BRIDGES AND SYPHONS
1. =Bridges.=--Bridges are of many kinds. In this book only those parts
of them are considered which are exposed to the stream. If a bridge has
piers, there must be some disturbance of the water. The disturbance
will be least when the area of the waterway of the bridge is at least
as great as that of the stream, and when its shape is as nearly as
possible the same. For small streams, a single span clearing the whole
stream may be adopted, especially when the channel is of soft material,
but for a large stream the cost of intermediate piers, even with a
certain amount of protection for them or with deep foundations, will be
more than counterbalanced by the smaller thickness of arch or depth of
girder.
Generally a bridge has vertical abutments which limit the waterway,
but it may have land-spans, and in this case the stream as it rises
can spread out. Piers and abutments should be so designed that abrupt
changes in the section of the stream are, as far as possible, avoided,
the piers being rounded or boat-shaped at both ends and the abutments
suitably curved (fig. 49). Boat-shaped piers, besides presenting the
neatest appearance, cause the least amount of disturbance.
A bridge can be made safe against scour either by giving deep
foundations to the piers and abutments or by adding a floor and, if
necessary, pitching. The former course is usually adopted and is
the best. But in a case in which the discharge of a stream is to be
increased or has been underestimated, it is often far easier to add a
floor to an existing bridge than to increase the span of the bridge. In
order to increase the waterway the floor can be “dished,” _i.e._ made
at a level lower than the bed of the stream[17] and gradually sloped
up--the slopes being pitched--both upstream and downstream of the
bridge, to meet the bed.
[Illustration: FIG. 49.]
In any case in which the water rises above the crown of the arch, the
bridge becomes a syphon, and a floor is probably necessary unless the
foundations are very deep, or unless the rise of water above the crown
is temporary.
[Illustration: FIG. 50.]
In the case of Indian rivers which have soft channels, and are
ordinarily of moderate width but are subject to occasional floods when
the width of the stream is multiplied several times and becomes very
great, it is the rule to make the span of a railway bridge far less
than this greater width. The stream during floods scours out a deep
channel through the bridge with great rapidity, and no heading up worth
mentioning occurs. The foundations of the piers are very deep, being
frequently 50 feet below the lowest point of the river bed which can
be found anywhere within several miles of the bridge. The span of the
bridge can be arrived at by considering a general cross-section of
the river as it is when in high flood, and assuming that scour to the
depth of the lowest point, found as just explained, will take place in
one-third of the span of the bridge. The span can then be so fixed as
to give no heading up. It is not assumed that there will be no increase
in velocity through the bridge. The velocity in the deep scoured
portions will be increased. The piers are protected by loose stone
(fig. 50). The spans vary from 100 to 250 feet. The bridge over the
river Chenab at Wazirabad had originally sixty-four spans of 145 feet
each. The number of spans has since been reduced to twenty-eight. With
a very long bridge, the current of the shifting stream is more likely
to strike the bridge obliquely, though this is not the chief reason for
reducing the length. Long spans, say 250 feet, have been found to be
better than shorter spans; the cost of the stone protection round the
piers is of course less (_Government of India Technical Paper_, No.
153, “River Training and Control on the Guide Bank System,” by Sir F.
J. E. Spring, C.I.E., 1904).
2. =Syphons and Culverts.=--Syphons are used to pass drainage channels
or other streams under canals or other lines of communication. In the
case of a masonry syphon under a stream which may be dry while the
syphon is full, the weight of the arch and its solid load must be not
less than the upward pressure of the water passing through the syphon.
The channel sometimes has a vertical drop at the upstream side (fig.
51) and a slope at the downstream side. The slope enables any solid
materials to be carried through, and facilitates cleaning out and
unwatering. The drop at the upstream side does not give rise to any
shock on the floor when the syphon is full, but a slope is preferable
if there is room for it, and it causes less loss of head.
[Illustration: FIG. 51.]
A culvert which is liable to run full and which has a steep approach
channel (fig. 52) may become suddenly drowned on the upstream side. As
soon as the water rises to the crown of the arch, the wet border of
the culvert increases and this reduces the velocity and discharge. The
water coming down the approach channel then rises abruptly, and the
increased section of the stream causes a reduced velocity of approach,
and this further reduces the discharge through the culvert. The heading
up continues until the difference in the upstream and downstream
water-levels is great enough to readjust matters (_Min. Proc. Inst.
C.E._, vol. clxxxvi.). The possibility of this heading up occurring
should be attended to in the design. In the case of a culvert in a
railway embankment where heavy floods have to be passed, the culvert
may be made bell-mouthed by a curved embankment constructed on its
upstream side.
[Illustration: FIG. 52.]
3. =Training Works.=--The object of the upstream and downstream
protections already described (CHAP. X.) is to prevent damage to the
structure owing to the disturbance caused by the structure itself.
When a river is given to shifting its course (CHAP. IV., _Art. 9_)
and cutting away its banks, protection of another kind is required.
The stream, if left to itself, may cut away one bank upstream of the
structure for a long distance, and eventually damage, or destroy by
undermining, the upstream pitching and the abutment itself. This is
known as outflanking. If in the neighbourhood of the line A B (fig.
53) there is nothing for the river to damage,--if, for instance, the
structure is a weir with a canal, if any, only on the opposite bank of
the river,--and if the land is of no particular value, the case could
conceivably be met by protecting the abutment on all sides, but even
then there might be a chance of the erosion of the bank continuing
until the stream had formed a connection at C with the downstream
reach. This, of course, in the case of a weir, would render the work
useless and might even destroy it.
[Illustration: FIG. 53.]
In the case of a bridge carrying a road or railway, or of a syphon or
aqueduct carrying a canal or other stream, it is wholly inadmissible
to allow the stream to cut away even as far as the point A for fear
of its severing the line of communication. Thus in every case it is
practically necessary to prevent any serious erosion of the bank
upstream of the structure. In ordinary cases it is sufficient to
protect the bank C D by any of the methods given in CHAP. VI., _Art.
3_, the protection being turned inwards, as shown at D, to prevent the
end of it being damaged.
In the case of railway bridges across the great shifting rivers of
India, protection used at one time to be afforded by various systems
of spurs. This has now been abandoned in favour of Bell’s guide banks
(fig. 54), which are found to be far more satisfactory. These guide
banks are discussed in the paper by Spring quoted above (_Art. 1_).
The spaces behind the guide banks become filled with water, at least
during floods, and are meant to be silted up. An opening in the
railway embankment should be provided at A, and another on the opposite
side of the river, to ensure a constant flow of water (CHAP. V., _Art.
3_), but they should not be large enough to cause high velocity. The
chief danger to which a guide bank is liable is outflanking when the
stream assumes the position shown. To guard against this danger it is
necessary to have very strong and massive heads to the guide banks.
When the bank of the eroding stream, downstream of the guide bank head,
becomes a semicircle or thereabouts, the stream takes a short-cut
across the sandbank, and to encourage this an artificial cut can be
dug, at the season of low water, on any suitable line.
[Illustration: FIG. 54.]
If the guide banks were made with an increased width of opening at
the upper end, this would reduce the chance of outflanking but would
increase the danger from a direct attack such as indicated, in the
figure, on the left bank. It has been suggested that the width at
the upstream end should be less than at the bridge, but this seems
undesirable. Probably the form shown in fig. 54 is the proper one. The
length of the guide bank upstream of the bridge is made about equal
to the span of the bridge between the two guide banks. If made less
than this, the river might cut into the line of railway. The length of
guide bank downstream of the bridge is generally 300 to 500 feet, being
greater as the velocity of the river is greater and the sand of its bed
finer.
[Illustration: FIG. 55.]
The Bengal Dooars Railway runs near the foot of the Bhutan Himalayas,
and crosses some broad river channels which, after the excessively
heavy rains which occur, are filled by streams of very high velocities.
One such channel or set of channels (fig. 55), more than half a mile
wide, is provided with a bridge whose waterway consists of ten spans
of 60 feet each. The railway embankment across the remainder of the
channel having been breached in many places in 1903, protection was
afforded by T-headed spurs and other groynes, the first arrangement,
which withstood the floods of 1904, being as shown in the figure. The
triangular apex of the A-shaped groyne, south-east of the bridge, was
added in 1905 because, in its absence, the water struck the bridge
obliquely. After the addition there was a great deposit of silt in
the neighbourhood of the four T-headed spurs. Next year the river,
in a great flood, rose over the top of the railway embankment near
these spurs, and finally caused a breach 600 feet wide. The embankment
was afterwards raised. The velocity through the bridge seems to have
approached 18 feet per second. The bridge had at first no floor. A
floor was added, but was much damaged by the floods (_Min. Proc. Inst.
C.E._, vol. clxxiii.). The level of the floor is not given, but it
would seem to have been desirable to make it at a very low level. The
rising of the stream over the railway embankment was attributed to the
silting up near the T-headed spurs. The addition of the triangular
portion above referred to would seem to have somewhat assisted this
process. If all the trouble could have been foreseen, it might have
been best to build an additional bridge 2000 feet south-east of the
existing bridge. The groynes were composed of the wire-network rolls,
described in CHAP. VI., _Art. 3_, piled pyramid fashion.