RECENT years have shown that the railway engineer is still as unafraid of the might of great rivers as in the days when he launched the Forth Bridge and first spanned the Falls of Niagara. His latest achievement in steel is the Lower Zambesi Bridge. This, the longest railway bridge in the world—though not the longest viaduct—measures nearly two miles from end to end, without counting approaches.
To do full justice to this great triumph in Africa demands in the writer an impossible combination of faculties—the rapturous admiration of youth, the awed wonder of a native, the knowledge and experience of a geographer, doctor, town planner, and many kinds of engineer, and the capacity for expression of a talented author.
Calling for a remarkable co-ordination of human industry, the task of building the Lower Zambesi Bridge was undertaken to provide something more than a mere spectacular addition to railway engineering. The great feat was accomplished as part of a comprehensive plan which will provide for the proper development of British Nyasaland, and for the simplification of its whole system of communication with the sea.
The best river transport can never handle such large quantities of merchandise as the railway, and the lower reaches of the Zambesi are unfavourable for shipping. From December to March in each year the flooded river inundates the country for miles around. From August to November, on the other hand, it shrinks until it is so shallow that only flat-bottomed barges can get across, and teams of native boys wade through the water pulling the barges with ropes. The two-mile crossing takes up to two hours to make, and a regular landing base cannot be maintained owing to the changes of the river.
At one time this river ferry provided the only link between the railway which ran south from Nyasaland to Chindio, on the north bank of the river, and the line which ran north from Beira to Murraca on the opposite bank. These two villages, Chindio and Murraca, are situated about ninety miles from the mouths of the Zambesi, and 160 miles from the Portuguese port of Beira, which lies to the south.
Since Beira is the only port which can conveniently handle the overseas trade of British Nyasaland, itself an inland territory, it will readily be seen how vital was the necessity for overcoming the break in the railway at the Zambesi River.
The building of the two-mile bridge was not undertaken without some trepidation, and much forethought had to be expended before the scheme got properly under way. British enterprise was responsible for the project, although the site of the bridge lay in Portuguese East Africa, some forty miles south of the Nyasaland boundary. After preliminary surveys, it was decided to bridge the river at Sena, twenty-five miles upstream from Murraca, where the primitive barges and stern-wheel steamers carried on the ferry service.
The preparations for the erection of the bridge included the construction of approach railway lines along new embankments, and the clearing of the bush to build a camp. The Cleveland Bridge and Engineering Co., Ltd., of Darlington. England, who secured the contract, placed orders for materials with nearly a hundred British firms. The necessary equipment included steel cranes, excavating gear, thousands of tons of steelwork, timber, cement, concrete mixers, diving apparatus, lubricating oil, electric light fittings, paint, machine tools, portable forges, scientific instruments, millions of bolts and nuts, medical equipment, water purification plant, portable offices, office furniture (specially treated to withstand the hot, damp climate), eight pontoons, twenty barges, and two light-draught stern-wheel steamers.
Some of the larger items, including the river barges, were dispatched in parts and reassembled on the banks of the Zambesi. Stone had to be obtained for concrete making ; two quarries were opened for this purpose on either side of the river. To fit out the 2-ft. gauge railways installed for running the quarries, further supplies of equipment were transhipped from England to Beira, and thence by the Trans-Zambesia Railway to the site of the proposed bridge.
These supplies included twenty-eight sets of switches and crossings, sixteen ball-bearing turntables, 130 tipping wagons, sixteen platform wagons, twenty rubble skips, twenty tipping buckets, and a number of petrol locomotives. Three 0-6-0 outside-framed Peckett steam, locomotives were also supplied from England for service on the 3 ft. 6 in. gauge main line approaching and crossing the bridge.
Not only did the engineers clear the bush to build a camp; they also changed the whole geography of the district and planted two small towns on opposing banks of the river. The European supervisors had four thousand native workers under their charge, at times even six thousand, and the task of housing, feeding—though scarcely clothing—this enormous number of natives called for skilled social organization. Particular attention was paid to the question of hygiene, and two hospitals, each with excellent surgery, dispensary, and specially trained staff, were erected near the Zambesi River.
Exhaustive precautions were taken, with encouraging results, against the malaria peril. The perfection of the drainage system helped very largely in this direction, while a permanent gang of boys was employed to wage an anti-malarial war. Their duties included the capturing of a number of mosquitoes at frequent intervals for examination by the medical officer, who was able to ascertain what percentage of them was of the malaria-carrying species.
Since the engineers and other Europeans had to spend four years in residence by the Zambesi, it was only reasonable that certain social amenities should be provided for their recreation during leisure hours. The indoor attractions comprised a recreation room, a squash-rackets court, a billiards table, and a reading-room. Out of doors there were tennis courts, a football ground, a cricket pitch, as well as a nine-hole golf course.
The remarkable preliminaries to the Lower Zambesi Bridge being completed, the year 1931 saw the beginning of constructional work. Perhaps of minor interest to the onlooker, but of surpassing importance to the engineer, are the foundations and piers of a bridge. Anyone giving the matter a passing thought must be impressed by the apparent impossibility of laying satisfactory foundations for piers which have to stand in water.
There are several methods of achieving this difficult feat. The usual procedure is to erect a cofferdam, this being an arrangement for laying dry a space below water-level. Cofferdams may be constructed of earth, timber, steel, or concrete, or of a combination of these items.
The earth cofferdam is the simplest kind, and is often used in shallow rivers with currents of low velocity. It consists of a bank of earth, containing a good deal of clay, placed round the pier foundation site to be enclosed, and of a thickness sufficient to furnish the required stability. The bank projects two or three feet above water, with a width of at least 3 ft. at the top, though this width will probably be a good deal larger unless one or two strengthening rows of wood or steel sheet-piling are inserted in the circular bank. When the bank has been built, it remains only to pump out the water from the interior, and excavation for the foundations may begin on the dry bed of the river.
A more elaborate cofferdam is formed by driving into the river-bed one or more concentric rings of contiguous wooden piles, braced internally with timber, to withstand the pressure of the surrounding water. If two or more circles of piling are used, the space of a foot or two between them is filled with concrete or clay-puddle. Steel sheeting is often substituted for wooden piles, and used with success.
The engineers do not attempt to make coffer-dams absolutely watertight ; this would be an expensive task. So long as the bed of the river can be kept dry with a reasonable amount of pumping, the conditions are considered satisfactory.
The nature of the Zambesi River permitted some of the pier foundations to be dredged without preliminary removal of the water, though cofferdams were usually adopted where rock replaced sand and mud at a distance not too great from the surface. Pontoons and barges, carrying concrete mixers, forges, cranes, and other impedimenta, were drawn round the site of each pier, forming suitable islands from which the constructional work could be conducted. During the dry season, as it happens, no water flows over the south side of the river-bed, so that railway lines here took the place of the pontoons and barges.
The laying of the foundations began with the lowering of an oval steel curb, shaped like a steamer funnel, and with cross-section measuring 36 ft. by 20 ft., into the bed of the river at the site of each pier. Within this structural shield the proper foundation well was next forced into the ground. This enormously strong steel-sided well, containing suitable dredge shafts leading to the bottom, was in most instances 120 ft. deep. Steam cranes lowered mechanical grabs, having a capacity up to 38 cu. ft., down the dredge shafts, the work of excavation proceeding simultaneously with the lowering of the well.
The well was built up in sections as the excavations deepened. At the same time it was filled with concrete, reinforced with vertical and horizontal steel rods. The two excavation shafts were, of course, not filled in till the sinking of the wells had been completed. The thirty-two wells were sunk to a minimum depth of 80 ft., and some of them to a depth of no ft., below low-water level. These figures, owing to the shallowness of low-water, practically represent the depth of the foundations below ground.
The wells were surmounted by imposing concrete piers, likewise reinforced. Two additional main piers, bringing the total up to thirty-four, were founded directly on rock on the left bank of the river.
The thirty-three main steelwork spans connecting the piers, and carrying a single line of 3 ft. 6 in. gauge track, each measures 262 ft. 6 in. in length Nineteen of the spans were set on a slight incline, mainly at 1 in 216, the permanent way being higher on the left bank of the river than on the right. The highest spans stand 27 ft. above high-water level.
From the right bank, or the Sena side of the river, the bridge is approached by a steel trestle viaduct, founded on ferro-concrete piles, with a total length of 1,805 ft. 8 in. There follow seven small spans, forming part of the main bridge, and then the thirty-three main spans already mentioned. The forty spans together measure 8,662 ft. 6 in. The six plate-girder spans constituting the left-bank approach together contribute 399 ft. to the total length. The overall length of the whole structure is 12,064 ft. 4 in. The last span was erected on October 12, 1934.
This, then, is the measure of the world's longest railway bridge, built at a cost of over £1,400,000. The provision of a footpath on the up-stream side of the bridge will also facilitate communication between the localities of Sena and Dona Anna.
The account which has been given of this engineering achievement will convey some idea of the complications and difficulties involved in the construction of major railway bridges in uncivilized territories.
It is a curious fact that the Lower Zambesi Bridge stands on a railway system which has a route mileage of only forty-two and a half, from Port Herald, in Nyasaland, to the southern end of the bridge. The old twenty-four miles stretch from Bawe to Chindio has been abandoned.
The Central Africa Railway, as it is called, is now operated by Nyasaland Railways Ltd., the company floated specially to finance the construction of the bridge. This company was empowered to purchase the Shire Highlands Railway, which ran from Port Herald to Blantyre, a distance of 113 miles, and to build an extension 160 miles long to Chipoka and Salima, at the foot of Lake Nyasa. An improved shipping service is being set up on this lake, which is 360 miles long. and a large part of Nyasaland will thus be brought within easy reach of the ocean port of Beira.
Although the bridge at Sena has not yet had time to enlarge the fame of the Zambesi River on its own account, this great African waterway has long been associated with one of the most spectacular engineering triumphs in the world. This is the well-known single-arch bridge which spans the Zambesi Gorge below Victoria Falls in Rhodesia. When Livingstone discovered these falls in 1885, the natives described them to him as the "smoke that sounds." This is a vivid description of the cauldron of mist which can be seen from the train as it passes 700 yards downstream from the foot of the falls. During the flood season 100,000,000 gallons of water tumble over the mile-long precipice every minute, and the fine spray is sometimes blown as far as the carriage windows.
The bridge is 650 ft. long, the main span accounting for 500 ft., with the furious waters seething 400 ft. below. The work reached completion in 1904, two years alter Cecil Rhodes's death. It represents a paramount link in the railway which this great statesman hoped would one day run from the Nile Delta to Capetown—a vision which has still to be realized.
The "taming of the Nile" is a favourite occupation with engineers, and probably another century will pass before the world's longest river has been bridged and dammed a sufficient number of times to satisfy all concerned.
The latest railway bridge across the Nile is in Uganda, near Jinja, a town standing on the north shore of Lake Victoria, a source of the river. Communication was previously maintained by a ferry which ran across the head of the lake a little above where the Nile rises. But when the Kenya and Uganda Railway decided to extend its main line westwards from Jinja to Kampala, the commercial capital of Uganda, the construction of a bridge over the Nile became imperative.
The railway extension began on January 1, 1929, and the bridge was opened for traffic on January 14, 1931. The single spandrel-braced arch measures 260 ft. 2 in., and there are two open-deck type spans of 100 ft. at the approaches. The bridge carries a single line of metre gauge track at approximately 69 ft. above flood-river level. Underneath this there is a fine 20 ft. motor roadway with footways. As the river bank at the Jinja end of the bridge is but a few feet above high-water level, a high bank had to be thrown up to carry both the railway and the road to the higher ground farther away from the river. A tall masonry pier was thus necessary to support the shore end of the 100-ft. Jinja approach span, as a smaller pier could not be founded with much safety in the newly-constructed embankment.
Unenlightened townsfolk of Jinja may have viewed with indignation the new bridge which the engineers began to put up when they first arrived. But this was only a temporary footbridge erected to provide easy communication between the two river banks during the construction of the main bridge ; the swirling waters made the use of boats dangerous. The cost of the Jinja rail and road bridge came to approximately £70,000.
One child in every five is born in India ; and a territory so densely populated as this requires an elaborate railway system, to serve its needs. The abundance of rivers in India gives engineers many opportunities for testing their bridge-building skill.
One of India's most famous bridges— the Lansdowne—deserves special attention, because of the contrast it presents to the African examples already mentioned. The bowstring girder spans of the Lower Zambesi Bridge were pieced together with the assistance of elaborate steelwork supports underneath, temporarily erected in the river between the piers. The Lansdowne Bridge in India, however, which carries the North Western Railway across the Indus from Sukkur to Rohri, is built on the cantilever principle. From the two high lattice towers, not unlike those of the Forth Bridge, which stand on either side of the river, were projected long steel members out towards the centre of the river. These were balanced by long arms extending in the opposite directions and secured to the banks. A great central span of 200 ft. was floated into the river to complete the gap between the trackways supported by the arms reaching from the great towers.
The Victoria Falls Bridge also was built on the cantilever principle, but nothing more dissimilar to the Lansdowne Bridge could be found than the graceful arch spanning the Zambesi Gorge. The Indian bridge has a clear span of 790 ft. from tower centres—no mean distance.
Of the many hundreds of railway bridges to be found in India, some are of notable length. These include the Upper Sone Bridge, 10,052 ft., on the East Indian Railway ; the Lower Godavari 'Bridge, 9,096 ft., on the Madras and Southern Mahratta Railway ; and the Mahanadi Bridge, 6,912 ft. on the Bengal Nagpur Railway. These mammoth bridges are carried in some part over land, and their continuous length over water is not so great as that of the Lower Zambesi Bridge.
Another great Indian bridge, opened by Lord Willingdon, the Viceroy, in December, 1931, spans the River Hooghly at Bally, near Calcutta. The Willingdon Bridge, as it is called, involved the use of 17,000 tons of steel, and cost nearly £3,700,000 to build. The steelwork was made at Calcutta. The seven giant 350-ft. spans were towed up the river on huge pontoons. There are two railway tracks between main girders and a roadway on either side carried on cantilevers.
The Willingdon Bridge offers a new outlet for Calcutta's trade with Bengal and other provinces.
The Kistna Bridge in Hyderabad, on H.E.H. the Nizam's State Railway, is typical of a large number of medium-sized bridges in India. The Kistna Bridge was built with plate girders and is of the "open-deck" type. A plate-girder bridge is so called because it is built up of girders made largely of steel plates, as opposed to lattice-work. If a plate-girder were sawn in two and then looked at endwise, it would resemble a large letter I. Each span of the Kistna Bridge consists of a double-girder, just wide enough to take the single track, and the complete lack of superstructure—signified by the term "open-deck"—gives the impression that the train might "lose its footing" while making its way over the seemingly perilous girders.
Though much of H.E.H. the Nizam's State Railway is laid to the broad 5 ft. 6 in. gauge, it was decided to lay the southern section from Secunderabad to Dronachellam, where it joins the Madras and Southern Mahratta Railway, to the metre gauge, that is 3 ft. 3-3/8 in. This extension necessitated the crossing of the Kistna River near Gadwal, and a bridge over half a mile long had to be designed. The earth and rock excavated so as to found the thirty-five masonry piers amounted to 557,392 cu. ft., and the thirty-four girder-spans weighed fifty tons each.
The erection of these 80-ft. girders was not a difficult task compared with the job of piecing together the complicated steelwork for the Nile and Zambesi bridges. Each double-girder was delivered in one piece from Secunderabad, with the wooden sleepers for the permanent way laid ready along the top. A travelling derrick, standing on the main line parallel to the siding to which the span was delivered, lifted the span clear of the wagons supporting it. The derrick was next propelled from the rear by a locomotive to the edge of the embankment approaching the bridge. The derrick then lowered its fifty-ton load so that the ends rested on the first two masonry piers.
After the span had been secured in position it was an easy matter to lay the rails upon the waiting sleepers, and the travelling derrick could thus proceed with the second span and lower it to reach the third pier. In this way the thirty-four spans were erected. The bridge was ready for traffic in 1923.
The seemingly precarious nature of the railway over the Kistna Bridge is not unusual, as a corresponding arrangement is sometimes found with bridges and viaducts of different designs in England. If it is the opposite of reassuring to cross one of these narrow, open-deck bridges by train, it is often positively alarming to perform the same operation on foot.
In extending praises to those who endanger themselves in the work of building railway bridges, it must be mentioned that a wire netting is sometimes stretched beneath a new span while construction proceeds. But this only partly amends the situation ; the sensation of dizziness still prevails, and it requires the skill of a trapeze artist to be sure of avoiding injury should a fall into the net occur.
The traveller by train from Zurich to Innsbruck travels through the six and a half miles Arlberg Tunnel, in the Austrian Tirol. Beyond this the line to Innsbruck is confronted with the Trisanna River, at a point not far above its confluence with the Rosanna.
It is refreshing to find how often the engineer, when called upon to design an instrument of utility, achieves a masterpiece of imperishable beauty. A fine example of his handiwork is the singularly handsome lattice-girder bridge spanning the Trisanna Gorge in the Tirol. The train is carried 282 ft. above the torrent by a single span of 390 ft.
To strengthen the bridge sufficiently to carry the modern electric locomotives and international rolling-stock now used over this route, a bowstring truss has been built on to the underside of the original bridge. The strengthening of bridges to meet altered traffic requirements is a task which comes the way of railway engineers in all quarters of the globe from time to time.
The weight which a new or rebuilt bridge can bear with safety is, naturally, known theoretically in advance, but practical tests with abnormally heavy burdens are made in addition, only the very smallest bridges being exempt from this precaution. The Sydney Harbour Bridge affords an example of the thoroughness with which testing is carried out. Before the bridge was opened to traffic, scores of locomotives were run on to all four railway, tracks, until these were packed solid from end to end. The locomotives weighed 4,000 tons—and all went well.
The coming of the Sydney Harbour Bridge robbed another New South Wales bridge of premier Australian fame. Fifty years ago, the estuary of the Hawkesbury River, between Newcastle and Sydney, alone remained to be conquered so as to complete a continuous chain of railways traversing four colonies and extending from Oodnadatta in South Australia to Cunnamulla in Queensland, a distance of 3,100 miles. On May 1, 1889, the Hawkesbury River Bridge was opened, the work having been carried out at a cost of £340,000 by American engineers, using English materials.
The inequalities of the bed of the estuary were such that, while some of the pier foundations were only just over 100 ft. in depth, yet in one instance a steel cylinder 150 ft. in height had to be sunk into the soft mud, for filling with concrete. The seven 416-ft. spans were erected on pontoons, provided with the necessary scaffolding, and then towed by a fleet of tugs to the site, to be dropped into position by the falling tide. Catastrophe nearly overtook the proceedings. One of the pontoons got adrift with its 1,000-tons load, fouled some rocks and went aground, temporarily developing a bad list with the falling tide. The divers, who had to work at the bottom of the river during the preparation of the foundations, were compelled to be continually on their guard against the fierce sharks which infest the coastal waters of Australia.
Some of the wildest and most picturesque scenery in the world lies in North Queensland. Not far from Cairns, by the Pacific coast, the railway passenger is carried through the heart of one of the most cherished beauty spots, thanks to the bridge installed beneath Stony Creek Falls. This is a graceful structure built on a curve, with the 150-ft. waterfall plunging and leaping into the gorge below ; up this gorge the 3 ft. 6 in. gauge trains have to toil.
Much has been written about wonderful bridges in the New World,
When the Union Pacific Railway pioneers were laying their trail westwards towards San Francisco, they found their line of progress impeded by the Great Salt Lake. To save expense and time, they carried their line round the obstruction. But the gradients and sharp curves involved high running costs, and a short cut of thirty-one and a half miles was eventually made straight across the lake. To the watcher at the train window, the dazzling blue water of the lake at the hour of sunset presents a sight hardly to be equalled anywhere else in the world. From isle to isle gulls, pelicans, and blue herons slowly wend their way in the still atmosphere.
The short cut across the Great Salt Lake saved the Union Pacific nearly forty-four miles, as compared with the old line round the north shore. No fewer than 2,824,700 lineal feet of timber were absorbed in the twelve miles of trestles, which raise the track 19 ft. above the water. The shallowness of the lake permitted eleven miles of the line to be run on an embankment, while part of the track was laid on a promontory which projects southwards towards the centre of the lake.
The Trans-Siberian Railway was likewise threatened with frustration, when under construction, at the hands of Lake Baikal. The mountainous nature of the lake's setting greatly delayed the building of a line round the south shore—the depth made a bridge impossible—and this arduous and costly piece of engineering took a number of years to achieve. The railway schemers foresaw that during this period of delay it would be vital to arrange tor a connecting link across the lake ; therefore, long before the permanent way came within sight of the lake shores, an order was placed with Messrs. Sir W. G. Armstrong Whitworth & Co., Ltd., of Newcastle, for a train ferry.
Although Lake Baikal is only some forty miles wide at its southern portion, it is 397 miles long and many thousands of feet deep. Fierce storms frequently pile up waves 6 ft. high. This in itself would not be an appreciable obstacle to a well-built ship. But the required vessel had also to be able to forge its way through thick ice, for Lake Baikal is frozen over for nearly five months in the year. Moreover, due to the fact that the vessel must carry express trains, it had to be capable ot a reasonable speed.
These features were successfully combined in a masterly ship, perhaps the most singular train ferry of all time. Messrs. Sir W. G. Armstrong Whitworth & Co., Ltd., received their order in 1896, and the vessel was constructed on the sectional principle at their Elswick Works, taken to pieces and packed on board an ocean steamer in less than six months. The conveyance of the 7,000 packages over 4,500 miles of land from St. Petersburg—now Leningrad—where the load was disembarked, to Lake Baikal, stands out as one of the most remarkable of engineering feats.
The parts ot the "Baikal" as the ship was appropriately christened, were transported in wagons to Krasnoyarsk, which at that time marked the eastern limit of the Trans-Siberian Railway. Eight hundred miles still had to be covered to reach Lake Baikal, and sledges proved the only solution. The consignments, 2,700 tons in weight, were slowly drawn by ponies to Irkutsk, near the lake. But difficulties were still not at an end, for a considerable journey upstream along the Angara River had to be completed before the building berth by the lake shore was reached. The heavy boilers, weighing twenty tons each, were particularly difficult consignments to handle.
In view of the primitive methods of transport available, it is not altogether surprising that the time taken to carry the dismantled "Baikal" from St. Petersburg to the great lake was two years and three months. The English engineers, sent out to accompany the vessel on the last critical stage of the journey and to superintend erection, were confronted with a certain amount of chaos when they arrived. Many were the occasions when they longed in vain for the equipment of a Newcastle shipyard and for the skilled men to be found in that distant region The Russian workmen at last completed the task of erecting the ship, and a miracle of man's invention was launched upon the lake.
The main features of the completed "Baikal" proved no less interesting than the events which led up to her making. The overall length was 290 ft., beam 57 ft., and displacement 4,250 tons. Twin engines, each of 1,250 horse-power, drove two screws in the stern, and a similar unit drove another screw placed well down below the ice-breaking bow. When working in winter this screw in the bow scooped out the water ahead, causing the ice above to lose much of its natural support, with the result that it cracked readily in front of the advancing stem, which was specially shaped to bear down upon the ice from above. At the water-line the ship was protected by a belt of steel plates, reinforced with heavy wooden beams 2 ft. thick. The rate of advance in winter conditions was three to six miles per hour, but in clear water the speed was up to fourteen knots. The engines burned wood fuel.
The train deck of the "Baikal" had a capacity for twenty-five large covered goods wagons of 28 ft. length overall on three lines of rail. The centre road was of special height and could be used by the big sleeping-cars of the Trans-Siberian Express.