THE GERMAN METHOD--EXCAVATING TUNNELS THROUGH SOFT GROUND (Continued);
BALTIMORE BELT LINE TUNNEL.
The German method of tunneling was first used in 1803 in constructing
the St. Quentin Canal. In 1837 the Königsdorf tunnel of the Cologne and
Aix la Chapelle R.R. was excavated by the same method. The success of
the method in these two difficult pieces of soft-ground tunneling led to
its extensive adoption throughout Germany, and for this reason it
gradually came to be designated as the German method. Briefly explained
the method consists in excavating first an annular gallery in which the
side walls and roof arch are built complete before taking out the center
core and building the invert.
[Illustration: FIG. 76.--Diagrams Showing Sequence of Excavation in
German Method of Tunneling.]
=Excavation.=--The excavation of tunnels by the German method is begun
either by driving two bottom side drifts or by driving a center top
heading. Fig. 76 shows the mode of procedure when bottom side drifts are
used to start the work. The two side drifts No. 1 are made from 7 ft. to
8 ft. wide, and about one-third the total height of the full section;
the width of each heading has to be sufficient for the construction of
the masonry and strutting, and for the passage of narrow spoil cars
alongside them. These drifts are increased in height to the springing
line of the arch by taking out the two drifts No. 2. Next the top center
heading No. 3 is driven, and finally the two haunch headings No. 4 are
excavated. The center core No. 5 is utilized to support the strutting
until the side walls and roof arch are completed, when it is broken down
and removed. In case of very loose material, where the first side drifts
cannot be carried as high as one-third the height of the section, it is
the common practice to make them about one-fourth the height, and to
take out the side portions of the annular gallery in three parts, as
shown by Fig. 76.
[Illustration: FIG. 77.--Diagram Showing Sequence of Excavations in
Water Bearing Material, German Method.]
The top center heading plan of commencing the excavation is usually
employed in firm materials or when a vein of water is encountered in the
upper part of the section. In the latter contingency a small bottom
drift _A_, Fig. 77, is first driven to serve as a drain; but in any case
the excavation proper of the tunnel consists in first driving the center
top heading No. 1, and then by working both ways along the profile
parts, Nos. 2, 3, 4, and 5 are removed. Part No. 6 is left to support
the strutting until the side walls and roof arch are built, when it is
also excavated.
=Strutting.=--When the excavation is begun by bottom side drifts these
drifts are strutted by erecting vertical posts close against the sides
of the drift and placing a cap-piece transversely across the roof of the
drift. The side posts are usually supported by sills placed across the
bottom of the drift. These frameworks of posts, cap, and sill are
erected at short intervals, and the roof, and, if necessary, the sides
of the drift between them, are sustained by means of longitudinal
poling-boards extending from one frame to the next. The cap-pieces of
the strutting for the bottom drifts serve as sills for the exactly
similar strutting of the heading next above. To support the additional
weight, and to allow the construction of the side walls, the strutting
of the bottom drifts is strengthened by inserting an intermediate post
between the original side posts of each frame. These intermediate posts
are not inserted at the center of the frames or bents, but close to the
wall masonry line as shown by Fig. 78. This eccentric position of the
post avoids any interference with the hauling, and also allows the
removal of the adjacent side post when the masonry is constructed.
[Illustration: FIG. 78.--Sketch Showing Work of Excavating and Timbering
Drifts and Headings.]
[Illustration: FIG. 79.--Sketch Showing Method of Roof Strutting.]
Two methods of strutting the soffit of the excavation are employed, one
being a modification of the longitudinal system employed in the English
method of tunneling described in a succeeding chapter, and the other a
modification of the Belgian system previously described. Fig. 79 shows
the method of employing the radial strutting of the Belgian system. At
the beginning the center top heading is strutted with rectangular bents
such as are employed for strutting the drifts. As this heading is
enlarged by taking out the haunch sections, radial posts are inserted,
as shown by Fig. 79, which also indicates the method of strutting the
side trenches when the excavation is carried downward from the center
top heading instead of upward from bottom side drifts.
=Masonry.=--Whatever plan of excavation or strutting is employed, the
construction of the masonry lining in the German method of tunneling
begins at the foundations of the side walls and is carried upward to the
roof arch. The invert, if one is required, is built after the center
core of earth is removed.
=Centering.=--Tunnel centers are generally employed in the German method
of tunneling, a common construction being shown by Fig. 80. It is
essentially a queen-post truss, the tie beam of which rests on a
transverse sill as shown by the illustration. The transverse sill is
supported along its central portion by the unexcavated center core of
earth, and at its ends either directly on the vertical posts or on
longitudinal beams resting on these posts. The diagonal members of the
queen-post truss form the bottom chords of small king-post trusses which
are employed to build out the exterior member of the center to a closer
approximation to the curve of the arch.
[Illustration: FIG. 80.--Sketch Showing Roof Arch Centers and Arch
Construction.]
=Hauling.=--When the bottom side drift plan of excavation is employed,
the spoil from the front of the drift is removed in narrow-gauge cars
running on a track laid as close as practicable to the center core.
These same cars are also employed to take the spoil from the drifts
above, through holes left in the ceiling strutting of the bottom drifts.
The spoil from the soffit sections may be removed by the same car lines
used in excavating the drifts, or a narrow-gauge track may be laid on
the top of the center core for this special purpose. In the latter case
the soffit tracks are usually connected by means of inclined planes
with the tracks on the bottoms of the side drifts. Generally, however,
the separate soffit car line is not used unless the material is of such
a firm character that the headings and drifts can be carried a great
distance ahead of the masonry work. With the center top heading plan of
beginning the excavation, the car track has, of course, to be laid on
the top of the center core. The center core itself is removed by means
of car tracks along the floor of the completed tunnel.
=Advantages and Disadvantages.=--Like the Belgian method of tunneling,
the German method has its advantages and disadvantages. Since the
excavation consists at first of a narrow annular gallery only, the
equilibrium of the earth is not greatly disturbed, and the strutting
does not need to be so heavy as in methods where the opening is much
larger. The undisturbed center core also furnishes an excellent support
for the strutting, and for the centers upon which the roof arches are
built. Another important advantage of the method is that the
construction of the masonry lining is begun logically at the bottom, and
progresses upward, and a more homogeneous and stable construction is
possible. The great disadvantage of the method is the small space in
which the hauling has to be done. The spoil cars practically fill the
narrow drifts in passing to and from the front, and interfere greatly
with the work of the carpenters and masons. Another objection to the
method is that the invert is the very last portion of the lining to be
built. This may not be a serious objection in reasonably compact and
stable materials, but in very loose soils there is always the danger of
the side walls being squeezed together before the invert masonry is in
position to hold them apart. Altogether the difficulties are of a
character which tend to increase the expense of the method, and this is
the reason why to-day it is seldom used even in the country where it was
first developed, and for some time extensively employed. For repairing
accidents, such as the caving in of completed tunnels, the German method
of tunneling is frequently used, because of the ease with which the
timbering is accomplished. In such cases the cost of the method used
cuts a small figure, so long as it is safe and expeditious.
BALTIMORE BELT LINE TUNNEL.
In the last few years a modification of the German method was used in
this country for the construction of several railroad tunnels. The
modification consists in excavating the two-side drifts up to the
springing line of the arch of the proposed tunnel. Then a central
heading, which is afterward enlarged to the whole section of the tunnel,
is excavated close to the crown. At the same time the masonry is
constructed from the foundation up in the side drifts. From the floor of
the upper section already excavated and strutted, the top of the masonry
of the drifts is reached by means of small side cuts; thus the lining is
made continuous up to the keystone. The central nucleus or bench is
removed after the tunnel has been lined.
The most important tunnel excavated by this method was the Baltimore
Belt Line tunnel described as follows:
The Baltimore Belt Ry. Co. was organized in 1890 by officials of the
Baltimore & Ohio, and Western Maryland railways, and Baltimore
Capitalists, to build 7 miles of double track railway, mostly within the
city limits of Baltimore. This railway was partly open cut and
embankment, and partly tunnel, and its object was to afford the
companies named facilities for reaching the center of the city with
their passengers and freight. To carry out the work the Maryland
Construction Co. was organized by the parties interested, and in
September, 1890, this company let the contract for construction to Ryan
& McDonald of Baltimore, Md. The chief difficulties of the work centered
in the construction of the Howard-street tunnel, 8350 ft. long, running
underneath the principal business section of the city.
=Material Penetrated.=--The soil penetrated by the tunnel was of almost
all kinds and consistencies, but was chiefly sand of varying degrees of
fineness penetrated by seams of loam, clay, and gravel. Some of the
clay was so hard and tough that it could not be removed except by
blasting. Rock was also found in a few places. For the most part,
however, the work was through soft ground, furnishing more or less
water, which necessitated unusual precautions to avoid the settling of
the street, and consequent damage to the buildings along the line. A
large quantity of water was encountered. Generally this water could be
removed by drainage and pumps, and the earth be prevented from washing
in by packing the space between the timbering with hay or other
materials. At points where the inflow was greatest, and the earth was
washed in despite the hay packing, the method was adopted of driving
6-in. perforated pipes into the sides of the excavation, and forcing
cement grout through them into the soil to solidify it. These pipes
penetrated the ground about 10 ft., and the method proved very efficient
in preventing the inflow of water.
=Excavation.=--The excavation was carried out according to the German
method of tunneling. Bottom side drifts were first driven, and then
heightened to the springing line of the roof arch. Next a center top
heading was driven, and the haunch sections taken out. The object of
beginning the excavations by bottom side drifts, was to drain the soil
of the upper part of the section. The center core was removed after the
side walls and roof arch were completed, its removal being kept from 50
ft. to 75 ft. to the rear of the advanced heading. The dimensions of the
side drifts proper were about 8 × 8 ft., but they were often carried
down much below the floor level to secure a solid foundation bed for the
side walls.
=Strutting.=--The side drifts were strutted by means of frames composed
of two batter posts resting on boards, and having a cap-piece extending
transversely across the roof of the drift. These frames were spaced
about 4 ft. apart. The excavation was advanced in the usual way by
driving poling-boards at the top and sides, with a slight outward and
upward inclination, so that the next frame could be easily inserted
leaving space enough between it and the sheeting to permit the next set
of poling-boards to be inserted. These poling-boards were driven as
close together as practicable so as to prevent as much as possible the
inflow of water and earth.
[Illustration: FIG. 81.--Sketch Showing Method of Excavating and
Strutting Baltimore Belt Line Tunnel.]
The center top heading was strutted in the same manner as were the side
drifts. The arrangement of the strutting employed in enlarging the
center top heading is shown clearly by Fig. 81, which also shows the
manner of strutting the side drifts and face of the excavation, and of
building the masonry.
=Centers.=--Both wood and iron centers were employed in building the
roof arch. The timber centering was constructed of square timbers, as
shown by Fig. 82. This construction of the iron centers is shown by Fig.
83. Each of the iron centers consisted of two 6 × 6 in. angles butted
together, and bent into the form of an arch rib. Six of these ribs were
set up 4 ft. apart. They were made of two half ribs butted together at
the crown, and were held erect and the proper distance apart by spacing
rods. The rearmost rib was held fast to the completed arch masonry, and
in turn supported the forward ribs while the lagging was being placed.
[Illustration: FIG. 82.--Roof Arch Construction with Timber Centers,
Baltimore Belt Line Tunnel.]
=Masonry.=--The side walls of the lining were built first in the bottom
side drifts, as shown by Fig. 81. They were generally placed on a
foundation of concrete, from 1 ft. to 2 ft. thick. As a rule the side
walls were not built more than 20 ft. in advance of the arch, but
occasionally this distance was increased to as much as 90 ft. The roof
arch consisted ordinarily of five rings of brick, but at some places in
especially unstable soil eight rings of brick were employed. The arch
was built in concentric sections about 18 ft. in length. All the timber
of the strutting above the arch and outside of the side walls was left
in place, and the voids were filled with rubble masonry laid in cement
mortar. It required about 125 mason hours to build an 18-ft. arch
section. Figs. 82 and 83 show various details of the masonry arch work.
[Illustration: FIG. 83.--Roof Arch Construction with Iron Centers,
Baltimore Belt Line Tunnel.]
Owing to the very unstable character of the soil, considerable
difficulty was experienced in building the masonry invert. The process
adopted was as follows: Two parallel 12 × 12 in. timbers were first
placed transversely across the tunnel, abutting against longitudinal
timbers or wedges resting against the side walls. Short sheet piles were
then driven into the tunnel bottom outside of these timbers, forming an
inclosure similar to a cofferdam, from which the earth could be
excavated without disturbing the surrounding ground. The earth being
excavated, a layer of concrete 8 ins. thick was placed, and the brick
masonry invert constructed on it. In less stable ground each of the
above described cofferdams was subdivided by transverse timbers and
sheet piling into three smaller cofferdams. Here the masonry of the
middle section was first constructed, and then the side sections built.
Where the ground was worst, still more care was necessary, and the
bottom had to be covered with a sheeting of 1¹⁄₄-in. plank held down by
struts abutting against the large transverse timbers. The invert masonry
was constructed on this sheeting. Refuge niches 9 ft. high, 3 ft. wide,
and 15 ins. deep were built in the side walls.
=Accidents.=--In this tunnel, owing to the quick striking of the
centers, it was found that the masonry lining flattened at the crown and
bulged at the sides. This was attributed to the insufficient time
allowed for the mortar to set in the rubble filling. Earth packing was
tried, but gave still worse results. Finally dry rubble filling was
adopted, with satisfactory results. There was necessarily some sinking
of the surface. This resulted partly from the necessity of changing and
removing of the timbers, and from the compression and springing of the
timbers under the great pressures. The crown of the arch also settled
from 2 ins. to 6 ins., due to the compression of the mortar in the
joints. The maximum sinking of the surface of the street over the tunnel
was about 18 ins.; it usually ran from 1 to 12 ins. Some damage was done
to the water and gas mains. This damage was not usually serious, but it
of course necessitated immediate repairs, and in some instances it was
found best to reconstruct the mains for some distance. At one point
along the tunnel where very treacherous material was found, the surface
settlement caused the collapse of an adjacent building, and necessitated
its reconstruction.