OPEN-CUT TUNNELING METHODS; TUNNELS UNDER CITY STREETS; BOSTON SUBWAY
AND NEW YORK RAPID TRANSIT.
OPEN-CUT TUNNELING.
When a tunnel or rapid-transit subway has to be constructed at a small
depth below the surface, the excavation is generally performed more
economically by making an open cut than by subterranean tunneling
proper. The necessary condition of small depth which makes open-cut
tunneling desirable is most generally found in constructing
rapid-transit subways or tunnels under city streets. This fact
introduces the chief difficulties encountered in such work, since the
surface traffic makes it necessary to obstruct the streets as little as
possible, and has led to the development of the several special methods
commonly employed in performing it.
Subways are usually constructed under and along important streets where
electric cars are running. The engineers have taken advantage of the
presence of these lines to facilitate the construction of subways. In
New York, for instance, the tracks of the electric lines were supported
by cast-iron yokes 4 or 5 ft. apart and were surrounded by concrete,
leaving only a large hollow space in the middle for the wires and
trolleys. The rails from 40 to 60 ft. long formed almost a solid
concrete structure for their entire length. The tracks and the street
surface were supported by horizontal beams inserted underneath the
tracks. These were the caps of bents constructed underground whose
rafters were finally resting on the subgrade of the proposed subway.
The various methods for constructing the subways may be classified as
follows: (1) The single wide trench method; (2) the single narrow
longitudinal trench method; (3) the parallel longitudinal trench method;
(4) the slice method.
=Single Longitudinal Trench.=--The simplest manner by which to construct
open-cut tunnels is to open a single cut or trench the full width of the
tunnel masonry. This trench is strutted by means of side sheetings of
vertical planks, held in place by transverse braces extending across the
trench and abutting against longitudinal timbers laid against the
sheeting plank. The lining is built in this trench, and is then filled
around and above with well-rammed earth, after which the surface of the
ground is restored. An especial merit of the single longitudinal trench
method of open-cut tunneling is that it permits the construction of the
lining in a single piece from the bottom up, thus enabling better
workmanship and stronger construction than when the separate parts are
built at different times. The great objection to the method when it is
used for building subways under city streets is, that it occupies so
much room that the street usually has to be closed to regular traffic.
For this reason the single longitudinal trench method is seldom
employed, except in those portions of city subways which pass under
public squares or parks where room is plenty.
This method was followed in the construction of the New York subway,
Section 2, along Elm St., a new street to be opened to traffic after the
subway had been completed, and at other points where local conditions
allowed it.
[Illustration: FIG. 112.--Diagram Showing Sequence of Construction in
Open-Cut Tunnels.]
A modification of this method was used in Contract Section 6, on upper
Broadway. The street at this point is very wide, so by opening a trench
as wide as the proposed four-track line of the subway there still
remained room enough for ordinary traffic. The electric car tracks were
supported by means of trusses 60 or 70 ft. long, which were laid in
couples parallel to the tracks and which rested on firm soil. The soil
under the car tracks was removed, beginning with transversal cuts to
receive the needles which were tied to the lower chord of the trusses by
means of iron stirrups. After the excavation had reached the subgrade,
posts were erected to support the needles thus forming bents upon which
the tracks rested. The trusses were removed and advanced to another
section of the tunnel, and, in the clear space left, the subway was
built from foundation up.
=The Single Narrow Longitudinal Trench.=--This method was used on
Contract Section 5, of the New York subway in order to comply with the
peculiar conditions of the traffic along 42nd St. On this street, on
account of the New York Central Station, there is a constant heavy
traffic, while pedestrians use the northern sidewalks almost
exclusively. A single longitudinal trench was then opened along the
south side, and from this trench all the work of excavation and
construction was carried on. At first the steel structure of the subway
was erected in the trench and then a small heading was driven and
strutted under and across the surface-car tracks. Afterward heavy
I-beams were inserted, which rested with one end on top of the steel
bents and the other end blocked to the floor of the excavation. These
I-beams were located 5 ft. apart and they supported the surface of the
street by means of longitudinal planks. The soil was removed from the
wide space underneath the I-beams and the subway was constructed from
the foundation up. When the structure had been completed, the packing
was placed between the roof of the structure and the surface of the
street, the I-beams withdrawn and the voids filled in.
=Parallel Longitudinal Trenches.=--The parallel longitudinal trench
method of open-cut tunneling consists in excavating two narrow parallel
trenches for the side walls, leaving the center core to be removed after
the side walls have been built. The diagram, Fig. 112, shows the
sequence of operations in this method. The two trenches No. 1 are first
excavated a little wider than the side wall masonry, and strutted as
shown by Fig. 113. At the bottoms of these trenches a foundation course
of concrete is laid, as shown by Fig. 114, if the ground is soft; or the
masonry is started directly on the natural material, if it is rock. From
the foundations the walls are carried up to the level of the springing
lines of the roof arch, if an arch is used; or to the level of its
ceiling, if a flat roof is used. After the completion of the side walls,
the portion of the excavation shown at No. 2, Fig. 112, is removed a
sufficient depth to enable the roof arch to be built. When the arch is
completed, it is filled above with well-rammed earth, and the surface is
restored. The excavation of part No. 3 inclosed by the side walls and
roof arch is carried on from the entrances and from shafts left at
intervals along the line.
[Illustration: FIG. 113.--Sketch Showing Method of Timbering Open-Cut
Tunnels, Double Parallel Trench Method.]
[Illustration: FIG. 114.--Side-Wall Foundation Construction Open-Cut
Tunnels.]
A modification of the method just described was employed in constructing
the Paris underground railways. It consists in excavating a single
longitudinal trench along one side of the street, and building the side
wall in it as previously described. When this side wall is completed to
the roof, the right half of part No. 2, Fig. 112, is excavated to the
line _AB_, and the right-hand half of the roof arch is built. The space
above the arch is then refilled and the surface of the street restored,
after which the left-hand trench is dug and the side wall and roof-arch
masonry is built just as in the opposite half. Generally the work is
prosecuted by opening up lengths of trench at considerable intervals
along the street and alternately on the left-and right-hand sides. By
this method one-half of the street width is everywhere open to traffic,
the travel simply passing from one side of the street to the other to
avoid the excavation. When the lining has been completed, the center
core of earth inclosed by it is removed from the entrances and shafts,
leaving the tunnel finished except for the invert and track
construction, etc.
Another modification of the parallel longitudinal trenches method was
used in the construction of the New York subway. A narrow longitudinal
trench was excavated on one side of the street near the sidewalk.
Meanwhile the pavement of half of the street was removed and a wooden
platform of heavy planks, supported by longitudinal beams which were
buried in the ground, was substituted. Then small cuts underneath the
car tracks were directed from the side trench and heavy beams or needles
were placed in these cuts, which also reached the longitudinal beams of
the wooden platform. The needles were wedged and blocked to the car
track structure and the beams. They were temporarily supported by cribs
built from underneath as the excavation progressed. When the subgrade
was reached, vertical and batter posts were inserted to support the
needles, thus forming regular timber bents underground. In the space
thus left open the subway was constructed to the middle of the street.
While the work was going on as described, another longitudinal narrow
trench was excavated at some distance on the other side of the street.
From this trench, the work of constructing the other half of the subway
was carried on in the manner just described. After the work had been
completed, the timbers removed, the voids filled in and the pavement of
the street restored, another equal section was attacked on both sides of
the street.
=Transverse Trenches.=--The transverse trench or “slice” method of
open-cut tunneling has been employed in one work, the Boston Subway.
This method is described in the specifications for the work prepared by
the chief engineer, Mr. H. A. Carson, M. Am. Soc. C. E., as follows:--
“Trenches about 12 ft. wide shall be excavated across the street to as
great a distance and depth as is necessary for the construction of the
subway. The top of this excavation shall be bridged during the night by
strong beams and timbering, whose upper surface is flush with the
surface of the street. These beams shall be used to support the railway
tracks as well as the ordinary traffic. In each trench a small portion
or slice of the subway shall be constructed. Each slice of the subway
thus built is to be properly joined in due time to the contiguous
slices. The contractor shall at all times have as many slice-trenches in
process of excavation, in process of being filled with masonry, and in
process of being back-filled with earth above the completed masonry, as
is necessary for the even and steady progress of the work towards
completion at the time named in the contract.”
In regard to the success of this method Mr. Carson, in his fourth annual
report on the Boston Subway work, says:
“The method was such that the street railway tracks were not disturbed
at all, and the whole surface of the street, if desired, was left in
daytime wholly free for the normal traffic.”
=Tunnels on the Surface.=--It occasionally happens when filling-in is to
take place in the future, or where landslides are liable to bury the
tracks, that a railway tunnel has to be built on the surface of the
ground. In such cases the construction of the tunnel consists simply in
building the lining of the section on the ground surface with just
enough excavation to secure the proper grade and foundation. Generally
the lining is finished on the outside with a waterproof coating, and is
sometimes banked and partly covered with earth to protect the masonry
from falling stones and similar shocks from other causes. A recent
example of tunnel construction of this character was described in
“Engineering News” of Sept. 8, 1898. In constructing the Golden Circle
Railroad, in the Cripple Creek mining district of Colorado, the line had
to be carried across a valley used as a dumping-ground for the refuse of
the surrounding mines. To protect the line from this refuse, the
engineer constructed a tunnel lining consisting of successive steel
ribs, filled between with masonry.
=Concluding Remarks.=--From the fact that the open-cut method of
tunneling consists first in excavating a cut, and second in covering
this cut to form an underground passageway, it has been named the
“cut-and-cover” method of tunneling. The cut-and-cover method of
tunneling is almost never employed elsewhere than in cities, or where
the surface of the ground has to be restored for the accommodation of
traffic and business. When it is not necessary to restore the original
surface, as is usually the case with tunnels built in the ordinary
course of railway work, it would obviously be absurd to do so except in
extraordinary cases. In a general way, therefore, it may be said that
the cut-and-cover method of construction is confined to the building of
tunnels under city streets; and the discussion of this kind of tunnels
follows logically the general description of the open-cut method of
tunneling which has been given.
TUNNELS UNDER CITY STREETS.
The three most common purposes of tunnels under city streets are: to
provide for the removal of railway tracks from the street surface, and
separate the street railway traffic from the vehicular and pedestrian
traffic; to provide for rapid transit railways from the business section
to the outlying residence districts of the city; and to provide conduits
for sewage or subways for water and gas mains, sewers, wires, etc.
Within recent years the greatest works of tunneling under city streets
have been designed and carried out to furnish improved transit
facilities.
=Conditions of Work.=--The construction of tunnels under city streets
may be divided into two classes, which may be briefly defined as shallow
tunnels and deep tunnels. Shallow tunnels, or those constructed at a
small depth beneath the surface, are usually built by one of the
cut-and-cover methods; deep tunnels, or those built at a great depth,
beneath the surface are constructed by any of the various methods of
tunneling described in this book, the choice of the method depending
upon the character of the material penetrated, and the local conditions.
In building tunnels under city streets the first duty of the engineer is
to disturb as little as possible the various existing structures and the
activities for which these structures and the street are designed. The
character of the difficulties encountered in performing this duty will
depend upon the depth at which the tunnel is driven. In constructing
shallow tunnels by the cut-and-cover method care has to be taken first
of all not to disturb the street traffic any more than is absolutely
necessary. This condition precludes the single trench method of open cut
tunneling in all places where the street traffic is at all dense, and
compels the engineer to use the methods employed in Paris and New York,
as previously described, or else the transverse trench or slice method
employed in the Boston Subway.
These methods have to be modified when the work is done on streets
having underground trolley and cable roads, and in which are located gas
and water pipes, conduits for wires, etc. Where underground trolley or
cable railways are encountered, a common mode of procedure is to
excavate parallel side trenches for the side walls, and turn the roof
arch until it reaches the conduit carrying the cables or wires. The
earth is then removed from beneath the conduit structure in small
sections, and the arch completed as each section is opened. As fast as
the arch is completed the conduit structure is supported on it. Where
pipes are encountered they may be supported by means of chains,
suspending them from heavy cross-beams, or by means of strutting, or
they may be removed and rebuilt at a new level. Generally the conditions
require a different solution of this problem at different points.
Another serious difficulty of tunneling under city streets arises from
the danger of disturbing the foundations of the adjacent buildings. This
danger exists only where the depth of the tunnel excavation extends
below the depth of the building foundations, and where the material
penetrated is soft ground. Where the tunnel penetrates rock there is no
danger of disturbing the building foundations. To prevent trouble of
this character requires simply that the excavation of the tunnel be so
conducted that there is no inflow of the surrounding material, which
may, by causing a settlement of the neighboring material, allow the
foundations resting on it to sink.
The Baltimore Belt tunnel, described in a preceding chapter, is an
example of the method of work adopted in constructing a tunnel under
city streets through very soft ground. This may be classed as a deep
tunnel. Another method of deep tunneling under city streets is the
shield method, examples of which are given in a succeeding chapter. Two
notable examples of cut-and-cover methods of tunneling are the Boston
Subway and the New York Rapid Transit Ry., a description of which
follows.
=Boston Subway.=--The Boston Subway may be defined as the underground
terminal system of the surface street railway system of the city, and as
such it comprises various branches, loops, and stations. The subway
begins at the Public Garden on Boylston St., near Charles St., and
passes with double tracks under Boylston St. to its intersection with
Tremont St., where it meets the other double-track branch, passing under
Tremont St. and beginning at its intersection with Shawmut Ave. From
their intersection at Tremont and Boylston streets the two double-track
branches proceed under Tremont St. with four tracks to Scollay Square.
At Scollay Square the subway divides again into two double-track
branches, one passing under Hanover St., and the other under Washington
St. At the intersection of Hanover and Washington streets the two
double-track branches combine again into a four-track line, which runs
under Washington St. to its terminus at Haymarket Square, where it comes
to the surface by means of an incline. The subway, therefore, has three
portals or entrances, located respectively at Boylston St., Shawmut
Ave., and Haymarket Square. It also has five stations and two loops, the
former being located at Boylston St., Park St., Scollay Square, Adams
Square, and Haymarket Square, and the latter at Park St. and Adams
Square. The total length of the subway is 10,810 ft.
_Material Penetrated._--The material met with in constructing the subway
was alluvial in character, the lower strata being generally composed of
blue clay and sand, and the upper strata of more loose soil, such as
loam, oyster shells, gravel, and peat. At many points the material was
so stable that the walls of the excavation would stand vertical for some
time after excavation. Surface water was encountered, but generally in
small quantities, except near the Boylston St. portal, where it was so
plentiful as to cause some trouble.
[Illustration: FIG. 115.--Wide Arch Section, Boston Subway.]
_Cross-Section._--The subway being built for two tracks in some places
and for four tracks in other places, it was necessary to vary the form
and dimensions of the cross-section. The cross-sections actually adopted
are of three types. Fig. 115 shows the section known as the wide-arch
type, in which the lining is solid masonry. The second type was known as
the double-barrel section, and is shown by Fig. 116. The third type of
section is shown by Fig. 117. The lining consists of steel columns
carrying transverse roof girders, the roof girders being filled between
with arches, and the wall columns having concrete walls between them.
The wide-arch type and the double-barrel type of sections were employed
in some portions of the Tremont St. line, where the traffic was very
dense, since it was possible to construct them without opening the
street. Much of the wide-arch line was constructed by the use of the
roof shield, which is described in the succeeding chapter on the shield
system of tunneling.
[Illustration: FIG. 116.--Double-Barrel Section, Boston Subway.]
_Methods of Construction._--Several different methods were employed in
constructing the subway. Where ample space was available, the single
wide trench method of cut-and-cover construction was employed, the earth
being removed as fast as excavated. In the streets, except where regular
tunneling was resorted to, the parallel trench or transverse trench
cut-and-cover methods were employed.
In the transverse trench method, trenches about 12 ft. wide were
excavated across the street, their length being equal to the extreme
transverse width of the tunnel lining, and their depth being equal to
the depth of the tunnel floor. These trenches were begun during the
night, and immediately roofed over with a timber platform flush with the
street surface. Under these platforms the excavation was completed and
the lining built. As each trench or “slice” was completed, the street
above it was restored and the platform reconstructed over the succeeding
trench or slice. During the construction of each slice the street
traffic, including the street cars, was carried by the timber platform.
[Illustration: FIG. 117.--Four-Track Rectangular Section, Boston
Subway.]
[Illustration: FIG. 118.--Section Showing Slice Method of Construction,
Boston Subway.]
In the parallel trench method, short parallel trenches were dug for the
opposite side walls, and also for the intermediate columns, and
completely roofed over during the night. Under this roofing the masonry
of the side walls and column foundations and the columns themselves were
erected. When the side walls and columns had been erected, the surface
of the street between them was removed, the roof beams laid, and a
platform covering erected, as shown by Fig. 118. This roofing work was
also done at night. The subsequent work of building the roof arches,
removing the remainder of the earth, and constructing the invert, was
carried on underneath the platform covering which carried the street
traffic in the meantime. The successive repetition of the processes
described constructed the subway.
Where the traffic was very dense on the street above, tunneling was
resorted to. For small portions of this work the excavation was done in
the ordinary way, using timber strutting, but much the greater portion
of the tunnel work was performed by means of a roof shield. In the
latter case, the side walls were first built in small bottom side drifts
and were fitted with tracks on top to carry the roof shield. The
construction and operation of this shield are described fully in the
succeeding chapter on the shield system of tunneling.
_Masonry._--The masonry of the inclined approaches to the subway
consists simply of two parallel stone masonry retaining walls. In the
wide-arch and double-barrel tunnel sections, the side walls are of
concrete and the roof arches are of brick masonry. In the other parts of
the subway the masonry consists of brick jack arches sprung between the
roof beams and covered with concrete, of concrete walls embedding the
side columns, and of the concrete invert and foundations for the
columns. Figs. 115 to 118 inclusive show the general details of the
masonry work for each of the three sections. The inside of the lining
masonry is painted throughout with white paint.
_Stations._--The design and construction of the stations for the Boston
Subway were made the subjects of considerable thought. All the stations
consist of two island platforms of artificial stone having stairways
leading to the street above. The platforms are made 1 ft. higher than
the rails. The station structure itself is built of steel columns and
roof beams with brick roof arches and concrete side walls. Its interior
is lined with white enameled tiles. The intermediate columns are cased
with wood, and have circular wooden seats at their bottoms. Each
stairway is covered by a light housing, consisting of a steel framework
with a copper covering and an interior wood and tile finish.
_Ventilation._--The subway is ventilated by means of exhaust fans
located in seven fan chambers, some of which contain two fans, and
others only one fan. Each of the fans has a capacity of from 30,000 to
37,000 cu. ft. of air per minute, and is driven by electric motor,
taking current from the trolley wires. This system of ventilation has
worked satisfactorily.
_Disposal of Rain Water._--The rain water which enters the subway from
the inclined entrances, together with that from leakage, is lifted from
12 ft. to 18 ft. by automatic electric pumps to the city sewers. The
subway has pump-wells at the Public Garden, at Eliot St., Adams Square,
and Haymarket Square. In each of these wells are two vertical submerged
centrifugal pumps made entirely of composition metal. In each chamber
above, are two electric motors operating the pumps. Each motor is
started and stopped according to the height of water by means of a float
and an automatic release starting box. The floats are so placed that
only one pump is usually brought into use. The other, however, comes
into service in case the first pump is out of order or the water enters
more rapidly than one pump can dispose of it. In the latter case, both
motors continue to run until the same low level has been reached.
Very little dampness except from atmospheric condensation is to be found
on the interior walls or roof of the subway, although numerous
discolored patches, caused by dampness and dust, may be seen on some
parts of the walls. Substantially all of the leakage comes through the
small drains in the invert leading from hollows left in the side walls.
Careful measurement was taken at the end of an unusually wet season to
determine the actual amount of leakage, and the total amount for the
entire subway was found to be about 81 gallons per minute.
_Estimated Quantities._--The estimated quantities of material used in
constructing the subway were as follows:
Excavation 369,450 cu. yds.
Concrete 75,660 „ „
Brick 11,105 „ „
Steel 8,105 tons
Granite 2,285 cu. yds.
Piles 117,925 lin. ft.
Ribbed tiles 12,440 sq. yds.
Plaster 88,190 „ „
Waterproofing (asphalt coating) 117,980 „ „
Artificial stone 6,790 „ „
Enameled brick 2,210 „ „
Enameled tiles 2,855 „ „
_Cost of the Subway._--The estimated cost of the subway made before the
work was begun was approximately $4,000,000, and the cost of
construction did not exceed $3,700,000. This includes ventilating and
pump chambers, changes of water and gas pipes, sewers and other
structures, administration, engineering, interest on bonds, and all cost
whatsoever. Dividing this number by the total length we obtain a cost
per linear foot of $342.30.
=New York Rapid Transit Railway.=--The project of an underground rapid
transit railway to run the entire length of Manhattan Island was
originated some years previous to 1890. In 1894, however, a Rapid
Transit Commission was appointed to prepare plans for such a road, and
after a large amount of trouble and delay this commission awarded the
contract for construction to Mr. John B. McDonald of New York City, on
Jan. 15, 1900.
_Route._--The road starts from a loop which encircles the City Hall
Park. Within this loop the tunnel construction is two-track; but where
the main line leaves the loop, all four tracks come to the same level,
and continue side by side thereafter except at the points which will be
noted as the description proceeds. Proceeding from the loop, the
four-track line passes under Center and Elm Streets. It continues under
Lafayette Place, across Astor Place and private property between Astor
Place and Ninth St. to Fourth Ave. The road then passes under Fourth and
Park Avenues until 42d St. is reached. At this point the line turns west
along 42d St., which it follows to Broadway. It turns northward again
under Broadway to the boulevard, crossing the Circle at 59th St. The
road then follows the boulevard until 97th St. is reached, where the
four-track line is separated into two double-track lines.
At a suitable point north of 96th St. the outside tracks rise so as to
permit the inside tracks, on reaching a point near 103d St., to curve to
the right, passing under the north-bound track, and to continue thence
across and under private property to 104th St. From there the two-track
tunnel goes under 104th St. and Central Park to 110th St., near Lenox
Ave.; thence under Lenox Ave to a point near 142d St.; thence across and
under private property and the intervening streets to the Harlem River.
The road passes under the Harlem River and across and under private
property to 149th St., which street it follows to Third Ave., and then
passes under Westchester Ave., where, at a convenient point, the tracks
emerge from the tunnel and are carried on a viaduct along and over
Westchester Ave., Southern Boulevard, and Boston Road to Bronx Park.
This portion of the line, from 96th St. to Bronx Park, is known as the
East Side Line.
From the northern side of 96th St. the outside tracks rise and after
crossing over the inside tracks they are brought together on a location
under the center line of the street and proceed along under the
boulevard to a point between 122d and 123d Streets. At this point the
tracks commence to emerge from the tunnel, and are carried on a viaduct
along and over the boulevard at a point between 134th and 135th Streets,
where they again pass into the tunnel under and along the boulevard and
Eleventh Ave. to a point about 1350 ft. north of the center line of
190th St. There the tracks again emerge from the tunnel, and are carried
on a viaduct across and over private property to Elwood St., and over
and along Elwood St. to Kingsbridge St. to Kingsbridge Ave., private
property, the Harlem Ship Canal and Spuyten Duyvil Creek, private
property, Riverdale Ave., and Broadway to a terminus near Van Cortland
Park. That portion of the line from 96th St. to the above-mentioned
terminus at Van Cortland Park is known as the West Side Line.
The total length of the rapid transit road, including the parts above
and below the surface ground of the streets, as well as both the East
and West Side Lines, is about 22¹⁄₂ miles.
_Material Penetrated._--The soil through which the road was excavated
was a varied one. The lower portion of the road, or the part including
the loop up to nearly Fourth St., was excavated through loose soil, but
from Fourth St. to the ends it was excavated in rock. The loose soil
forming the southern part of Manhattan Island is chiefly composed of
clay, sand, and old rubbish--a soil very easy to excavate. Water was met
at some points, but not in such quantities as to be a serious
inconvenience. From Fourth St. to the ends of both the east and west
side lines, the soil was chiefly composed of rock of gneissoid and
mica-schistose character, these rocks prevailing nearly throughout the
whole of Manhattan Island. The rock, as a rule, was not compact, but
full of seams and fissures, and at many points it was found
disintegrated and alternated with strata of loose soils, and even
pockets of quicksand were met with along the line of the road.
_Cross-Sections._--The section of the underground road is of three
different types,--the rectangular, the barrel-vault, and the circular.
The rectangular section. Fig. 119, is used for the greater part of the
road, of which a portion is for four tracks and a portion for two
tracks. The dimensions adopted for the four tracks are 50 × 13 ft., and
for the double tracks 25 × 13 ft. The barrel-vault section, composed of
a polycentric arch, having the flattest curve at the crown, has been
adopted for the tunnels under Park Avenue--while the semicircular arch
is used for all the other portions of the road to be tunneled. The
circular section of 15-ft. diameter is used under the Harlem River, and
being for single track, two parallel tunnels were built side by side.
[Illustration: FIG. 119.--Double-Track Section, New York Rapid Transit
Railway.]
The main line from the City Hall loop to about 102d St. consists of four
tracks built side by side in one conduit, except for that portion under
the present Fourth Ave. tunnel where two parallel double-track tunnels
are employed. The West Side Line will consist of double tracks laid in
one conduit, except across Manhattan St. and beyond 190th St., where it
is carried on an elevated structure. The East Side Line consists of a
double-track tunnel driven from 102d St., and the boulevard under
Central Park to 110th St. and Lenox Ave., and two parallel circular
tunnels excavated under the Harlem River,--the other portions of the
road being double-track, subway and elevated structure.
_Methods of Excavation._--Both the double-and four-track subway were
built by using the different varieties of the cut-and-cover method. The
single wide-trench method was used for the construction of the
double-track line and also for the construction of the four-track line
where the local conditions allowed it. The single narrow-trench method
was used for the construction of the four-track subway at 42d St., to
meet with the peculiar conditions of the traffic. Almost the total
length of the four-track line of the subway was built by means of the
two parallel side trenches. The slice method, so successfully employed
in the Boston Subway, was used only on 42d St. west of 6th Avenue.
_Lining._--The lining of the subway is of concrete, carried by a
framework of steel. The floor consists of a foundation layer of concrete
at least eight inches thick on good foundation, but thicker, according
to conditions, where the foundation is bad. On top of this is placed
another layer of concrete, with a layer of waterproofing between the
two. In this top layer are set the stone pedestals for the steel
columns, and the members making up the tracks.
In the four-track subway, the steel framework consists of transverse
bents of columns, and I-beams spaced about five feet apart along the
tunnel. The three interior columns of each bent are built-up bulb-angle
and plate columns of H-section. The wall columns are I-beams, as are
also the roof beams; between the I-beams, wall columns, and roof beams
there is a concrete filling, so that the roof of the subway will be made
up of concrete arches resting on the flanges of the I-beams of the roof.
The concrete used is of one part Portland cement, two parts sand, and
four parts broken stones. The double-track subway is built in the same
way, except that only one column is placed between the tracks for the
support of the roof.
All the concrete masonry of the roof, foundations, and side walls
contains a layer of waterproofing, so as to keep perfectly dry the
underground road, and prevent the percolation of water. This
waterproofing is made up as follows: On the lowest stratum of concrete,
whose surface is made as smooth as possible, a layer of hot asphalt is
spread. On this asphalt are immediately laid sheets or rolls of felt;
another layer of hot asphalt is then spread over the felt, and then
another layer of felt laid, and so on, until no less than two, and no
more than six, layers of felt are laid, with the felt between layers of
asphalt. On top of the upper surface of asphalt the remainder of the
concrete is put in place so as to reach the required thickness of the
concrete wall.
[Illustration: FIG. 120.--Park Avenue Deep Tunnel Construction, New York
Rapid Transit Railway.]
_Tunnels._--When the distance between the roof of the proposed structure
and the street was 20 ft. or over, the Standard Subway construction was
replaced by tunnels. Three important tunnels have been constructed along
the line of the New York Rapid Transit and these are located between 33d
and 42d Streets on Park Ave., under Central Park northeast of 104th St.
and under Broadway north of 152d St. The Park Ave. construction (Fig.
120) consists of two parallel double-track tunnels, located on each side
of the street, and about 10 ft. below the present tunnel. The soil being
composed of good rock, the tunnels were driven by a wide heading, and
one bench, since no strutting was required, and the masonry lining, even
of the roof, was left far behind the front of the excavation. The
masonry lining consists of concrete walls and brick arches. The tunnels
under Central Park and under Broadway being driven through a similar
rock, the same method of excavation and the same manner of lining was
used.
The tunnel under the Harlem River was driven through soft ground; and it
was constructed as a submarine tunnel, according to the caisson process.
The tunnels were lined with iron made up of segments, with radial and
circumferential flanges. Concrete was placed inside and flush with the
flanges.
[Illustration: FIG. 121.--Harlem River Tunnel, New York Rapid Transit
Railway.]
The tracks, both in the subway and tunnels, are an intimate part of the
concrete flooring. The rail rests on a continuous bearing of wooden
blocks, laid with the grain running transversely with respect to the
line of the rail, and held in place by two channel iron guard rails. The
guard rails are bolted to metal cross-ties embedded in the concrete.
_Viaduct._--A considerable portion of the double-track branch lines
north of 103d St. is viaduct, or elevated structure. The viaduct
construction on the West Side Line extends, including approaches, from
122d St. to very near 135th St. Of this distance, 2018 ft. 8 ins. are
viaduct proper, consisting of plate girder spans carried by trestle
bents at the ends, and by trestle towers for the central portion. The
columns of the bents and towers are built-up bulb-angle and plate
columns of H-section of the same form as those used in the bents inside
the subway. The approaches proper are built of masonry. The elevated
line proper consists of plate girder spans, supported on plate cross
girders carried by columns.
_Stations._--Many stations are built along the line. These are located
on each side of the street. The entrances at the stations consist of
iron framework, with glass roofs covering the descending stairways. The
passageways leading down are walled with white enameled bricks and
wainscoted with slabs of marble. The stations for the local trains are
located on each side of the road close to the walls, since the outside
tracks are reserved for the local trains, while the middle ones are
reserved for the expresses. The few stations for the express trains are
located between the middle and outside tracks. Stations are provided
with all the conveniences required, having toilet rooms, news stands,
benches, etc., and are lighted day and night by numerous arc lamps.
_General._--The contractor completed the work in four years. No
difficulty was encountered in doing this, since the great extension of
the road and the great width of the avenues under which it runs allowed
work all along the line at the same time. The work, briefly summarized,
comprises the following items:--
Length of all sections, ft. 109,570
Total excavation of earth, cu. yds. 1,700,228
Earth to be filled back, cu. yds. 773,093
Rock excavated, cu. yds. 921,128
Rock tunneled, cu. yds. 368,606
Steel used in structure, tons 65,044
Cast iron used, tons 7,901
Concrete, cu. yds. 489,122
Brick, cu. yds. 18,519
Waterproofing, sq. yds. 775,795
Vault lights, sq. yds. 6,640
Local stations, number 43
Express stations, number 5
Station elevators, number 10
Track total, lin. ft. 305,380
„ underground, lin. ft. 245,514
„ elevated, lin. ft. 59,766
In addition to the construction of the railway itself, it was necessary
to construct or reconstruct certain sewers, and to adjust, readjust,
and maintain street railway lines, water pipes, subways, and other
surface and subsurface structures, and to relay street pavements.
The total cost of the work, according to the contract signed by Mr.
McDonald, was $35,000,000. Dividing this amount by the total length of
the road, which is 109,570 lineal feet, we have the unit cost a lineal
foot $315, or a little less than unit of cost of the Boston Subway,
which was $342 per lineal foot.
The road belongs to the city. The contractor acts as an agent for the
city in carrying out the work, and he is the leaser of the road for
fifty years. The work was paid for as soon as the various parts of the
road were completed, and the money was obtained from an issue of city
bonds. During the fifty years’ lease the contractor will pay the
interest plus 1% of the face value of the bonds. This 1% goes to the
sinking-fund, which within the fifty years at compound interest forms
the total sum required for the redemption of bonds.
This first New York Subway has been extended to Brooklyn, and more lines
will be built so as to form a complete underground railway system to
accommodate the ever-increasing traveling crowd of the American
metropolis. No new method of construction has been devised yet. The only
variation from the illustrated methods has been where the subway is
built underneath the Elevated Road which had to be strongly supported
during the construction of the subway. This has been done in two
different ways, either by supporting the columns of the Elevated Road by
means of two wooden A-frames abutting at the top and leaving a large
space close to the foot of the column where a pit was excavated to the
required depth of the subway, or by attaching the columns to long iron
girders placed longitudinally and resting with both ends in firm soil.
CHAPTER XVII.
SUBMARINE TUNNELING: GENERAL DISCUSSION.--THE SEVERN TUNNEL.
GENERAL DISCUSSION.
Submarine tunnels, or tunnels excavated under the beds of rivers, lakes,
etc., have been constructed in large numbers during the last quarter of
a century, and the projects for such tunnels, which have not yet been
carried to completion, are still more numerous. Among the more notable
completed works of this character may be noted the tunnel under the
River Severn and those under the River Thames in England, the one under
the River Seine in France, those under the St. Clair, Detroit, Hudson,
Harlem and East Rivers, and the one under the Boston Harbor for
railways, that under the East River for gas mains, that under Dorchester
Bay, Boston, for sewage, and those under Lakes Michigan and Erie for the
water supply of Milwaukee, Chicago, Buffalo, and Cleveland in America.
For the details of the various projected submarine tunnels of note,
which include tunnels under the English and Irish Channels, under the
Straits of Gibraltar, under the sound between Copenhagen in Denmark and
Malmö in Sweden, under the Messina Straits between Italy and Sicily, and
under the Straits of Northumberland between New Brunswick and Prince
Edward Island, and those connecting the various islands of the Straits
of Behring, the reader is referred to the periodical literature of the
last few years.
Previous to attempting the driving of a submarine tunnel it is necessary
to ascertain the character of the material it will penetrate. This fact
is generally determined by making diamond-drill borings along the line,
and the object of ascertaining it is to determine the method of
excavation to be adopted. If the material is permeable and the tunnel
must pass at a small depth below the river bed, a method will have to be
adopted which provides for handling the difficulty of inflowing water.
If, on the other hand, the tunnel passes through impermeable material at
a great depth, there will be no unusual trouble from water, and almost
any of the ordinary methods of tunneling such materials may be employed.
Shallow submarine tunnels through permeable material are usually driven
by the shield method or by the compressed air method, or by a method
which is a combination of the first and second.
It is not an uncommon experience for a submarine tunnel to start out in
firm soil and unexpectedly to find that this material becomes soft and
treacherous as the work proceeds, or that it is intersected by strata of
soft material. The method of dealing with this condition will vary with
the circumstances, but generally if any considerable amount of soft
material has to be penetrated, or if the inflow of water is very large,
the firm-ground system of work is changed to one of the methods employed
for excavating soft-ground submarine tunnels. The Milwaukee water supply
tunnel, described elsewhere, is a notable example of submarine tunnels,
began in firm material which unexpectedly developed a treacherous
character after the work had proceeded some distance. Occasionally the
task of building a submarine tunnel in the river bed arises. In such
cases the tunnel is usually built by means of cofferdams in shallow
water, and by means of caissons in deep water.
Submarine tunnels under rivers are usually built with a descending grade
from each end which terminates in a level middle position, the
longitudinal profile of the tunnel corresponding to the transverse
profile of the river bottom. Where, however, such tunnels pass under the
water with one end submerged, and the other end rising to land like the
water supply tunnels of Chicago, Milwaukee, and Cleveland, the
longitudinal profile is commonly level, or else descends from the shore
to a level position reaching out under the water.
The drainage of submarine tunnels during construction is one of the most
serious problems with which the engineer has to deal in such works. This
arises from the fact that, since the entrances of the tunnel are higher
than the other parts, all of the seepage water remains in the tunnel
unless pumped out, and from the possibility of encountering faults or
permeable strata, which reach to the stream bed and give access to water
in greater or less quantities. Generally, therefore, the excavation is
conducted in such a manner that the inflowing water is led directly to
sumps. To drain these sumps pumping stations are necessary at the shore
shafts, and they should have ample capacity to handle the ordinary
amount of seepage, and enough surplus capacity to meet probable
increases in the inflow. For extraordinary emergencies this plant may
have to be greatly enlarged, but it is not usual to provide for these at
the outset unless their likelihood is obvious from the start. The
character and size of the pumping plants used in constructing a number
of well-known tunnels are described in Chapter XII.
In this book submarine tunnels will be classified as follows: (1)
Tunnels in rock or very compact soils, which are driven by any of the
ordinary methods of tunneling similar materials on land; (2) tunnels in
loose soils, which may be driven, (_a_) by compressed air, (_b_) by
shields, or (_c_) by shields and compressed air combined; (3) tunnels on
the river bed, which are constructed, (_a_) by cofferdams, or (_b_) by
caissons. To illustrate tunnels of the first class, the River Severn
tunnel in England has been selected; to illustrate those of the second
class, the several tunnels discussed in the chapter on the shield system
of tunneling and the Milwaukee tunnel is sufficient; to illustrate those
of the third class, the Van Buren Street tunnel in Chicago, the Harlem,
the Seine and the Detroit River tunnels are selected.
THE SEVERN TUNNEL.
The Severn tunnel, which carries the Great Western Railway of England,
beneath the estuary of a large river, is 4 miles 642 yards long. It
penetrates strata of conglomerate, limestone, carboniferous beds, marl,
gravel, and sand at a minimum depth of 44³⁄₄ ft. below the deepest
portion of the estuary bed. The following description of the work is
abstracted from a paper by Mr. L. F. Vernon-Harcourt.[12]
[12] Proceedings Inst. C. E., vol. cxxi.
The first work was the sinking of a shaft, 15 ft. in diameter, lined
with brickwork, on the Monmouthshire bank of the Severn, 200 ft. deep.
After the Monmouthshire shaft had been sunk, a heading 7 ft. square was
driven under the river, rising with a gradient of 1 in 500 from the
shaft on the Monmouthshire shore, so as to drain the lowest part of the
tunnel. Near to the first, a second shaft was sunk and tubbed with iron,
in which the pumps were placed for removing the water from the tunnel
works, connection being made by a cross-heading with the heading from
the first shaft. There was also a shaft on the Gloucestershire shore;
and two shafts inland from the first on the Monmouthshire side, to
expedite the construction of the tunnel. Headings were then driven in
both directions along the line of the tunnel, from the four shafts; and
the drainage heading from the old shaft was continued, in the line of
the tunnel, under the deep channel of the estuary, and up the ascending
gradient towards the Gloucestershire shore, till, in October, 1879, it
had reached to within about 130 yards of the end of the descending
heading from the Gloucestershire shaft. During this period, though the
work had progressed slowly, no large quantity of water had been met with
in any of the headings, in spite of their already extending under almost
the whole width of the estuary. On October 18, 1889, however, a great
spring was tapped by the heading which was being driven landwards from
the old shaft, about 40 ft. above the level of the drainage heading;
and the water poured out from this land spring in such quantity that,
flowing along the heading, falling down the old shaft, and thus finding
its way into the drainage heading and the long heading of the tunnel
under the estuary in connection with it, it flooded the whole of the
workings in communication with the old shaft, which it also filled
within twenty-four hours from the piercing of the spring.
To obtain increased security against any influx of water from the deep
channel of the estuary into the tunnel, the proposed level portion of
the tunnel, rather more than a furlong long under this part, was lowered
15 ft. by increasing the descending gradient on the Monmouthshire side
from 1 in 100 to 1 in 90, and lowering the proposed rail level on the
Gloucestershire side 15 ft. throughout the ascent, so as not to increase
the gradient of 1 in 100 against the load. A new shaft, 18 ft. in
diameter, was sunk slightly nearer the estuary on the Monmouthshire
shore than the old one; two shafts also were sunk on the land side of
the great spring for pumping purposes; and additional pumping machinery
was erected. The flow from the spring into the old shaft was arrested by
a shield of oak fixed across the heading; and at last, after numerous
failures and breakdowns of the pumps, the headings were cleared of
water, after a diver, supplied with a knapsack of compressed oxygen, had
closed a door in the long heading under the estuary; and the works were
resumed nearly fourteen months after the flooding occurred. The great
spring was then shut off from the workings by a wall across the heading
leading to the old shaft; and, owing to the lowering of the level of the
tunnel, a new drainage heading had to be driven from the bottom of the
new shaft at a lower level, which was made 5 ft. in diameter, and lined
with brickwork, whilst the old drainage heading was enlarged to 9 ft. in
diameter, and lined with brickwork, so as to aid in the permanent
ventilation of the tunnel. The lowering of the level, moreover,
converted the bottom tunnel headings into top headings, so that along
more than a mile of the tunnel the semicircular arch, 26 ft. in
diameter, was built first, and then, after lowering the headings, the
invert was laid and the side walls were built up. Bottom headings were
driven along the remainder of the tunnel, and the work was expedited by
means of break-ups. Ventilation was effected in the works by a fan 18
inches in diameter and 7 ft. wide, fixed at the top of the new deep
shaft; the rock was bored by drills worked by compressed air; the
blasting was eventually effected exclusively by tonite, owing to its
being freer from deleterious fumes than any other explosive; and the
workings were lighted by Swan and Brush electric lamps. The tunnel is
lined throughout with vitrified brickwork, between 2¹⁄₄ ft. to 3 ft.
thick, set in cement, and has an invert 1¹⁄₂ ft. to 3 ft. in thickness;
the lining was commenced towards the end of 1880, the headings under the
river were joined in September, 1881, and the last length of the tunnel,
across the line of the great spring, was completed in April, 1885.
Water came in from the river through a hole in a pool of the estuary,
close to the Gloucestershire shore, in April, 1881, during the lining of
a portion of the tunnel, but fortunately before the headings were
joined. This influx was stopped by allowing the water to rise in the
tunnel to tide-level, to prevent the enlargement of the hole, which was
then filled up at low water with clay, weighted on the top with clay in
bags. The great spring broke out again in October, 1883, and flooded the
works a second time; but within four weeks the water had been pumped out
and the spring again imprisoned. During this period an exceptionally
high tide, raised still higher by a southwesterly gale, inundated the
low-lying land on the Monmouthshire side of the estuary, and, flowing
down one of the inland shafts, flooded a section of the tunnel, but the
pumps removed this water within a week.
In order to construct the portion of tunnel traversing the line of the
great spring, the water was diverted into a side heading below the level
of the tunnel, leading to the old shaft, whence it was pumped, and the
fissure below the tunnel was filled with concrete, over which the
invert was built. An attempt to imprison the spring, on the completion
of this length of tunnel, having resulted in imposing an excessive
pressure on the brickwork, leading to fractures and leakage, a shaft, 29
ft. in diameter, was sunk at the side of the tunnel at this point in
1886, and pumps were erected powerful enough to deal with the entire
flow of the spring.
The tunnel was opened for traffic in December, 1886, and gives access to
a double line of railway, connecting the lines converging to Bristol
with the South Wales railway and the western lines. The pumping power
provided at the shaft connected with the great spring, and at four other
shafts, is capable of raising 66,000,000 gallons of water per day, the
maximum amount pumped from the tunnel being 30,000,000 gallons a day.
The ventilation of the tunnel is effected by fans placed in the two main
shafts on each bank of the estuary, and the fan in the Monmouthshire
shaft is 40 ft. in diameter, and 12 ft. wide. The tunnel gives passage
to a large traffic, numerous through-trains between the north and
southwest of England making use of it.
CHAPTER XVIII.
SUBMARINE TUNNELING (Continued); THE COMPRESSED AIR METHOD.--THE
MILWAUKEE WATER-WORKS TUNNEL.
Tunnels excavated at shallow depth from the bed of the river are liable
to cave in under the great weight of the water and material above the
roof. Besides, the progress of the work will be greatly interfered with
by the water which may reach the tunnel passing through the loose soil
in large quantities. To contend with these two sources of trouble,
different methods of constructing subaqueous tunnels have been devised;
they are: by compressed air, by shield, and finally by a combination of
these two methods, viz., by shield and compressed air.
The compressed air method was suggested by Mr. Haskin, the promoter and
the first builder of the Hudson River tunnel. In 1874, when he began to
sink the shaft for the construction of his tunnel, several subaqueous
tunnels had already been driven by means of shields. Mr. Haskin had
ideas of his own, and thought he could dispense with the shield and
could trust to compressed air, since he was firmly convinced that
compressed air alone could expel the water and temporarily support the
roof of the excavation prior to the building of the lining masonry. In
other words, he expected to substitute a core of compressed air for the
core of earth removed. In the patent granted him for this method of
tunneling, he expresses himself as follows: “The distinguishing feature
of my system is that, instead of using temporary facings of timber or
other rigid material, I rely upon the air pressure to resist the caving
in of the wall and infiltration of water until the masonry wall is
completed. The pressure is, of course, to be regulated by the
exigencies of the occasion. The effect of such a pressure has been found
to drive water in from the surface of the excavation, so that the sand
becomes dry.”
The compressed air method was soon found to be inefficient, even in the
construction of the Hudson tunnel where the roof of the excavation was
supported by timbering in the manner indicated in the pilot system. Thus
large subaqueous railway tunnels cannot be driven exclusively by the
compressed air method; still it has been successfully employed in the
construction of small tunnels driven for aqueduct purposes. But the use
of compressed air marked a great progress in the art of submarine
tunneling.
THE MILWAUKEE WATER-WORKS TUNNEL.
The following description of the Milwaukee Water-Works Tunnel is an
example of subaqueous tunnels driven through good soil in the usual
manner employed in land tunnels; but afterward when treacherous material
was encountered, the work was continued by means of compressed air.
The new water supply intake tunnel for the city of Milwaukee, Wis., is
one of the most difficult examples of tunnel construction which American
engineering practice has afforded. The difficulties were in a large
measure unexpected when the work was decided upon and put under way. The
tunnel began and ended in a hard, impervious clay, practically a rock,
and all the preliminary investigations led to the conclusion that the
same favorable material would be encountered for its entire length. With
such material a brick-lined tunnel 7¹⁄₂ ft. in diameter presented no
unusual problems; but after about 1640 ft. had been excavated from the
shore end the tunnel ran out of the hard clay, and for the next 600 ft.
or more a variety of water-bearing material was encountered, which tried
the skill and patience of the engineer to their utmost. Other
difficulties were indeed met with, but these were of minor importance in
comparison with that of safely and successfully penetrating the
water-bearing drift.
The work of sinking the shore shafts and excavating the first 1600 ft.
of tunnel did not prove especially difficult. A hard, compact, and
rock-like clay, bearing very little moisture, was encountered all along,
and was blasted and removed in the ordinary manner. The only mishap
which occurred with this portion of the work was the destruction of the
contractor’s boiler plant by fire on Jan. 12, 1891, which allowed the
tunnel to fill with water, and delayed work about a month. By Oct. 21,
1891, 1640 ft. had been driven, averaging about 6²⁄₃ ft. per day, all in
the hard clay. No timbering had been necessary, and except for the first
100 ft. of the tunnel there was very little seepage. On the afternoon of
Oct. 21 water was observed coming out from one of the drill holes in the
heading, but no attention was paid to it. Shortly after a blast was
fired, and was immediately followed by a rush of water from the heading.
An unsuccessful attempt was made to check the flow, and the pumps were
started; but they were unable to keep the water down, and after seven
hours’ hard work the tunnel was abandoned. By the next morning the
tunnel and shaft were full of water.
Several attempts were made to empty the tunnel; but the limited pumping
capacity was not equal to the task, and it was finally decided to
install larger pumps. The pumping had, however, shown that about 1000
gallons of water a minute was coming through the leak. With the
increased pumping plant the tunnel was finally laid dry Feb. 13, 1892.
Upon examination the head of the drift was found to be in the same
undisturbed condition in which it was left when the water broke in three
months before.
A brick bulkhead was built into the end of the brickwork of the tunnel,
and provided with a timber door for passage, and two 10-in. pipes for
the outlet of the water. With these openings closed, the flow was
checked sufficiently to allow the placing of pumps at the bottom of the
shore shaft. Meanwhile the pressure of the water against the bulkhead
caused dangerous leakage, and so after the pumps were in position the
10-in. pipes were opened, relieving the pressure and allowing the water
its normal rate of flow. Trouble with the pumps now arose, and after
various stoppages and breaks the discharge pipe finally fell, disabling
the whole plant. It became necessary to close the 10-in. pipes in the
bulkhead and draw up the pumps. This allowed the tunnel to again fill
with water.
After thoroughly overhauling the pumping machinery, the contractor again
laid the tunnel dry on March 19; and after the pumps had been
permanently placed so as to take care of the water, an examination of
the work was made. It was found that the water was coming from the
north, and with the hope of avoiding the difficulties of the old
heading, it was decided to make a détour of the south. On April 16 work
was begun at a point about 90 ft. back from the face, and deflecting the
line about 38° toward the south. About 38 ft. from the angle of junction
a brick bulkhead with two 8-in. openings was built into the new bore.
The work progressed successfully for about 75 ft., when water was again
encountered; and upon pushing forward the heading, gravel and sand came
in such quantities that it was found impracticable to continue the work
further. On June 1 the bulkhead was permanently closed, and the work in
this direction was abandoned.
A further and closer examination was now made of the heading first
abandoned. Upon breaking through the rock-like clay it was found that
the water came from an underground stream flowing from the north through
a well defined channel in red clay. This channel was about 13 ft. above
the grade of the tunnel; and above it in every direction visible was a
bed of hard, dry, red clay, while immediately in front of the face of
the work was a bank of coarse gravel. Fig. 122 is a sketch of the
channel and stream where they entered the work. In this last drawing the
photograph has been followed exactly, no particular being exaggerated in
the slightest. The water from this stream was clear and pure; and a
chemical analysis showed that it was not lake water, but must come from
some separate source.
[Illustration: FIG. 122.--Sketch Showing Underground Stream, Milwaukee
Water-Works Tunnel.]
While the engineer did not consider the difficulty of proceeding along
the old line insurmountable, it was decided to be less difficult on the
whole to go back from 150 ft. to 175 ft. and deflect the line to the
north and upward, so as to pass over the underground entrance. Instead
of allowing the water to flow at its normal rate and take care of it by
pumping, the contractors desired to reduce the pumping, and to this end
they constructed a bulkhead just west of the deflection toward the south
with a view of shutting off the water. The water, however, accumulated
with a pressure of some 50 lbs. per sq. in. and penetrated the filling
around the brick lining of the tunnel, preventing the cutting through of
the lining for the new line. A second bulkhead was then built about 20
ft. west of the first, but with not much better results, for upon
closing it the water was found to leak through the brickwork for a long
distance west. Finally on Aug. 2, 1892, the contractors lifted their
pumps and allowed the tunnel to fill again with water.
No further work was done on the tunnel by the contractors, although they
continued work on the lake shaft for some months. Difficulties had,
however, arisen here, which will be described further on; and finally a
disagreement arose between the contractors and the city over the delay
in prosecuting the tunnel work and over one or two other questions,
which resulted in the City Council suspending their contract and
ordering the Board of Public Works to go ahead with the work.
The first step to be taken by the engineer was to purchase adequate
pumping machinery and empty the tunnel. This was effected Jan. 17, 1894;
and as soon as practicable thereafter the two bulkheads were removed and
the tunnel cleaned, tram-car tracks laid, and everything prepared for
work. It was now determined to go ahead on the original line of the
tunnel if possible, and the bulkhead here was removed and work begun.
Meanwhile, a safety bulkhead had been built to replace the first one
torn away. This was provided with a door and drainage pipes. Work was
begun on the original heading, but had proceeded only a little way when
the water broke in, driving out the workmen. This was removed three or
four times, when the flow suddenly increased to 3000 gallons per minute.
An examination of the lake bottom above the break showed that it had
settled down, indicating that the new break connected with the lake
bottom, and making further work along the original line out of the
question.
The question now arose what it was best to do. It was impracticable to
use a shield, as the material ahead of the break required blasting, and
the pressure from above was enormous. On account of its expense and
difficulty of application the freezing process did not seem advisable,
and the plenum process was likewise out of the question on account of
the great pressure which would be required at this depth. The détour to
the south which had been made by the contractor had been unsuccessful,
and had left the ground in a treacherous condition. To depress the
tunnel was not advisable, for it was not by any means certain that the
bed of gravel could be avoided in that way; and, moreover, it would be
necessary to ascend again further on, and thus leave a trap which would
effectually cut off escape to those at work on the face if water again
broke into the tunnel.
It was finally decided that the old plan of deflecting the line toward
the north and upward so as to pass over the underground stream should be
tried. A hole was therefore cut through the tunnel lining 1433 ft. from
the shore, and work was begun on a détour of 20° toward the north and an
upward grade of 10%. Fair progress was made on this new line, gradually
ascending into solid rock, until May 10, when the test borings, which
were constantly made in every direction from the face, showed that sand
was being approached. A brick bulkhead was therefore built into the
masonry as a safeguard, should it happen that water was encountered in
large quantities. As the borings seemed to indicate that the top surface
of the rock underlying the sand was nearly level, the lower half of the
tunnel was first excavated, leaving about 18 ins. of the rock to serve
as a roof (Sketch _a_, Fig. 123), and the brick invert was built for a
distance of 52 ft. The rock roof was then carefully broken through for
short distances at a time, and short sheeting driven ahead into the
sand, which proved to be a very fine quicksand flowing through the
smallest openings. Extreme care had to be taken in this work, but little
by little the brickwork was pushed ahead until at a distance of 90 ft.
from the point where the sand was first met, and 208 ft. from the old
tunnel, the sand stopped and the heading entered a hard clay.
All this work had been done on an ascending grade, and the ascent was
continued about 40 ft. farther in the clay. By this time a sufficient
elevation was gained to pass over the underground stream, and the tunnel
line was changed to head toward the lake shaft, and the grade reduced to
a level. The underground stream was passed without trouble and the
tunnel continued for a distance of 54 ft. without difficulty. On July 10
the clay in the heading suddenly softened, and before the miners could
secure it by bracing, the water rushed in, followed by gravel, filling
up solidly some 34 ft. of the tunnel before it was stopped by a timber
bulkhead hastily built.
[Illustration: ~Longitudinal Section Showing Method of Construction in
Rock Covered with Quicksand.~
~Sketch “a”.~
~Section A-B-C-D.~
~Sketch “c”.~
~Longitudinal Section of Tunnel.~
~Sketch “b”.~
~Cross Section Showing Manner of Constructing Lining around Boulder.~
~Sketch “d”.~
FIG. 123.--Sketch Showing Methods of Lining, Milwaukee Water-Works
Tunnel.]
Upon examining the lake bottom a cavity over 60 ft. deep and 10 ft. in
diameter was found directly over the end of the tunnel, which had been
caused by the gravel breaking into the tunnel. Having now reached an
elevation where it was possible to use compressed air, it was determined
to put in double air-locks and use the plenum process. The locks were
built, and some 670 cu. yds. of clay were dumped into the hole in the
lake bottom. On Aug. 4 the air-locks were tried with 26 lbs. air
pressure; but, upon a temporary release of the pressure, the water
passed around the locks and back of the tunnel lining for some distance,
and even forced through the lining, carrying considerable clay and fine
sand with it. Upon sounding the lake bottom it was found that the cavity
had again increased to a depth of 65 ft., whereupon an additional 600
cu. yds. of clay were dumped into it.
On account of the water leaking through the brickwork, the only dry
place to cut through the brickwork and build in an air-lock was just
ahead of the brick bulkhead. This lock was completed Aug. 27, and to
avoid encountering the danger of the direct connection with the lake at
the end of the drift, it was decided to make another détour to the
north. On Aug. 28, therefore, the brick on the north side of the tunnel
12 ft. back from the end of the brickwork was cut through under 25 lbs.
air pressure, and work proceeded in good, hard clay. The original
air-lock was cut out and a new lock built into this clay about 34 ft.
from the last détour, to be used in case of further difficulties. After
building the tunnel for about 80 ft. from the détour, the soundings
again indicated the approach to gravel and water, and on Oct. 14 the
water broke through from the bottom in such volume and with such force
that the men ran out, closing every air-lock and the valves of every
drain in their haste to escape, until the brick bulkhead was reached. It
was with great difficulty that the portion of the tunnel up to the last
air-lock was recovered and cleaned out.
It was now recognized that a pressure of from 38 to 40 lbs. of air would
be needed to hold this water, and accordingly another compressor was
added to the plant. With a pressure of 36 lbs. the water was driven out
and the work again started. At this time also an additional 350 cu. yds.
of clay were dumped into the hole in the lake bottom. Altogether, 1620
cu. yds. of clay had been put into this hole.
Loose gravel and boulders, some of immense size, were now encountered,
and the work became exceedingly difficult on account of the great escape
of air. The interstices between the gravel and boulders were not filled
with silt or sand, but contained water. Moreover, this material extended
upward to the lake bottom, as was shown by the escape of air at the
surface of the lake. For an area of several hundred square feet the
surface of the water resembled a pot of boiling water. At times the air
would escape very rapidly; and again only a few bubbles would show.
It need hardly be said that the work in this gravel was very slow. It
was impossible to blast or to tear out the large boulders whole, as so
much surface would be exposed that an inrush of water would take place
despite the air pressure. The method of procedure was to excavate a
heading and build the brick roof arch first, and then to take out the
bench and build the invert. Fig. 123 gives a number of sketches showing
how the work was done. A short piece of heading was taken out, the top
and face of the bench being meanwhile plastered with clay (Sketches _b_
and _c_, Fig. 123) to reduce the escape of air, and then the roof arch
was built and supported on side sills resting on the bench. Bit by bit
the roof arch was pushed forward until some little distance had been
completed, then the heading was plastered with clay and the bench taken
out little by little and the invert built. All the gravel except the
small area upon which work was actually in progress was kept thoroughly
plastered with clay; and as the air escaped through the completed
brickwork very rapidly, water was allowed to cover a portion of the
invert (see Sketch _c_, Fig. 123), so as to reduce the area of escape.
When a large boulder was reached, which lay partly within and partly
without the tunnel section, the lining was built out and around it, as
shown in Sketch _d_, Fig. 123. The boulder was then broken and taken
out. All through this gravel bed the cross-section of the lining is made
irregular by the construction of these pockets in the lining to get
around boulders. Sometimes they were on one side and sometimes on the
other, or on both, or at the top or bottom. In fact, there was no
regularity. Despite the hazard and danger of this work, continual
progress was made, though sometimes it was only 4 ft. of completed
tunnel per week, working night and day; and, if some cases of caisson
disease be excepted, the only mishap occurring was a fire which got into
the timber packing behind the lining and caused some trouble. From the
gravel the tunnel ran into clay and quicksand, and then into hard, dry
clay similar to that encountered near the shore. Some difficulty was had
with the quicksand, but it was successfully overcome; and when the hard
clay was struck, the trouble, as far as the work from the shore shaft
was concerned, was virtually over.
Meanwhile, a different set of afflictions had come upon the engineer and
contractors in sinking the lake shaft and driving the heading toward
shore. This shaft was intended to be built by sinking a cast-iron
cylinder 10 ft. in diameter, made up of sections bolted together. Work
was begun July 5, 1892, and the sinking was accomplished first by
weighting the cylinder, and afterwards by pumping out the sand and water
within it until the pressure from the outside broke through under the
cutting edge and forced the sand into the cylinder, allowing it to sink
a little. From 10 to 30 cu. yds. of sand were carried into the cylinder
each time, and finally it was feared that if the process continued, the
crib, which had been previously erected, would be undermined. On Sept.
6, therefore, the contractors were ordered to discontinue this method of
work. No change was made, however, until Oct. 1, when the cylinder had
reached a depth of 68 ft., and by this time there was quite a large
cavity underneath the crib. This was refilled, and the cylinder pumped
out, and excavation begun inside of it. On Oct. 11 a 2¹⁄₂-ft. deep ring
of brickwork was laid underneath the cutting edge; but in trying to put
in another ring beneath the first, two days later, the sand and water
broke through the bottom, driving the men out, and filling the cylinder
to a depth of 16 ft. with sand. The pumps were started, but the water
could not be lowered to a greater depth than 60 ft.
At the request of the contractors, the city engineer had a boring made
at the center of the shaft to determine the character of the material to
be further penetrated. This boring showed that sand mixed with loam and
gravel would be found for a depth of 26 ft., then would come 15 ft. of
red clay, and finally a layer of hard clay like that penetrated by the
shore end of the tunnel. About the middle of December the contractors
made another attempt to pump the shaft, but finding that the water came
in at the rate of 25 gallons a minute, abandoned the attempt. In the
latter part of February preparations were made to put an air-lock in the
shaft and use compressed air. Hardly had the work been begun by this
system when, on April 20, 1893, a terrific easterly storm swept the top
of the crib bare of the buildings and machinery, and drowned all but one
of the 15 men at work there.
This disaster delayed the work for some time, but in June the
contractors erected a new building and new machinery, and resumed work.
Very little progress was made; and the air escaped so rapidly that it
loosened the sand surrounding the shaft and reduced the friction to such
an extent that on July 28 the entire cylinder lifted bodily about 6 ft.,
and sand rushed in, filling the lower part of the cylinder to within 45
ft. of the lake surface. No further work was done by the contractors
although they submitted a proposition to sink a steel cylinder inside
the cast-iron cylinder and extending from 5 ft. above datum to 100 ft.
below datum for $300 per ft. This proposition was refused by the city;
and since work on the tunnel proper had been abandoned by the
contractors some time before, as had already been described, the city
suspended their contract on Oct. 19.
On Oct. 30 a contract was made with Mr. Thos. Murphy of Milwaukee, Wis.,
to sink a steel cylinder inside the old iron cylinder. The water was
first pumped out of the old cylinder, and a timber bulkhead built at
the bottom. On this the steel cylinder was built, and then the bulkhead
was removed. Air pressure was put on, and the excavation proceeded
successfully until the bottom layer of clay was met with, when all
chances for trouble ceased.
The cylinder, as it was completed, penetrated 9 ft. into the hard clay,
and was underpinned with brickwork for a depth of 29 ft. or more, to a
point 4 ft. below the grade line of the tunnel. At the lower end, the
section of the shaft was changed from a circle to a square. Later the
steel cylinder was lined with brick.
On March 28, 1894, an agreement was made with Mr. Thos. Murphy to
construct the tunnel from the lake shaft toward the shore. Except that
considerable water was encountered, which, owing to inadequate pumping
machinery, filled the tunnel and shaft at two different times, and had
to be removed, no very great difficulty was had with this part of the
work.
On July 28, 1895, the headings from the lake and shore shafts met.
Meanwhile the cast-iron pipe intake, the intake crib, etc., had been
completed, and practically all that remained to be done was to clean the
tunnel and lift the pumping machinery at the shore shaft. During the
cleaning, the air pressure had been kept up on account of the leakage
through the brick lining, and, indeed, the pressure was kept up until
the last possible moment, and everything made ready for removing the
air-locks, bulkheads, pumps, etc., in the least possible time. The pumps
were the last to come out.
CHAPTER XIX.
SUBMARINE TUNNELING (Continued).
THE SHIELD SYSTEM.
=Historical Introduction.=--The invention of the shield system of
tunneling through soft ground is generally accredited to Sir Isambard
Brunel, a Frenchman born in 1769, who emigrated to the United States in
1793, where he remained six years, and then went to England, in which
country his epoch-making invention in tunneling was developed and
successfully employed in building the first Thames tunnel, and where he
died in 1849, a few years after the completion of this great work. Sir
Isambard is said to have obtained the idea of employing a shield to
tunnel soft ground from observing the work of ship-worms. He noticed
that this little animal had a head provided with a boring apparatus with
which it dug its way into the wood, and that its body threw off a
secretion which lined the hole behind it and rendered it impervious to
water. To duplicate this operation by mechanical means on a large enough
scale to make it applicable to the construction of tunnels was the plan
which occurred to the engineer; and how closely he followed his animate
model may be seen by examining the drawings of his first shield, for
which he secured a patent in 1818. Briefly described, this device
consisted of an iron cylinder having at its front end an auger-like
cutter, whose revolution was intended to shove away the material ahead
and thus advance the cylinder. As the cylinder advanced the perimeter of
the hole behind was to be lined with a spiral sheet-iron plating, which
was to be strengthened with an interior lining of masonry. It will be
seen that the mechanical resemblance of this device to the ship-worm, on
which it is alleged to have been modeled, was remarkably close.
In the same patent in which Sir Isambard secured protection for his
mechanical ship-worm he claimed equal rights of invention for another
shield, which is of far greater importance in being the prototype of the
shield actually employed by him in constructing the first Thames tunnel.
This alternative invention, if it may be so termed, consisted of a group
of separate cells which could be advanced one or more at a time or all
together. The sides of these cells were to be provided with friction
rollers to enable them to slide easily upon each other; and it was also
specified that the preferable motive power for advancing the cells was
hydraulic jacks. To summarize briefly, therefore, the two inventions of
Brunel comprehended the protecting cylinder or shield, the closure of
the face of the excavation, the cellular division, the hydraulic-jack
propelling power, and cylindrical iron lining, which are the essential
characteristics of the modern shield system of tunneling. The next step
required was the actual proof of the practicability of Brunel’s
inventions, and this soon came.
Those who have read the history of the first Thames tunnel will recall
the early unsuccessful attempts at construction which had discouraged
English engineers. Five years after Brunel’s patent was secured a
company was formed to undertake the task again, the plan being to use
the shield system, under the personal direction of its inventor as chief
engineer. For this work Brunel selected the cellular shield mentioned as
an alternative construction in his original patent. He also chose to
make this shield rectangular in form. This choice is commonly accounted
for by the fact that the strata to be penetrated by the tunnel were
practically horizontal, and that it was assumed by the engineer that a
rectangular shield would for some reason best resist the pressures which
would be developed. Whatever the reason may have been for the choice,
the fact remains that a rectangular shield was adopted. The tunnel as
designed consisted of two parallel horseshoe tunnels, 13 ft. 9 ins. wide
and 16 ft. 4 ins. high and 1200 ft. long, separated from each other by a
wall 4 ft. thick, pierced by 64 arched openings of 4 ft. span, the whole
being surrounded with massive brickwork built to a rectangular section
measuring over all 38 ft. wide and 22 ft. high.
The first shield designed by Brunel for the work proved inadequate to
resist the pressures, and it was replaced by another somewhat larger
shield of substantially the same design, but of improved construction.
This last shield was 22 ft. 3 ins. high and 37 ft. 6 ins. wide. It was
divided vertically into twelve separate cast-iron frames placed close
side by side, and each frame was divided horizontally into three cells
capable of separate movement, but connected by a peculiar articulated
construction, which is indicated in a general way by Fig. 124. To close
or cover the face of the excavation, poling-boards held in place by
numerous small screw-jacks were employed. Each cell or each frame could
be advanced independently of the others, the power for this operation
being obtained by means of screw-jacks abutting against the completed
masonry lining. Briefly described, the mode of procedure was to remove
the poling-boards in front of the top cell of one frame, and excavate
the material ahead for about 6 ins. This being done, the top cell was
advanced 6 ins. by means of the screw-jacks, and the poling-boards were
replaced. The middle cell of the frame was then advanced 6 ins. by
repeating the same process, and finally the operation was duplicated for
the bottom cell. With the advance of the bottom cell one frame had been
pushed ahead 6 ins., and by a succession of such operations the other
eleven frames were advanced a distance of 6 ins., one after the other,
until the whole shield occupied a position 6 ins. in advance of that at
which work was begun. The next step was to fill the 6-in. space behind
the shield with a ring of brickwork.
[Illustration: FIG. 124.--Longitudinal Section of Brunel’s Shield, First
Thames Tunnel.]
The illustration, Fig. 124, is the section parallel to the vertical
plane of the tunnel through the center of one of the frames, and it
shows quite clearly the complicated details of the shield construction.
Two features which are to be particularly noted are the suspended
staging and centering for constructing the roof arch, and the top plate
of the shield extending back and overlapping the roof masonry so as to
close completely the roof of the excavation and prevent its falling.
Notwithstanding its complicated construction and unwieldy weight of 120
tons, this shield worked successfully, and during several months the
construction proceeded at the rate of 2 ft. every 24 hours. There were
two irruptions of water and mud from the river during the work, but the
apertures were effectually stopped by heaving bags of clay into the
holes in the river bed, and covering them over with tarpaulin, with a
layer of gravel over all. The tunnel was completed in 1843, at a cost of
about $5600 per lineal yard, and 20 years from the time work was first
commenced, including all delays.
[Illustration: FIG. 125.--First Shield Invented by Barlow.]
The next tunnel to be built by the shield system was the tunnel under
London Tower constructed by Barlow and Greathead and begun in 1869. In
1863 Mr. Peter W. Barlow secured a patent in England for a system of
tunnel construction comprising the use of a circular shield and a
cylindrical cast-iron lining. The shield, as shown by Fig. 125, was
simply an iron or steel plate cylinder. The cylinder plates were thinned
down in front to form a cutting edge, and they extended far enough back
at the rear to enable the advance ring of the cast-iron lining to be set
up within the cylinder. In simplicity of form this shield was much
superior to Brunel’s; but it seems very doubtful, since it had no
diametrical bracing of any sort, whether it would ever have withstood
the combined pressure of the screw-jacks and of the surrounding earth in
actual operation without serious distortion, and, probably, total
collapse. It should also be noted that Barlow’s shield made no provision
for protecting the face of the excavation, although the inventor did
state that if the soil made it necessary such a protection could be
used. The patent provided for the injection of liquid cement behind the
cast-iron lining to fill the annular space left by the advancing
tail-plates of the shield. Although Barlow made vigorous efforts to get
his shield used, it was not until 1868 that an opportunity presented
itself. In the meantime the inventor had been studying how to improve
his original device, and in 1868 he secured additional patents covering
these improvements. Briefly described, they consisted in partly closing
the shield with a diaphragm as shown by Fig. 126. The uninclosed portion
of the shield is here shown at the center, but the patent specified that
it might also be located below the center in the bottom part of the
shield. The idea of the construction was that in case of an irruption of
water the upper portion of the shield could be kept open by air
pressure, and work prosecuted in this open space until the shield had
been driven ahead sufficiently to close the aperture, when the normal
condition of affairs would be resumed. This was obviously an improvement
of real merit. The partial diaphragm also served to stiffen the shield
somewhat against collapse, but the thin plate cutting-edges and most of
the other structural weaknesses were left unaltered. To summarize
briefly the improvements due to Barlow’s work, we have: the construction
of the shield in a single piece; the use of compressed air and a partial
diaphragm for keeping the upper part of the shield open in case of
irruptions of water; and the injection of liquid cement to fill the
voids behind the lining.
[Illustration: ~Longitudinal Section.~
~Cross Section.~
FIG. 126.--Second Shield Invented by Barlow.]
Turning now to the London Tower tunnel work, it may first be noted that
Barlow found some difficulty in finding a contractor who was willing to
undertake the job, so little confidence had engineers generally in his
shield system. One man, however, Mr. J. H. Greathead, perceived that
Barlow’s device presented merit, although its design and construction
were defective, and he finally undertook the work and carried it to a
brilliant success. The tunnel was 1350 ft. long and 7 ft. in diameter,
and penetrated compact clay. Work was begun on the first shore shaft on
Feb. 12, 1869, and the tunnel was completed the following Christmas, or
in something short of eleven months, at a cost of £14,500.
The shield used was Barlow’s idea put into practical shape by Greathead.
It consisted of an iron cylinder, or, more properly, a frustum of a cone
whose circumferential sides were very slightly inclined to the axis, the
idea being that the friction would be less if the front end of the
shield were slightly larger than the rear end. The shell of the cone was
made of ¹⁄₂-in. plates. The thinned plate cutting-edge of Barlow’s
shield was replaced by Greathead with a circular ring of cast iron.
Greathead also altered the construction of the diaphragm by arranging
the angle stiffeners so that they ran horizontally and vertically, and
by fastening the diaphragm plates to an interior cast-iron ring
connected to the shell plates. This was a decided structural
improvement, but it was accompanied with another modification which was
quite as decided a retrogression from Barlow’s design. Greathead made
the diaphragm opening rectangular and to extend very nearly from the top
to the bottom of the shield, thus abandoning the element of safety
provided by Barlow in case of an irruption of water. Fortunately the
material penetrated by the shield for the Tower tunnel was so compact
that no trouble was had from water; but the dangerous character of the
construction was some years afterwards disastrously proven in driving
the Yarra River tunnel at Melbourne, Australia. To drive his shield
Greathead employed six 2¹⁄₂-in. screw-jacks capable of developing a
total force of 60 tons. The tails of the jack bore against the completed
lining, which consisted of cast-iron rings 18 ins. wide and ⁷⁄₈ in.
thick, each ring being made up of a crown piece and three segments. The
different segments and rings were provided with double (exterior and
interior) flanges, by means of which they were bolted together. The
soil behind the lining was filled with liquid cement injected through
small holes by means of a hand pump.
[Illustration: FIG. 127.--Shield Suggested by Greathead for the Proposed
North and South Woolwich Subway.]
[Illustration: FIG. 128.--Beach’s Shield Used on Broadway Pneumatic
Railway Tunnel.]
The remarkable success of the London Tower tunnel encouraged Barlow to
form in 1871 a company to tunnel the Thames between Southwark and the
City, and Greathead, in 1876, to project a tunnel under the same
waterway known as the North and South Woolwich Subway. Barlow’s
concession was abrogated by Parliament in 1873, without any work having
been done. Greathead progressed far enough with his enterprise to
construct a shield and a large amount of the iron lining when the
contractors abandoned the work. From the brief description of his shield
given by Greathead to the London Society of Civil Engineers, it
contained several important differences from the shield built by him for
the London Tower tunnel, as is shown by Fig. 127. The changes which
deserve particular notice are the great extension of the shield behind
the diaphragm, the curved form of the diaphragm, and the use of
hydraulic jacks. Greathead had also designed for this work a special
crane to be used in erecting the cast-iron segments of the lining.
[Illustration: FIG. 129.--Shield for City and South London Railway.]
While these works had been progressing in England, Mr. Beach, an
American, received a patent in the United States for a tunnel shield of
the construction shown by Fig. 128, which was first tried practically in
constructing a short length of tunnel under Broadway for the nearly
forgotten Broadway Pneumatic Underground Railway. This shield, as is
indicated by the illustration, consisted of a cylinder of wood with an
iron-cutting-edge and an iron tail-ring. Extending transversely across
the shield at the front end were a number of horizontal iron plates or
shelves with cutting-edges, as shown clearly by the drawing. The shield
was moved ahead by means of a number of hydraulic jacks supplied with
power by a hand pump attached to the shield. By means of suitable valves
all or any lesser number of these jacks could be operated, and by thus
regulating the action of the motive power the direction of the shield
could be altered at will. Work was abandoned on the Broadway tunnel in
1870. In 1871-2 Beach’s shield was used in building a short circular
tunnel 8 ft. in diameter in Cincinnati, and a little later it was
introduced into the Cleveland water-works tunnel 8 ft. in diameter. In
this latter work, which was through a very treacherous soil, the shield
gave a great deal of trouble, and was finally so flattened by the
pressures that it was abandoned. The obviously defective features of
this shield were its want of vertical bracing and the lack of any means
of closing the front in soft soil.
[Illustration: FIG. 130.--Shield for St. Clair River Tunnel.]
[Illustration: ~Longitudinal Section.~
~Cross Section.~
FIG. 131.--Shield for Blackwall Tunnel.]
With the foregoing brief review of the early development of the shield
system of tunneling, we have arrived at a point where the methods of
modern practice can be studied intelligently. In the pages which follow
we shall first illustrate fully the construction of a number of shields
of typical and special construction, and follow these illustrations with
a general discussion of present practice in the various details of
shield construction.
[Illustration: ~Transverse Section.~
~Longitudinal Section.~
FIG. 132.--Elliptical Shield for Clichy Sewer Tunnel, Paris.]
[Illustration: ~Longitudinal Section.~
~Cross Section.~
FIG. 133.--Semi-elliptical Shield for Clichy Sewer Tunnel.]
Mr. Raynald Légouez, in his excellent book upon the shield system of
tunneling, considers that tunnel shields may be divided into three
classes structurally, according to the character of the material which
they are designed to penetrate. In the first class he places shields
designed to work in a stiff and comparatively stable soil, like the
well-known London clay; in the second class are placed those constructed
to work in soft clays and silts; and in the third class those intended
for soils of an unstable granular nature. This classification will, in a
general way, be kept by the writer. As a representative shield of the
first class, the one designed for the City and South London Railway is
illustrated in Fig. 129. The shields for the London Tower tunnel, the
Waterloo and City Railway, the Glasgow District Subway, the Siphons of
Clichy and Concorde in Paris, and the Glasgow Port tunnel, are of the
same general design and construction. To represent shields of the second
class, the St. Clair River and Blackwall shields are shown in Figs. 130
and 131. The shields for the Mersey River, the Hudson River, and the
East River tunnels also belong to this class. To represent shields of
the third class, the elliptical and semi-elliptical shields of the
Clichy tunnel work in Paris are shown by Figs. 132 and 133. The
semi-circular shield of the Boston Subway is illustrated by Fig. 134.
[Illustration: ~Half Transverse Section A-B.~
~Half Rear-End Elevation.~
~Details of Casting Supporting Ends of Jacks.~
~Details of Castings under Ends of Girders.~
~Longitudinal Section C-D.~
FIG. 134.--Roof Shield for Boston Subway.]
=Prelini’s Shield.=--In closing this short review mention will be made
of a new shield designed and patented by the Author and shown in Fig.
135. It is an articulated shield composed of two separated shields whose
outer shells overlap each other. The shields are connected together by
means of hydraulic jacks attached all around the two diaphragms. Between
these diaphragms is a large inclosed space called a safety chamber,
where the men can withdraw in case of accidents and where the air can be
immediately raised to the required pressure. This is an advantage in
case of blow-outs, because the flooding of the tunnel is prevented,
while the accident is limited to only a few feet from the front. On
account of the shield being advanced half at a time it is always under
control and is thus better directed through grade and alignment.
Besides, this shield will not rotate around its axis and consequently it
can be built of any shape, thus permitting the construction of
subaqueous tunnels of any cross-section and even with a wider
foundation, which is impossible to-day with the ordinary rotating
shields of circular cross-section.
[Illustration: FIG. 135.--Transversal and Longitudinal Section of
Prelini’s Shield.]
SHIELD CONSTRUCTION.
=General Form.=--Tunnel shields are usually cylindrical or
semi-cylindrical in cross-section. The cylinder may be circular,
elliptical, or oval in section. Far the greater number of shields used
in the past have been circular cylinders; but in one part of the sewer
tunnel of Clichy, in Paris, an elliptical shield with its major axis
horizontal, was used, and the German engineer, Herr Mackensen, has
designed an oval shield, with its major axis vertical. A semi-elliptical
shield was employed on the Clichy tunnel, and semi-circular shields were
used on the Baltimore Belt Line tunnel and the Boston Subway in America.
Generally, also, tunnel shields are right cylinders; that is, the front
and rear edges are in vertical planes perpendicular to the axis of the
cylinder. Occasionally, however, they are oblique cylinders; that is,
the front or rear edges, or both, are in planes oblique to the axis of
the cylinder. One of these visor-shaped shields was employed on the
Clichy tunnel.