for the foundations of large piers in deep water. Practically they may
be considered as cylinders on a large scale, with the difference that
whereas cylinders are generally continued up to the under side of the
girders of the superstructure, caissons are only carried up to a short
distance above the water-level. A caisson forms a strong water-tight
iron cofferdam, from which the water can be excluded, and a masonry or 122
brickwork pier constructed inside. It may be made all in one piece to
correspond to the form of the pier, or in separate pieces to form one
whole, each being sunk independent of the other, and connected
together afterwards. Being built up of plates cut to the proper size
and shape, it is a very simple matter to rivet on additional tiers of
plates as the caisson is lowered deeper and deeper into the bed of the
river. The lower length is made with a cutting edge to penetrate the
ground; the exterior is made without any projection larger than the
rivet heads, and the interior is strengthened with T-irons or double
L-irons at the joints, and strong cross-bracing to resist the
pressure of the water. About 7 or 8 feet above the cutting edge a
strongly framed iron floor is riveted to the vertical sides, and
strengthened by plate-iron under-brackets placed at short distances.
The excavators work in the space below the floor, and the excavated
material is passed up through openings formed in the floor at
convenient points to suit the working. The methods of lowering a
caisson are the same as for lowering a cylinder. If the pneumatic
system has to be adopted, then two or more air-tight tubes of liberal
dimensions (say 5 to 8 feet diameter), according to the size of the
caisson, must be attached to the floor, and on the top of each of
these tubes air-locks must be secured for the removal of men and
materials. The masonry or brickwork of the pier is built upon the iron
floor, and a portion of this building work is usually carried on
during the sinking of the caisson to obtain weight to assist in the
lowering. When down to the proper depth, the space below the floor is
properly cleared of _débris_ and water, and then carefully filled in
with cement concrete.
Some caissons are made with vertical sides throughout their entire
height; others have an outward taper for 15 or 20 feet on the lower
end. The former are not only simpler in construction, but are more
easily kept in a vertical position during the sinking. Caissons are
usually put together in some convenient place near the edge of the
water, and then conveyed on pontoons to the sites of the piers. Great
care is required in lowering them into position in the bed of the
river, and guide-piles, guy-chains, and other appliances are
frequently necessary to keep them vertical during the sinking.
The form, dimensions, thickness of plates, cross-bracing, and general
arrangement will depend upon the size and depth of the pier to be 123
constructed. Caissons for heavy work on difficult or treacherous
ground require great care, not only in their construction, but also in
placing them in exact position, and in sinking them correctly to their
proper depth. A tilted caisson is a most difficult subject to handle,
and entails heavy expenditure to restore it to a true vertical
position. By making careful borings, the engineer can ascertain very
closely the depth to which the caisson will have to be lowered to
obtain a good firm foundation. With this information the caisson can
be so constructed that the upper portion, termed the temporary
caisson, commencing a few feet above the bed of the river, can be
detached, and removed at the completion of the work from the lower or
permanent portion sunk below the ground line.
Fig. 118 gives sketches of a wrought-iron plate-caisson applied to a
deep-water river pier, and lowered to its full depth by the pneumatic
process; dotted lines show the air-tubes through which the excavated
material is hoisted and emptied into barges alongside.
Many large and important pier foundations have been constructed on the
system of brick cylinders or wells, particularly in India, where the
foundations for large river viaducts have to be carried down to great
depths through thick deposits of soft material. These wells are built
upon V-shaped curbs to facilitate the penetration when sinking. Fig.
119 is a section of a well with a wrought-iron curb, and Fig. 120 is a
similar well with a wooden curb. The wrought-iron curb is made in
segments for convenience of transport, the pieces forming the complete
ring being bolted or riveted together at the site of the foundations.
The wooden curb is composed of several thick layers of hard wood
planking cut to the proper shape, and laid with broken joints, the
whole being bound together with suitable bolts and spikes. In some
cases the lower or cutting edge of the wooden curb is strengthened or
protected by a sheathing of wrought-iron plates.
[Illustration: Fig. 118]
[Illustration: Fig. 119, 120, 121, 122, 123]
Well foundations are usually put in when the rivers are at their
lowest, and reduced to a few small channels in the great width of
dried-up river bed. This condition enables the greater portion of the
curbs to be conveniently and accurately placed in position on dry
ground, or on ground which, although soft and muddy, is not covered
with water. Should the site of one of the wells occur in one of the
small channels, the stream can be diverted to one side, and a small 126
artificial island made to receive the curb above water-level. When a
curb is fairly fixed in position, the work of building the brick well
can be commenced. With the wrought-iron curb the triangular cavity
between the vertical plate and sloping plate must be filled with
concrete to form a level base for the first course of brickwork. The
wooden curb being composed of horizontal layers of timber, is ready to
receive the brickwork without further preparation. To strengthen and
keep the brickwork firmly tied together, strong wrought-iron vertical
tie-rods, 1¼ or 1½ inch in diameter, are generally built into the
work--as shown in the sketches--at distances about four feet apart.
The lower end of the bottom tier of tie-rods is secured to the curb,
and the upper end passed through a strong wrought-iron plate-ring,
which is continuous all round the brickwork. A long deep nut is
screwed down over the top or screwed end of tie-rod until the
plate-ring is down tight on the brickwork. The tightening nuts are
made sufficiently deep to receive the lower ends of a second series of
vertical tie-rods, which in like manner pass through another
wrought-iron plate-ring on the next section of brick well, and the
same arrangement is continued for the full height of the well. The
lengths of the tie-rods will depend upon the lengths of the section of
brickwork to be built at a time, and may vary from 10 to 15 feet.
As the work of building proceeds the curb and brick well will sink
gradually into the ground, and down to a certain depth, varying
according to the material of the river bed, the weight of the brick
well itself will effect the penetration and lowering. Beyond this
depth the lowering must be done by scooping or dredging the material
from the inside of the well, and placing heavy weights of old railway
iron or other convenient masses on the top. When one section or length
of well has been sunk down, then another set of tie-rods are inserted
into the deep nuts, and another section of brickwork commenced. The
operation of lowering is rather tedious, as all the weights have to be
hoisted up on to the top of the length in hand, and piled so as to
leave space for lifting out the material dredged from the interior;
and then, when the length has been lowered, all the weights must be
removed before the brickwork can be resumed on another length. Where
the river bed consists of soft material, the excavation inside the
well can generally be effected by suitable dredges or scoops worked 127
from the surface or top of brickwork. Should trees or other
obstructive masses be met with embedded in the strata, it will be
necessary to employ divers to remove them piecemeal out of the way of
the curb.
When the brick well has been lowered down to the full depth, and is
thoroughly bedded in a stratum of strong material, the test weights
should be left on for some time to ascertain if there is any further
sinking. After all the weights have been removed the bottom of the
well can be dredged out clean, and the interior filled in with
concrete to such height as may be considered necessary.
Brick wells must be watched carefully to ensure that they sink down in
a perfectly vertical position. Any inclination away from the
perpendicular must be corrected at once by means of guys and struts,
the same as in sinking iron cylinders. The principal difficulty will
be with the first 20 or 25 feet.
The diameter of the well will depend upon the weight it has to carry,
and its height from river bed to under side of girders. The wells may
be either circular or polygonal in section, and built singly or in
pairs, as shown in sketches (Fig. 121).
Many piers and abutments of bridges in shallow or moderately deep
water are built by means of coffer-dams of timber and clay puddle. The
coffer-dam forms a water-tight wall round the site of the foundation,
from which the water is pumped out, and the excavation carried down to
the depth required. In very shallow water it is sometimes sufficient
to drive only a single row of piles, and form a bank of good clay
puddle on the outside, as shown in Fig. 122. In deep water it is
necessary to drive a double row of piles, 3 or 4 or more feet apart,
and fill in the space between with clay puddle, as shown in Fig. 123.
The piles for coffer-dam work should be carefully selected, of good
timber straight, and correctly sawn on the contact faces. Guide-piles
are first driven in proper line and position round the intended
foundation. To these strong horizontal double waling pieces are
securely bolted, one on each side of the guide-pile, one pair near the
top, and the other pair as low down as can be placed. The sheeting
piles, which are lowered down between the horizontal waling or guiding
pieces, are driven as close to one another as possible, being assisted
in doing so by the sheet-pile shoe, shown on Fig. 124, which is made
not with a point like an ordinary pile shoe (Fig. 125), but with a 128
cutting edge slightly inclined, so that in driving the tendency of the
pile is to drift towards the pile previously driven. Sometimes the
outer row of piles consists of whole balks, and the inner row of half
balks; the size of the piles must, however, be regulated by the depth
and current of the water. When both rows of piles have been completed,
the space between should be dredged out, and then filled with
carefully prepared clay puddle. To enable the puddle to adapt itself
thoroughly to the wooden sides, it is desirable to remove the inside
walings after all the piles are driven, as any internal projections
interfere with the proper punning and settling of the puddle. The
swelling of the puddled clay has a tendency to force apart the two
rows of piles, and to counteract this as much as possible, iron
tie-rods should be passed through from side to side every few feet,
and screwed up against large washers placed on the outside of the
outer walings. Strong struts or cross-bracing of timber must be placed
from side to side inside the coffer-dam to resist the pressure of the
water in the river. This cross-bracing can be removed gradually as the
work of building progresses upwards, and be replaced with short struts
wedged in against the sides of the finished courses.
In cases where the ground is soft, and when it is not considered
prudent to excavate the foundations deeper for fear of disturbing the
stability of the coffer-dam piles, rows of large, square bearing-piles
may be driven in the floor of the foundation, as shown in Fig. 111.
The tops of these bearing-piles must all be sawn off to the same
level, and a platform of strong double planking securely fixed to the
piles to receive the foundation course of concrete, masonry, or
brickwork. The spaces around the tops of the piles and the under side
of the timber platform should be filled in with good cement concrete.
The interior of the coffer-dam is kept dry by constant pumping, either
by hand pumps or steam pumps, according to the volume of water finding
its way into the foundations. When the finished pier or abutment has
been carried up above the river water-level, the coffer-dam is no
longer required, and may be removed. Sometimes, to save the timber,
the piles are drawn by means of strong tackle fitted up for the
purpose; but in doing this there is considerable risk of disturbance
to the foundations, and it is better to leave the piles in the ground
and employ divers to cut off the tops a little above the bed of the 129
river.
In preparing the design for a large foundation it is absolutely
necessary to first ascertain by careful borings the description of
material upon which that foundation must be placed, so as to
proportion the area of bearing surface to the weight to be sustained.
Some materials will naturally carry more weight than others, and
although the engineer cannot always select the material he would
prefer, he can, however, control the superficial area of the
foundations. Much valuable information has been obtained both from
experiments and from comparisons of actual practice, and the following
memoranda may be useful for reference, as indicating the pressures per
superficial foot which may be safely put on various materials:--
Moderately stiff clay 2½ tons.
Chalk 4 ”
Solid blue clay 5 ”
Compact gravel and close sand 6 ”
Solid rock 12 ”
Doubtless the above weights have been exceeded in many cases, but it
is better to be on the safe side, and leave a good margin for
stability.
Large subaqueous foundations for heavy piers and abutments are costly
and tedious, and especially so when the pneumatic process has to be
adopted. Special appliances and well-trained, experienced workmen are
requisite, and if all the men and materials have to pass through the
air-locks, the progress of the work must necessarily be slow. When the
foundations have been completed up to the level of the water, the
construction can be pushed on more rapidly, as the work of
scaffolding, hoisting, and building, can all be carried on in the open
air.
Amongst the very many types of arch-work and girder-work adopted for
railway purposes, the following examples from actual practice may be
useful for reference:--
[Illustration: Fig. 126, 127]
Fig. 126 represents small 24-foot span, low viaduct arching suitable
for a line passing through towns or villages, where ground is valuable
and the area to be covered must be kept as small as possible. The
arches may be utilized for stables, stores, or roads of communication
between the lands and properties intersected by the railway. The
segmental form gives a better headway underneath than the
semicircular, besides containing less material in the arching proper,
and requiring a smaller amount of centering. Every precaution should 131
be taken to prevent water percolating through any portion of the
arching, or haunching, and a thick layer of good asphalte should be
placed over the entire upper surface, and carried well up the lower
portion of the parapet walls, as shown on the sketch. The cast-iron
pipes with rose heads form a very efficient means of taking away the
rain-water which filters through the ballast and filling. The pipes
should be carried down in chases, or recesses, built in the fronts of
the piers, to protect them as much as possible from injury in the
yards below. Rose heads, pierced with holes, and surrounded with small
stones hand-laid, serve well to conduct the water into the pipes.
Where the arching is of considerable length, recesses or refuges for
the platelayers may be obtained by substituting a short length of
cast-iron-plate parapet, instead of the stone or brick parapet, over
some of the piers, as indicated in the sketch.
Fig. 127 shows a similar description of arching for spans of 30 feet.
The above two examples represent plain substantial work, but if
circumstances warrant more external finish, this can readily be added
without interfering with the general arrangement. In a similar manner,
if considered preferable, the arches may be made semicircular or
elliptical.
In the sketches shown of the arched over-line and under-line bridges,
the arching and coping of parapets are in brick, and the remainder of
the work in stone. In very many cases brick will be found cheaper and
more expeditious for arching than stone, unless the quarries turn out
stone in blocks which can be conveniently trimmed for arching. All
bricks used for arch-work should be hard and well burnt, and special
care should be taken in the selection of those to form the under-side
course, which will be exposed to the atmosphere. For moderate spans
arches have been successfully constructed of concrete. For this
description of work the materials should be carefully gauged and mixed
together, and the finished work should be allowed to stand some time
on the centres to allow the concrete to become thoroughly set.
[Illustration: Fig. 131]
In Fig. 102, the cutting being deep, almost up to the level of the
public road, the foundations of the wing walls are built in steps,
resulting in a minimum of masonry below the finished ground line.
Where the cutting is shallow, and the public road has to be brought up
to the bridge on an embanked approach, the greater portion of the wing 133
walls will have to be built up from the solid or original ground, and
there will be a large amount of masonry below the finished ground
line, as indicated in Fig. 128.
In some cases of over-line bridges it is necessary to curve the wing
walls to correspond to the road which turns off to the right or left
after crossing the railway, as shown in Fig. 129; or the wing walls
may have to form two separate curves where the road branches off in
two directions after leaving the bridge, as shown in Fig. 130.
Fig. 131 shows plan, elevation, and cross-section of an under-line
arch bridge, considerably on the skew, carrying a railway over a
river. The wing walls are curved, and very similar in type to some of
those in preceding examples. The river bed and ground alongside being
of solid rock, good foundations were obtained at a very moderate cost.
On many railways constructed in the beginning as single lines only,
the over-line bridges have been built for double line. The additional
cost in the outset has been small, compared with the great expenditure
which would be incurred afterwards in reconstructing the bridges to
suit a double line.
The general arrangement of abutments and wing walls shown in the
foregoing examples will apply to similar classes of bridges where
girder-work is adopted instead of arching.
There are many ways of forming the floor or deck of a girder bridge
intended to carry a railway over a road or stream. In some cases it
will be imperative to have a thoroughly water-tight floor to prevent
rain-water percolating through to the roadway below; while in others,
such as bridges over streams, and secondary roads, this special
provision will not be necessary, and a lighter and more economical
floorway can be adopted. A strong wrought-iron or steel-plate
flooring, with its corresponding filling and ballasting, means not
only so much additional cost in the flooring proper, but also so much
additional dead weight to be carried by the main girders.
[Illustration: Fig. 132, 133, 134, 135, 136, 141]
Fig. 132 is a sketch of rolled joist-iron I-girders and timber floor
frequently adopted for small farm roads and cattle creeps of 10 or 12
feet span. A beam of timber is fitted in between the two rolled
joist-irons, and the three pieces securely fastened together with
strong iron bolts placed about 3 feet apart. These small compound
girders rest on bearing-plates of wrought or cast iron, and are held 135
together and to gauge by tie-rods, as shown. The rails are spiked or
bolted down on to the timber beams, and the flooring formed of strong
planking.
Fig. 133 shows an arrangement of plate girders for a 16-foot opening
over a stream. The girders are placed immediately under the rails, and
are tied together by plate-iron cross-bracing the same depth as the
main girders. The flooring consists of 4-inch planking laid with
¾-inch spaces, on which are laid longitudinal rail-bearers 14 inches
wide by 7 inches thick.
Fig. 134 is a sketch of a somewhat similar arrangement for a
lattice-girder bridge, 45 feet span, carrying a single line of railway
over a river. The main girders are tied together by lattice-work
cross-bracing. The floorway consists of 5-inch planking, laid with
¾-inch spaces, on which is placed the 14 feet by 7 feet longitudinal
rail-bearers. Plate-iron outside brackets are riveted to the main
girders to carry the ends of the planking and light tube-iron parapet.
Fig. 135 illustrates an example of trough girders, constructed to
carry a double-line railway over a country road 25 feet wide, where
the space from under side of girder to rail-level is small. The
girders are constructed in pairs, with short, shallow cross-girders at
3 feet 6 inch centres, riveted in between them to carry longitudinal
timbers on which the rails are laid. Bottom plates, 5/8 inch thick,
unite the two girders for the length of their bearing on the
abutments, and a similar plate, 9 inches wide, unites them at the
centre; the remainder of the span is left open to prevent the lodgment
of rain-water. Three strong tie-rods are placed to keep the girders to
gauge. Curved wrought-iron ballast-plates are used between the
running-rails, and plank flooring forms the rest of the covering.
[Illustration: Fig. 137]
Fig. 136 is a sketch of a plate-girder bridge over a country road 28
feet wide, with the load carried on the lower flange of girder. Three
main girders carry the double line of railway, the centre one having
double the strength of each of the outside girders. On the top of the
cross-girders, strong angle irons are riveted to serve as guides and
supports for the longitudinal timbers which carry the rails. Every
third cross-girder has raised ends to give increased lateral stability
to the main girders. A close cast-iron plate parapet forms a screen to
the roadway. Wrought-iron ballast-plates are used between the
running-rails, and the remainder of the flooring is of timber.
Fig. 137 gives the particulars of one 60-foot span of a viaduct 137
carrying a double line of railway over tidal water. The main girders
are placed one under each line of rails, and all the four are strongly
tied together by lattice-work bracing the full depth of the girders.
The outside footpaths for the platelayers are carried on strong
brackets, riveted to the main girders. Longitudinal timbers, coped
with angle iron, are placed as outside guards, alongside each rail,
for the full length of the viaduct. Wrought-iron ballast-plates are
placed between the running-rails. The remainder of the footways
consist of timber planking, laid with half-inch spaces, and covered
with a layer of small pebbles as a protection against fire.
Fig. 138 shows a very similar arrangement in a viaduct carrying a
single line of railway across a river. The two main lattice
girders--66 feet span--are placed at 9-foot centres, to obtain greater
stability. The cross-girders are extended to carry the outside
footpaths and handrailing. Outside guards are placed alongside each
rail as in the preceding example. Wrought-iron ballast-plates are
fixed all along between the running-rails, and timber planking used
for the rest of the floorway.
Fig. 139 gives cross-section of a lattice-girder bridge, 82 feet span,
carrying a single line of railway over a river, with the load carried
on the lower flange. The cross-girders are placed at 4 feet 3 inch
centres. Wrought-iron ballast-plates compose the floorway between the
rails, and timber planking covers the rest of the bridge. Plate
diaphragms, or stiffeners, of the form shown at A, A, A, A,
are riveted to the main girders at five places in their length.
Fig. 140 shows cross-section of a lattice-girder bridge of 200 feet
span, carrying a single line of railway over a river, the load being
placed on the lower flange. The floorway consists of plate-iron
cross-girders, spaced at 4-foot centres, on which are placed the
longitudinal rail-bearers and planking, the latter being covered with
a layer of clean pebbles for the width between the running-rails. As
the depth of the main girders was sufficient to admit of overhead
bracing, strong plate-iron diaphragms, of the form shown on the
sketch, were riveted to the main girders at every 50 feet. These
diaphragms thoroughly brace the two girders together, and effectually
prevent any tendency to side-canting, at the same time imparting an
effective appearance to the bridge.
[Illustration: Fig. 138, 139, 143]
Fig. 141 shows cross-section of a plate-girder bridge, of 36 feet 139
span, carrying six lines of way across a street. Strong plated
cross-girder bracing, at 4 feet 8¼ inch centres, is riveted to the
main girders, and the top is covered with old Barlow rails, 12 inches
wide, and weighing 90 lbs. per lineal yard. A layer of asphalte, about
1½ inches thick, is carefully laid all over the upper surface of these
rails to make a thoroughly water-tight floor. Clean gravel is placed
on the top, on which are laid the sleepers and rails of the permanent
way. Rain-water passes through the gravel into the hollows of the
Barlow rails, and finds its way into suitable drains provided at each
abutment. This arrangement not only prevents the falling of drip-water
into the street below, but permits of the alterations of the lines of
way, or putting in of cross-over roads on the surface above. The
outside main girders are made deeper, and are surmounted by close
cast-iron parapets.
Fig. 142 gives the particulars of a three-span plate-girder bridge,
constructed to carry a double line of railway over two other railways
and a canal, the load being placed on the lower flange. Two main
girders are used for each line of way. Strong plated cross-girders are
placed at 5 feet 3 inch centres, and on the top of these is laid a
flooring of old Barlow rails, terminating at the sides with sloping
wing-plates riveted to the cross-girders and main girders, the entire
surface being covered with an inch and a half layer of asphalte. Good
gravel ballast is placed on the top, on which are laid the sleepers
and rails. One central main girder of sufficient strength would have
been as efficient as the two central girders, but there was a
practical difficulty which prevented its adoption. The new girder-work
was built to replace an old structure of peculiar arrangement, and to
keep the traffic going on one line there was no alternative but to
make each line of way complete in itself.
[Illustration: Fig. 140, 144]
Fig. 143 illustrates an example of jack arches in concrete built
between strong plate-girders. The span of the girders was only 16
feet, but the opening or roadway was of considerable length, and
passed under a portion of a busy station yard. The girders are placed
at 6-foot centres, and tied together in pairs by 1¼-inch tie-rods,
three to the span, spaces of 6 inches in plan being allowed between
each set of the rods. The concrete was curved up to the top plate of
the girder, as shown, and the entire surface covered with a thick
layer of asphalte, on which were placed the ballast and permanent way. 141
Brickwork might have been used for the jack-arching, but concrete was
considered more convenient.
Fig. 144 shows the cross-section of a truss-girder bridge of 123 feet
span, carrying a double line of railway over a wide thoroughfare, the
load being placed on the lower flange. There are two main girders,
each 12 feet 6 inches deep in the centre, and 8 feet deep at the ends.
Plate cross-girders are placed at 4 feet 6 inch centres, on which is
riveted longitudinal plate-iron troughing, extending across the bridge
and terminating at the sides with wing-plates, as shown. The entire
floor is covered with a thick layer of asphalte previous to filling in
with ballast to receive the permanent way. Plate stiffeners are
adopted in this bridge very similar to those in Fig. 139.
Fig. 145 gives plan, elevation, and cross-section of a plate-girder
bridge of 95 feet span, carrying a double line of railway over a very
busy street. There are two curved-top main girders, each 10 feet 9
inches deep in the centre, and 6 feet 7½ inches deep at the ends. The
arrangement of cross-girders, longitudinal plate-iron troughing, and
permanent way, is very similar to that in the preceding example, but
the side wing-plates are carried up higher, and are riveted up to the
web-plate of main girder, forming continuous stiffeners from end to
end of the main girders. A light, ornamental, close cast-iron parapet
is bolted on to the top of the curved, or upper, boom of the main
girder, the top line of the parapet being carried out parallel to the
bottom boom of girder. This bridge crosses the street very obliquely,
and, although cast-iron columns were allowed at the edge of the
footpaths, the main spans are unavoidably large. When designing the
above bridge, the writer had to adopt a girder that would form a
screen, to provide a deck, or floor-way, which would be not only
water-tight, but also deaden as much as possible the sound or
vibration of passing trains, and at the same time give some ornamental
appearance to the girders and parapets. This bridge carries a constant
service of heavy trains; it is perfectly dry underneath, and is
remarkably free from noise or vibration.
[Illustration: Fig. 142]
Fig. 146 shows cross-section of a plate-girder bridge of 40 feet span,
carrying a double-line railway over a street, in a situation where the
depth from top of rails to under side of girders had to be made as
small as possible. Three main girders were used, the centre one being 143
double the strength of each of the outside girders. Instead of
ordinary cross-girders, transverse plate-iron troughing was adopted,
very similar in section to the longitudinal iron troughing in Fig.
145, but stronger. The troughing rested on the angle iron of bottom
flange of main girder, and was riveted to the vertical web-plates of
main girders, shallow additional vertical plates being inserted
alongside web-plates to prevent any drip-water or moisture coming in
contact with the main web-plates. The entire surface of the troughing
was well covered with asphalte before filling the hollows with gravel
ballast. An ordinary transverse wooden sleeper was placed in each
hollow, and on these sleepers the rails were secured as shown. In this
case--as in others of transverse troughing--the rain-water had to be
conveyed away from the hollow of each trough by a separate outlet into
longitudinal gutters shown at A, B, and continued on to the
abutments.
Transverse troughing is always more troublesome than longitudinal
troughing, as both ends of each trough must be effectually closed to
prevent the drainage water leaking out on to the web-plates, or angles
of the main girders. With longitudinal troughing the water is readily
carried away from each hollow, to cross drains constructed at the
piers, or abutments.
Fig. 147 shows cross-section of a truss-girder bridge, 120 feet span,
carrying a single line of railway over a river. The cross-girders are
placed at 10-foot centres to correspond to the vertical members of the
main truss-girder. Longitudinal plate-iron rail-girders are riveted in
between the cross-girders, and the entire floor is covered with curved
wrought-iron ballast plates, as shown. The rails are carried on
longitudinal timbers, which are bolted on to the rail-girders. Angle
iron brackets, riveted on the top of the cross-girders, keep the rail
timbers in position and gauge.
[Illustration: Fig. 145]
In each of the above examples, where longitudinal rail timbers are
adopted, flange rails are shown, as many engineers prefer to have a
continuous bearing for the rails on bridges, in case of rail fracture.
There is nothing, however, to prevent the chair road being laid on
longitudinal timbers, and for this purpose the writer has used chairs
of the ordinary pattern, specially cast with side lugs to grip the
timber, as shown in Fig. 148. Chairs of this form have a very firm 145
hold on the longitudinal timber, and the side lugs check any tendency
of the splitting or opening of the wood when putting in the spikes or
screw bolts.
Fig. 149 shows cross-section of a plate-girder over-line bridge, 32
feet span, carrying a private road, 12 feet wide, over a double-line
railway. The road traffic being small, the floorway was constructed of
creosoted planking carried on rolled I-iron cross-girders placed at 3
feet 8 inch centres, and riveted to the main girders. The horse-tread
track was provided with a second layer of planking, laid transversely,
to take up the wear, cross battens, 4 inches by 2 inches, being placed
at 12-inch centres, and sand spread between to give good foothold. A
light lattice-work parapet was bolted on to the top of the main
girders.
Fig. 150 gives cross-section of a plate-girder over-line bridge, 30
feet span, carrying a private road, 20 feet wide, over a double-line
railway. The main girders are tied together by lattice-work bracing,
spaced at 7-foot centres. Curved wrought-iron plates are laid across
from girder to girder, and butt against a narrow horizontal plate,
which forms part of the upper boom. The curved plates are riveted on
to the top of girder, and form a continuous iron floor, or deck, from
side to side of the bridge. Upon this iron floor is laid an ordinary
asphalte roadway. The outside girders are made deeper, and carry an
ornamental cast-iron parapet. In some bridges of a similar
construction, the roadway is formed of creosoted wooden block paving,
on a foundation of asphalte.
[Illustration: BRIDGE CARRYING THE D. W. AND W. RAILWAY (LOOP LINE)
OVER AMIENS STREET, DUBLIN. [_To face p. 144._]
[Illustration: Fig. 146, 147, 158, 159, 148]
Fig. 151 shows cross-section of a plate-girder over-line bridge, 28
feet span, carrying a public road, 35 feet wide, over a double-line
railway. The main girders, 2 feet 4 inches deep, are placed at 5 feet
2 inch centres, and are tied together by plate-iron cross-bracing 2
feet deep. Jack-arches of brickwork, 9 inches thick, are built in
between the main girders, the haunching being filled in with concrete.
The entire surface is covered over and made watertight with asphalte,
on which is laid the metalling of the roadway. The outside girders are
made considerably deeper, and have strong cast-iron-plate parapets
bolted on to the top booms. There is no doubt that jack-arching of
brickwork or concrete makes a very strong and permanent floorway, but
its dead weight is very great, and its adoption is not to be recommended 147
where iron or steel plate troughing can be obtained at a moderate
price.
Fig. 152 gives cross-section of plate-girder over-line bridge, 41 feet
6 inch span, carrying a public road, 25 feet wide, over three lines of
way. Two main girders are used, of sufficient depth to form parapets
or screens for the finished roadway. Plate cross-girders, placed at 6
feet 6 inch centres, are riveted to the web-plate and lower angle
irons of main girders; and on these is placed a flooring of plate-iron
longitudinal troughing to carry the metalled roadway.
Fig. 153 gives the particulars of a plate-girder over-line bridge,
carrying an important public road, 35 feet wide, over several main
lines and sidings. The carriage-way is carried by two girders placed
at 25-foot centres, and on the lower boom of these are riveted
lattice-work cross-girders to receive the plate-iron longitudinal
troughing and roadway. The footpath girders are set at a higher level,
and the load placed on the lower flange. The curved side brackets
merely act as bracing between the carriage-way girders and footpath
girders. A cast-iron-plate parapet is bolted on to the top of each of
the footpath girders, making a close screen, 6 feet high, above the
footpath. Lattice-work cross-girders were adopted for the convenience
of supporting small water mains and gas mains below the road-level.
The roadway is formed of ordinary metalling, and the footpaths of
asphalte pavement; the kerbing is of granite, and the side
water-tables of crushed granite concrete.
[Illustration: Fig. 149, 150, 151, 152]
Fig. 154 is a cross-section of a small uncovered lattice-girder
footbridge 41 feet span, and 5 feet wide, suitable for small roadside
stations. The top and bottom flange consist each of two angle irons,
those in the bottom flange being placed table side upwards, so as to
bring the entire section of both angle irons fairly into play, and
also to provide a better bearing for the channel-iron cross-girders
which carry the planking of the footway. When planking is carried on
the inside of light angle iron, as in Fig. 155, a severe strain is
produced at the point A; this is entirely obviated by placing the
bottom angle irons table side upwards, as in Fig. 156. Three of the
channel-iron cross-girders are extended outwards, and to the ends of
these are riveted tee-iron stiffeners to steady the main girders. In
some cases stamped, or ribbed, wrought-iron plates are used for a
footway, but, although more durable, they do not give such a secure or 149
agreeable foothold as timber. The ascent or descent of the bridge may
consist either of steps and landings, or of ramps, according to
circumstances or expediency. Sometimes these bridges are made with
curved tops, terminating in steps when nearing the steps, or ramps. It
is very questionable whether such an arrangement is a good one or a
safe one. There is always a feeling of insecurity when walking over a
sloping surface broken up by steps, and experience points out that it
is better to continue the footway level right across to the place
where the passenger must change his direction to go down the stairs or
ramp.
Fig. 157 gives cross-section of a covered lattice-girder footbridge,
62 feet 6 inches span, and 10 feet wide, suitable for an important
station. The upper boom of girder consists of two angle irons and top
plate, and the bottom boom of two channel irons. The cross-girders are
rolled joist-irons resting on the top tables of the channel irons.
Four of the cross-girders are extended outwards, and carry plate-iron
outside vertical brackets to stiffen the main girders. Three-inch
longitudinal planking is laid down from end to end of the bridge, and
on this is laid 1¼-inch transverse flooring, in narrow widths, to form
the walking deck. The footbridge is lighted from the sides by
continuous glazed sashes fixed in strong wooden framework, as shown.
The roof is covered with canvas bedded in white lead, and painted in
the same way as an ordinary carriage roof.
The above examples of under-line and over-line bridges are given more
with a view of illustrating some of the many different descriptions of
flooring, rather than to point out or suggest the type of main girder
to carry the load. The description and size of the main girders can be
varied to suit the span of the bridge, the requirements of the
traffic, and the opinion of the designer. For spans up to 50 feet it
will generally be found that web-plate girders are both simpler and
cheaper than lattice or truss girders; at the same time, there are
occasions where plate girders can be advantageously adopted for very
much larger spans, as, for instance, in the example given in Fig. 145,
where the deep plate girders form a most efficient screen.
[Illustration: Fig. 153]
Figs. 160 to 194 give diagram sketches of a few out of the many forms
of open, or truss, girders which have been adopted for large spans.
There are many types from which to make a selection, each one
possessing its own special features and advocates. In working out the 151
details of any, or all of them, there are some points which should
always be kept in mind when deciding the distribution of material in
the main booms. Rain-water, or moisture of any kind, is the great
enemy of wrought-iron or steel work, and therefore the plates, angles,
tees, or channel sections, should be so arranged as to afford the
least possible facility for the collection or lodgment of water. With
open, level booms, as in Figs. 137, 139, 140, 144, and 145, the
rain-water cannot collect, but runs off at the sides, and the plates
are quickly dried by the sun and wind. With trough booms, as in Fig.
158, the collected rain-water can only get away through holes drilled
for the purpose in the bottom plates. These holes are liable to become
choked up, but even when open they rarely carry off all the
accumulated water; some of it remains to corrode the plates, and is
only dried up by evaporation. The inside of trough booms should be
constantly inspected, and the exposed plates more frequently painted
than the rest of the girder. In a similar manner, in small double-web
lattice girders, with the lattice-bars inserted between two angle
irons, as in Fig. 159, the rain-water finds its way into the spaces at
A, A, in spite of the most careful packing or filling with cement
or asphalte. Numbers of small girders of this latter type have had to
be taken out after a comparative short life, in consequence of the
great corrosion and wearing away of the lower ends of the lattice-bars
and angle irons into which they were inserted.
It is most essential, also, that all portions of the girder-work
should be conveniently accessible for inspection and painting.
Complicated connections, and parts which are difficult to examine, are
liable to be overlooked, or, at the best, only painted in a very
imperfect manner. Neglected corners soon create deterioration, the
paint scales off, corrosion commences, and the working section is
gradually reduced. A discovered weakness in some of the important
parts points to an early condemnation of the entire structure. The
difficulty of access to the interior of box or tubular girders,
especially those of small or moderate dimensions, is a great objection
to that type of girder. Experience has pointed out that open girders,
free and exposed to the light and air, can be so much more effectually
inspected and painted.
[Illustration: Fig. 154, 155, 156, 157]
[Illustration: Fig. 160, 161, 162, 163, 164, 165, 166, 167, 168]
[Illustration: Fig. 169, 170, 171, 172, 173, 174, 175, 176]
[Illustration: Fig. 177, 178, 179, 180, 181, 182, 183]
[Illustration: Fig. 184, 185, 186, 187, 188, 189, 190, 191]
[Illustration: Fig. 192, 193, 194]
Perhaps one of the most anxious tasks which falls to the lot of an
engineer is the renewal of under-line bridges and viaducts on a
working line. On a new line in course of construction the entire site 158
of the work is at the disposal of the erectors, and the building of a
bridge or viaduct can be carried on with a freedom which cannot be
obtained on an open line. On a working railway, the train service must
be kept going, irrespective of renewals, and very often the best that
can be done is to reduce the double line to single line working at the
site of the operations. It is not always expedient or possible to make
a temporary bridge and diverted line for traffic purposes, as the
expenditure to be incurred might be too great to warrant the outlay,
or there may be local difficulties to effectually prevent the
introduction of a provisional structure. The taking down of one half
of the old structure may necessitate the removal of stays and bracing
affecting the stability of the half remaining to carry the traffic,
and thus render temporary shoring and bracing necessary. The erection
of the new work in such a limited space has to be watched with great
care; all cranes, lifting appliances, and scaffolding must be kept
clear of vehicles moving over the running-line, and very frequently it
is found prudent to cease erecting operations during the passage of a
train.
In very many cases of renewals, the description and arrangement of the
old structure will materially influence or control the design for the
new one, and the details of the latter must be schemed out so as to
disturb as little as possible the stability of the old work remaining
as the working road.
The following list gives the lengths of the main spans of some railway
bridges, and may be found useful for reference:--
LENGTHS OF MAIN SPANS OF SOME LARGE RAILWAY BRIDGES.
-----------------------------------+-------+----------------
Name. | Span. | Description.
-----------------------------------+-------+----------------
| feet. |
Forth Bridge | 1,710 | Cantilever.
Niagara | 821 | Suspension.
Sukkur | 820 | Cantilever.
Poughkeepsie, U.S.A. | 548 | Cantilever.
Douro | 525 | Arch.
St. Louis | 520 | Arch.
Cincinnati | 515 | Linville truss.
Haarlem | 510 | Arch.
Kuilemburg | 492 | Lattice bow.
St. John’s River | 477 | Cantilever.
Niagara | 470 | Cantilever.
Britannia | 460 | Tube.159
Ohio River, Pennsylvania | 442 | Pratt through truss. 159
Saltash | 434 | Tube and girder.
Hawkesberry Viaduct | 410 | Compound truss.
Conway | 400 | Tube.
Vistula | 397 | Lattice.
Spey River, Garmouth, N.B. | 350 | Bowstring.
St. Laurence | 330 | Tube.
Hamburg | 316 | Double bow.
Cologne | 313 | Lattice.
Runcorn | 305 | Lattice.
Sunderland | 300 | Bowstring.
Rondout Bridge, Buffalo | 264 | Pratt through truss.
Newark Dyke (New) | 259 | Lattice bow.
Tay Bridge (New) | 245 | Lattice bow.
Ohio River, Louisville | 245 | Fink truss.
Beaver Bridge, Pennsylvania | 230 | Pratt deck truss.
Craigellachie Bridge | 200 | Lattice.
Rohrbach Bridge, St. Gothard River | 197 | Wrought-iron arch.
Windsor Bridge | 187 | Bowstring.
Victoria Bridge over Thames | 175 | Wrought-iron arch.
Shannon River Bridge | 165 | Bowstring.
Carron Bridge over Spey | 150 | Cast-iron arch.
Preston Viaduct | 102 | Cast-iron arch.
Trent River Bridge | 100 | Cast-iron arch.
-----------------------------------+-------+----------------------
Retaining Walls.--Instances frequently occur during the construction
of a railway where it is advisable, if not absolutely necessary, to
substitute retaining walls in preference to forming the slopes of
cuttings and embankments.
The excavation of a cutting may be greatly reduced in quantity by
introducing low retaining walls, as in Fig. 195, and the saving in the
material to be removed will be all the more important in those cases
where cutting is in excess of embankment.
The amount of filling for an embankment and the land on which it has
to be formed may both be considerably diminished by building a low
retaining wall, say 6 or 7 feet high, at the foot of the slope, as
shown in Fig. 196. Such a retaining wall makes a most efficient fence
and well defined boundary of property.
[Illustration: Fig. 195, 196, 197, 198, 199, 200, 201]
The policy of adopting low retaining walls in cases like the above 161
will depend mainly upon the cost of building materials as compared
with the cost of earthwork and land.
Where land is very valuable, and where residential property, streets,
or roads must be interfered with as little as possible, the retaining
walls may have to be carried up to the level of the original surface
of the ground, as in Fig. 197, which is shown as for a cutting 25 feet
deep. The walls may be built of masonry, brickwork, or concrete, or a
combination of them, and the dimensions or thickness will depend upon
the description of material to be supported. Weeping holes, or small
pipe drains, should be formed in the walls, a little above formation
level, to take away any water which may collect at the back.
Where the cutting is through soft, wet, treacherous clay, liable to
slip or expand, it may be necessary to insert arched thrust girders
extending from side to side, as in Fig. 198, so that the outward
pressure against the one wall may counteract against the outward
pressure of the other. The thrust girders should be placed at from 10
to 15 feet centres, and be well braced together in plan to enable them
the better to resist any tendency of bulging out of the walls.
A similar arrangement of high retaining wall may be introduced in
embankment to lessen the encroachment on streets or public roads, as
shown in Fig. 199.
In making a railway through thickly populated towns, it is generally
preferable to construct the line on arches rather than on earthwork
filling between two high retaining walls. The numerous openings are
available for future streets, or means of communication from one side
to the other, and the arches themselves can be profitably utilized for
stables, stores, offices, and workshops.
Fig. 200 shows a narrow rocky pass with deep rapid river on the one
hand and high cliffs on the other, the only available ledge being
already occupied by a public highway. By building a retaining wall, as
indicated on the sketch, and excavating a little out of the cliff,
space may be obtained for a line of railway; or the arrangement may
have to be reversed, and the retaining wall for the railway built
along the margin of the river, as in Fig. 201.
In both the cases, Figs. 200 and 201, not only must there be a number
of weeping holes left in the lower part of the wall, but there must be
sufficient well-built drains and culverts under the filling and through 162
the wall to carry away all ordinary or flood water coming down from
the cliffs and hills above. Where a retaining wall is built along the
margin of a river, the lower portion, which will be in contact with
the water when the river is full, should be constructed of selected
large heavy stones to withstand the scouring action of the water, and
any brushwood or floating timber which may be brought down by flood
water.
Where retaining walls are built to support wet clay, or in embanked
places on wet side-lying ground, the efficiency of the work will be
much increased by constructing a layer, two or three feet in
thickness, of dry, flat, bedded stones carefully hand-laid, from the
foundation to the top of the wall, as shown in Fig. 199.
These dry stones form a continuous vertical drain to take away water
from any part of the earthwork down to the outlets left in the lower
portion of the wall.
The building of retaining walls entirely of dry stone is very
questionable economy, and entails a constant expenditure in
maintenance and renewal. The working out of one stone loosens the
surrounding portion of the wall, and if not at once repaired, a length
of the wall will fall down, bringing with it a large quantity of the
earthwork.
If readily obtained, large heavy stones should be selected for the
coping of retaining walls, so as to minimize as much as possible the
chance of their disturbance or displacement. Where lighter stones have
to be used, or bricks laid on edge, they should be bedded and pointed
in cement.
In many places it is necessary to form wide and massive foundations of
concrete on which to build the retaining wall; and in some cases of
soft, treacherous ground, timber piling may be necessary.
Tunnels.--It would be difficult to assign a date to the first
examples of subterranean works constructed for utilitarian purposes.
Nature had furnished so many grand specimens of caves, grottoes, and
underground passages formed in the solid rock, that man soon grasped
the principle, and essayed to carry out similar works on his own
account. The early attempts would probably be limited to forming
places of shelter, storage or security. Advantage would be taken of
those rocks which from their locality, accessibility, and compactness
of material, promised favourable results. The appliances being few and
primitive, the work of construction would be laborious and slow. So 163
long, however, as the workers restricted their operations to the solid
rock, they had merely to contend against the hardness of the material,
as the opening or passage-way, once made, required no further support
or attention; but as the wave of progress swept onward, man was
compelled to deviate from the lines originally followed by nature, and
had to form his subterranean pathway through softer material, where
the workings required substantial support. The search for minerals of
various kinds led to the driving of long headings or galleries
underground, and as these had frequently to penetrate through strata
of a soft and yielding character, strong timber framework had to be
introduced to afford stability to the works, and safety to the
workers. For ordinary mining operations, strong rough timber supports
may meet all requirements, and may last until the heading is worked
out and abandoned; but for subterranean passages or tunnels which are
intended to form permanent means of communication, the strongest and
most durable materials must be used to protect the interior as far as
possible from deterioration or decay. Heavy timbering might be
sufficient for mere temporary purposes, but substantial masonry or
brickwork side walls and arching became necessary for permanent work
in those portions where the tunnel required artificial support.
The first tunnels of any importance were most probably those
constructed for canal purposes. Many of them were of considerable
magnitude, and in some instances were from two to three miles in
length. They were substantially lined with masonry or brickwork at all
places where the tunnel passed through soft material or loose rock,
and from the solid nature of the work, and the many years they have
been in existence, they thoroughly testify to the ability of the
constructors.
The introduction of railways involved the making of a large number of
tunnels, perhaps more so in the beginning, when it was thought that
the use of the locomotive would be confined to very moderate
gradients, and when engineers hesitated to adopt the steeper inclines
and sharper curves which form the practice of modern times. Another
element of consideration also consisted in the fact that the first
railways were designed to connect the most populous and busiest
districts, where the prospects of heavy traffic would appear to
warrant a large outlay for works of construction. As the system spread
and railways extended further away from the important centres, the 164
probabilities of traffic would become less promising, and efforts
would be made to keep down cost of construction, and avoid tunnel work
as much as possible.
It is not easy to define where cutting should end and tunnelling
begin. There is no practical difficulty in making a cutting 50, 60, or
70 feet deep, with slopes to suit the material excavated, and the
estimated cost per yard forward may even compare favourably with the
cost of average tunnel-work. But there are other questions which must
be kept in view--the time required to form the cutting, the space to
be obtained on which to deposit the enormous quantity of excavated
material, and the probable difficulty in obtaining the large area of
land necessary for the cutting.
Before deciding the actual position of a tunnel, both as to line and
level, it is necessary to obtain the most reliable information
possible regarding the strata through which it has to pass. In
addition to the geological indications on the surface and in the
locality, borings should be made, and trial holes or shafts sunk along
the proposed centre line of the work, and from these an approximately
accurate longitudinal section can be laid down on paper, showing the
respective layers of material to be cut through, and the angle at
which they lie. With these particulars before him, the engineer may,
in some cases, consider it more prudent to change the position of the
tunnel in preference to incurring specially difficult or tedious work
in dealing with some recognized unfavourable material. Occasionally
the route may be slightly varied and better material obtained, but
very frequently there is little to be gained except by a wide
deviation from the original line.
Solid rock, except for the slow progress, is perhaps the most
favourable material for tunnelling, as the timbering, side walling,
and arching can be almost, if not entirely, dispensed with.
Loose rock, although more readily removed, necessitates strong
timbering to prevent large masses breaking away and falling into the
tunnel.
Some clays are very compact and tenacious, and will stand well with
moderate timbering, but even these should not be left long before
following up with the side walls and arching.
Many clays give much trouble by expanding, or swelling out, when the
excavation penetrates the layer, and although extra strong timbering 165
may be used, and be placed closer together, the logs and planks are
frequently bulged out and broken by the action of the clay. Specially
strong supports are required for this description of clay, and extra
thickness of material in the permanent work of side walls and arching.
Solid unbroken beds of chalk are not difficult to cut through: the
material is easy to work, and the excavation will stand with ordinary
timbering; but where the chalk is broken and intersected with deep
pockets of gravel and sand, the operations are very much impeded. The
loose material, once set free by cutting through the confining barrier
of chalk, will quickly fall into and fill up the excavation if not
held back by strong timbering. Side walls and arching are generally
necessary for tunnels through chalk.
Soft wet clay, quicksands, or other strata having springs of water
percolating through them, are serious obstacles in the way of
expeditious tunnelling. No sooner is one cube yard of this soft
material removed than another slides down, or is washed down, to take
its place. When once the excavation taps the water-bearing strata,
large volumes of water will find their way into the workings, and must
be conveyed away to the mouth of the tunnel, or pumped up through the
nearest shaft. The timbering of the sides and roof through this
description of working is very tedious, and attended also with a
considerable amount of risk. The absence of really solid ground on
which to place or shore up the supports, taxes the skill of the
excavators, and very often, when a short length has been made
apparently secure, it will come down with a run, compelling all hands
to beat a hasty retreat. The permanent lining through such treacherous
material should follow the excavation very closely, and special care
should be exercised in building the walls, arching and invert.
In the excavation through stratified rocks it is necessary to note
carefully the lie of the strata, whether horizontal, vertical, or
shelving, as with each one the excavators are exposed to risks,
against which every precaution should be taken. A large horizontal
slab of solid-looking rock will suddenly break and fall down without
any warning. A heavy mass from a vertical layer, perhaps unkeyed, or
loosened, by an adjacent blasting operation, drops down when least
expected; and pieces from the high side of the shelving layers detach
themselves and slide into the working in a most unaccountable manner.
No attempt should be made to carry a tunnel through material which has 166
been disturbed or at all affected by any natural slip or cleavage, as
although the strata may be hard and compact in themselves, they have
really no solid or fixed foundation. The sliding away, once initiated,
is certain to continue, and, accelerated by the tunnelling operations,
will most likely, sooner or later, crush in the tunnel and sweep away
every vestige of the work. Amongst the great mountain ranges these
natural disturbances are by no means rare, and it will be wiser to
keep away from their locality, even at the expense of a longer tunnel.
Unfortunately, instances are on record of tunnels made, or in course
of construction, through hillsides which had already commenced to
slide away from the more solid rock, and the ultimate results were a
further sliding away and total destruction of the work.
The lower slopes and outlying portions of high mountains are the most
exposed to these natural slips, and they should be most carefully
studied before commencing any tunnelling operations through them.
To facilitate drainage, it is essential that a railway tunnel should
be laid down with a gradient or gradients falling in the direction of
one or both ends of the tunnel. In nearly all tunnels a considerable
amount of water finds its way in through the weeping-holes left for
that purpose in the side walls, and must be carried away in suitable
drains. If the quantity of water be small, ordinary water-tables, one
on each side, may be sufficient; but for large volumes of water it
will be necessary to build substantial side-drains, or an ample
culvert below the level of the rails.
The gradients in a tunnel should be moderate, and not by any means
excessive, or likely to tax the hauling powers of the locomotives.
When an engine is working nearly to the utmost of its power on a steep
tunnel incline, and the speed has become very slow, the exhaust
vapours or gases from the funnel strike the arching with great force,
and are deflected down on to the footplate in such dense volumes as to
almost suffocate the driver and fireman. The writer will never forget
two or three trying experiences in foreign tunnels, when he and the
engine-staff were compelled to leave the footplate and climb forward
to the front of the funnel, leaving the engine to work its way alone.
Except for very short tunnels it is wiser to have easy inclines, and
to restrict the steep gradients to the open line, where the very slow 167
travelling, or even the coming to a stand from “slipping,” may not
produce unpleasant or alarming consequences.
In tunnels of any length it is usual, where possible, to construct
shafts extending from the surface of the ground overhead down to the
tunnel below. These shafts serve the double purpose of enabling the
excavation to be carried on at an increased number of faces, and act
as permanent ventilators after completion. In some cases the shafts
are sunk exactly over the centre line of the tunnel, in others a few
yards away from the centre line. The latter arrangement, if not quite
so convenient for hoisting material while carrying on the excavations,
has certainly the great after advantage that anything falling or
maliciously thrown down the shaft cannot strike a passing train. The
short side-gallery, or space between the tunnel and the shaft,
provides a good refuge for workmen employed in repairs, and a
convenient site for storing a few materials advisable to keep on hand.
Occasionally favourable opportunities present themselves for making
horizontal shafts. For a portion of its length the tunnel may be
located at no very great distance from the precipitous sides of some
deep mountain ravine, or run near to the cliffs on the sea-coast, and
advantage can be taken to drive a lateral heading or gallery through
which the material from the tunnel excavation may be conveyed and
thrown out into the gorge or seashore below.
In many cases the surface of the ground rises so abruptly from the
faces of the tunnel and ascends to so great a height, that shafts of
any kind are entirely out of the question, and the whole of the work
must be carried on from the two ends. The rate of progress is
consequently much slower, and the ventilation more difficult. In a
shaftless tunnel of considerable length, and with a frequent train
service, the question of providing suitable appliances for promoting
artificial ventilation is of the utmost importance.
When the centre line of the tunnel has been accurately set out on the
ground, and the levels of the different parts of the work decided, the
construction of the shafts and the driving of the headings can be
commenced. Working shafts intended to serve for permanent ventilation
are generally made nine or ten feet or more in diameter, and are
usually lined with substantial brickwork or masonry. When the well-like 168
excavation has been carried down a few yards, or as far as it can be
taken without the risk of the earth falling in upon the sinkers, a
strong curb of hard wood or iron of the same diameter as the finished
shaft is laid down perfectly level and to exact position, and on this
curb the ring or lining of brickwork or masonry is built up to the
level of the ground. The first length finished, the excavation
downwards is resumed, and the interior lining continued, either by
allowing the first length to slide down as the material below is
gradually removed, and building further lining on the top, or by
excavating and propping up the curbing with strong timbers below. When
working to the latter method, stout wooden props of convenient length,
stepped on to sole-pieces, are adjusted to the under side of the
wooden curb above, the material is then removed for the thickness of
the brickwork or masonry, and another curb accurately set to level and
position; on this is built a length of lining up to the first curb.
This work of under-building or under-pinning must be carried out with
great care and in segments; no props must be removed until the curb
immediately above is well supported by the new lining, and the inside
of the lining must be watched and tested to prevent any tilting. All
spaces at the back of the work must be filled in and well packed with
hard dry material. As the shaft is continued downwards the mode of
working may have to be varied; different descriptions of material may
be encountered, and blasting, shoring, and pumping may each in turn be
necessary.
When down to the full depth, the lower length of the shaft will have
to be securely supported by strong timbers, until it can be properly
built into and incorporated with the arching of the tunnel or side
gallery.
The completion of the shaft enables the workings to be commenced on
each side, the excavated material can be hoisted to the surface, and
building material lowered down. When the tunnel works are finally
finished, the lining of the shaft should be carried up until it is 15
or 20 feet above the level of the surface of the ground, and a
dome-shaped iron grating placed on the top as a protection against
stones or other articles which malicious persons might attempt to
throw down the shaft.
Some shafts are only intended for the temporary purpose of lifting the
excavations from below, or lowering building materials down, and when 169
the work is completed they are filled in again and closed. These
service shafts are generally made square in section, and are merely
lined with wood. Strong vertical timbers are placed at the four
corners, to which horizontal double cross-pieces are bolted, thick
planking being placed vertically at the back of these cross-pieces to
support the sides of the excavation.
The _heading_ of a tunnel is a narrow passage or gallery cut
through from end to end of the works in the direction of the centre
line. Where there are shafts, the cutting of the heading can be pushed
on from several points, and be completed much more rapidly than when
the working is restricted to the two ends. Headings are usually made
just sufficiently large for the miners to work, say about 5 feet 6
inches high by about 3 feet wide, the object being rather to expedite
the driving of the driftway than to remove large masses of material.
They must be set out with great accuracy, and be constantly checked as
the driving is in progress. When completed from end to end, the centre
line can be checked throughout, and the course actually taken compared
with the course intended. If there has been much variation in the
narrow pioneer pathway, either in line or level, the amount of the
divergence must be rectified when ranging the final centre line for
the full-size excavation.
Tunnels cannot always be delayed until the heading is cut through for
the entire length. In many cases the heading, the full-size
excavation, and the permanent lining have all to be carried on at the
same time, but as the work of the heading is smaller in extent, that
portion of the operations can usually be kept well in advance of the
others. The critical moment arrives when the headings from opposite
directions meet, as any deviation or want of coincidence must be
adjusted in the portion of the tunnel still remaining to be opened out
to full size. Some tunnels of moderate length have been constructed
without any heading at all, the excavation being taken out to the full
dimensions from the commencement.
The heading of a tunnel assists not only in the correct alignment of
the work, but furnishes at the same time an accurate knowledge of the
strata passed through. It is also of service for ventilation,
communication, and drainage.
In some cases the heading is driven at the bottom of the tunnel
section, as in Fig. 211, and in others at the top, as in Figs. 202 and 170
204. Many of the earlier tunnels were constructed on the former
system, while of late years the latter method has been very largely
adopted. The bottom heading may perhaps in some instances be more
efficacious for drainage, but it is very liable to be frequently
choked up when taking out the excavation to the full size, and the
lower surface is much cut up by the movement and conveyance of
materials. Another disadvantage arises from the necessity of removing
such a large amount of the cutting approaching the tunnel entrance
before a beginning can be made to the bottom heading. The top heading
has the advantage that it requires less removal of open cutting
previous to its commencement, and, being high up in position, there is
less chance of its being stopped up by falling material, the finished
excavations being carried out on the sides and below the heading.
Where the headings are cut through solid rock, stiff shale, or compact
chalk, little or no supports are necessary, but where they pass
through clay or loose material, timbering will be required for sides,
roof, and floor. Rough round poles, about 6 inches in diameter, are
generally used for verticals, and are firmly secured to transverse
sole-pieces, and on the top of these verticals strong transverse
top-sills are fastened by means of rough tenons or checks. Strong
boards are inserted at the back of this framework to keep the earth
from falling into the working. The distance apart of the verticals
will depend upon the description of material excavated; in very soft
places they will have to be placed very close together, but where
fairly sound and tenacious they may be placed at about 3-foot centres.
The excavated material must be conveyed away to the entrance of the
heading in small hand-trucks running on planks or light rails.
The widening out of the excavation to the full size will be a
repetition on a large scale of the work carried out in the heading,
with the difference that, the exposed surfaces being of so much
greater extent, extra care and precautions must be taken with the
framework and shoring of the timbering.
[Illustration: Fig. 202, 203]
The form and arrangement of the timbering, as well as the number,
sizes, and positions of the pieces, must be determined by the material
of the excavation and the contour line of the finished arching or
lining. The framework, which would be sufficient to support ordinary
soft material, must be largely augmented both in quantity and scantling 172
to meet the requirements for wet treacherous clay.
Figs. 202 and 203 give end view and longitudinal section of timber
framework frequently adopted for average tunnel work. The positions of
the different pieces will explain themselves and the duty they have to
perform. The main struts, or raking pieces, which have to sustain
great pressure, may be shored against the finished lengths of masonry
or brickwork. The timbering of the sides can be removed as the lining
proceeds, but in many cases the round logs and boards near the crown
cannot be withdrawn, and have to be left in the work, the space
between the top of the arching and under side of the boards being
firmly packed with brickwork, masonry, or dry rubble stonework.
As the tunnel lining is generally carried forward in short lengths,
following up the main excavations, the centering for the arching
should be of such description that it can be readily transferred or
moved forward as the work proceeds. The form of the centering, and the
spacing of its upright supports, must admit of sufficient width for
one or more lines of rails for the waggons required to remove the
excavated _débris_ and convey the building materials used in the
lining.
Picks, bars, and shovels are the tools used in the excavation of the
softer material and loose disintegrated rock, but for the hard rock,
blasting will be necessary. The tunnel opening being comparatively
small, only moderate blasting charges can be used with safety, and
these must be placed so as to break up the rock-bed in a suitable
manner for working, and without shaking or damaging the already
completed excavation. Ordinary hand-drills, or _jumpers_, may be used
for forming the charge holes, a number of them being at work at the
same time, and the charges fired very closely one after the other. As
the blasting operations necessitate the retiring of the miners to a
considerable distance, out of the way of flying fragments, and the
remaining away until the foul air has been dispelled, it is advisable
to fire off several charges about the same time, and thus minimize as
much as possible the stoppage to the drilling and clearing away the
loosened material.
[Illustration: Fig. 204, 205, 206, 207, 208, 209, 210, 211]
Mechanical drills, worked by compressed air or other motive-power, are
now very extensively used where the rock is solid and continuous. They
are much more expeditious than the hand drills, but they are costly in 174
their installation, and also in their working and maintenance.
In some tunnels, where the material has been firm and dry, the upper
portion of the excavation has been first removed, and the masonry and
brickwork lining built in position down to about the springing of the
arch, the remainder of the excavation being afterwards taken out, and
the side walls built by means of shoring and underpinning.
In other tunnels the complete section has been excavated and timbered,
and the work of building commenced from the foundation of the side
walls. A strong continuous invert from side wall to side wall is
necessary where passing through soft swelling clay or loose strata
intersected with small streams of water. Where the material is very
solid and dry, it is not necessary to introduce inverts, but the
foundations of the side walls should be laid at such a depth below
rail-level as not to be affected by drain-water running through the
tunnel.
The side walls and arching may be either of masonry or brickwork, but
should be of the best description, especially for the facework. For
brick arching only the best hard-burnt bricks should be used, and the
inner or exposed ring should consist of selected hard fire-bricks to
withstand the heat and gases escaping from the funnels of the
locomotives. The thickness of the side walls and arching will depend
upon the description of material to be supported. In some places a
comparative thin lining may be sufficient, while in others extra
thickness must be given to resist the great pressure exerted by
expanding clay and loose wet strata.
Weeping-holes, or small drain-pipes, placed low down must be left in
the side walls every three or four yards, or closer in very wet
places, to allow the water collected at the back of the walls to
escape into the side drains of tunnel. In building the arch portion
every effort should be made to have close solid work without any open
joints or spaces through which the water may run, and the crown of the
arch and a few feet down on each side should be coated with cement or
asphalte to lead all water away from the top to the sides. Water
dripping from the under side of the arch on to the line is a great
destructor of the permanent way materials, especially the fastenings;
and bolts, nuts, fish-plates, and spikes placed in a wet dripping
tunnel will not last half the time they would out in the open line,
where they would have the sun and wind to dry them.
[Illustration: Fig. 212, 213, 214, 215, 216, 217, 218, 219]
Small arched recesses or niches should be formed in the side walls at 176
convenient distances to serve as refuges for platelayers or others
working in the tunnels.
It is most essential that the space between the masonry and brickwork
lining and the facework of the excavation should be carefully filled
in and hard packed, so as to prevent the possibility of pieces of rock
or other material falling on to the top of the arch. The neglect of
this precaution may lead to a casualty years after the tunnel has been
completed.
It would be impossible to over-rate the importance of a constant
faithful supervision of the building of the lining, especially the
arching. The work has to be carried on by workmen in cramped
positions, with imperfect light, and surrounded by all kinds of
obstacles and inconveniences, and unless a detailed inspection be
rigidly maintained, a carelessness in the selection of the materials,
and a laxity in the workmanship, will be the inevitable result.
Figs. 204 to 219 are sections of tunnels which have been constructed
for double and single line railways. The sections give the normal form
and dimensions adopted in each case, although there may have been many
portions of the work where unfavourable or treacherous material
necessitated an increase in the thickness of the side walls, or of the
arching, or of both. The types vary in accordance with the opinions of
the designers as to the most suitable section for the purpose, and
range from the comparatively thin lining and vertical side walls shown
on Fig. 207, to the almost circular form and very thick lining shown
on Fig. 216. The latter is the section which experience has proved to
be the best to sustain the enormous all-round pressure exerted by
certain descriptions of swelling clay.
Careful judgment will be required to decide which parts of a rock
tunnel may be left unlined. The apparently solid-looking portions are
oftentimes deceptive, and numbers of instances are on record of large
pieces of rock falling down in tunnels which for many years had been
considered as thoroughly secure. Where there is any doubt it is better
and safer to put in a lining, even if only to the extent of an arching
springing from side walls of solid rock, as shown on Fig. 206. A
moderate additional expenditure at the time of construction may
prevent a serious catastrophe afterwards.
[Illustration: Fig. 220, 221, 222, 223]
The faces or entrances to tunnels may be constructed with curved wing 178
walls, as in Fig. 220, or with straight wing walls, as in Figs. 221,
222, and 223. Where the approach cutting is in rock, the latter form
is generally adopted.
It would be misleading to put down any average price for tunnel-work.
So much depends upon the locality, the description of material to be
excavated, the cost of masonry or brickwork, and the cost of labour.
Added to these come the unforeseen troubles of slips and water-laden
strata, creating difficulties which baffle the miners for a time, and
add enormously to the expenditure. Some tunnels for double line have
been constructed in good ground, and under favourable circumstances as
to building materials and labour, for as low as £32 per lineal yard;
while others, carried out under adverse conditions, have cost as much
as £150 per lineal yard. A medium somewhere between the two should
represent the cost of tunnel-work through ground which does not
present any special difficulty. At the same time it must be borne in
mind that simple tunnelling which can be done in one locality for £50
or £60 per lineal yard, would be increased 20, 30, or 40 per cent. in
another, where building material for the lining is scarce and
expensive.
Tunnel-work abroad will generally cost more than the same work at
home. The native labourers may perhaps be procured at low rates, but
the skilled workmen must be brought from a distance, and will obtain
high wages.
Another form of tunnel-work, generally termed the covered-way system,
is frequently adopted in towns and places where land and space are
very valuable. This method consists in the excavating and removing of
earthwork to admit of the building of the side walls and arching of a
suitable tunnel-way, and then filling in over the top to a depth of
three or four feet, or up to the level of the original surface of the
ground. This work may be carried out by either removing the entire
width of the earthwork before the commencement of any building
operations, or by first forming two deep, well-shored trenches, in
which to build the side walls up to about arch-springing. In bad
ground the latter arrangement has the advantage, as the shoring and
strutting to hold up the sliding material is limited to the widths of
the two narrow trenches, and the centre block of earthwork is left
untouched as a support to the strutting. When the side walls have been
built sufficiently high the upper portion of the centre block of
earthwork can be removed to allow of the erection of centering and 179
building of the arching, and afterwards the remaining portion of
earthwork can be removed at convenience. In this manner a tunnel-way
may be constructed under streets, gardens, and even under buildings.
Being nearly all done in the open, the work is more under control than
in an ordinary tunnel, but it is usually very costly. Temporary or
diverted roads must be arranged; the excavated material must generally
all be removed by carts, sometimes to long distances; and provision
must be made for diverting the network of sewers, gas, and water pipes
which are intercepted along the route.
Fig. 224 is a sketch of covered-way with brick arching. Fig. 225
illustrates another type where cast-iron girders and jack-arches of
brickwork were introduced on account of the small headway. In soft
yielding clay it is necessary to construct strong inverts, as
indicated in the sketches. Recesses for the platelayers should be
provided every ten or fifteen yards.
The above systems of covered-way were largely adopted in the
construction of the underground portions of the Metropolitan Railway
and District Railways in and around London.
[Illustration: Fig. 224, 225, 226]
In addition to the ordinary type of tunnel formed by first excavating
the material and then lining the opening with brickwork or masonry,
tunnels of moderate size have been constructed of cast-iron tubes,
similar in section to Fig. 226. The tubes were cast in short segments,
bolted together inside, the outer circumference, or surface in contact
with the earth or clay, being left free from projections of any kind.
By making the segments with bolt-holes exact to template, they were
readily fitted together in the work, and a thin layer of suitable
packing material placed between the bolting-flanges sufficed to render
the tubes water-tight. The tunnelling was carried on by means of a
short length of slightly larger tube, or cap, made of plate-iron or
steel, which fitted over the leading end of the main tube. The front
end of this cap was made very strong, and provided with doors through
which the miners could work. A series of hydraulic presses attached to
the cap were brought to bear on the bolting-flange of the last
completed ring, and as the excavated matter was removed by the miners
from the front the cap was forced forward by the hydraulic presses,
and another ring of cast-iron segments inserted. On the City and South
London Railway, constructed on the above system, the small annular
space formed round the cast-iron tube by the operation of the sliding 181
cap was filled in with cement grouting by means of an ingenious
machine designed for the purpose.
Large tunnels under rivers or tidal estuaries must each be dealt with
according to the particular circumstances of depth below stream-bed,
material to be cut through, length of tunnel, and gradient. The chief
obstacle to be contended against in so much of the river tunnel-work
is the large volume of water which pours into the workings through
fissures in rock or seams of gravel and sand, necessitating the
constant use of most powerful pumps. In ordinary land tunnels the
gradients are generally laid out to fall towards one or both
entrances, and any water finding its way into the excavations may be
led away to the entrances by drains or pipes. On the other hand, in a
river tunnel the gradients generally fall away from the entrances down
towards the centre of the river, and all water coming in must be
pumped out and raised up to at least the level of the river. In places
where the water comes streaming in from many points, any failure or
stoppage of the pumps would place the lives of the miners, and the
security of the work itself, in great jeopardy. Iron shields, or
protection chambers for the miners advancing the excavation, have been
used with great success in carrying on work through loose wet strata
which appeared to defy all other means of progress. Solid rock, chalk,
or compact clay, may present no difficulty so far as they go, but a
continued dip in the gradient, or a line of fault, may suddenly change
the entire course of operations, and require the immediate use of the
most powerful pumping machinery and protective appliances. The special
features of each case will demand special precautions, and the
judgment and inventive powers of the engineer will be severely tested
in coping with the difficulties with which he is surrounded.