SPECIAL TREACHEROUS GROUND METHOD; ITALIAN METHOD; QUICKSAND TUNNELING;
PILOT METHOD.
ITALIAN METHOD.
The Italian method of tunneling was first employed in constructing the
Cristina tunnel on the Foggia & Benevento R.R. in Italy. This tunnel
penetrated a laminated clay of the most treacherous character, and after
various other soft-ground methods of tunneling had been tried and had
failed, Mr. Procke, the engineer, devised and used successfully the
method which is now known as the Italian or Cristina method. The Italian
method is essentially a treacherous soil method. It consists in
excavating the bottom half of the section by means of several successive
drifts, and building the invert and side walls; the space is then
refilled and the upper half of the section is excavated, and the
remainder of the side walls and the roof arch are built; finally, the
earth filling in the lower half of the section is re-excavated and the
tunnel completed. The method is an expensive one, but it has proved
remarkably successful in treacherous soils such as those of the Apennine
Mountains, in which some of the most notable Italian tunnels are
located. It is, moreover, a single-track tunnel method, since any soil
which is so treacherous as to warrant its use is too treacherous to
permit an opening to be excavated of sufficient size for a double-track
railway, except by the use of shields.
=Excavation.=--The plan of excavation in the Italian method is shown by
the diagram Fig. 99. Work is begun by driving the center bottom heading
No. 1, and this is widened by taking out parts No. 2. Finally part No. 3
is removed, and the lower half of the section is open. As soon as the
invert and side wall masonry has been built in this excavation, parts
No. 2 are filled in again with earth. The excavation of the center top
heading No. 4 is then begun, and is enlarged by removing the earth of
part No. 5. The faces of this last part are inclined so as to reduce
their tendency to slide, and to permit of a greater number of radial
struts to be placed. Next, parts No. 6 are excavated, and when this is
done the entire section, except for the thin strip No. 7, has been
opened. At the ends of part No. 7 narrow trenches are sunk to reach the
tops of the side walls already constructed in the lower half of the
section. The masonry is then completed for the upper half of the
section, and part No. 7 and the filling in parts No. 2 are removed. The
various drifts and headings and the parts excavated to enlarge them are
seldom excavated more than from 6 ft. to 10 ft. ahead of the lining.
[Illustration: FIG. 99.--Diagram Showing Sequence of Excavation in
Italian Method of Tunneling.]
[Illustration: FIG. 100.--Sketch Showing Strutting for Lower Part of
Section.]
=Strutting.=--The bottom center drift, which is first driven, is
strutted by means of frames consisting of side posts resting on floor
blocks and carrying a cap-piece. Poling-boards are placed around the
walls, stretching from one frame to the next. As soon as the invert is
sufficiently completed to permit it, the side posts of the strutting
frames are replaced by short struts resting on the invert masonry as
shown by Fig. 100. To permit the old side posts to be removed and the
new shorter ones to be inserted, the cap-piece of the frame is
temporarily supported by inclined props arranged as shown by Fig. 103.
When parts No. 2 are excavated the roof is strutted by inserting the
transverse caps _a_, Fig. 100, the outer ends of which are carried by
the system of struts _b_, _c_, _d_, and _e_. The longitudinal
poling-boards supporting the ceiling and walls are held in place by the
cap _a_ and the side timber _e_. To stiffen the frames longitudinally of
the tunnel, horizontal longitudinal struts are inserted between them.
The excavation of the upper half of the tunnel section is strutted as in
the Belgian method, with radial struts carrying longitudinal roof bars
and transverse poling-boards. On account of the enormous pressures
developed by the treacherous soils in which only is the Italian method
employed, the radial strutting frames and crown bars must be of great
strength, while the successive frames must be placed at frequent
intervals, usually not more than 3 ft. After the masonry side walls have
been built in the lower part of the excavation, longitudinal planks are
laid against the side posts of the center bottom drift frames, to form
an enclosure for the filling-in of parts No. 2. The object of this
filling is principally to prevent the squeezing-in of the side walls.
[Illustration: FIGS. 101 and 101A.--Sketches Showing Construction of
Centers, Italian Method.]
=Centers.=--Owing to the great pressures to be resisted in the
treacherous soils in which the Italian method is used, the construction
of the centers has to be very strong and rigid. Figs. 101 and 101A show
two common types of center construction used with this method. The
construction shown in Fig. 101 is a strong one where only pressures
normal to the axis of the tunnel have to be withstood, but it is likely
to twist under pressures parallel to the axis of the tunnel. In the
construction shown by Fig. 101A, special provision is made to resist
pressures normal to the plane of the center or twisting pressures, by
the strength of the transverse bracing extending horizontally across the
center.
[Illustration: FIG. 102.--Sketch Showing Invert and Foundation Masonry,
Italian Method.]
=Masonry.=--The construction of the masonry lining begins with the
invert, as indicated by Fig. 100, and is carried up to the roof of parts
No. 2, as already indicated, and is then discontinued until the upper
parts Nos. 4, 5, and 6 are excavated. The next step is to sink side
trenches at the ends of part No. 7, which reach to the top of the
completed side walls. This operation leaves the way clear to finish the
side walls and to construct the roof arch in the ordinary manner of such
work in tunneling. Since this method of tunneling is used only in very
soft ground which yields under load, the usual practice is to construct
the invert and side walls on a continuous foundation course of concrete
as indicated by Fig. 102. The lining is usually built in successive
rings, and the usual precautions are taken with respect to filling in
the voids behind the lining. The thickness of the lining is based upon
the figures for laminated clay of the third variety given in Table II.
=Hauling.=--The system of hauling adopted with this method of tunneling
is very simple, since the excavation of the various parts is driven only
from 6 ft. to 10 ft. ahead, and the work progresses slowly to allow for
the construction of the heavy strutting required. To take away the
material from the center bottom drift, narrow-gauge tracks carried by
cross-beams between the side posts above the floor line are employed.
This same narrow-gauge line is employed to take away a portion of parts
No. 2, the remaining portion being left and used for the refilling after
the bottom portion of the lining has been built, as previously
described. The upper half of the section being excavated, as in the
Belgian method, the system of hauling with inclined planes to the tunnel
floor below, which is a characteristic of that method, may be employed.
It is the more usual practice, however, since the excavation is carried
so little a distance ahead and progresses so slowly, to handle the spoil
from the upper part of the section by wheelbarrows which dump it into
the cars running on the tunnel floor below. Hand labor is also used to
raise the construction materials used in building the upper section. The
tracks on the tunnel floor, besides extending to the front of the
advanced bottom center drift, have right and left switches to be
employed in removing the refilling in parts No. 2, the spoil from the
upper part of the section, and the material of part No. 7. Fig. 103 is a
longitudinal section showing the plan of excavation and strutting
adopted with the Italian method.
[Illustration: FIG. 103.--Sketch Showing Longitudinal Section of a
Tunnel under Construction, Italian Method.]
=Modifications.=--It often happens that the filling placed between the
side walls and the planking, which is practically the space comprised by
parts No. 2, is not sufficient to resist the inward pressure of the
walls, and they tip inward. In these cases a common expedient is to
substitute for the earth filling a temporary masonry arch sprung
between the side walls with its feet near the bottom of the walls, and
its crown just below the level of their tops, as shown by Fig. 107. This
construction was employed in the Stazza tunnel in Italy. In this tunnel
the excavation was begun by driving the center drift, No. 1, Fig. 104,
and immediately strutting it as shown by Fig. 105. The other parts, Nos.
2 and 3, completing the lower portion of the section, were then taken
out and strutted. While part No. 2 was being excavated at the bottom,
and the center part of the invert built, the longitudinal crown bars
carrying the roof of the excavation were carried temporarily by the
inclined props shown by Fig. 106. After completing the invert and the
side walls to a height of 2 or 3 ft., a thick masonry arch was sprung
between the side walls, as shown in transverse section by Fig. 107, and
in longitudinal section by Fig. 106. This arch braced the side walls
against tipping inward, and carried short struts to support the crown
bars. The haunches of the arch were also filled in with rammed earth.
The upper half of the section was excavated, strutted, and lined as in
the standard Italian method previously described. When the lining was
completed, the arch inserted between the side walls was broken down and
removed.
[Illustration: FIG. 104.--Sketch Showing Sequence of Excavation, Stazza
Tunnel.]
[Illustration: FIG. 105.--Sketch Showing Method of Strutting First
Drift, Stazza Tunnel.]
[Illustration: FIGS. 106 and 107.--Sketches Showing Temporary Strutting
Arch Construction, Stazza Tunnel.]
=Advantages and Disadvantages.=--The great advantage claimed for the
Italian method of tunneling is that it is built in two separate parts,
each of which is separately excavated, strutted, and lined, and thus can
be employed successfully in very treacherous soils. Its chief
disadvantage is its excessive cost, which limits its use to tunnels
through treacherous soils where other methods of timbering cannot be
used.
QUICKSAND TUNNELING.
When an underground stream of water passes with force through a bed of
sand it produces the phenomenon known as quicksand. This phenomenon is
due to the fineness of the particles of sand and to the force of the
water, and its activity is directly proportional to them. When sand is
confined it furnishes a good foundation bed, since it is practically
incompressible. To work successfully in quicksand, therefore, it is
necessary to drain it and to confine the particles of sand so that they
cannot flow away with the water. This observation suggests the mode of
procedure adopted in excavating tunnels through quicksand, which is to
drain the tunnel section by opening a gallery at its bottom to collect
and carry away the water, and to prevent the movement or flowing of the
sand by strutting the sides of the excavation with a tight planking.
The sand having to be drained and confined as described, the ordinary
methods of soft-ground tunneling must be employed, with the following
modifications:
(1) The first work to be performed is to open a bottom gallery to drain
the tunnel. This gallery should be lined with boards laid close and
braced sufficiently by interior frames to prevent distortion of the
lining. The interstices or seams between the lining boards should be
packed with straw so as to permit the percolation of water and yet
prevent the movement of the sand.
(2) As fast as the excavation progresses its walls should be strutted
by planks laid close, and held in position by interior framework; the
seams between the plank should be packed with straw.
(3) The masonry lining should be built in successive rings, and the work
so arranged that the water seeping in at the sides and roof is collected
and removed from the tunnel immediately.
=Excavation.=--The best and most commonly employed method of driving
tunnels through quicksand is a modification of the Belgian method. At
first sight it may appear a hazardous work to support the roof arch, as
is the characteristic of this method, on the unexcavated soil below,
when this soil is quicksand, but if the sand is well confined and
drained the risk is really not very great. Next to the Belgian method
the German method is perhaps the best for tunneling quicksand. In these
comparisons the shield system of tunneling is for the time being left
out of consideration. This method will be described in succeeding
chapters. Whenever any of the systems of tunneling previously described
are employed, the first task is always to open a drainage gallery at the
bottom of the section.
Assuming the Belgian method is to be the one adopted, the first work is
to drive a center bottom drift, the floor of which is at the level of
the extrados of the invert. This drift is immediately strutted by
successive transverse frames made up of a sill, side posts, and a cap
which support a close plank strutting or lining, with its joints packed
with straw. Between the side posts of each cross-frame, at about the
height of the intrados of the invert, a cross-beam is placed; and on
these cross-beams a plank flooring is laid, which divides the drift
horizontally into two sections, as shown by Fig. 108; the lower section
forming a covered drain for the seepage water, and the upper providing a
passageway for workmen and cars. The bottom drift is driven as far ahead
as practicable, in order to drain the sand for as great a distance in
advance of the work as possible. After the construction of the bottom
drainage drift the excavation proper is begun, as it ordinarily is in
the Belgian method by driving a top center heading, as shown by Fig.
108. This heading is deepened and widened after the manner usual to the
Belgian method, until the top of the section is open down to the
springing lines of the roof arch. To collect the seepage water from the
center top heading it is provided with a center bottom drain constructed
like the drain in the bottom drift, as shown by Fig. 108. When the top
heading is deepened to the level of the springing lines of the roof
arch, its bottom drain is reconstructed at the new level, and serves to
drain the full top section opened for the construction of the roof arch.
This top drain is usually constructed to empty into the drain in the
bottom drift.
[Illustration: FIG. 108.--Sketch Showing Preliminary Drainage Galleries,
Quicksand Method.]
[Illustration: FIG. 109.--Sketch Showing Construction of Roof Strutting,
Quicksand Method.]
=Strutting.=--The method of strutting the bottom drift has already been
described. For the remainder of the excavation the regular Belgian
method of radial roof strutting-frames is employed, as shown by Fig.
109. Contrary to what might be expected, the number of radial struts
required is not usually greater than would be used in many other soils
besides quicksand. Single-track railway tunnels have been constructed
through quicksand in several instances where the number of radial props
required on each side of the center did not exceed four or five. It is
necessary, however, to place the poling-boards very close together, and
to pack the joints between them to prevent the inflow of the fine sand.
In strutting the lower part of the section it is also necessary to
support the sides with tight planking. This is usually held in place by
longitudinal bars braced by short struts against the inclined props
employed to carry the roof arch when the material on which they
originally rested is removed. This side strutting is shown at the right
hand of Fig. 110.
[Illustration: FIG. 110.--Sketch Showing Construction of Masonry Lining,
Quicksand Method.]
=Masonry.=--As soon as the upper part of the section has been opened the
roof arch is built with its feet resting on planks laid on the
unexcavated material below. This arch is built exactly as in the regular
Belgian method previously described, using the same forms of centers and
the same methods throughout, except that the poling-boards of the
strutting are usually left remaining above the arch masonry. To prevent
the possibility of water percolating through the arch masonry, many
engineers also advise the plastering of the extrados of the arch with a
layer of cement mortar. This plastering is designed to lead the water
along the haunches of the arch and down behind the side walls. In
constructing the masonry below the roof arch the invert is built first,
contrary to the regular Belgian method, and the side walls are carried
up on each side from the invert masonry. Seepage holes are left in the
invert masonry, and also in the side walls just above the intrados of
the invert. At the center of the invert a culvert or drain is
constructed, as shown by Fig. 110, inside the invert masonry. This
culvert is commonly made with an elliptical section with its major axis
horizontal, and having openings at frequent intervals at its top. The
thickness of the lining masonry required in quicksand is shown by Table
II.
=Removing the Seepage Water.=--After the tunnel is completed the water
which seeps in through the weep-holes left in the masonry passes out of
the tunnel, following the direction of the descending grades. During
construction, however, special means will have to be provided for
removing the water from the excavation, their character depending upon
the method of excavation and upon the grades of the tunnel bottom. When
the excavation is carried on from the entrances only, unless the tunnel
has a descending grade from the center toward each end, the tunnel floor
in one heading will be below the level of the entrance, or, in other
words, the descending grade will be toward the point where work is going
on, while at the opposite entrance the grade will be descending from the
work. In the latter case the removal of the seepage water is easily
accomplished by means of a drainage channel along the bottom of the
excavation. In the former case the water which drains toward the front
is collected in a sump, and if there is not too great a difference in
level between this sump and the entrance, a siphon may be used to remove
it. Where the siphon cannot be used, pumps are installed to remove the
water. When the tunnel is excavated by shafts the condition of one high
and one low front, as compared with the level at the shaft, is had at
each shaft. Generally, therefore, a sump is constructed at the bottom of
the shaft; the culvert from the high front drains directly to the shaft
sump, while the water from the low-front sump is either siphoned or
pumped to the shaft sump. From the shaft sump the water is forced up the
shaft to the surface by pumps.
THE PILOT METHOD.
The pilot system of tunneling has been successfully employed in
constructing soft-ground sewer tunnels in America by the firm of
Anderson & Barr, which controls the patents. The most important work on
which the system has been employed is the main relief sewer tunnel built
in Brooklyn, N.Y., in 1892. This work comprised 800 ft. of circular
tunnel 15 ft. in diameter, 4400 ft. 14 ft. in diameter, 3200 ft. 12 ft.
in diameter, and 1000 ft. 10 ft. in diameter, or 9400 ft. of tunnel
altogether. The method of construction by the pilot system is as
follows:
Shafts large enough for the proper conveyance of materials from and into
the tunnel are sunk at such places on the line of work as are most
convenient for the purpose. From these shafts a small tunnel,
technically a pilot, about 6 ft. in diameter, composed of rolled boiler
iron plates riveted to light angle irons on four sides, perforated for
bolts, and bent to the required radius of the pilot, is built into the
central part of the excavation on the axis of the tunnel. This pilot is
generally kept about 30 ft. in advance of the completed excavation, as
shown by Fig. 111. The material around the exterior of the pilot is then
excavated, using the pilot as a support for braces which radiate from it
and secure in position the plates of the outside shell which holds the
sand, gravel, or other material in place until the concentric rings of
brick masonry are built. Ribs of T-iron bent to the radius of the
interior of the brick work, and supported by the braces radiating from
the pilot, are used as centering supports for the masonry. On these ribs
narrow lagging-boards are laid as the construction of the arch proceeds,
the braces holding the shell plates and the superincumbent mass being
removed as the masonry progresses. The key bricks of the arches are
placed in position on ingeniously contrived key-boards, about 12 ins. in
width, which are fitted into rabbeted lagging-boards one after another
as the key bricks are laid in place. After the masonry has been in place
at least twenty-four hours, allowing the cement mortar time to set, the
braces, ribs, and lagging which support it are removed. In the meantime
the excavation, bracing, pilot, and exterior shell have been carried
forward, preparing the way for more masonry. The top plates of the shell
are first placed in position, the material being excavated in advance
and supported by light poling-boards; then the side-plates are butted to
the top and the adjoining side-plates. In the pilot the plates are
united continuously around the perimeter of the circle, while in the
exterior shell the plates are used for about one-third of the perimeter
on top, unless treacherous material is encountered, when the plates are
continued down to the springing lines of the arch. This iron lining is
left in place. The bottom is excavated so as to conform to the exterior
lines of the masonry. The excavation follows so closely to the outer
lines of the normal section of the tunnel that very little loss occurs,
even in bad material; and there is no loss where sufficient bond exists
in the material to hold it in place until the poling-boards are in
position.
[Illustration: ~Bracing.~
~Arch Construction.~
~Longitudinal Section.~
FIG. 111.--Sketch Showing Pilot Method of Tunneling.]
In the Brooklyn sewer tunnel work, previously mentioned, the pilot was
built of steel plates ³⁄₈ in. thick, 12 ins. wide, and 37¹⁄₂ ins. long,
rolled to a radius of 3 ft. Steel angles 4 × 4¹⁄₂ ins. were riveted
along all four sides of each plate, and the plates were bolted together
by ³⁄₄-in. machine-bolts. The plates weighed 136 lbs. each, and six of
them were required to make one complete ring 6 ft. in diameter. In
bolting them together, iron shims were placed between the horizontal
joints to form a footing for the wooden braces for the shell, which
radiate from the pilot. The shell plates of the 15-ft. section of the
tunnel were of No. 10 steel 12 ins. wide and 37 ins. long, with steel
angles 2¹⁄₂ × 2¹⁄₂ × ³⁄₈ ins., riveted around the edges the same as for
the pilot, and put together with ⁵⁄₈-in. bolts. These plates weighed 61
lbs. each, and eighteen of them were required to make one complete ring
15 ft. in diameter. The plates for the 12-ft. section were No. 12 steel
12 ins. wide with 2 × 2 × ¹⁄₄-in. angles. Seventeen plates were required
to make a complete ring.