable ways, several for sewers in Melbourne, and two under the Seine at Paris for sewer siphons.
The Rotherhithe tunnel, under the Thames, for a roadway,
with a length of 4863 ft. between portals, of which about
1400 ft. are directly under the river, has
the largest cross-section of any subaqueous tube of this
type in the world (see fig. 3). It was begun in 1904 and
finished in 1908, Maurice Fitzmaurice being the engineer of
design and construction, and Price & Reeves the contractors. It
penetrates sandy and shelly clay overlying a seam of limestone
beneath which are pebbles and loamy sand. A preliminary
tunnel for exploration, 12 ft. in diameter, was driven across
the river, the top being within 2 ft. of the following main
tunnel. The top of the main tunnel excavation in the middle
of the river was only 7 ft. from the bed of the Thames, and
a temporary blanket of filled earth, usually allowed in similar
cases, was prohibited owing to the close proximity of the docks.
The maximum progress in one day was 12.5 ft., and the
average in six days 10.4 ft. The air compressors were together
capable of supplying 1,000,000 cub. ft. of air per hour.
Some tunnels of marked importance of this type?to be
operated solely with electric cars?have been built under the
East and Hudson rivers at New York. Two tubes of 15 ft.
interior diameter and 4150 ft. long penetrate gneiss and
gravel directly under the East River between the Battery
and Brooklyn. They were begun in 1902, with Wm. B.
Parsons and George S. Rice as engineers, and were finished
in December 1907, under the direction of D. L. Hough of the
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(Upload an image to replace this placeholder.)
The Thames '1'mnel (Brunel), 18:5-1842.
Hudson River (Haskin), 1879.
Wlterloo & City Rail- $3
Wty, Thames. River Spree, Berlxn
Hudscn Rizsgellldorton Sr. Q-M /
Blnckwall Tunnel. Thames. 1 tube. f 44
Baker St. & Waterloo Greenwich Footway,
Railwny. Thames. ' a tubes Tunnel. | tube. .W East Boston Tunnel under'HarbuUr. r tube.
Harlem River. 2 tubes. " ':"
""' -11: . ff
Rocherhixhe, Thames. x tube ; .
Hudson and East Rivers. Pennsylvania
Detroit River '|'unnel. 2 lubet.
Scale of Feet
no 5 o xo
Fig. 3.-Cross Sections of Tunnels under
River Seine. Pub. 1 lube
zo 30
Rivers and Harbour!
New York Tunnel Company. They carry subway trains. In
one of the blow-outs of compressed air a Workman was blown
through the gravel roof into the river above. He lived until
the next day. Two other tubes of the same size built also through
gneiss and gravel between 1905 and 1907 by the Degnon
Contracting Company, with R. A. Shailer as the contractors
engineer, go from 42nd Street to Long Island City.
Four much larger tubes (see ng. 3) built in 1904 to 1909, for
the Pennsylvania railroad, with Alfred Noble as chief engineer,
S. Pearson & Son as contractors, and E. W. Moir as general
manager, cross from 32nd and 33rd Streets to Long Island.
The maximum average progress per day (one heading) for the
best month's work was: rock, 4-1 ft.; rock and earth, 3-8 ft.;
earth, with full sand face, 12-8 ft. The best methods of preventing
blow-outs were found to consist of employing clay blankets
(sometimes 2 5 ft. thick) on the river bed, which could be carried
up to 20 ft. depth of water, and of filling the pores of the sand
and gravel with blue lias lime or cement grout. The maximum
air pressure was 38 lb per sq. in. In the case of sand face with
poor leaky cover the usual practice was to make the air pressure
equal to that of water from the surface down to about a quarter
the distance below the top of the shield. The average amount
of free air supplied per man per hour was approximately 2300
cub. ft. On the Hudson river side two tubes of the same
size as those in the East river are for the Pennsylvania trains
to New Jersey. Two tubes from Morton Street to New jersey,
begun by Haskin, already referred to, are for subway trains, and
so are the most southerly of all on the Hudson side, viz. the
two from Cortlandt Street to under the Pennsylvania station
in jersey City.
The two tubes from Morton Street were completed under
the direction of Charles M. Jacobs, who was also chief engineer
of the four other Hudson River tubes. The contractors for the
Hudson tubes for the Pennsylvania road were the O'Rourke
Contracting Company. Skilful treatment was required to
overcome the difficulties on the New York side of the Hudson
in all the tubes where the face excavation was partly in rock
and partly in soft earth. Most of their length, however, was
through silt, and in this the tunnelling was the easiest and
most rapid that has ever been carried out in sub aqueous work,
50 lineal ft. per day being sometimes accomplished. A large
proportion of the silt which under ordinary processes would
be taken into the tunnel through the shield, carried to the shore
and got rid of by expensive methods, was by the latter process
merely displaced as the shield with nearly or quite closed
diaphragm was pushed ahead.
The East Boston tunnel, the first important example of a
shield-built monolithic concrete arch, from the Boston Subway
to East Boston, is 1-4 m. long, 3400 ft. being under the
harbour. One mile was excavated by tunnelling with roof shields
about 29 ft. wide, through clay containing pockets of sand and
gravel. The engineer was H. A. Carson, and the contractors the
Boston Tunnel Construction Company and Patrick McGovern.
Some 25 m. of waterworks brick-lined tunnels have been built
since 1864, mostly in clay, under the Great Lakes, without the
use of shields, though in the later ones compressed air was
utilized. A large portion of the latest Cleveland tunnel, 9 ft.
interior diameter, was built at the rate of 17 ft. per day at a
cost of about $18 per ft. During this work three explosions
of inflammable gases occurred, in which nineteen men were
killed and others were injured. Later a fire at the shaft in the
lake caused the death of ten men. Work was thereafter completed
under the engineering direction of G. H. Benzenberg.
Less serious accidents, principally explosions of marsh gas,
occurred in many of the other tunnels. In one case (at Milwaukee
under Benzenberg) drift material was penetrated,
with large boulders and coarse afnd fine gravel, and without
any sand or clay filling, apparently in direct communication
with the lake bottom. At times the necessary air pressure
was 42 lb per sq. in.-Subaqueous
Tunnels ma/ie by sinking Tubes, 'Caissons, &'c.?In
1845 De la Haeye, in England, doubtless having in mind the
tedious and difficult work of the Thames tunnel, proposed to
make tunnels under water by sinking large tubes on a previously
prepared bed and connecting them together. Since then many
inventors have proposed similar schemes. In 1866 Belgrand
sank twin plate-iron pipes, 1 metre diameter and 156 metres
long, under the Seine at Paris for a sewer siphon, and there have
since been numerous examples of sunk cast-iron sub aqueous
water-pipes. It is believed that the first tunnel of this class,
large enough for men to move upright in, was by H. A. Carson,
assisted by W. Blanchard and F. D. Smith, in 1893-1894, in the
outer portion of Boston harbour, for the metropolitan sewer
outlet. The later tubes were about 9 ft. exterior diameter,
in sections each 52 ft. long, weighing about 210,000 lb, made of
brick and concrete, with a skin of wood and water-tight bulkheads
at each end. A trench was dredged in the harbour bed
and saddles were accurately placed to support the tubes. The
latter, made in cradles above water alongside a wharf, were
lowered by long vertical screws moved by steam power, and were
towed e to #2 m. to their final positions. After sufficient water
had been admitted they were lowered to their saddles by travelling
shears on temporary piles. 'The temporary joints between
consecutive sections were made by rubber gaskets between
flanges which were bolted together by divers. The later operations
were back filling the trench over the pipes, and in each
section pumping out the water, removing its bulkheads,
and making good the masonry between consecutive bulkheads,
this masonry being inside the flanges. T his work,
about 1500 ft. in length, was done without contractors, by
labourers and foremen under the immediate control of
the engineers, and was found perfectly tight, straight and
sound. cg
1'he double-track railroad tunnel at Detroit, made in 1906-IQOQ,
under the direction of an advisory board consisting of
W. J. Wilgus (chairman), H. A. Carson and W. S. Kinnear (the
last-named being chief engineer), is 1% m. long, with a portion
directly under the river of e m. The method used under the
river (proposed by Wilgus) is an important variation on the
Boston scheme. -A trench was dredged with a depth equal to
the thickness of the tunnel below the river bed and about 70 ft.
below the river surface, and grill ages were accurately placed
in it to support the ends of thin steel tube-forms, inside of
which concrete was to be moulded and outside of which deposited.
These tubes, each about 23 ft. in diameter and 262' 5 ft.
long, were in pairs (one tube for each track), and were
connected sidewise and surrounded by thin steel diaphragms
12 ft. apart. Planking, to limit the concrete, was secured
outside the diaphragms (see fig. 3). The forms were made
tight, bulk headed at their ends, floated into place, -sunk by
admitting water, set on the grill ages, and the ends of successive
pairs connected together by bolts through rubber gaskets and
fianges. The succeeding pair of tubes was not lowered until
concrete had been deposited through the river around the
tubes of the preceding pair. The following steps were to remove
the water from one pair of tubes, mould inside a lining
of concrete 20 in. thick, remove the contiguous bulkheads,
and repeat again and again the processes described until, the
sub aqueous tunnel was complete.
The New York Rapid Transit tunnel under Harlem river,
built 1904-1905, has two tubes, each about 1 5 ft. diameter and
400 ft. long, with a surrounding shell of cast iron itself surrounded
by concrete. The outside width of concrete is about 33 ft.
Its top is 28 ft. below high' water and about 3 ft. below the bed
of the river. D. D. McBean, the sub-contractor, dredged a
trench in the river to within 7 or 8 ft. of the required depth.
He then enclosed a space of the width of the tunnel from shore
to mid-stream with 12-in. sheet piling, which was evenly cut
off some 2 ft. above the determined outside top of the tunnel.
On top of this piling he sank and tightly fitted a fiat temporary
roof of timber 3 ft. thick in sections, and covered this 'with
about 5 ft. of dredged mud. ~ Water was expelled from this
sub aqueous chamber by compressed air, .after which 'the remaining
earth was easily taken out, and the iron and concrete
tunnel walls were then built in the chamber. For the remaining
part of the river the foregoing process was varied by cutting off
the sheet piling at mid-height of the tunnel and making the upper
half of the tunnel, which was built above and lowered in sections
through the water, serve as the roof of the chamber in
which the lower half of the tunnel was built. ' - V
The tunnels of the Metropolitain railway of Paris (F. Bienvenue,
engineer-in-chief) under the two arms of the Seine,
between Place Chatelet and Place Saint Michel, were made by
means of compressed-air caissons sunk beneath the river bed,
were next made by the aid of temporary small caissons sunk
through about 26 ft. of earth under the river. The tops of the
side walls were made even with the end walls. A steel rectangular
coffer-dam (figs. 5 and 6) was sunk to rest with rubber
or clay joint on these surrounding walls. The coffer-dam had
shafts reaching above the surface of the water, so that the earth
core was easily taken out (after removing the water) in free air.
The adjacent chambers under the caissons were then connected
together. Three caissons, of a total length of 396 ft., were
used under the larger arm, and two, of an aggregate length
M ounldin Tunnels for Railways.
Average
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T 1 L Length. Internal Width and Material progress per Approximate
“une ° ocatlon' (miles) Height. penetrated. day =24 hrs. “1st per,
7 i (lin. yds). lm- Y<'≫
Mont Cenis (1 tunnel). Modane, France and 7-98 26 ft. 3 in. X 24 ft. £
Bardonecchia, Italy. 7 in. (horseshoe). Granitic 2~57 226
St Gotthard (1 tunnel) Goschenen and Airolo in 9~3 26 ft. 3 in. >< 24 ft.
Switzerland. 7 in. (horseshoe). Granitic 6~O1 143
Arlberg (1 tunnel) . . Innsbruck and Bludenz 6~36 25 ft. 3 in. wide
in Tirol. - 9-o7 108
Simplon (2 tunnels) Brigue, Switzerland and 12-3 16 ft. 5 in. X 19 ft. Gneiss, mica schist, 11~63 148
Iselle, Italy.
6 in. each (min.).
limestone and
disintegrated mica
schist rock.
FIG. 4.-Perspective showin manner of enclosin s ace between
L. Chagnaud being the contractor. They were built of plates;
of sheet steel and masonry, with temporary steel diaphragms
in the ends, filled with concrete, making a cross wall with'a≫
level top about even with the outside top of the tunnel and
about 2 ft. below the bottom of the Seine. The caissons were
sunk on the line of the tunnel so that adjacent ends (and the
walls just described) were nearly 5 ft. apart with-at that stage
-fa core of earth between them. Side walls joiningithe 'end
walls and thus enclosing the earth core on four sides (fig. 4)
(From Enginaring N nas, New York.)
tunnel caissons for the Metropohtain under the Seine at Paris.
of 132 ft., under the smaller arm of the Seine. The cost of
the tunnel was 7000 francs per lineal metre.
William Sooy Smith published in Chicago, in 1877, a description
of a scheme for building a tunnel under the Detroit
river by sinking caissons end to end, each caisson to be secured
to the adjoining one by tongued and grooved guides, and a
nearly water-tight connexion between the two-to be made by
means of an annular inflated hose.
Tunnelling through M ountaim.~Where a great thickness of
rock overlies a tunnel through a mountain, it may be necessary
to do the work wholly from the two ends without intermediate
shafts. The problem largely resolves itself into devising the
most expeditious way of excavating and removing the rock.
Experience has led to great advances in speed and economy, as
may be seen from examples in the above table.
In 1857 the first blast was fired in connexion with the Mont
Cenis works; in 1861 machine drilling was introduced; and in
1871 the tunnel was opened for traffic. With the exception of
about 300 yds. the tunnel is lined throughout with brick or
stone. During the first four years of hand labour the average
progress was not more than 9 in. per day on each side of the
Alps; but with compressed air rock-drills the rate towards
1 the end was five times greater.
In 1872 the St Gotthard
tunnel was begun, and in
as 1881 the first locomotive
ran through it. Mechanical
drills were used from the
beginning; Tunnelling was
cairrie on ly driving) in
a vanceato a 1 a out
square: t en enlarging
WaYS' and .many
sinking the excavation to
invert level (see figs. 7 and
Air for working the
rock-drills was compressed
to seven atmospheres by
turbines -of about 2000
The driving of the Arlberg
tunnel was be un in1880 and
the work Wag completed in
FIG. 5.?Transverse Section. FIG. 6.?Longitudinal Section. little more than three yeatS
Coffer-dam superimposed over joints between caissons in tunnels for the Metropolitain
under the Seine at Paris.
The main heading was driven
along the bottom of the
tunnel and shafts were opened up 25 to 70 yds. apart, from which
smaller headings were driven right and left. The tunnel was
enlarged to its full section at different points simultaneously in
lengths of 8 yds., the excavation cf each occupying about twenty
days, and the masonry fourteen days. Ferroux percussion air-d rills
and Brandt rotary hydraulic drills were used, the performance of
the latter being especially satisfactory. After each blast a fine
spray of water was injected, which assisted the ventilation
FIGS. 7 and 8.-Method of excavation in St Gotthard Tunnel.
materially. In the St Gotthard tunnel the discharge of the
air-drills was relied on for ventilation. In the Arlberg tunnel
over 8000 cub. ft. of air per minute were thrown in by ventilators.
To keep pace with the miners, 900 tons of excavated
material had to be removed, and 350 tons of masonry
introduced, daily at each end of the tunnel, which necessitated
the transit of 450 wagons. The cost per lineal yard varied
according to the thickness of masonry lining and the distance
from the mouth of the tunnel. For the first thousand yards
from the entrance the prices per lineal yard were £11 Ss. for
the lower heading; £7 12s. for the upper one; £30 IOS. for the
unlined tunnel; £45 for the tunnel with a thin lining of masonry;
and £124 5s. with a lining 3 ft. thick at the arch, 4 ft. at the
sides, and 2 ft. 8 in. at the invert.
The Simplon tunnel was begun in 1898 and completed in
1905. It is over 30 % longer than the St Gotthard, and the
greatest depth below the surface is 7005 ft. A novel method
was introduced in the shape of two parallel bores (56 ft. apart,
connected at intervals of 660 ft. by oblique galleries), which
greatly facilitated ventilation, and resulted in increased economy
and rapidity of construction, while ensuring the health of the
men. One of these galleries was made large enough for a single track
railroad, and the second is to be enlarged and similarly
used. The death-rate in the Simplon tunnel was decreased as
compared with the St Gotthard from 800 in eight years to 60
in seven years. Had one wide tunnel been made instead of two
narrow ones, it would have been difficult to maintain its
integrity; even with the narrow cross-section employed the floor
was forced up at points in the solid rock from the great weight
above, and had to be secured by building heavy inverts of
masonry. Temperatures were reduced to 89° F. by spraying
devices, although the rock temperatures ranged from 129° to
130° F. At one point 4374 yds. from the portal of Iselle the
“Great Spring” of cold water was struck; it yielded 10,564
gallons per minute at 600 lb pressure per sq. in., and reduced
the temperature to 55-4° F., the lowest point recorded. A
spring of hot water was met on the Italian side which discharged
into the tunnel 1600 gallons per minute with a temperature of
113° F. The maximum flow of cold water was 17,081 gallons
per minute, and of hot water 4330 gallons per minute. These
springs often necessitated a temporary abandonment of the
work. Water power from the Rhone at the Swiss and from the
Diveria at the Italian end provided the power for operating all
plant during the construction of most of the work. Among the
able engineers connected with this work must be mentioned
Alfred Brandt, a man of remarkable energy and ability, whose
drills were used with much success. He died early in the work,
of injuries received from falling rock.
A group of tunnels-the Tauern, Barengraben, Wocheiner and
Bosrtick-was undertaken by the Austrian government in
connexion with new Alpine railroads to increase the commercial
territory tributary to the seaport of Trieste, which at onetime
was' greater than Hamburg. The principal tunnel of this group
is under the main body of the Tauern mountain. The bottom
drifts met on the 21st of July 1907. The difficulties resulted
mostly from mountain debris and springs. There are four
minor tunnels between Schwarzach, St Veit, and the north
portal of the Tauern, and nineteen between the south portal
and the south slope at Mollbriicken.
The electric railway from the Eiger glacier to near the summit
of the Jungfrau includes a tunnel 1% m. long, 3-6 metres wide
and 3-8 metres high, with a midway station, from which a large
part of northern Switzerland can be seen. From the Jungfrau
terminus, at an elevation of 13,428 ft., the summit, 242 ft.
higher, will be reached by an elevator.
The Hoosac tunnel was the first prominent tunnel in America.
It was begun in 1855 and iinished in 1876, after many interruptions.
It was memorable for the original use in America of
air-drills and nitroglycerin. The Pennsylvania railroad tunnels
crossing New York City under 32nd and 3 3rd Streets are of unusual
size. 0wing≫to the close proximity of large buildings and
other structures special methods were adopted for mining the
rock to lessen the vibrations by explosions. At 33rd Street
and 4th Avenue the tunnels pass directly under two of the
Rapid Transit system, above which there is another belonging
to the Metropolitan Traction Company, so that there are three
tunnels at different levels under the street.
Among other rock tunnels may be mentioned the Albula,
through a granite ridge of the Rhaetian Alps, for arsingle-track
narrow-gauge railroad, 3-6 m. long; tunnels on the Midland
railway, near Totley in Derbyshire, ≫over 3-5 m. long, largely
in shale, and at Cowburn, over 2 m. long, in shale and harder
rock, each 27 ft. wide and 20-5 ft. high inside; the Suram, on
the Trans-Caucasus railway, for double track, 2-47 m. long,
through soft rock; the tail-race tunnel for the Niagara Falls
Water Power Company, 1~3 m. long, IQ ft. wide and 2I ft. high,
through argillaceous shale' and limestone, costing about
$1,250,000; the Tequixquiac outlet to the drainage system for
the city of Mexico, costing $6,760,000; the Cascade, Washington,
part of the Great Northern railroad system, saving 9 m.
in distance; and the Gunnison, irrigating 147,000 acres in
Colorado.
Tunnelling in Towns.-Where tunnels have to be carried
through soft soil in proximity to valuable buildings special
precautions have to be taken to avoid settlement. A successful
example of such work is the tunnel driven in 1886 for the Great
Northern Railway Company under the Metropolitan Cattle
>|? "" °'°" 7 ""*' °" ';'-1
eat... - ?s.6 ?f; Qi
/ -~-' |1i||i:; . .:: ':;:|, s|,1 § ia
I ' I I
I | I |;
I
1 E 2
A | Q
1 m 4. =
§ '=
Q ~ ≫ 1
1 E =
1 g E
I
1, |= . .
, lillglll- ||-n n u gl,
~ .'V / i WvV* 5 5;
FIG. 9.-Paris Metropolitain Tunnel, longitudinal horizontal
section.
5.
Market, London. This was done by the crown-bar method,
the bars being built in with solid brickwork. The subsidence
in the ground was from 1 to about 3% in. Several buildings
were tunnelled under without any structural damage.
London has now some Q0 m. of tunnels for railways, mostly
operated by electric traction. Most of those which have been
constructed since 1890 have been tunnelled by the use of cylindrical
shields and walls of cast iron. Shields about 23 ft. in
diameter were used in constructing the stations on the Central
London railway, and one 52 ft. 4 in. in diameter and only
9 ft. 3 in. long was used for a short distance on the Clapham
extension oi the City and South London railway.
4, ?¢~' ~.≫ -≫~ ? .Y v -.
general, the upper half of the tunnel was executed first (figs. 9
and ro) and the lower part completed by underpinning.
Figs. rr, 12 and 13 illustrate a case of tunnelling near important
buildings in Boston in 1896, with a roof-shield 29 ft. 4in. in
external diameter. The vertical sidewalls were first made in
small drifts, the roof-shield running on top of these, and the
core was taken out latervand the invert or floor of the tunnel
put in last. Each hydraulic press of the shield reacted against
a small continuous cast-iron rod imbedded in the brick arch.
In some large sewerage tunnels in Chicago the shields were
pushed from a wall of oak planks, 8 in. thick, surrounding the
brick walls of the sewer.
FIG. IO.-Paris Metropolitain Tunnel, longitudinal vertical section.
Paris has an elaborate plan for underground railways some
5o rn. in length, a considerable number of which have been
constructed since 1898 under the engineering direction of F.
Bienveniie. Instead of using completely cylindrical shields
and cast-iron walls, as in London, roof-shields (boucliers de
vofite) were employed for the construction of the upper half of
the tunnel, and masonry walls were adopted throughout. In
Ventilation aj Tunnels.-The simplest method for ventilating
a railway tunnel is to have numerous wide openings to
daylight at frequent intervals. If these are the full width of
the tunnel, at least 20 ft. in length, and not farther 'apart
than zoo yds., it can be naturally ventilated. Such arrangements
are, however, frequently impracticable, and .then recourse
must be had to mechanical means. I
unusual!
Fig. 11.-Boston Subway, first and second
FIG. 12.-Boston Subway, third phase.
A i FIG. 13.-Boston Subway, longitudinal vertical section through shield.
The first application of mechanical or fan ventilation to railway
tunnels was made in the Lime Street tunnel of the London and
North-Western railway at Liver ool, which has since been re laced
by an open cutting. At a later diate fans were applied to the § evern
and Mersey tunnels.
The principle ordinarily acted upon, where mechanical ventilation
has been adopted, is to exhaust the vitiated air at a point midway
between the portals of a tunnel, by means of a shaft with which is
connected a Ventilating fan of suitable power and dimensions. In
the case of the tunnel under the river Mersey (fig. I4) such a shaft
could not be provided, owing to the river being overhead, but a
ventilating heading was driven from the middle of the river (at which
point entry into the tunnel was effected) to each shore, where a fan
$0 ft. in diameter was placed. In this way the vitiated air is drawn
rom the lowest point of the railway, while fresh air Hows in at the
stations on each side to replenish the partial vacuum, as indicated
by arrows in the accompanying longitudinal section of the tunnel.
The principle was that fresh air should enter at each station and
“ split " each way into the tunnel, and that thus the atmosphere
on the station platforms should be maintained in a condition of
urity.
p The fans in the Mersey tunnel are somewhat similar to the well known
Guibal fans, with the exception of an important alteration
in the shutter. With the Guibal shutter, the top of the opening
(Fmm n diagram in Proc. Insl. Ci1:.Eng.)
FIG. 14.-Longitudinal Section of the Mersey Tunnel, showing Method of Ventilation.
into the chimney from the fan has a line parallel to that of the fanshaft
and of the fan-blades, and, as a consequence, .as eazch blade
passes this shutter, the stoppage of the discharge- of the air is instantaneous,
and the sudden change of the pressure of the air on the face
of the blade whilst discharging and the reversal- of the pressure,
due to the vacuum inside the fan-casing, cause the vibration hitherto
inseparable from this type of ventilator. As an illustration of the
eliect of the pulsatory action of the Guibal shutters the following
figures may be given: a fan having ten arms and running, say, sixty
revolutions per minute, and working twenty-f0ur hours per day,
ives (10 X 60 >< 60 X 24 ==) 864,000 blows per day transmitted
from the tip of the fan-vanes to the fan-shaft; the shaft is thus in
a constant state of tremor, and sooner or later reaches its elastic
limit, and the conse uent injury to the general structure of the fan
is obvious. This di§ culty is avoided by cutting a /-shaped opening
in the shutter, thus gradually decreasing the aperture and allowing
the air to pass into the chimney in a continuous stream instead of
intermittently. The action of this regulating shutter increases
the durability and efficiency of the fans in an important degree.
In towns like Liverpool and Birkenhead any pulsatory action would
be readily felt by the inhabitants, but with the above, arrangement
it is difficult to detect any sound whatever, even when standing close
to the buildings containing the fans. The admission of the air on
both sides is found in practice to conduce to smooth running and to
the reduction of the side-thrust which occurs when the air is
admitted on one side only. The fans are five in number: two are
40 ft. in diameter by 12 ft. wide, and two 30 ft. indiameterby 1oft.
wide. one of each size being erected at Liverpool and at Birkenhead
respectively. In addition, there is a high-speed fan 16 ft. in
diameter in Liverpool which throws 300,000 cub. ft.
The following table gives the result of
the ventilating fans of the Mersey railway
experiments made with
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liaenilton Street, Y
irkenhead IO II3 1~3o 1895 214,135
Shore Road, -j
pl Birkgnhead . 12 41 2-50 32881 134,685
I ames rrect,
J Livegpool . ., 12 72 2-45 2465 178,880
ames treet,
B lfgivgrpool . 10 60 2-30 2062 123,720
0 . treet,
Liverpool . . - 300,000
|, € .
Total 951,420,
s-.. ' ' I
The central point of the Severn tunnel (hg. 15) lies toward the
Monmouthshire bank of the river, and ventilation is effected from
that point by means of one fan placed on the surface at Sudbrooke,
Monmouth, at the top of a shaft which is connected with a horizontal
Vent//ating Fan
~ 40112 [get Rive -I
~ ?≫ r Severn ? ~?≫
M0l1m°l¢h5h1l' C 4 69: The Shoats l Gloucestershire
Level
5°?rata/:engfh uf Tunnel 4 miles 624 yurdS~ ? sl
FIG. 15.-Section of Severn Tunnel (Fox).
heading leading to the centre. This fan, which is 40 ft. in diameter
by 12 ft. in width, removes from the tunnel some 400,000 cub. ft.
per minute, and draws in an equivalent volume of fresh air from the
two ends.
About 1896 an excellent system was introduced by Signor Saccardo,
the well-known Italian engineer, which to a great extent has minimized
the difficulty of ventilating long tunnels under mountain-ranges
where shafts are not available. This system, which is not applicable
to tunnels in which underground stations exist, is illustrated in
fig. 16, which represents its application to the single-line tunnel
through the Apennines at Pracchia. This tunnel is one of fiftytwo
single-line tunnels, with a gradient of I in 40, on the main line
between Florence and Bologna, built by Thomas Brassey. There
is a greaft deal of traffic which has to be worked by heavy locomotives.
Before the installation of a Ventilating system under any condition
of wind the state of this tunnel, about 3000 yds, in length, was bad;
I In the case of this circular drift-way a velocity of 4000 ft. per
minute was subsequently attained.
2 Quick-running fan.
but when the wind was blowing in at the lower end at the same time
that a heavy goods or passenger train was ascending the gradient
the condition of affairs became almost insupportable. The engines,
working with the regulators full open, often emitted. large quantities
of both smoke and steam, which travelled concurrently with the
train. The goods trains had two engines, one in front and another
at the rear, and when, from the humidity in the tunnel, due to the
(From the Proc. I ml. Civ. Eng.)
FIG. 16.-Diagram illustrating the Saecardo System for
Ventilating Tunnels.
steam, the wheels slipped and possibly the train stopped, the state
of the air was indescribable. A heavy train with two engines,
conveying a royal party and their suite, arrived on one occasion
at the upper exit of the tunnel with both engine men and both firemen
insensible; and on another occasion, when a heavy passenger
train came to a stop in the tunnel, all the occupants were seriously
affected.-In
applying the Saccardo system, the" tunnel 'was extended for
I5 or 20 ft. by a structure either of timber or brickwork, the inside
line of which represented the line of maximum construction, and this
was allowed to project for about 3 ft. into the tunnel. The space
between this line and the exterior constituted the chamber into
which air was blown by means of a fan. Considering the length
of tunnel it might at first be thought there would be some tendency
for the air to return through the open mouth, but nothing of the
kind happened. The whole of the air blown by the fan, 164,000
cub. ft. per minute, was augmented by the induced current yielding
46,000 cub. ft. per minute, making a total of 210,000 cub. ft.; and
this volume was blown down the gradient against the ascending
train, so as to free the driver and men in charge of the train from
the products of combustion at the earliest possible moment. Prior
to the installation of this system the drivers and firemen had to be
clothed in thick woollen garments, pulled on over their ordinary
clothes, and wrapped round and round the neck and over the head;
but in spite of all these precautions they sometimes arrived at the
upper end of the tunnel in a state of insensibility. The fan, however,
immensely improved the condition of the air, which is now pure
and fresh. 1
In the case of the St Gotthard tunnel, which is 9% m. in length
and 26 ft. wide with a sectional area of 603 s ft., the Saccardo
system was installed in 1899 with most benecial results., The
railway is double-tracked and worked by steam locomotives, the
cars being lighted by gas. The Ventilating plant is situated at
Goschenen at the north end of the tunnel and consists of two large
fans operated by water power. The quantity of air passed into
the narrow mouth of the tunnel is 413,000 cub. ft. per minute at
a velocity of 686 ft., this velocity being much reduced as the full
A sample of the air taken from
section of the tunnel is reached.
a carriage contained IO'I9 parts of carbonic acid gas per 10,000
volumes.
In the Simplon tunnel, where electricity is the motive power,
A steel sliding door is arranged
mechanical ventilation is installed.
at each entrance to be raised and lowered by electric power. After the entrance of a train the door is lowered and fresh air forced into the tunnel at considerable pressure from the same end by fans. The introduction of electric traction has simplified the problem of Ventilating intra-urban railways laid in tunnels at a greater or less distance below the surface, since the absence of smoke and products of combustion from coal and coke renders necessary 0nly such a quantity of air as is required by the passengers and staff. For supplying air to the shallow tunnels which form the underground portions of the Metropolitan and District railways in London, ocen staircases. blow-holes and sections of uncovered track are relied on. When the lines were worked by steam locomotives they afforded notorious examples of bad ventilation, the proportion of
carbonic acid amounting to from 15 or 20 to 60, 70 and even 89 parts in 10,000. But since the adoption of electricity as the motive power the atmosphere of the tunnels has much improved, and two samples taken from the cars in 1905 gave 11-27 and 14-07 parts of carbonic acid in 10,000.
When deep level “tube” railways were first constructed in London, it was supposed that adequate ventilation would be obtained
through the lift-shafts and staircases at the stations, with the aid of the scouring action of” the trains which, being of nearly the same cross-section as the tunnel, would, it was supposed, drive the air in front of them out by the openings at the stations they were approaching, while drawing fresh air in behind them at the stations
they had left. This expectation, however, was disappointed, and it was found necessary to employ mechanical means. On the Central
London railway, which runs from the Bank of England to Shepherd's Bush, distance of 6 m., the Ventilating plant installed in 1902 consists of a 300 h.p. electrically driven fan, which is placed at Shepherd's
Bush and draws in fresh air from the Bank end of the line and at other intermediate points. The fan is 5 ft. wide and 20 ft. in
diameter, and makes 145 revolutions a minute, its capacity being 100,000 cub. ft. a minute. It is operated from 1 to 4 a.m., and the openings at all the intermediate stations being closed it draws fresh
air in at the Bank station. The tunnel is thus cleared out about 2¾ times each night and the air is left in the same condition as it is outside. The fan is also worked during the day from 11 a.m. to 5 p.m., the intermediate doors being open; in this way the atmosphere is improved for about half the length of the line and the cars are cleared out as they arrive at Shepherd's Bush. Samples of the air in the tunnel taken when the fan was not running contained
7·07 parts of carbonic acid in 10,000, while the air of a full car contained 10·7 parts. The outside air at the same time contained
4·4 parts. A series of tests made for the London County Council in 1902 showed that the air of the cars contained a minimum of
9·60 parts and a maximum of 1447 parts. In some of the later tube railways in London-such as the Baker Street and Waterloo, and
the Charing Cross and Hampstead lines-electrically driven exhaust fans are provided at about half-mile intervals; these each extract 18,500 cub. ft. of air per minute from the tunnels, and discharge it from the tops of the station roofs, fresh air being conveyed to the points of suction in the tunnels.
The Boston system of electrically operated subways and tunnels is ventilated by electric fans capable of completely changing the air in each section about every fifteen minutes. Air admitted at portals and stations is withdrawn midway between stations. In the case of the East Boston tunnel, the air leaving the tunnel under the middle of the harbour is carried to the shore through longitudinal ducts (fig. 3) and is there expelled through fan-chambers.
In the southerly 5 m. of the New York Rapid Transit railway, which runs in a four-track tunnel of rectangular section, having
an area of 650 sq. ft., and built as close as possible to the surface of the streets, ventilation by natural means through the open staircases at the stations is mainly relied upon, with satisfactory results
as regards the proportions of carbonic acid found in the air. But when intensely hot weather prevails in New York the tunnel air is sometimes 5° hotter still, due to the conversion of electrical energy into heat. This condition is aggravated by the fine diffusion through the air of oil from the motors, dust from the ballast and particles
of metal ground off by the brake shoes, &c.
Volume of Air Required for Ventilation.-The consumption of coal by a locomotive during the passage through a tunnel having
been ascertained, and 29 cub. ft. of poisonous gas being allowed for each pound of coal consumed, the volume of fresh air required to maintain the atmosphere of the tunnel at a standard of purity of 20 parts of carbon dioxide in 10,000 parts of air is ascertained as follows: The number of pounds of fuel consumed per mile, multiplied by 29, multiplied by 500, and divided by the interval in minutes between the trains, will give the volume of air in cubic feet which must be introduced into the tunnel per minute. As an illustration, assume that the tunnel is a mile in length, that the
consumption of fuel is 32 lb per mile, and that one train passes through the tunnel every five minutes in each direction; then the volume of air required per minute will be
- ?l?752 fb X 29 wb” ft' X 500=185,600 cub. ft.
2% minutes.-Corrosion
of Rails in Tunnels.-Careful tests made in the Box and Severn tunnels of the Great Western railway, to ascertain if possible
the loss that takes place in the weight of rails owing to the presence of corrosive gases, gave the following results:-
Box TUNNEL (1 m. 66 chains in length).
Percentage of Wear per annum.
lb per yard
Down line, gradient falling 1 in 100- % per annum.
At east mouth ....... . 0'439= 0'377
28 chains from east mouth
48 chains from east mouth
1 m. 8 chains from east mouth
At west mouth .... .
I'800=I'540
2'IIO= 1 ~810
2 -880 =2.480
0-64e=e~==1
Up line, , gradient rising 1 in 100- ≫
0~620 =o-575
1 -500 = 1 ~380
I 520~= 1 '3I0
o~680=o~587
SEVERN TUNNEL (4 m. 28% chains in length).
Percentage of Wear per annum.
lb per yard
Down line, outside and quite clear of tunnel, % per annum.
Bristol end, gradient falling 1 in 100 0'28O=0'24O
Up line, outside and quite clear of tunnel,
Newport end, gradient falling 1 in 90 O'440=O'390
At Bristol mouth, gradient falling 1 in 100 1 -200=1 -020
33 chains from Bristol mouth, gradient falling
1 in 100 . . . - .. 2-160=1-860
3 m. 15% chains from Bristol mouth, gradient
rising 1 in 90 . ..... . 1 ~900= 1 -630
At Newport mouth ..... 0'3I0=0'270
Down and up line under main-shaft level 3 -200=2'750
At east mouth . . .
I m. 8 chains from east mouth
I m. 28 chains from east mouth
At west mouth . .
It will be seen that the maximum wear and corrosion together
reached the extraordinary weight of 2% lb per yard of rail per yeara
very serious amount that involved reat expenditure The wear
occurred over the whole of the rail, Tut the top, over which the
engine and train passed, wore at a greater rate, presumably on
account of the surface being kept bright and the gases being able
to act on it. The Great Vlfestern Company tried the experiment in
the Severn tunnel of boxing up the rails, so that the ballast
approached their surface within 1 in. or 1% in. It was found,
however, that-in the case, at any rate, of the limestone ballast the
cure was almost worse than the disease, the result being a
maximum wear of 2% lb and an average wear of just under 2 Ib
per ard of rail per year. The average on the open line would
be about 0.25 lb in the same time.
See
Proc. Inst. Civ. Eng.
; also works on tunnelling by Drinker,
Simms, Stauffer and Prelini, and on tunnel shields, &c., by Copperthwaite.
(
H. A. C.
)