CHAPTER XI
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.
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.
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.
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.
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.
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.
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.
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.