The Forth Bridge/Commencement of Work
Commencement of Work.
Soon after the contract was signed in December, 1882, a start was made with the preparatory and temporary work.
Offices and stores as well as a workshop, hereafter known as No. 1 shed, had been erected by Mr. Arrol in connection with Sir Thomas Bouch's suspension bridge, and these were taken possession of and considerably added to from time to time.
To enable the contractors to make an early start with the permanent work, that is, the building of the masonry piers, it was necessary that the positions of these should be fixed without delay. A base line about 4000 ft. in length was laid down along the high ground on the south shore, starting from a point in the centre line of the bridge, passing along the North British Railway for some distance and terminating opposite the east breakwater at Port Edgar. Along the breakwater a timber gangway was erected, and near the far end of it, at a distance of about 3000 ft. from the base line, an observatory was built. Three points on the centre line of the bridge, one on the Queensferry shore, one on Inchgarvie, and one on the Fife shore were marked down, and their distances stated by the Ordnance Survey of Great Britain. Various other stations, about twenty in number, were laid down as required, and by means of these a most careful triangulation was made and the centres of the three main piers finally fixed. In order to verify, if possible, the correctness of the distances apart of the three principal stations on the centre line of the bridge, which had been obtained by calculation based upon the triangulation, a measurement of the north span of 1700 ft. from the centre of the north circular piers on Inchgarvie to the south circular piers on Fife was made in the summer of 1884.
In a straight portion of the North British Railway a distance of 1700 ft. had been carefully measured and marked and transferred to high posts at the side of the cutting. Upon these posts notched knife-edges were placed at the two extremities. A fine steel wire about 120 in. in thickness was laid along the span and drawn over the knife-edges with a certain amount of stress put upon it, previously agreed upon. Thus drawn up the wire left a certain amount of sag in the centre, which was carefully measured by level and noted. Two narrow copper tags were then soldered on, to mark the end points. The wire was then coiled up and kept ready for use. The temperature also was noted.
On the two shores immediately under the piers which marked the stations, places had been prepared for levels, by means of which the amount of sag in the wire could be fixed. On a calm, cloudy day, with the temperature about the same, the wire was taken across the north channel and laid down upon the prepared knife-edges on the piers, and with the same amount of stress put upon it, and with the same amount of sag allowed, the two copper tags soldered on should have coincided with the notches in the knife-edges, provided the distance was correct.
It was, however, found somewhat short, and, although carefully tried several times over, the result did not vary.
Since the span has been completed the measurements made along the girders have reduced the error to about 1 in. in the north span and 6 in. in the south span, both being shorter than intended. The work of triangulation was carried out by Mr. R. E. Middleton, M.I.C.E., and a full record of his labours is given in a pamphlet published by E. and F. N. Spon, London, about two years ago.
For several reasons the positions of the four circular granite piers on Fife were first fixed, and soon the work there was in full swing. The greater portion of the old coastguard station, which was situated on the site now occupied by the north-east pier, had to be removed, and an entirely new station built on the rock further to the north, and about 40 ft. higher. The rock extending over the site of the pier had to be levelled down to about 7 ft. above high water in order to obtain ground for laying down plant, machinery, and materials. The old battery slip, the ancient landing-place for the ferry boats which slopes down from above high water to a few feet above low water, was covered over with an extensive and substantial timber staging, on which cranes were erected for unloading and moving material, and which also served for landing the workmen and for mooring barges and steamboats. (See Plates IX., XI., XII., and XIII.) Further inland, in convenient situations, wooden huts for the accommodation of the workmen and their families were erected, to which were added, later on, a canteen, stores, and dining and reading-rooms.
At South Queensferry the preparatory work was of a much more extensive character. The ground here rises rapidly from the shore at a gradient of about 1 in 2 until it reaches an elevation of about 100 ft. above the sea, and thence continues to rise with a gentle slope in a southerly direction. As a great deal of level ground was required here for workshops, drill roads, and spaces for temporarily fitting together portions of the steel-work, it became necessary to level the ground in terraces. Thus the general level of No. 1 Shed is about 12 ft. below the offices, while No. 2 Shed is about 6 ft. above these, the drill roads another 5 ft. higher, and so on. No. 1 Shed was considerably enlarged, and No. 2 Shed at once commenced, as were also the drill roads on which the tubular members had to be put together and drilled. Further to the south still some forty wooden huts were erected, together with stores for the sale of food and clothing, boots, and groceries, and a canteen with dining and reading-rooms.
To these were presently added sixteen houses substantially built in bricks for the accommodation of foremen and members of the staff, and about sixty tenements at Queensferry for leading hands and gangers.
Next to the drill roads a carpenter's and joiner's shop was erected with a saw-bench and a pattern shop, and a large drawing loft, 200 ft. long by 60 ft. wide, with blackened floor upon which full-sized drawings were prepared and full-sized templates made for drilling, planing, bending, &c., of portions of the superstructure. Telephonic communication between offices, stores, workshops, and the Queensferry, Inchgarvie, and Fife centres, was established by means of a cable laid across the Forth.
On the west side, the ground was bounded by the North British Railway to Queensferry, which here runs in a deep cutting. Two temporary bridges—girders of the ill-fated Tay Bridge—were thrown across at a distance of about 200 yards apart, and as the progress of the work required further extensions, ground was taken on the other side of the lines, until finally between 50 and 60 acres were occupied by the works on both sides of the line and right up to the Edinburgh-road. All this ground during the busy years of 1886 to 1889 was covered with girder-work under construction, and presented a striking scene both day and night.
Down by the shore to the east of the central line of the bridge a sawmill was erected, and later on a large cement store. The Queensferry jetty was commenced early in 1883, and completed in the spring of 1884, shortly before the first caisson was launched. It is a little over 2100 ft. long, by 50 ft. wide. It runs parallel with the centre line of the bridge at a distance from it of 60 ft. to about 100 ft. beyond the cantilever end pier, whence it passes by a gentle S curve right into the centre line, and terminates in a strong crosshead which embraces the four masonry supports of the Queensferry Pier. Extensions of this jetty were made during the building of the foundations and lower portions of the approach viaduct piers, but were removed again after the girders had been completed. A similar extension with landing stage for steamboats, and storage ground for building material was made round the cantilever end pier, and remained during the whole time.
The jetty is built on piles driven in the silt and soft clay down to the boulder clay. Where rock occurs, the uprights are secured on level points by being bolted to stout iron pins which are sunk in holes drilled for the purpose. There are six piles mostly 12 in. by 12 in. baulks, set in a row transversely, three on either side being bound together by half-timber cross bracings on opposite sides and the whole connected by a full timber crownhead set on top. These trestles are set about 20 ft. apart, and are held together by raking struts passing from the top of one trestle to about low-water mark of the next. Passing from trestle to trestle are 12 in. by 6 in. rolled joists about 25 ft. long, weighing 56 lb. to the lineal foot; these are set from 4 ft. to 5 ft. apart, according to position, and transversely on these the planking 3 in. or 4 in. thick, as the case may be, is laid, which forms the flooring. The level of the top of the planking is about 8 ft. above high water. The jetty was erected by means of an ordinary traveller carrying an overhanging piledriver, which set out the piles 20 ft. in advance and drove them in. The uprights were then cut to level, the crownhead set on, the diagonal bracings and raking struts fixed, and the traveller moved forward on double longitudinals. The joists were laid down behind the traveller and the flooring made up. As this mode of working did not progress as quickly as desired, a pile-driving barge was placed near the far end and worked towards the shore. In the position of the jetty forming the T head, the piles required to be very long, as the average depth at high water was about 32 ft. and the depth of soft mud or silt, some 24 ft. , requiring piles of from 65 ft. to 70 ft. in length, spliced in the ordinary way. Pointed pile shoes made of malleable iron were fixed to the lower ends.
The shore end of the jetty was connected with the under works by an inclined road laid on timber trestles, the gradient being about 1 in 6½, with an iron girder bridge of about 65 ft. span across the Edinburgh-road. There was a single line of rails and a footpath to one side with a covered box running alongside for carrying the hydraulic pressure pipes, and also a supply of water down the jetty to Queensferry Pier. The incline was worked by a wire rope, 1¼ in. in diameter, drawn by a winding engine on the top. Down this incline all material was sent which arrived by North British Railway, whether fuel, oil, or other stores or plant and machinery, or, finally, steel-work which had been prepared in the shops and yards. Upon the jetty itself a number of service lines were laid down with sufficient room between, to store immense quantities of building material. Along the east side two lines were laid down which conveyed all the material coming down the incline to three or four heavy steam cranes, by which it was lowered into the the barges and conveyed to the landing stages on Inchgarvie or Fife. While the jetty was in course of construction, launching ways were laid down in a sheltered bay about a hundred yards to the east below, and in front of, the sawmill. They consisted of parallel rows of timbers, of sufficient width to take two caissons of 70 ft. diameter side by side. They were laid to the natural slope of the ground, which is here part rock, part shingle, and has an easy gradient of about 1 in 11. The launching ways will be further referred to in connection with the building of the caissons.
Machinery and plant of every kind and description commenced now to arrive on the ground. A siding had been laid down from the works to South Queensferry station, about half a mile distant, and another siding in the cutting alongside the shops. Cranes were set up in all convenient positions, and thus a large amount of material arriving by train could be unloaded close to, and distributed among the different shops. By this arrangement the various sections of raw steel—whether plates, bars, angles, tees, or others—could at once be delivered near the heating furnaces or drilling or planing machines where they required to be dealt with.
From this time forward for fully five years, the plant kept on increasing at an astounding rate, most of it being of a special character, and purposely designed for these works. The list of the plant on the works towards the end of 1888 is a voluminous document, and is represented in the accounts by a very large sum, not far short of half a million sterling.
Inchgarvie.—The island of Inchgarvie, which Providence has so kindly placed in the middle of the Firth, is a peak of whinstone rock, about 850 ft. long at high, and 1500 ft. at low tide, and on an average not more than 60 ft. wide above water. The castle stands on the highest part of the rock about 40 ft. above high water, and the square keep is about 30 ft. high; on the top of this the wind gauges are fixed. The distance from Inchgarvie to the Fife shore is about 1600 ft., while to the Queensferry shore it is rather more than double. Although owned by the Dundas family, whose seat is in Linlithgowshire, the authorities on the north shore claim it as belonging to the County of Fife.
The castle was built some time subsequent to 1490, on the 20th of March of which year, King James IV. of Scotland granted a license to John Dundas of Dundas to erect a fortress on the island of Inchgarde, since corrupted to Inchgarvie. The object was to afford protection to all shipping seeking refuge from the pirates with which the North Sea seems to have been infested in those days, and for giving such protection, Dundas was authorised to levy a toll of 6d. per ton. One of those persons without belief in the good old days has suggested that the Laird of Dundas not only kept a garrison on Inchgarvie for the protection of the shipping, but that he also provided the pirates in the North Sea for chasing them up the Firth; though with regard to this both history and charter are silent.
The four circular piers are situated at the western extremity, wholly submerged at high water, although a broad ledge of rock is uncovered by the tide at low water. This portion of the island is called Craig Spurry, and upon its north-western point stands a brick pier, about 33 ft. square, and about 7 ft. above high water, the only piece of permanent work built in connection with Sir Thomas Bouch's gigantic suspension bridge. Upon it was erected, some three years ago, a lighthouse with a revolving light giving flashes about every five seconds, visible for many miles both up and down the Firth.
A spring of fresh water was said to have existed on the island, and some 40 yards east of the castle, a square well, partly cut out of the solid rock, partly formed by a brick wall, was found. It was pumped out and carefully examined, but no borehole or other inlet could be discovered, and it proved to be simply a storage tank in which the rainwater from the overlying portions of the rock collected.
On Inchgarvie the first work in connection with the cantilever bridge was carried out in the erection, during the summer of 1882, of the wind gauges on the top of the castle, and already described.
About the middle of April, 1883, the construction of a landing stage was commenced, of iron girders and iron columns pinned to the rock. At the same time the square keep of the castle, some outbuildings, and the whole of the battlements, were roofed in to afford space for the most necessary shops, offices, and stores, to which was added later on, a substantial cottage and a kitchen, and sleeping accommodation for ninety foreign workmen occupied in the sinking of the pneumatic caissons.
All over the west end of the island, the rock was cut down to the general stage-level of 7 ft. above high water, and as much ground was made up as could be obtained, by filling up with the debris of excavation and of removal of rock. Some 100 yards of sea-wall had to be built as a protection against the heavy breakers rolling in from the east, and every square foot of space thus gained was of great value thereafter, when an immense amount of material required to be stored.
In the Act of Parliament authorising the erection of the bridge, the whole of the island had been included in the Parliamentary limits, but at first only the area within the four circular masonry piers, and a reasonable amount of ground for working purposes was acquired. It was soon found that this was quite insufficient for the requirements of the works, and notice of compulsory purchase by the Forth Bridge Company, was given to the proprietor. The matter came to arbitration and was settled in 1884. A sum of 1500l. had been paid to the proprietor for the right of placing four masonry supports on the island, and in terms of the arbitration a further sum of 2800l. was paid. The island is now, therefore, the property of the Forth Bridge Railway Company.
It having been considered advisable to cover the whole area between the piers with staging, a commencement was made without delay. Owing to the nature of the bottom, which was all solid rock, it was found more expedient and safer in view of the exposed position, to make this staging of iron, and not of timber. The area of this staging ultimately amounted to more than 10,000 square yards, and the weight of iron used in its construction was between 1100 and 1200 tons. In view of this large expenditure for temporary purposes, it was deemed right to proceed on some system in the laying out the main line of girders. As the horizontal tubes, the horizontal girders between skewbacks, and the double cross of diagonal horizontal wind-bracing, required to be carried by the staging before they could be connected with the skewbacks and rivetted up, the main lines of the iron girders of the staging followed the centre lines of these members, the remaining spaces being filled in sufficiently strong to carry any weight which was likely to be put upon them during the erection of the superstructure. As this staging stood well the very severe tests to which it was subjected from various causes, and only yielded to a slight extent in places where it had been absurdly overloaded, it may not be amiss to give a few particulars as to its construction, shown in Fig. 20.
The upright columns or supports consisted of four angle-bars 4 in. by 4 in. by 1⁄2 in., braced together on all four sides by double crossbars of flat section, 3 in. by 1⁄2 in., the column being 2 ft. square. A cast-iron shoe, to which the four corner angles were bolted, formed the foot of the column, and it had a large boss in the centre with a 4 in. round hole through which a pin was passed going into the rock to a depth of about 3 ft.
The longitudinal main girders were 5 ft. in depth, and consisted of four angles 6 in. by 4 in. by 1⁄2 in., two in the bottom boom and two in the top boom, with gussets inserted between every 5 ft. apart for attaching the diagonal bracings, which were angles 6 in. by 3 in. by 1⁄2 in. placed alternately on one side or the other. The girders were carried by, and bolted to, cross-angles attached to the supporting columns. The girders carried safely a distributed load of about 6 cwt. to 7 cwt. per square foot up to 30 ft. clear span between supports. The columns were arranged in clusters of four, forming a square, the sides of which were 14 ft. taken at the centres of columns, and such clusters were placed immediately under the points of intersection in the girders of the structure overhead, or in fact in any place where much weight had to be carried. The four columns so placed were braced to each other by crosses of flat bars of heavy section 31⁄2 in. wide by 1 in. thick, the centre of the cross being formed by a heavy ring, the flat bars terminating at that point in 11⁄2 in. round with a thread cut upon them which passed into the ring. By a nut on either side of the ring the flat bar—the other end of which was bolted across the column—could be tightened or slackened as desired. Two or three sets of these bracings, according to the length of column, were bolted to each of the four sides of the cluster, the lower ones reaching in most cases to within a foot-or two of the bottom of the column having to be put on by divers. Generally speaking these clusters were placed about every 45 ft., centre to centre, and for intermediate supports, where necessary, single columns under each girder were considered sufficient, and these would receive transversely a pair of angle-iron cross-bracings between high-water and low-water mark.
The longitudinal girders run through the uprights, spaced 14 ft. apart, and they were braced laterally about every 25 ft. by a pair of cross-angle bracings carried from top boom on one side to bottom boom on the other side and vice versâ.
All the girders were made in regular lengths of bars, breaking joint at 5 ft.; they were marked by template and punched, and all parts except those for closing lengths were interchangeable. All holes were punched for 7⁄8-in. bolts. The girders weighed just 1⁄4 cwt. per foot lineal, including all bracing, the columns about 64 lb. per foot lineal, including bolts.
The whole staging was erected by overhang with derrick cranes in the following way: Starting from a cluster, the next columns forward would be lowered till they nearly touched the ground, the diver would then descend and examine the bottom, selecting a nearly level bit, or otherwise dressing the rock roughly. A few bags of concrete were then lowered to him, with which he made the bed into which the column was lowered. It was then strutted by one or two light battens to the already fixed work, a 4-in. jumper lowered in the centre of the column, and a hole jumped by means of a spring pole some 25 ft. long. The jumper was lowered through a tube 41⁄2 in. in diameter, which fitted into a socket left in the cast-iron shoe at the bottom, and acted as guide. The hole was jumped about 3 ft. to 3 ft. 6 in. deep; the jumper was then withdrawn, and a wrought-iron pin, 4 in. in diameter and 5 ft. long was dropped down the tube. The latter was then lifted up, and the diver descended to ascertain whether the pin had well entered into the rock. When the columns were fast, a length of girder was laid upon them on each side, and the crane advanced another section.
Across the longitudinal girders, 12-in. rolled joists weighing 56 lb. to the lineal foot were laid, being 21 ft. to 25 ft. in length, and therefore overhanging the girders from 3 ft. to 5 ft. The joists were bolted to the girders by hook bolts and ordinary draw-washers, and were spaced 5 ft. apart. The decking consisted of 3 in. by 9 in. planking, bolted to the top flanges of the rolled joists by bolts and draw-washers from underneath.
The staging, as first projected, was commenced in the autumn of 1883 and finished before the end of 1884, but additions became necessary from time to time to accommodate the immense quantities of material, and at times the 10,000 square yards of staging were found quite inadequate to meet the demands upon it.
All portions of the staging which had to be used as landing places for materials—and of such there were a good many—had the iron columns strengthened by bolting on heavy timbers, with half-timber fendering upon that for wear and tear. Horizontal timber struts were also set up against the backs of those columns, to guard them against bending in by the action of the boats and barges upon them in rough weather.
By the time the staging was up round the north piers on Inchgarvie—these were already well forward—arrangements had to be made for the reception of the two heavy caissons for the south piers. Facing the south, therefore, two semi-circular openings were left about 73 ft. in diameter, an extra number of columns were placed in the half-circle and well braced with the others next behind, and the whole row of columns between the north and south piers, on either side, were joined together at about low-water mark by a continuous line of double angles to take the thrust from and give resistance to any possible bumping of the heavy caissons during the time they required to remain afloat.
Meanwhile plant and machinery were sent here as to the other centres, a number of offices, large workshops, stores and shelters for the men were built; also engine-houses for pumping engines, air compressors, hydraulic accumulators, electric light machinery, and much more.