The Scotsman Thursday 14th June 1866

THE FORTH RAILWAY BRIDGE

The delay and inconvenience caused by the intervention of the Forth and Tay ferries, on the route from Edinburgh to the North, have long been a source of complaint with the public, while the working expenses have been so heavy as to the induce the North British Railway Company, who a few years ago became proprietors of the route, to consider whether a more suitable means of communication could be established. Bridges across the Firths were seen as the only effective substitutes for the ferry boats; and gigantic though the task appeared, the directors of the railway resolved to undertake the carrying of the iron road across the waters of the Forth and Tay.

Powers were last year obtained from Parliament to proceed with the erection of the Forth bridge, and the works required to connect it to the already existing parts of the North British system; but the Tay bridge is meantime held in abeyance.

It was expected that a convenient site for the Forth bridge would be obtained at Queensferry; but though the channel is narrowest at that point, the depth of water was too great to build in, being 240 feet. Another objection was the fact that any bridge crossing there would require to be of immense height to allow the passage of large ships which occasionally seek shelter in St Margaret’s Hope.

Mr Bouch, C.E., engineer of the railway company, sought a site further up the Forth; and, after careful consideration, selected a point five miles above Queensferry, where the channel is considerable wider, but not so deep. On the south side the bridge will be connected with the Glasgow and Edinburgh line by a branch extending to a point about midway between Winchburgh and Linlithgow; and on the north side a short branch will join it to the network of lines adjoining Dunfermline. A convenient connection will thus be established with the mineral fields of Fife, the importance of which will be much enhanced. Another advantage is that the journey from Edinburgh to Perth will be shortened by about twenty miles; but this shortening would be due to the bridge in an indirect way only as the chief curtailment would be made by a line leading from Gateside Station on the Fife and Kinross Railway, to a point near Bridge of Earn. For the construction of this line powers were obtained on 1863, and it will be proceeded with even should the bridge scheme be abandoned.

Though the site chosen for the bridge is, all things considered, the best that could be got on the Firth, it presents engineering challenges that will require the exercise of the most consummate skill to overcome. The bottom is composed of silt carried down by the Forth, and some of the borings went down through 120 feet of this material.

Before proceeding with the entire undertaking, it was resolved to test the practicability of constructing on this soft foundation, and at a reasonable cost, a bridge that would have sufficient stability to bear heavy traffic. To this purpose Mr Bouch has assiduously applied himself, and he considers that the result holds out a reasonable prospect of success.

Preparations are far advanced for commencing the erection of one of the five central piers according to the plan proposed to be adopted for all the five; and should the experimental pier give satisfaction, the other parts of the bridge will be proceeded with; but if the pier should prove a failure, the undertaking will, there is reason to fear, be abandoned.

As the mode of construction employed in the experimental pier possesses some features of novelty, it may be interesting to give a detailed description.
In order to obtain a good footing on the silt in the bottom of the Firth and at the same time not to give too much weight to the structure, the pier is to have a wooden base of considerable extent. This base which has already been constructed, and will be launched today at Burntisland, may be compared to a huge tray, on the centre of which the masonry of the pier will be built. The tray is composed of logs of Memel timber, firmly bolted together to form a solid mass, 80 feet long, 60 feet wide and 7 feet thick.

[Note – Memel (now Klaipėda) in Lithuania was a major timber producing region from the 18^th century]

Its superficial area is 4800 [square] feet, and the weight it will have to sustain will be equal to 15 cwt per [square] foot. This pressure is unusually light in such a structure, it being no uncommon things for the foundations of a bridge-pier to have to sustain 10 tons per [square] foot; but in the case of the Forth bridge, as already stated, it was necessary to distribute the pressure over a large surface. From the centre of the wooden base the pier will be built up of brick to a height of 12 feet above the water level, and as the water is 40 feet deep, this will give a height of 52 feet.

The ground-plan of the brickwork somewhat resembles a figure 8 in outline, the only difference being that it is not so much contracted in the central part. The greatest diameter of the masonry will be 50 feet, reduced by stages to 27 feet, and the thickness will be 7 feet. The brickwork will be built in a caisson, the floor of which is already laid on the wooden platform or base. The caisson will take the shape of the masonry, and is constructed of half-inch iron plates so as to be easily removed when the masonry is finished.

As soon as the preliminary arrangements are completed, and a certain amount of the brickwork put in, the base will be towed to the site of the pier and securely moored between two barges specially constructed for the purpose. Each barge is of 700 tons burthen, and the interior is fitted up as a dwelling place and workshops for the men to be employed at the pier, while the decks will serve as platform on which to deposit as required the material for the construction of the pier.

After the base is moored in its position, the masonry will be proceeded with; and as the work proceeds and the platform settles down, the walls of the caisson will be carried up gradually, so as always to form a sufficient barrier against the ingress of water. On the wooden base, and placed at regular intervals round the outside of the caisson, are twelve strong iron cylinders, each eight feet in diameter; and in the hollow centre of the masonry, two similar cylinders twelve feet in diameter. These will, like the caisson, be constructed in sections as the pier settles down into the water; and when the platform reaches the bottom, will stand a few feet above the surface at high water.

The cylinders are not a permanent part of the work, their purpose being this:- When the wooden base reaches the ground, the cylinders will be loaded with 10,000 tons of pig-iron, which is two and a half times the weight that the pier will have to bear when the superstructure is completed, and a train standing upon it. It is calculated that his weight will be sufficient to press the pier into the silt to a depth of six or seven feet, so that the surface of the wooden base will be nearly level with the surface of the silt.

Once this is effected, the cylinders will be emptied and taken up; and the masonry having by this time reached the surface and been allowed to consolidate, the walls of the caisson will be removed, and, with the cylinders, applied to the construction of the next pier. The cylinders will also be used for another purpose than that described.

When the platform settles down, it is possible that it may not take a level position, in which case men can be sent into the cylinders to excavate, through ingeniously contrived openings, some of the silt on the high side or end.

After the base has been adjusted in this manner, a large quantity of slag will be deposited on and around the wooden platforms, and as this consolidates, considerable strength will be imparted to the substructure.

Above the low-water level the piers will be faced with whinstone, and on a platform of that material the columns of the bridge will stand. The principle of construction adopted in the piers is similar to that which was followed in the case of the old Westminster Bridge.

Leaving the Experimental Pier completed so far as the masonry is concerned, we shall now proceed to describe the bridge as it will appear when completed, if it ever be so. The entire length of the bridge will be 3887 yards, or about two miles and one-fifth; and it will be supported on sixty-one piers. Commencing at the south side, there will be fourteen openings of 100 foot span, six of 175 feet, fifteen of 200 feet; then come the four central navigable openings of 500 feet each, next two of 200 feet, four of 173 feet, four of 150 feet, and twenty of 100 feet.

The height of the openings will gradually rise from 65 feet at the sides to 135 feet in the centre above high-water level.

The abutments will consist of 30 yards of stone arching on one side and 94 yards on the other.

The piers of the spans on either side of the four central openings will be of simpler construction than those described above. Each will consist of four cast-iron piles, two feet in diameter and one inch thick, arranged in pairs, and protected by a malleable-iron pile of similar diameter placed on each side. These piles are furnished with disks at the lower end which will be embedded to a considerable depth in the silt. At the low-water line the piles will be united by a platform, which will form a base for the columns on which the girders will rest. Each pier will sustain two columns, composed of cast iron, ten feet in diameter, one inch thick, and built up in lengths of ten feet.

The girders which are on the lattice principle will be immense structures, especially the central ones. Each of the four great girders will weigh 1170 tons, which is 592 tons less than the weight of the tubes of the Britannia Bridge, though the span is 49 feet more. The depth of these girders will be 64 feet, and the width between their sides is 18 feet.

There will be no cross-girders, the fabric being sufficiently strengthened by overhead lattice work, and flooring of wood one foot in thickness. This flooring will be a few feet above the lower edge of the four great openings, but on either side of these the form of the bridge will undergo a change, so that while in the case of the central openings the train may be said to run along the bottom of the girders inside, in the case of the side openings it will run along the top. The object of this arrangement is to increase the height of the central openings, so that the largest vessels may be able to pass through; and this having been provided for, it would be unnecessary and inexpedient to carry the side girders to greater height than would be required to allow their upper edges to reach the level of the rails which pass through the central girders.

Over the smaller openings the weight of the girders will be much reduced, and will diminish towards either end of the bridge. The girders will be constructed on land, and floated on pontoons to their respective piers. The work of elevating them to their lofty positions will, as may be imagined, be a work of great difficulty, and will require special and novel appliances.

As the girders or sections are put up, they will be joined, so as to form one continuous web of iron from side to side of the Firth. By this plan great strength will be given to the bridge.

The greatest height from the base to the summit of the bridge will be 212 feet. From the difficulties attending the construction of the central piers, it is not expected that they can be completed in a shorter time than four years, and of course a considerable time longer will be required to finish the work.

The estimated cost of the bridge is £470,543, of which sum £267,105 is allowed for the central spans. No contract has been entered into for the construction of the pier, nor will there be any until the results of the experiment at present in progress becomes known. The iron work required for the caisson and cylinders has been supplied by Messrs Hopkins, Jilkes, & Co. (Limited). Middlesboro’-on-Tees; and the wood platform has been put together by workmen connected with the railway.

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