THE FULL SECTION METHOD OF TUNNELING: ENGLISH METHOD; AMERICAN METHOD;
AUSTRIAN METHOD.
ENGLISH METHOD.
The English method of tunneling through soft ground, as its name
implies, originated in England, where, owing to the general prevalence
of comparatively firm chalks, clays, shales, and sandstones, it has
gained unusual popularity. The distinctive characteristics of the method
are the excavation of the full section of the tunnel at once, the use of
longitudinal strutting, and the alternate execution of the masonry work
and excavation. In America the method is generally designated as the
longitudinal bar method, owing to the mode of strutting, which has
gained particular favor in America, and is commonly employed here even
when the mode of excavation is distinctively German or Belgian in other
respects.
[Illustration: FIG. 84.--Diagram Showing Sequence of Excavation in
English Method of Tunneling.]
=Excavation.=--Although, as stated above, the distinctive characteristic
of the English method is the excavation of the full section at once, the
digging is usually started by driving a small heading or drift to locate
and establish the axis of the tunnel, and to facilitate drainage in wet
ground. These advance galleries may be driven either in the upper or in
the lower part of the section, as the local conditions and choice of the
engineer dictate. Whether the advance gallery is located at the top or
at the bottom of the section makes no difference in the mode of
enlarging the profile. This work always begins at the upper part of the
section. A center top heading is driven and strutted by erecting posts
carrying longitudinal bars supporting transverse poling-boards. This
heading is immediately widened by digging away the earth at each side,
and by strutting the opening by temporary posts resting on blocking, and
carrying longitudinal bars supporting poling-boards. This process of
widening is continued in this manner until the full roof section, No. 1,
Fig. 84, is opened, when a heavy transverse sill is laid, and permanent
struts are erected from it to the longitudinal bars, the temporary posts
and blocking being removed. The excavation of part No. 2 then begins by
opening a center trench and widening it on each side, temporary posts
being erected to support the sill above. As soon as part No. 2 is fully
excavated, a second transverse sill is placed below the first, and
struts are placed between them. The excavation of part No. 3 is carried
out in exactly the same manner as was part No. 2. The lengths of the
various sections, Nos. 1, 2, and 3, generally run from 12 ft. to 20 ft.,
depending upon the character of the soil.
=Strutting.=--The strutting in the English method of tunneling consists
of a transverse framework set close to the face of the excavation, which
supports one end of the longitudinal crown bars, the other ends of which
rest on the completed lining. The transverse framework is composed of
three horizontal sills arranged and supported as shown by Fig. 85. The
bottom sill _A_ is carried by vertical posts resting on blocking on the
floor of the excavation. From the bottom sill vertical struts rise to
support the middle sill _B_. The top sill, or miners’ sill _C_, is
carried by vertical posts or struts rising from the middle sill _B_. The
vertical struts are usually round timbers from 6 ins. to 8 ins. in
diameter; and the sills are square timbers of sufficient section to
carry the vertical loads, and generally made up of two posts
scarf-jointed and butted to permit them to be more easily handled. In
firm soils the struts between the sills are all set vertically, but
those at the extreme sides of the roof section are inclined. In loose
soils, however, where the sides of the excavation must be shored, the
V-bracing shown by Fig. 85 is employed between one or more pairs of
sills as the conditions necessitate. The manner of holding the
transverse framework upright is explained quite clearly by Fig. 85;
inclined props extending from the completed masonry to the sills of the
framework being employed. Two props are used to each sill. Sometimes, in
addition to the props shown, another nearly horizontal prop extends from
the crown of the arch masonry to the middle piece of the strutting.
[Illustration: FIG. 85.--Sketches Showing Construction of Strutting,
English Method.]
Referring to Fig. 85, it will be observed that the longitudinal crown
bars are above the extrados of the roof arch. When, therefore, the
lining masonry has been completed close up to the transverse framework,
the latter is removed, leaving the crown bars resting on the arch
masonry; and excavation, which has been stopped while the masonry was
being laid, is continued for another 12 ft. to 20 ft., and the
transverse framework is erected at the face, and braced or propped
against the completed lining as shown by Fig. 85. The next step is to
place the crown bars, and this is done by pulling them ahead from their
original position over the masonry of the completed section of the roof
arch. It will be understood that the crown bars are not pulled ahead
their full length at one operation, but are advanced by successive short
movements as the excavation progresses, their outer ends being supported
by temporary posts until the transverse framework is built at the face
of the excavation.
=Centers.=--Two standard forms of centers are employed in the English
method of tunneling, as shown by Figs. 86 and 87. Both consist of an
outer portion, constructed much like a typical plank center, which is
strengthened against distortion by an interior truss framework. The
elemental members of this truss framework take the form of a queen-post
truss, as is shown more particularly by Fig. 86. In Fig. 87 the
queen-post truss construction is less easily distinguished, owing to the
cutting of the bottom tie-beam and other modifications, but it can still
be observed. The possibility of cutting the tie-beam as shown in Fig.
87, without danger, is due to the fact that the lateral pressures on the
haunches of the center counteract the tendency of the center to flatten
under load, which is usually counteracted by the tie-beam alone. The
object of cutting the tie-beam is to afford room for the props running
from the completed masonry to the transverse framework of the strutting
as shown by Fig. 85.
[Illustration: FIGS. 86 and 87.--Sketches of Typical Timber Roof-Arch
Centers, English Method.]
Generally four or five centers are used for each length of arch built.
They are set up so that the tie-beams rest on double opposite wedges
carried by a transverse beam below. This transverse beam in turn rests
on another transverse beam which is supported by posts carried on
blocking on the invert masonry. It is usually made with a butted joint
at the middle to permit its removal, since it is so long that the
masonry has to be built around its extreme ends. The lagging is of the
usual form, and rests on the exterior edges of the curved upper member
of the centers.
=Masonry.=--In the English method of tunneling, the masonry begins with
the construction of the invert, and proceeds to the crown of the arch.
The lining is built in lengths, or successive rings, corresponding to
the length of excavation, which, as previously stated, is from 12 ft. to
20 ft. Each ring or length of lining terminates close to the transverse
strutting frame erected at the face of the excavation. Work is first
begun on the invert at the point where the preceding ring of masonry
ends, and is continued to the transverse strutting frame at the front of
the excavation. As fast as the invert is completed, work is begun on the
side walls. In very loose soils the longitudinal bars supporting the
sides of the excavation are removed after the side walls are built; but
in firmer soils they may be taken out one by one just ahead of the
masonry, or in very firm soils it may be possible to remove them
entirely before beginning the side walls. In all cases it is necessary
to fill the space between the masonry and the walls of the excavation
with riprap or earth. To build the roof arch the centers are first
erected as described above, and the crown bars are removed as previously
described by pulling them ahead after the arch ring is completed. As
with the side walls, the vacant space between the arch ring and the roof
of the excavation must be filled in. Usually earth or small stones are
used for filling; but in very loose soils it is sometimes the practice
not to remove the poling-boards, but to support them by short brick
pillars resting on the arch ring and then to fill around these pillars.
=Hauling.=--To haul away the material and take in supplies, tracks are
laid on the invert masonry. Generally the permanent tracks are laid as
fast as the lining is completed. A short section of temporary track is
used to extend this permanent track close to the work of the advanced
drift.
=Advantages and Disadvantages.=--The great advantage of the English
method of tunneling is that the masonry lining is built in one piece
from the foundations to the crown, making possible a strong, homogeneous
construction. It also possesses a decided advantage because of the
simple methods of hauling which are possible: there being no differences
of level to surmount, no hoisting of cars nor trans-shipments of loads
are necessary. The chief disadvantage of the method is that the
excavators and masons work alternately, thus making the progress of the
work slower perhaps than in any other method of tunneling commonly
employed under similar conditions. This disadvantage is overcome to a
considerable extent when the tunnel is excavated by shafts, and the work
at the different headings is so arranged that the masons or excavators
when freed from duty at one heading may be transferred to another where
excavation or lining is to be done as the case may be. Another
disadvantage of the English method arises from the excavation of the
full section at once, which in unstable soils necessitates strong and
careful strutting, and increases the danger of caving. The fact also
that the arch ring has to carry the weight of the crown bars, and their
loading at one end while the masonry is green, increases the chances of
the arch being distorted.
=Conclusion.=--The English method of tunneling in its entirety is
confined in actual practice pretty closely to the country from which it
receives its name. A possible extension of its use more generally is
considered by many as likely to follow the development of a successful
excavating machine for soft material. The space afforded by the opening
of the full section at once, especially adapts the method to the use of
excavators like, for example, the endless chain bucket excavator used
on the Central London Ry., and illustrated in Fig. 11. The method also
furnishes an excellent opportunity for electric hauling and lighting
during construction.
The English method of tunneling has been used in building the Hoosac,
Musconetcong, Allegheny, Baltimore and Potomac, and other tunnels in
America. The names of the European tunnels built by this method are too
numerous to mention here.
AMERICAN METHOD.
In this country tunnels through loose soils are excavated according to
the “Crown Bar” or American Method. This consists in opening the whole
section of the tunnel before the construction of the lining as in the
English Method. It differs from the English method, however, in that
many timber structures are erected for the support of the roof, and that
the excavation and construction of the lining are far apart, so allowing
the miners and the masons to work continuously and without interfering
with each other.
[Illustration: FIG. 88.--Sequence of Excavation in the American Method.]
[Illustration: ~Section A-B.~
FIG. 89.--Strutting the Heading in the American Method.]
[Illustration: ~Section C-D.~
FIG. 90.--Temporary Timbering of the Roof in the American Method.]
[Illustration: ~Section E-F.~
FIG. 91.--Showing Crown Bars Supported by Segmental Arches.]
=Excavation.=--The diagram in Fig. 88 shows the sequence of excavation.
The work begins by driving a central heading usually 7 × 8 ft., strutted
by means of vertical or batter posts and cap-piece. Fig. 89,[11] the
props resting on foot blocks. Between the cap-pieces of the consecutive
frames are placed planks driven upward at a slightly inclined angle.
After the heading has been excavated and strutted, the floor is lowered
by removing the part marked 2 in the figure. The two batter posts
supporting the cap-piece are now substituted by two longer ones resting
on the floor of part 2 and abutting against longitudinal beams which
are inserted underneath the cap-pieces. These longitudinal beams are
called crown bars. The new batter posts are resting either on foot
blocks or sills according to the quality of soil and they are strongly
wedged to the crown bars. On each side of these crown bars are inserted
poling-boards or planks close to each other, which are driven downward.
The part marked 3 in the figure is removed by enlarging the cut 1 × 2 on
both sides. The plank, inserted above the crown bar, is driven in either
preceding or following the excavation and another crown bar is inserted
at the end of this plank. This second crown bar is supported by a prop
whose other end abuts against the foot of the rafter strutting the
heading. Between this crown bar and the roof of the excavation, other
planks are placed transversally to the axis of the tunnel and are driven
in until they are supported by a new crown bar, etc. The various props
supporting the crown bars are placed radially or in a fan-like manner,
similar to the characteristic arrangement of the timbering in the
Belgian method. Bracers to strengthen the timbering and the roof of the
excavation are inserted longitudinally between the various posts and
transversally between the crown bars, Fig. 90. As a rule, only three or
four of these radial structures are temporarily erected. A trench is
excavated at the side of the part marked 3 in the figure to receive the
wall plate which is a heavy timber laid on the floor parallel to the
longitudinal axis of the tunnel. On the wall plates are erected the
arched timber sets composed of five or seven segments of hewn timbers so
as to form a polygonal frame which is wedged to the crown bars and
which will support the arch of the roof. After one of these segmental
timber sets is erected the temporary radial structure is removed and the
upper section of the tunnel is cleared of any obstruction as the
pressures are transferred to the wall plates, Fig. 91. The bench marked
4 in the figure is taken away and the vertical props inserted under the
wall plates, Fig. 92.
[11] Figs. 89 to 91 are taken from a paper by S. W. Hopkins in
_Harvard Engineering Journal_, April, ’03, on the Fort George tunnel.
[Illustration: ~Section G-H.~
~Longitudinal Section.~
FIG. 92.--Transversal and Longitudinal Section of a Tunnel Excavated and
Strutted According to the American Method.]
=Strutting.=--The longitudinal strutting is used in connection with the
American method of tunneling. In fact, the strutting consists of a
series of longitudinal bars supporting planks laid transversally to the
axis of the tunnel and abutting against the roof of the excavation.
These crown bars during the excavations and immediately after are
temporarily supported by radial timbers forming almost a fan-like
structure, but this is soon substituted by a permanent one composed of a
polygonal timber frame of five or seven segments which are cut to
dimensions. The batter posts of the heading, the radial posts of the
temporary timber structure and the crown bars are all round timbers from
10 to 12 ins. in diameter. All the other timbers are square edged, the
usual dimensions being 10 × 10 ins. or 12 × 12 ins. with the exception
of the wall plates which are 14 × 14 ins. The dimensions of the various
members of the strutting and the distance apart of the different frames
vary with the quality of the soil. For instance, in ordinary loose
soils the frames are placed between 4 to 6 ft., but in very soft soils
they are erected only 3 or 3¹⁄₂ ft. apart.
Chiefly in the southwest, in tunnels excavated according to the American
method, the timbering has been left as regular lining and it was only
after many years when this temporary structure had decayed or was burned
down, that the tunnels were lined with masonry. But in many instances
the whole timber structure was left in place even when the tunnel was
lined with masonry immediately after the excavation had been made. This
was usually done when the tunnel was lined with concrete masonry. In
such a case the timbering was left to support the pressures of the roof
while the concrete was plastic and before it hardened.
=Centers.=--In the American method the whole section of the tunnel is
open before the construction of the lining, thus the masonry can be
built from the foundations up. The centers are designed so as to support
only the weight of the masonry during its construction and not the
pressures of the tunnel as in the other methods and consequently they
are of light construction. The centers described in the Murray Hill
tunnel, page 123, may be advantageously used in building the concrete
lining in tunnels through loose soils excavated by the American method.
=Hauling.=--The excavation of the heading and the upper section of the
tunnel is usually far ahead of the bench, consequently the hauling of
both the débris and the building materials is made at two different
levels, viz., on the bench and on the floor of the tunnel. When the face
of the heading and the excavation of the bench are not more than 50 ft.
apart, the hauling can be conveniently done on the tunnel floor, while
the materials and débris on the upper section of the tunnel are hauled
by wheelbarrows or light cars propelled by handpower. For a greater
distance, however, it is more convenient to use light cars running on
narrow-gauge tracks all through the tunnel. In this case the tracks on
the tunnel floor and on top of the bench are connected by means of an
inclined platform where the cars may ascend and descend without
interfering with the excavation of the bench. Here, as a rule, tunnels
have been excavated in soils considered good, generally through rock,
while loose soils have been encountered only in small sections. The same
method of excavation for whatever material is encountered is certainly
very convenient, as it affords a great regularity in the work; hence its
extensive use. A great disadvantage of this method is the double
strutting, viz., the polygonal and the longitudinal strutting succeeding
each other, whereas one of them could be easily spared. Another defect
is that it requires a larger amount of excavation, in case the strutting
is left in place.
AUSTRIAN METHOD.
The Austrian full-section method of tunneling through soft ground was
first used in constructing the Oberau tunnel on the Leipsic and Dresden
R.R., in Austria in 1837. It consists in excavating the full section and
building up the lining masonry from the foundations as in the English,
but with the important exception that the invert is built last instead
of first in all cases except where the presence of very loose soil
requires its construction first. A still more important difference in
the two methods is that the excavation is carried out in smaller
sections and is continuous in the Austrian method instead of alternating
with the mason work as it does in the English method.
=Excavation.=--The excavation in the Austrian method begins by driving
the bottom center drift No. 1, Fig. 93, rising from the floor of the
tunnel section nearly to the height of the springing lines of the roof
arch. When this drift has been driven ahead a distance varying from 12
ft. to 20 ft. or sometimes more, the excavation of the center top
heading No. 2 is driven for the same distance. The next operation is to
remove part No. 3, thus forming a central passage the full depth of the
tunnel section at the center. This trench is enlarged by removing parts
Nos. 4, 5, 6, 7, and 8 in the order named until the full section is
opened. A modification of this plan of excavation is shown by Fig. 94
which is used in firm soils.
[Illustration: FIGS. 93 and 94.--Diagrams Showing Sequence of Excavation
in Austrian Method of Tunneling.]
=Strutting.=--Each part of the section is strutted as fast as it is
excavated. The center bottom drift first excavated is strutted by laying
a transverse sill across the floor, raising two side posts from it, and
capping them with a transverse timber having its ends projecting beyond
the side posts and halved as shown by Fig. 95. The top center heading
No. 2, which is next excavated, is strutted by means of two side posts
resting on blocking and carrying a transverse cap as also shown by Fig.
95. Sometimes the side posts in the heading strutting-frames are also
carried on a transverse sill as are those of the bottom drift. This
construction is usually adopted in loose soils. When the sill is
employed, the middle part, No. 3, is strutted by inserting side posts
between the bottom of the top sill and the cap of the frame in the drift
below. When, however, the posts of the top heading frame are carried on
blocking, it is the practice to replace them with long posts rising from
the cap of the bottom drift frame to the cap of the top heading frame.
Further, when the intermediate sill is employed at the bottom level of
the top heading it projects beyond the side posts and has its ends
halved.
[Illustration: FIGS. 95 to 97.--Sketches Showing Construction of
Strutting, Austrian Method.]
After the completion of the center trench strutting the next task is to
strut parts Nos. 4 and 5. This is done by continuing the upper sill by
means of a timber having one end halved to join with the projecting end
of the sill in position. This extension timber is shown at _a_, Fig. 96.
The next operation is to place the timber _b_, having one end resting on
the cap-piece of the top heading frame and the other beveled and resting
on the top of the sill _a_ near the end. The timber _b_ is laid tangent
to the curve of the roof arch, and to support it against flexure the
strut _c_ is inserted as shown. To support the thrust of this strut the
additional post _d_ is inserted and the original bottom heading frame is
reinforced as shown. The next step is to insert the strut _e_, and when
this and the previous construction are duplicated on the opposite side
of the tunnel section we have the strutting of the parts Nos. 1 to 5;
inclusive, complete. Part No. 6 is then removed and strutted by
extending the bottom drift cap-piece by a timber similar to timber _a_
above, and then by inserting a side strut between the outer ends of
these two timbers, as indicated by Fig. 97. As the final parts. Nos. 7
and 8, are removed, the inclined prop _a_, Fig. 97, is inserted as
shown. When the soil is loose some of the members of the framework are
doubled and additional bracing is introduced as shown by Fig. 97.
The frames just described are placed at intervals of about 4 ft. along
the excavation, and are braced apart by horizontal struts. Some of the
longitudinal bearing beams, as at _b_, Fig. 97, also extend through two
or three frames, and help to tie them together. Finally, the
longitudinal poling-boards extending from one frame to the next along
the walls of the excavation serve to connect them together. The short
transverse beam _c_, Fig. 90, located just above the floor of the
invert, serves to carry the planking upon which the train car tracks are
laid. Besides the timber strutting peculiar to the Austrian method, the
Rziha iron strutting described in a previous chapter is frequently used
in tunneling by the Austrian process.
[Illustration: FIG. 98.--Sketch Showing Manner of Constructing the
Lining Masonry, Austrian Method.]
=Centers.=--The two forms of centers used in the English method of
tunneling are also used in the Austrian method. One of the methods of
supporting these centers is shown by Fig. 98. The tie-beam of the center
rests on longitudinal timbers carried by the strutting frames and
intermediate props. In single-track tunnels it is the frequent practice
also to carry the ends of the tie-beams in recesses left in the side
wall masonry, with intermediate props inserted to prevent flexure at
the center. When the Rziha iron strutting is employed, it also serves
for the centering upon which the arch masonry is built.
=Masonry.=--In the Austrian system of tunneling, the lining is built
from the foundations of the side walls upward to the crown of the roof
arch in lengths in consecutive rings equal to the lengths of the
consecutive openings of the full section, or from 12 ft. to 20 ft. long.
Except in infrequent cases in very loose materials the invert is the
last part of the masonry to be built, since to build it first requires
the removal of the strutting which cannot easily or safely be
accomplished until the side walls and roof arch are completed. As the
side wall foundations are built, however, their interior faces are left
inclined, as shown by Figs. 97 and 98, ready for the insertion of the
invert, and are meanwhile kept from sliding inward by the insertion of
blocking between them and the bottom of the strutting. Fig. 98 shows the
nature of this blocking, and also the manner in which the side wall and
roof arch masonry is carried upward. Finally when the roof arch is keyed
and the centers are struck, the strutting is taken down and the invert
is built.
=Advantages and Disadvantages.=--The principal advantages claimed for
the Austrian method of tunneling are: (1) The excavation being conducted
by driving a large number of consecutive small galleries, which are
immediately strutted, there is little disturbance of the surrounding
material; (2) the polygonal type of strutting adopted is easily erected
and of great strength against symmetrical pressures; (3) the masonry,
being built from the foundations up, is a single homogeneous structure,
and is thus better able to withstand dangerous pressures; (4) the
excavation is so conducted that the masons and excavators do not
interfere, and both can work at the same time. The disadvantages which
the method possesses are: (1) The strutting while very strong under
symmetrical pressures, either vertical or lateral, is distorted easily
by unsymmetrical vertical or lateral pressures, and by pressure in the
direction of the axis of the tunnel; (2) the construction of the invert
last exposes the side walls to the danger of being squeezed together,
causing a rotation of the arch of the nature discussed in describing the
Belgian method of tunneling.