EXCAVATING TUNNELS THROUGH SOFT GROUND; GENERAL DISCUSSION; THE BELGIAN
METHOD.
GENERAL DISCUSSION.
It may be set down as a general truth that the excavation of tunnels
through soft ground is the most difficult task which confronts the
tunnel engineer. Under the general term of soft ground, however, a great
variety of materials is included, beginning with stratified soft rock
and the most stable sands and clays, and ending with laminated clay of
the worst character. From this it is evident that certain kinds of
soft-ground tunneling may be less difficult than the tunneling of rock,
and that other kinds may present almost insurmountable difficulties.
Classing both the easy and the difficult materials together, however,
the accuracy of the statement first made holds good in a general way.
Whatever the opinion may be in regard to this point, however, there is
no chance for dispute in the statement that the difficulty of tunneling
the softer and more treacherous clays, peats, and sands is greater than
that of tunneling firm soils and rock; and if we describe the methods
which are used successfully in tunneling very unstable materials, no
difficulty need be experienced in modifying them to handle stable
materials.
=Characteristics of Soft-Ground Tunneling.=--The principal
characteristics which distinguish soft-ground tunneling are, first, that
the material is excavated without the use of explosives, and second,
that the excavation has to be strutted practically as fast as it is
completed. In treacherous soils the excavation also presents other
characteristic phenomena: The material forming the walls of the
excavation tends to cave and slide. This tendency may develop
immediately upon excavation, or it may be of slower growth, due to
weathering and other natural causes. In either case the roof of the
excavations tends to fall, the sides tend to cave inward and squeeze
together, and the bottom tends to bulge or swell upward. In materials of
very unstable character these movements exert enormous pressures upon
the timbering or strutting, and in especially bad cases may destroy and
crush the strutting completely. Outside the tunnel the surface of the
ground above sinks for a considerable distance on each side of the line
of the tunnel.
=Methods of Soft-Ground Tunneling.=--There are a variety of methods of
tunneling through soft ground. Some of these, like the quicksand method
and the shield method, differ in character entirely, while in others,
like the Belgian, German, English, Austrian, and Italian methods, the
difference consists simply in the different order in which the drifts
and headings are driven, in the difference in the number and size of
these advance galleries, and in the different forms of strutting
framework employed. In this book the shield method is considered
individually; but the description of the Belgian, German, English,
Austrian, Italian, and quicksand methods are grouped together in this
and the three succeeding chapters to permit of easy comparison.
THE BELGIAN METHOD OF TUNNELING THROUGH SOFT GROUND.
[Illustration: FIGS. 68 and 68A.--Diagrams Showing Sequence of
Excavations in the Belgian Method.]
The Belgian method of tunneling through soft ground was first employed
in 1828 in excavating the Charleroy tunnel of the Brussels-Charleroy
Canal in Belgium, and it takes its name from the country in which it
originated. The distinctive characteristic of the method is the
construction of the roof arch before the side walls and invert are
built. The excavation, therefore, begins with the driving of a top
center heading which is enlarged until the whole of the section above
the springing lines of the arch is opened. Various modifications of the
method have been developed, and some of the more important of these will
be described farther on, but we shall begin its consideration here by
describing first the original and usual mode of procedure.
=Excavation.=--Fig. 68 is the excavation diagram of the Belgian method
of tunneling. The excavation is begun by opening the center top heading
No. 1, which is carried ahead a greater or less distance, depending upon
the nature of the soil, and is immediately strutted. This heading is
then deepened by excavating part No. 2, to a depth corresponding to the
springing lines of the roof arch. The next step is to remove the two
side sections No. 3, by attacking them at the two fronts and at the
sides with four gangs of excavators. The regularity and efficiency of
the mode of procedure described consist in adopting such dimensions for
these several parts of the section that each will be excavated at the
same rate of speed. When the upper part of the section has been
excavated as described, the roof arch is built, with its feet supported
by the unexcavated earth below. This portion of the section is excavated
by taking out first the central trench No. 4 to the depth of the bottom
of the tunnel, and then by removing the two side parts No. 5. As these
side parts No. 5 have to support the arch, they have to be excavated in
such a way as not to endanger it. At intervals along the central trench
No. 4, transverse or side trenches about 2 ft. wide are excavated on
both sides, and struts are inserted to support the masonry previously
supported by the earth which has been removed. The next step is to widen
these side trenches, and insert struts until all of the material in
parts No. 5 is taken out.
When the material penetrated is firm enough to permit, the plan of
excavation illustrated by the diagram, Fig. 68A, is substituted for the
more typical one just described. The only difference in the two methods
consists in the plan of excavating the upper part of the profile, which
in the second method consists in driving first the center top heading
No. 1, and then in taking out the remainder of the section above the
springing lines of the arch in one operation, while in the first method
it is done in two operations. The distance ahead of the masonry to which
the various parts can be driven varies from 10 ft. to, in some cases,
100 ft., being very short in treacherous ground, and longer the more
stable the material is.
=Strutting.=--The longitudinal method of strutting, with the
poling-boards running transversely of the tunnel, is always employed in
the Belgian method of tunneling. In driving the first center top
heading, pairs of vertical posts carrying a transverse cap-piece are
erected at intervals. On these cap-pieces are carried two longitudinal
bars, which in turn support the saddle planks. As fast as part No. 2,
Fig. 68, is excavated, the vertical posts are replaced by the batter
posts _A_ and _B_, Fig. 69. The excavation of parts No. 3 is begun at
the top, the poling-boards _a_ and _b_ being inserted as the work
progresses. To support the outer ends of these poling-boards, the
longitudinals _X_ and _Y_ are inserted and supported by the batter posts
_C_ and _D_. In exactly the same way the poling-boards _c_ and _d_, the
longitudinals _V_ and _W_, and the struts _E_ and _F_, are placed in
position; and this procedure is repeated until the whole top part of the
section is strutted, as shown by Fig. 69, the cross struts _x_, _y_,
_z_, etc., being inserted to hold the radial struts firmly in position.
The feet of the various radial props rest on the sill _M N_. These
fan-like timber structures are set up at intervals of from 3 ft. to 6
ft., depending upon the quality of the soil penetrated.
[Illustration: FIG. 69.--Sketch Showing Radial Roof Strutting, Belgian
Method.]
[Illustration: FIG. 70.--Sketch Showing Roof Arch Center, Belgian
Method.]
=Centers.=--Either plank or trussed centers may be employed in laying
the roof arch in the Belgian method, but the form of center commonly
employed is a trussed center constructed as shown by Fig. 70. It may be
said to consist of a king-post truss carried on top of a modified form
of queen-post truss. The collar-beam and the tie-beam of the queen-post
truss are spaced about 7 ft. apart, and the posts themselves are left
far enough apart to allow the passage of workmen and cars between them.
The tie beam of the king-post truss is clamped to the collar-beam of the
queen-post truss by iron bands. On the rafters of the two trusses are
fastened timbers, with their outer edges cut to the curve of the roof
arch. These centers are set up midway between the fan-like strutting
frames previously described. They are usually built of square timbers.
The tie beams are usually 6 × 6 in., and the struts and posts 4 × 4 in.
timbers. The reason for giving the larger sectional dimensions to the
tie beams, contrary to the usual practice in constructing centers, is
that it has to serve as a sill for distributing the pressure to the
foundation of unexcavated soil which supports the center. Sometimes a
sub-sill is used to support the center upon the soil; and in any case
wedges are employed to carry it, which can be removed for the purpose of
striking the center. After the arch is completed, the centers may be
removed immediately, or may be left in position until the masonry has
thoroughly set. In either case the leading center over which the arch
masonry terminates temporarily is left in position until the next
section of the arch is built.
=Masonry.=--The masonry of the roof arch, which is the first part built,
is of necessity begun at the springing lines, and the first course rests
on short lengths of heavy planks. These planks, besides giving an even
surface upon which to begin the masonry, are essential in furnishing a
bearing to the struts inserted to support the arch while the earth below
them, part No. 5, Fig. 68, is being excavated. As the arch masonry
progresses from the springing lines upward, the radial posts of the
strutting are removed, and replaced by short struts resting on the
lagging of the centers, which support the crown bars or longitudinals
until the masonry is in place, when they and the poling-boards are
removed, and the space between the arch masonry and walls of the
excavation is filled with stone or well-rammed earth.
Considering now the side wall masonry, it will be remembered that in
excavating the part No. 5, Fig. 68, of the section, frequent side
trenches were excavated, and struts inserted to take the weight of the
masonry. These struts are inserted on a batter, with their feet near the
center of the tunnel floor, so that the side wall masonry may be carried
up behind them to a height as near as possible to the springing lines of
the arch. When this is done the struts are removed, and the space
remaining between the top of the partly finished side wall and the arch
is filled in. This leaves the arch supported by alternate lengths or
pillars of unexcavated earth and completed side wall. The next step is
to remove the remaining sections of earth between the sections of side
wall, and fill in the space with masonry. Fig. 71 is a cross-section,
showing the masonry completed for one-half and the inclined props in
position for the other half; and Fig. 72 is a longitudinal section
showing the pillars of unexcavated earth between the consecutive sets of
inclined struts and several other details of the lining, strutting, and
excavating work.
[Illustration: FIG. 71.--Sketch Showing Method of Underpinning Roof Arch
with the Side Wall Masonry.]
[Illustration: FIG. 72.--Longitudinal Section Showing Construction by
the Belgian Method.]
The invert masonry is built after the side walls are completed. This is
regarded as a defect of this method of tunneling, since the lateral
pressures may squeeze the side walls together and distort the arch
before the invert is in place to brace them apart. To prevent as much as
possible the distortion of the arch after the centers are removed, it is
considered good practice to shore the masonry with horizontal beams
having their ends abutting against plank, as shown by Fig. 71. These
horizontal beams should be placed at close intervals, and be supported
at intermediate points by vertical posts, as shown by the illustration.
Since the roof arch rests are for some time supported directly by the
unexcavated earth below, settlement is liable, particularly in working
through soft ground. This fact may not be very important so long as the
settlement is uniform, and is not enough to encroach on the space
necessary for the safe passage of travel. To prevent the latter
possibility the centers are placed from 9 ins. to 15 ins. higher than
their true positions, depending upon the nature of the soil, so that
considerable settlement is possible without any danger of the necessary
cross-section being infringed upon. In conclusion it may be noted that
the lining may be constructed in a series of consecutive rings, or as a
single cylindrical mass.
=Hauling.=--Since in this method of tunneling the upper part of the
section is excavated and lined before the excavation of the lower part
is begun, the upper portion is always more advanced than the lower. To
carry away the earth excavated at the front, therefore, an elevation has
to be surmounted; and this is usually done by constructing an inclined
plane rising from the floor of the tunnel to the floor of the heading,
as shown by Fig. 72. This inclined plane has, of course, to be moved
ahead as the work advances, and to permit of this movement with as
little interruption of the other work as possible, two planes are
employed. One is erected at the right-hand side of the section, and
serves to carry the traffic while the left-hand side of the lower
section is being removed some distance ahead and the other plane is
being erected. The inclination given to these planes depends upon the
size of the loads to be hauled, but they should always have as slight a
grade as practicable. Narrow-gauge tracks are laid on these planes and
along the floor of the upper part of the section passing through the
center opening mentioned before as being left in the centers and
strutting.
In excavating the top center heading there is, of course, another rise
to its floor from the floor of the upper part of the section. Where, as
is usually the case in soft soils, this top heading is not driven very
far in advance, the earth from the front is usually conveyed to the rear
in wheelbarrows, and dumped into the cars standing on the tracks below.
In firm soils, where the heading is driven too far in advance to make
this method of conveyance adequate, tracks are also laid on the floor of
the heading, and an inclined plane is built connecting it with the
tracks on the next level below. In place of these inclined planes, and
also in place of those between the floor of the tunnel and the level
above, some form of hoisting device is sometimes employed to lift the
cars from one level to the other. There are some advantages to this
method in point of economy, but the hoisting-machines are not easily
worked in the darkness, and accidents are likely to occur.
[Illustration: FIG. 73.--Diagram Showing Sequence of Excavation in
Modified Belgian Method.]
In the advanced top heading and in the upper part of the section
narrow-gauge tracks are necessarily employed, and these may be continued
along the floor of the finished section, or the permanent broad-gauge
railway tracks may be laid as fast as the full section is completed. In
the former case the permanent tracks are not laid until the entire
tunnel is practically completed; and in the latter case, unless a third
rail is laid, the loads have to be transshipped from the broad- to the
narrow-gauge tracks or _vice versa_. It is the more general practice to
use a third rail rather than to transship every load.
=Modifications.=--Considering the extent to which the Belgian method of
tunneling has been employed, it is not surprising that many
modifications of the standard mode of procedure have been developed. The
modification which differs most from the standard form is, perhaps, that
adopted in excavating the Roosebeck tunnel in Germany. This method
preserves the principal characteristic of the Belgian method, which is
the construction of the upper part of the section first; but instead of
building the side walls from the bottom upward, they are built in small
sections from the top downward. The excavation begins by driving the
center top heading No. 1, Fig. 73, whose floor is at the level of the
springing lines of the roof arch, and then the two side parts No. 2 are
excavated, opening up the entire upper portion of the section in which
the roof arch is built, as in the regular Belgian method. The next step
is to excavate part No. 3, shoring up the arch at frequent intervals.
Between these sets of shoring the side walls are built, resting on
planks on the floor of part No. 3, and then the sets of shores are
removed and replaced by masonry. Next part No. 4 is excavated, shored,
and filled with masonry as was part No. 3. In exactly the same way parts
5, 6, 7, and 8 are constructed in the order numbered. To prevent the
distortion of the arch during the side-wall construction it is braced by
horizontal struts, as indicated above in Fig. 71.
=Advantages.=--The advantages of the Belgian method of tunneling may be
summarized as follows: (1) The excavation progresses simultaneously at
several points without the different gangs of excavators interfering
with each other, thus securing rapidity and efficiency of work; (2) the
excavation is done by driving a number of drifts or parts of small
section, which are immediately strutted, thus causing the minimum
disturbance of the surrounding material; (3) the roof of the tunnel,
which is the part of the lining exposed to the greatest pressures, is
built first.
[Illustration: FIG. 74.--Sketch Showing Failure of Roof Arch by Opening
at Crown.]
=Disadvantages.=--The disadvantages of the Belgian method of tunneling
may be summarized as follows: (1) The roof arch which rests at first on
compressible soil is liable to sink; (2) before the invert is built
there is danger of the arch and side walls being distorted or sliding
under the lateral pressures; (3) the masonry of the side walls has to be
underpinned to the arch masonry.
=Accidents and Repairs.=--One of the most frequent accidents in the
Belgian method of tunneling is the sinking of the roof arch owing to
its unstable foundation on the unexcavated soil of the lower portion of
the section. The amount of settlement may vary from a few inches in firm
soil to over 2 ft. in loose soils. To counteract the effect of this
settlement it is the general practice to build the arch some inches
higher than its normal position. When the settlement is great enough to
infringe seriously upon the tunnel section, repairs have to be made; and
the only way of accomplishing them is to demolish the arch and rebuild
it from the side walls. It is usually considered best not to demolish
the arch until the invert has been placed, so that no further
disturbance is likely to occur once the lining is completed anew.
The rotation of the arch about its keystone, or the opening of the arch
at the crown, by the squeezing inward of the haunches by the lateral
pressures, is another characteristic accident. Fig. 74 shows the nature
of the distortion produced; the segments of the arch move toward each
other by revolving on the intradosal edges of the keystone, which are
broken away and crushed together with the operation, while the
extradosal edges are opened. It is to prevent this occurrence that the
horizontal struts shown in Fig. 71 are employed. The manner of repairing
this accident differs, depending upon the extent of the injury. When the
intradosal edges of the keystone are but slightly crushed, the repairing
is done as directed by Fig. 75. When the keystone is completely crushed,
however, the indications are that the material of the keystone, usually
brick, is not strong enough to resist the pressures coming upon it, and
it is advisable to substitute a stronger material in the repairs, and a
stone keystone is constructed as shown by Fig. 75. The middle stone of
this keystone extends through the depth of the arch ring, and the two
side stones only half-way through, their purpose being merely to resist
the crushing forces which are greatest at the intrados. Sometimes, when
the pressures are unsymmetrical, the arch ring breaks at the haunches as
well as the crown, as shown by Fig. 75, which also indicates the mode of
repairing. This consists in demolishing the original arch, and
rebuilding it with stone voussoirs inserted in place of the brick in
which the rupture occurred.
[Illustration: FIG. 75.--Sketches Showing Methods of Repairing Roof Arch
Failures.]