GENERAL METHODS OF EXCAVATION: SHAFTS: CLASSIFICATION OF TUNNELS.
A number of different modes of procedure are followed in excavating
tunnels, and each of the more important of these will be considered in a
separate chapter. There are, however, certain characteristics common to
all of these methods, and these will be noted briefly here.
[Illustration: FIG. 14.--Diagram Showing Sequence of Excavation for St.
Gothard Tunnel.]
[Illustration: FIG. 15.--Diagram Showing Manner of Determining
Correspondence of Excavation to Sectional Profile.]
=Division of Section.=--It may be asserted at the outset that the whole
area of the tunnel section is not ordinarily excavated at one time, but
that it is removed in sections, and as each section is excavated it is
thoroughly timbered or strutted. The order in which these different
sections are excavated varies with the method of excavation, and it is
clearly shown for each method in succeeding chapters. As a single
example to illustrate the proposition just made, the division of the
section and the sequence of excavation adopted at the St. Gothard tunnel
is selected (Fig. 14). The different parts of the section were excavated
in the order numbered; the names given to each part, and the number of
holes employed in breaking it down, are given by the table on page 35.
Whatever method is employed, the work always begins by driving a
heading, which is the most difficult and expensive part of the
excavation. All the other operations required in breaking down the
remainder of the tunnel section are usually designated by the general
term of enlargement of the profile. The various operations of excavation
may, therefore, be classified either as excavation of the heading or
enlargement of the profile.
=Excavation of the Heading.=--There is considerable confusion among the
different authorities regarding the exact definition of the term
“heading” as it is used in tunnel work. Some authorities call a small
passage driven at the top of the profile a heading, and a similar
passage driven at the bottom of the profile a drift; others call any
passage driven parallel to the tunnel axis, whether at the top or at the
bottom of the profile, a drift; and still others give the name “heading”
to all such passages. For the sake of distinctness of terminology it
seems preferable to call the passage a heading when it is located at the
top of the profile, and a drift when it is located near the bottom.
Headings and drifts are driven in advance of the general excavation for
the following purposes: (1) To fix correctly the axis of the tunnel; (2)
to allow the work to go on at different points without the gangs of
laborers interfering with each other; (3) to detect the nature of
material to be dealt with and to be ready in any contingency to overcome
any trouble caused by a change in the soil; and (4) to collect the
water. The dimensions of headings in actual practice vary according to
the nature of the soil through which they are driven. As a general rule
they should not be less than 7 ft. in height, so as to allow the men to
work standing, and have room left for the roof strutting. The width
should not be less than 6 ft., to allow two men to work at the front,
and to give room for the material cars without interfering with the wall
strutting. Usually headings are made 8 ft. wide. The length of headings
in practice varies according to circumstances. In very long tunnels
through hard rock the headings are sometimes excavated from 1000 ft. to
2000 ft. in advance, in order that they may meet as soon as possible and
the ranging of the center line be verified, and so that as great an
area of rock as possible may be attacked at the same time in the work of
enlarging the profile. In short tunnels, where the ranging of the center
line is less liable to error, shorter headings are employed, and in soft
soils they are made shorter and shorter as the cohesion of the soil
decreases. When the material has too little cohesion to stand alone, the
tops and sides of the heading require to be supported by strutting. To
prevent caving at the front of the heading, the face of the excavation
is made inclined, the inclination following as near as may be the
natural slope of the material.
=Enlargement of the Profile.=--The enlargement of the profile is
accomplished by excavating in succession several small prisms parallel
to the heading, and its full length, which are so located that as each
one is taken out the cross-section of the original heading is enlarged.
The number, location, and sequence of these prisms vary in different
methods of excavation, and are explained in succeeding chapters where
these methods are described. To direct the excavation so as to keep it
always within the boundaries of the adopted profile, the engineer first
marks the center line on the roof of the heading by wooden or metal
pegs, or by some other suitable means by which a plumb line may be
suspended. He next draws to a large scale a profile of the proposed
section; and beginning at the top of the vertical axis he draws
horizontal lines at regular intervals, as shown by Fig. 15, until they
intersect the boundary lines of the profile, and designates on each of
these lines the distance between the vertical axis and the point where
it intersects the profile. It is evident that if the foreman of
excavation divides his plumb line in a manner corresponding to the
engineer’s drawing, and then measures horizontally and at right angles
to the vertical center plane of the tunnel the distance designated on
the horizontal lines of the drawing, he will have located points on the
profile of the section, or in other words have established the limits of
excavation.
[Illustration: FIG. 16.--Polar Protractor for Determining Profile of
Excavated Cross-Section.]
In the excavation of the Croton Aqueduct for the water supply of New
York city, an instrument called a polar protractor was used for
determining the location of the sectional profile. It was invented by
Mr. Alfred Craven, division engineer of the work. This instrument
consists of a circular disk graduated to degrees, and mounted on a
tripod in such a manner that it may be leveled up, and also have a
vertical motion and a motion about the vertical axis. The construction
is shown clearly by Fig. 16. In use the device is mounted with its
center at the axis of the tunnel. A light wooden measuring-rod tapering
to a point, shod with brass and graduated to feet and hundredths of a
foot, lies upon the wooden arm or rest, which revolves upon the face of
the disk, and slides out to a contact with the surface of the
excavation at such points as are to be determined. If the only
information desired is whether or not the excavation is sufficient or
beyond the established lines, the rod is set to the proper radius, and
if it swings clear the fact is determined. If a true copy of the actual
cross-section is desired, the rod is brought into contact with the
significant points in the cross-section, and the angles and distances
are recorded.
The general method of directing the excavation in enlarging the profile
by referring all points of the profile to the vertical axis is the one
usually employed in tunneling, and gives good results. It is considered
better in actual practice to have the excavation exceed the profile
somewhat than to have it fall short of it, since the voids can be more
easily filled in with riprap than the encroaching rock can be excavated
during the building of the masonry. In tunnels where strutting is
necessary the excavation must be made enough larger than the finished
section to provide the space for it. In soft-ground tunnels it is also
usual to enlarge the excavation to allow for the probable slight sinking
of the masonry. The proper allowance for strutting is usually left to
the judgment of the foreman of excavation, but the allowance for
settlement must be fixed by the engineer.
SHAFTS.
Shafts are vertical walls or passages sunk along the line of the tunnel
at one or more points between the entrances, to permit the tunnel
excavation to be attacked at several different points at once, thus
greatly reducing the time required for excavation. Shafts may be located
directly over the center of the tunnel or to one side of it, and, while
usually vertical, are sometimes inclined. During the construction of the
tunnel the shafts serve the same purpose as the entrances; hence they
must afford a passageway for the excavated materials, which have to be
hoisted out, and also for the construction tools and materials which
have to be lowered down them. They must also afford a passageway for
workmen, draft animals, and for pipes for ventilation, water, compressed
air, etc. The character of this traffic indicates the dimensions
required, but these depend also upon the method of hoisting employed.
Thus, when a windlass or horse gin is used, and the materials are
hoisted in buckets of small dimensions, the dimensions of the shaft may
also be small; but when steam elevators are employed, and the material
is carried on cars run on to the platform of the elevator, large
dimensions must be given to the shaft. Generally the parts of the shaft
used for different purposes are separated by partitions. The elevator
for workmen and the various pipes are placed in one compartment, while
the elevator for hoisting the excavated material and lowering
construction material is placed in another.
Shafts may be either temporary or permanent. They are temporary when
they are filled in after the tunnel is completed, and permanent when
they are left open to supply ventilation to the tunnel. Permanent shafts
are usually made circular, and lined with brick, unless excavated in
very hard and durable rock. When sunk for temporary use only, shafts are
usually made rectangular with the greater dimension transverse to the
tunnel. They are strutted with timber. A pump is generally located at
the bottom of the shaft to collect the water which seeps in from the
sides of the shaft and from the tunnel excavation. The dimensions of
this pump will of course vary with the amount of water encountered, as
will also the capacity of the pump for forcing it up and out of the
shaft, which has always to be kept dry.
The majority of engineers prefer to sink shafts directly over the center
line of the tunnel. Side shafts are employed chiefly by French
engineers. The chief advantage of the former method is the great
facility which it affords for hoisting out the materials, while in favor
of the latter method is the non-interference of the shaft with the
operations inside the tunnel. Were it not that the side shaft requires
the introduction of a transverse gallery connecting it with the tunnel,
it would be on the whole superior to the center shaft; but the side
gallery necessitates turning the cars at right angles, and consequently
the use of a very sharp curve or a turntable to reach the shaft bottom,
which is a disadvantage that may outweigh its advantages in some other
respects. It is impossible to state absolutely which of these methods of
locating shafts is the best; both present advantages and disadvantages,
and the use of one or the other is usually determined more by the local
conditions than by any general superiority of either.
When side shafts are employed they are sometimes made inclined instead
of vertical. This form is used when the depth of the shaft is small. By
it the hauling is greatly simplified, since the cars loaded at the front
with excavated material can be hauled directly out of the shaft and to
the dumping-place, surmounting the inclined shaft by means of continuous
cables. The short galleries connecting the side shafts with the tunnel
proper usually have a smaller section than the tunnel, but are excavated
in exactly the same manner. Another form of side shaft sometimes used is
one reaching to the surface when the tunnel runs close to the side of
cliff, as is the case with some of the Alpine railway tunnels.
CLASSIFICATION OF TUNNELS.
Tunnels are classified in various ways, but the most logical method
would appear to be a grouping according to the quality of the material
through which they are driven; and this method will be adopted here. By
this method we have first the following general classification: (1)
Tunnels in hard rock; (2) tunnels in ordinary loose soil; (3) tunnels in
quicksand; (4) open-cut tunnels; and (5) submarine tunnels. It is hardly
necessary to say that this classification, like all others, is simply
an arbitrary arrangement adopted for the sake of order and convenience
in treating the subject.
=Tunnels in Hard Rock.=--With the numerous labor-saving methods and
machines now available, hard rock is perhaps the safest and easiest of
all materials through which to drive a tunnel. Tunnels through hard rock
may be excavated, either by a drift or by a heading. The difference
depends upon whether the advance gallery is located close to the floor
or near the soffit of the section.
=Tunnels in Loose Soils.=--In driving tunnels through loose soils many
different methods have been devised, which may be grouped as follows:
(1) Tunnels excavated at the soffit--Belgian method; (2) tunnels
excavated along the perimeter--German method; (3) tunnels excavated in
the whole section--English, Austrian and American methods; (4) tunnels
excavated in two halves independent of each other--Italian method.
(1) Excavating the tunnel by beginning at the soffit of the section, or
by the Belgian method, is the method of tunneling in loose soils most
commonly employed in Europe at the present time. It consists in
excavating the soffit of the section first; then building the arch,
which is supported upon the unexcavated ground; and finally in
excavating the lower portion of the section, and building the side walls
and invert.
(2) In excavating tunnels along the perimeter an annular excavation is
made, following closely the outline of the sectional profile in which
the lining masonry is built, after which the center core is excavated.
In the German method two drifts are opened at each side of the tunnel
near the bottom. Other drifts are excavated, one above the other, on
each side to extend or heighten the first two until all the perimeter is
open except across the bottom. The masonry lining is then built from the
bottom upwards on each side to the crown of the arch, and then the
center core is removed and the invert is built.
(3) This method, as its name implies, consists in taking out short
lengths of the whole sectional profile before beginning the building of
the masonry. In the English method the invert is built first, then the
side walls, and finally the arch. The excavators and masons work
alternately. The Austrian method differs in two particulars from the
English: the length of section opened is made great enough to allow the
excavators to continue work ahead of the masons, and the side walls and
roof are built before the invert. In the American method the whole
section of the tunnel is open at once: excavators and masons work
simultaneously, but a very large quantity of timbering is required.
(4) The Italian method is very seldom employed on account of its
expensiveness, but it can often be used where the other methods fail. It
consists in excavating the lower half of the section, and building the
invert and side walls, and then filling the space between the walls in
again except for a narrow passageway for the cars; next the upper part
of the section is excavated, as in the Belgian method, and the arch is
built; and finally the soil in the lower part is permanently removed.
=Tunnels in Quicksand.=--Tunnels through quicksand are driven by one of
the ordinary soft-ground methods after draining away the water, or else
as submarine tunnels.
=Open-Cut Tunnels.=--Open-cut tunnels are those driven at such a small
depth under the surface that it is more convenient to excavate an open
cut, build the tunnel masonry inside it, and then refill the open
spaces, than it is to carry on the work entirely underground. In firm
soils the usual mode of operation is to excavate first two parallel
trenches for the side walls, then remove the core, and build the arch
and the invert. In unstable soils, since the invert must be built first,
it is usual to open up a single wide trench. In infrequent cases where a
tunnel is desired in a place which is to be filled in, the masonry is
built as a surface structure, which in due time is covered.
=Submarine Tunnels.=--The mode of procedure followed in excavating
submarine tunnels depends upon whether the material penetrated is
pervious or impervious to water. In impervious material any of the
ordinary methods of tunneling found suitable may be employed. In
pervious material the excavation may be accomplished either by means of
compressed air to keep the water out of the excavation, or by means of a
shield closing the front of the excavation, or by a combination of these
two methods. Tunnels on the river bed are built by means of coffer dams
which inclose alternate portions of the work, by sinking a continuous
series of pneumatic caissons and opening communication between them, and
by sinking the tunnel in sections constructed on land.
{_In hard rock._ {By drifts.
{ {By a heading.
{ {_By upper half:_ }
{ { the arch is built }Belgian method.
{ { before the side walls. }
{ {
{ {_By the perimeter:_ }
{ { excavated and lined }
{ { before the central }German method.
{ { nucleus is removed. }
{ {
{_In loose soil._{_By whole section:_ {English method.
{ { the lining begins after{Austrian method.
{ { the whole section is {American method.
{ { excavated. {
{ {
{ {_By halves:_ }
{ { the lower half is }
{ { excavated and lined, }Italian method.
{ { followed by the work }
{ { of the upper half. }
METHODS OF{
EXCAVATING{_In quicksand._
TUNNELS. {
{ {In resistant soils. {By two lateral
{ { {narrow trenches.
{_Open-cut_ {
{_tunnels._ {In loose soils. {By one very large
{ { {trench.
{ {
{ {Built up. By slices.
{
{ {At great depths under }By any method.
{ {the river bed. }
{ {
{ { {By shield.
{ {At small depths {By compressed
{ {under the river {air.
{_Submarine_ {bed. {By shield and
{_tunnels._ { {compressed air.
{ {
{ { {By coffer dams.
{ { {By pneumatic
{ {On the river bed. {caissons.
{ { {By built-up
{ { {sections.
The above diagram gives in compact form the classification of tunnels
according to materials penetrated and methods of excavation adopted,
which have been described more fully in the succeeding paragraphs. It
may be noted here again that this is a purely arbitrary classification,
and serves mostly as a convenience in discussing the different classes
of tunnels without confusion.