METHODS OF LINING TUNNELS.
Tunnels in soft soils and in loose rock, and rock liable to
disintegration, are always provided with a lining to hold the walls and
roof in place. This lining may cover the entire sectional profile of the
tunnel, or only a part of it, and it may be constructed of timber, iron,
iron and masonry, or, more commonly, of masonry alone.
=Timber Lining.=--Timber is seldom employed in lining tunnels except as
a temporary expedient, and is replaced by masonry as soon as
circumstances will permit. In the first construction of many American
railways, the necessity for extreme economy in construction, and of
getting the line open for traffic as soon as possible, caused the
engineers to line many tunnels with timber, which was plentiful and
cheap. Except for their small cost and the ease and rapidity with which
they can be constructed, however, these timber linings possess few
advantages. It is only the matter of a few years when the decay of the
timber makes it necessary to rebuild them, and there is always the
serious danger of fire. In several instances timber-lined tunnels in
America have been burned out, causing serious delays in traffic, and
necessitating complete reconstruction. Usually this reconstruction has
consisted in substituting masonry in place of the original timber
lining. In a succeeding chapter a description will be given of some of
the methods employed in replacing timber tunnel linings with masonry.
Various forms of timber lining are employed, of which Fig. 44 and the
illustrations in the chapter discussing the methods of relining
timber-lined tunnels with masonry are typical examples.
[Illustration: ~Cross Section.~
~Longitudinal Section.~
FIGS. 43 and 44.--A Typical Form of Timber Lining for Tunnels.]
=Iron Lining.=--The use of iron lining for tunnels was introduced first
on a large scale by Mr. Peter William Barlow in 1869, for the second
tunnel under the River Thames at London, England, and it has greatly
extended since that time. The lining of the second Thames tunnel
consisted of cylindrical cast-iron rings 8 ft. in diameter, the abutting
edges of the successive rings being flanged and provided with holes for
bolt fastenings. Each ring was made up of four segments, three of which
were longer than quadrants, and one much smaller forming the “key-stone”
or closing piece. These segments were connected to each other by flanges
and bolts. To make the joints tight, strips of pine or cement and hemp
yarn were inserted between the flanges. Since the construction of the
second Thames tunnel, iron lining has been employed for a great many
submarine tunnels in England, Continental Europe, and America, some of
them having a section over 28 ft. in diameter. Where circular iron
lining is employed, the bottom part of the section is leveled up with
concrete or brick masonry to carry the tracks, and the whole interior
of the ring is covered with a cement plaster lining deep enough
thoroughly to embed the interior joint flanges. In the succeeding
chapter describing the methods of driving tunnels by shields several
forms of iron tunnel lining are fully described.
=Iron and Masonry Lining.=--During recent years a form of combined
masonry and iron lining has been extensively employed in constructing
city underground railways in both Europe and America. Generally this
form of lining is built with a rectangular section. Two types of
construction are employed. In the first, masonry side walls carry a flat
roof of girders and beams, which carry a trough flooring filled with
concrete, or between which are sprung concrete or brick arches.
Sometimes the roof framing consists of a series of parallel I-beams laid
transversely across the tunnel, and in other cases transverse plate
girders carry longitudinal I-beams. In the second type of construction
the roof girders are supported by columns embedded in the side walls.
Where the tunnel provides for two or four tracks, intermediate column
supports are in some cases introduced between the side columns. In this
construction the roofing consists of concrete filled troughs or of
concrete or brick arches, as in the construction first described.
Examples of combined masonry and iron tunnel lining are illustrated in
the succeeding chapter on tunneling under city streets.
=Masonry Lining.=--The form of tunnel lining most commonly employed is
brick or stone masonry. Concrete and reinforced concrete masonry lining
has been employed in several tunnels built in recent years. The masonry
lining may inclose the whole section or only a part of it. The floor or
invert is the part most commonly omitted; but sometimes also the side
walls and invert are both omitted, and the lining is confined simply to
an arch supporting the roof. The roof arch, the side walls, and the
invert compose the tunnel lining; and all three may consist of stone or
brick alone, or stone side walls may be employed with brick invert and
roof arch. Rubble-stone masonry is usually employed, except at the
entrances, where the masonry is exposed to view. Here ashlar masonry is
usually used. The stone selected for tunnel lining should be of a
durable quality which weathers well. Where bricks are used they should
be of good quality. Owing to the comparative ease with which brick
arches can be built, they are generally used to form the roof arch, even
where the side walls are of stone masonry. Masonry lining may be built
in the form of a series of separate rings, or in the form of a
continuous structure extending from one end of the tunnel to the other.
The latter method of construction produces a stronger structure; but in
case of failure by crushing, the damage done is likely to be more
widespread than where separate rings are employed, one or two of which
may fail without injury to the others adjacent to them. The construction
is also somewhat simpler where separate rings are employed, since no
provision has to be made for bonding the whole lining into a continuous
structure. Where a series of separate rings is employed, the length of
each ring runs from 5 ft. up to 20 ft., it depending upon the character
of the material penetrated, and the method of construction employed. For
the purpose of detailed discussion the construction of masonry lining
may be divided into four parts,--the side-wall foundations, the side
walls themselves, the roof arch, and the invert.
Concrete and reinforced concrete linings are now extensively used on
account of cheapness and facility of handling, but they have the great
disadvantage of resisting pressure after they become hard, which is some
time after being placed. The strutting should, therefore, be left to
support the roof so as to prevent direct pressure on the fresh material.
The roof, as a rule, is supported by longitudinal planks held in
position by five or seven segments of arched frames placed across the
tunnel. A large quantity of timber and carpenter work is thus entirely
wasted and these costly items, in many cases, make the concrete lining
of a tunnel more expensive than the one built of brick and stone. To
avoid these inconveniences tunnels have been successfully lined with
concrete on the side walls and concrete blocks in the arches. These
blocks have been built by hand and molded in the shape of the arch
voussoirs.
=Foundations.=--In tunnels through rock of a hard and durable character
the foundations for the side walls are usually laid directly on the
rock. In loose rock, or rock liable to disintegration, this method of
construction is not generally a safe one, and the foundation excavation
should be sunk to a depth at which the atmospheric influences cannot
affect the foundation bed. In either case the foundation masonry is made
thicker than that of the side walls proper, so as to distribute the
pressure over a greater area, and to afford more room for adjusting the
side-wall masonry to the proper profile. In yielding soils a special
foundation bed has to be prepared for the foundation masonry. In some
instances it is found sufficient to lay a course of planks upon which
the masonry is constructed, but a more solid construction is usually
preferred.
This is obtained by placing a concrete footing from 1 ft. to 2 ft. deep
all along the bottom of the foundation trench, or in some cases by
sinking wells at intervals along the trench and filling them with
concrete, so as to form a series of supporting pillars.
[Illustration: FIG. 45.--Diagram Showing Forms Adopted for Side-Wall
Foundations.]
The form given to the foundation courses and lower portions of the side
walls varies. Where a large bearing area is required, the back of the
wall is carried up vertically as shown by the line _AB_, Fig. 45,
otherwise the rear face of the wall follows the line of excavation _AC_.
For similar reasons the front face of the wall may be made vertical, as
at _FG_, or inclined, as at _FH_. The line _FE_ indicates the shelf
construction designed to support the feet of the posts used to carry the
arch centers during the construction of the roof arch.
=Side Walls.=--The construction of the side walls above the foundation
courses is carried out as any similar piece of masonry elsewhere would
be built. To direct the work and insure that the inner faces of the
walls follow accurately the curve of the chosen profile, leading frames
previously described are employed.
=Roof Arch.=--For the construction of the roof arch, the centers
previously described are employed. Beginning at the edges of the center
on each side, the masonry is carried up a course at a time, care being
taken to have it progress at the same rate on both sides, so that the
load brought onto the centering is symmetrical. As soon as the centers
are erected, the roof strutting is removed, and replaced by short props
which rest on the lagging of the centers and support the poling-boards.
These props are removed in succession as the arch masonry rises along
the curve of the center, and the space between the top of the arch
masonry and the ceiling of the excavation is filled with small stones
packed closely. The keystone section of the arch is built last, by
inserting the stones or bricks from the front edge of the arch ring,
there being no room to set them in from the top, as is the practice in
ordinary open-arch construction. The keying of the arch is an especially
difficult operation, and only experienced men skilled in the work should
be employed to perform it. The task becomes one of unusual difficulty
when it becomes necessary to join the arches coming from opposite
directions.
=Invert.=--In all but one or two methods of tunneling, the invert is the
last portion of the lining to be built. In the English method of
tunneling, the invert is the first portion of the lining to be built,
and the same practice is sometimes necessary in soft soils where there
is danger of the bottoms of the side walls being squeezed together by
the lateral pressures unless the invert masonry is in place to hold them
apart. The ground molds previously described are employed to direct the
construction of the invert masonry.
=General Observations.=--In describing the construction of the roof
arch, mention was made of the stone filling employed between the back of
the masonry ring and the ceiling of the excavation. The spaces behind
the side walls are filled in a similar manner. The object of this stone
filling, which should be closely packed, is to distribute the vertical
and lateral pressures in the walls of the excavation uniformly over the
lining masonry. As the masonry work progresses, the strutting employed
previously to support the walls of the excavation has to be removed.
This work requires care to prevent accident, and should be placed in
charge of experienced mechanics who are familiar with its construction,
and can remove it with the least damage to the timbers, so that they may
be used again, and without causing the fall of the roof or the caving of
the sides by removing too great a portion of the timbers at one time.
=Thickness of Lining Masonry.=--It is obvious, of course, that the
masonry lining must be thick enough to support the pressure of the earth
which it sustains; but, as it is impossible to estimate these pressures
at all accurately, it is difficult to say definitely just what thickness
is required in any individual case. Rankine gives the following formulas
for determining the depths of keystone required in different soils:
For firm soils
( _r_²)
_d_ = √(0.12----),
( _s_ )
and for soft soils,
( _r_²)
_d_ = √(0.48----),
( _s_ )
where _d_ = the depth of the crown in feet, _r_ = the rise of the arch
in feet, and _s_ = the span of the arch in feet. Other writers, among
them Professor Curioni, attempt to give rational methods for calculating
the thickness of tunnel lining; but they are all open to objection
because of the amount of hypothesis required concerning pressures which
are of necessity indeterminate. Therefore, to avoid tedious and
uncertain calculations, the engineer adopts dimensions which experience
has proven to be ample under similar conditions in the past. Thus we
have all gradations in thickness, from hard-rock tunnels requiring no
lining, and tunnels through rocks which simply require a thin shell to
protect them from the atmosphere, to soft-ground tunnels where a masonry
lining 3 ft. or more in thickness is employed. Table II. shows the
thickness of masonry lining used in tunnels through soft soils of
various kinds.
The thickness of the masonry lining is seldom uniform at all points, as
is indicated by Table II. Figs. 46 and 47 show common methods of varying
the thickness of lining at different points, and are self-explanatory.
[Illustration: FIGS. 46 and 47.--Transverse Sections of Tunnels Showing
Methods of Increasing the Thickness of the Lining at Different Points.]
=Side Tunnels.=--When tunnels are excavated by shafts located at one
side of the center line, short side tunnels or galleries are built to
connect the bottoms of the shafts with the tunnel proper. These side
tunnels are usually from 30 ft. to 40 ft. long, and are generally made
from 12 ft. to 14 ft. high, and about 10 ft. wide. The excavation,
strutting, and lining of these side tunnels are carried on exactly as
they are in the main tunnel, with such exceptions as these short
lengths make possible. Table III. gives the thickness of lining used for
side tunnels, the figures being taken from European practice.
=Culverts.=--The purpose of culverts in tunnels is to collect the water
which seeps into the tunnel from the walls and shafts. The culvert is
usually located along the center line of the tunnel at the bottom. In
soft-ground tunnels it is built of masonry, and forms a part of the
invert, but in rock tunnels it is the common practice to cut a channel
in the rock floor of the excavation. Both box and arch sections are
employed for culverts. The dimensions of the section vary, of course,
with the amount of water which has to be carried away. The following are
the dimensions commonly employed:
+----------------+--------+--------+----------+-----------+
|KIND OF CULVERT.| HEIGHT | WIDTH |THICKNESS | THICKNESS |
| |IN FEET.|IN FEET.|OF WALLS |OF COVERING|
| | | | IN FEET. | IN FEET. |
+----------------+--------+--------+----------+-----------+
|Box culvert |1 to 1.5|1 to 1.5|0.8 to 1.2| 0.3 |
|Arch culvert |1 to 1.5|1 to 1.5|0.8 to 1.2| 0.4 |
+----------------+--------+--------+----------+-----------+
It should be understood that the dimensions given in the table are those
for ordinary conditions of leakage; where larger quantities of water are
met with, the size of the culverts has, of course, to be enlarged. To
permit the water to enter the culvert, openings are provided at
intervals along its side; and these openings are usually provided with
screens of loose stones which check the current, and cause the suspended
material to be deposited before it enters the culvert. In cases where
springs are encountered in excavating the tunnel, it is necessary to
make special provisions for confining their outflow and conducting it to
the culvert. In all cases the culverts should be provided with catch
basins at intervals of from 150 ft. to 300 ft., in which such suspended
matter as enters the culverts is deposited, and removed through covered
openings over each basin. At the ends of the tunnel the culvert is
usually divided into two branches, one running to the drain on each
side of the track.
[Illustration: FIG. 48.--Refuge Niche in St. Gothard Tunnel.]
=Niches.=--In short tunnels niches are employed simply as places of
refuge for trackmen and others during the passing of trains, and are of
small size. In long tunnels they are made larger, and are also employed
as places for storing small tools and supplies employed in the
maintenance of the tunnel. Niches are simply arched recesses built into
the sides of the tunnel, and lined with masonry; Fig. 48 shows this
construction quite clearly. Small refuge niches are usually built from 6
ft. to 9 ft. high, from 3 ft. to 6 ft. wide, and from 2 ft. to 3 ft.
deep. Large niches designed for storing tools and supplies are made from
10 ft. to 12 ft. high, from 8 ft. to 10 ft. wide, and from 18 ft. to 24
ft. deep, and are provided with doors. Refuge niches are usually spaced
from 60 ft. to 100 ft. apart, while the larger storage niches may be
located as far as 3000 ft. apart. The niche construction shown by Fig.
48 is that employed on the St. Gothard tunnel.
=Entrances.=--The entrances, or portals, of tunnels usually consist of
more or less elaborate masonry structures, depending upon the nature of
the material penetrated. In soft-ground tunnels extensive wing walls are
often required to support the earth above and at the sides of the
entrance; while in tunnels through rock, only a masonry portal is
required, to give a finish to the work. Often the engineer indulges
himself in an elaborate architectural design for the portal masonry.
There is danger of carrying such designs too far for good taste unless
care is employed; and on this matter the writer can do no better than to
quote the remarks of the late Mr. Frederick W. Simms in his well-known
“Practical Tunneling”:
“The designs for such constructions should be massive to be suitable
as approaches to works presenting the appearance of gloom, solidity,
and strength. A light and highly decorated structure, however elegant
and well adapted for other purposes, would be very unsuitable in such
a situation; it is plainness combined with boldness, and massiveness
without heaviness, that in a tunnel entrance constitutes elegance,
and, at the same time, is the most economical.”
[Illustration: FIG. 49.--East Portal of Hoosac Tunnel.]
Fig. 49 is an engraving from a photograph of the east portal of the
Hoosac tunnel, which is an especially good design. The portals of the
Mount Cenis tunnel were built of samples of stone encountered all along
the line of excavation. The stones were cut and dressed and utilized for
walls and voussoirs. The only ornament that is usually allowed on the
portals is the date of the opening of the tunnel prominently cut in the
stone above the arch.
TABLE II.
Showing Thickness of Masonry Lining for Tunnels through Soft Ground.
+------------------------------+------------+-----------+------------+
| CHARACTER OF MATERIAL. | KEYSTONE. | SPRINGERS.| INVERT. |
+------------------------------+------------+-----------+------------+
| | Ft. | Ft. | Ft. |
|Laminated clay, first variety |2.15 to 3 |2.75 to 3.5| 1.6 to 2.5|
|Laminated clay, second variety|3 to 4.5 |3.5 to 5.5| 2.5 to 4 |
|Laminated clay, third variety |4.5 to 6.5 |5.5 to 8.1| 4 to 4.5|
|Quicksand |2 to 3.28|2 to 4.1| 1.33 to 2.5|
+------------------------------+------------+-----------+------------+
TABLE III.
Showing Thickness of Masonry Lining for Side Tunnels through Soft
Ground.
+------------------------------+----------+----------+-----------+
| CHARACTER OF MATERIAL. | KEYSTONE.|SPRINGERS.| INVERT. |
+------------------------------+----------+----------+-----------+
| | Ft. | Ft. | Ft. |
|Laminated clay, first variety |1.6 to 2.3|1.8 to 3 |1.5 to 2 |
|Laminated clay, second variety|2.3 to 3 |3 to 4.1|2 to 2.6 |
|Laminated clay, third variety |3 to 4 |4.1 to 5 |2.6 to 3.29|
|Quicksand |1.6 to 2.5|1.3 to 2 |1.3 to 2 |
+------------------------------+----------+----------+-----------+