The Forth Bridge/Accident to No. 4, or Queensferry North-West Caisson

by Wilhelm Westhofen

Accident To No. 4, or Queensferry North-West Caisson.

This caisson was built and launched in the usual way and towed into position at the north-west corner of the Queensferry jetty in December, 1885. It was secured to the staging in the usual, way, allowing it to rise and fall with the tide pending the putting on and rivetting of the remaining courses of plates at top, and the otherwise completing it for the sinking process. Owing to the inconsiderable depth of water, the cutting edge touched the mud soon after half ebb, and embedded itself at low water. On New Year's Day, 1884, an exceptionally high tide occurred, followed by an equally exceptional low ebb, and the caisson sank deeply into the mud, assuming, no doubt, somewhat of a list or dip in accordance with the natural slope of the mud. The cutting edge entered so deeply into the more solid portion of the mud or silt that, on the flood tide returning, the water could not get underneath, and the caisson failed to rise with the tide. Not being built high enough, the water soon flowed in, and filled it completely; and on ebbing, as the lower sluice valves could not be got at, the caisson became top-heavy, and tilted still more over towards the low side. At last, it began to slide in the same direction, and moved thus for about 20 ft., when it stopped. In this position the lowest portion of the top plates remained fully 6 ft. under low water, and it became necessary to arrange to add to its height before pumping could be resorted to. A number of divers were employed to bolt on two or three tiers of plates.

Fig. 46. Tilted Caisson at South Queensferry.

Arrangements were made to stiffen the caisson internally by timber struts on top, and to keep on stiffening in the same manner as the pumps succeeded in reducing the water level inside the caisson. Unfortunately, the pumping was carried on at too great a rate for the carpenters who were to put in the strutting, and the thin plating could not support the pressure of the water from outside. The plates gave way, and a great rent, right across the lower side of the caisson, some 25 ft. to 30 ft. long, was the result. After discussing many plans suggested for getting out of this difficulty, it was resolved to construct a sheath or barrel of 12-in. timber balks all round the caisson, to bolt as many of these balks as could be got at firmly through the iron skin, and to make the best joint that could be got either by puddle clay or cement grouting, or a combination of both. All this work below had to be done by divers, and therefore required a considerable time in doing. In the first instance, a heavy circular timber frame was placed inside the caisson near the top and strutted in all directions. A number of balks were first placed all round, and, where necessary, driven down into the soil as far as required. Strong iron hoops or belts were laid round all these. and then the filling between these guide timbers carried on, all of them being V grooved on the one side, and the other side V shaped to fit the groove. Finally, wedge piles or closing piles were driven in between the other balks, and as many as could be got at were bolted tight up to the skin of the caisson where it was sound. Nearly ten months had elapsed since the accident occurred before the timber barrel was in such a condition that pumping out could be proceeded with (see Fig. 40). The ground under the high side was now dredged away, and both air and water were forced down the air-shafts into the chamber to wash or force out as much as possible from underneath. Pumping was then commenced, all leaks as they showed being dealt with in any suitable manner, further stiffening inside resorted to where necessary, and thus, step by step, the lost ground fought for. Weight was also added on the high side by commencing to build a brick casing against the outer shell. Patiently this work was persevered with, and at last, one Sunday morning, October 19, 1885, some what unexpectedly, the caisson raised itself out of its muddy bed and floated once more. It was at once drawn into position, and no time was lost in commencing the sinking process. As it was impossible to place a new shell in the place where it was rent, it was decided to leave this alone, and not to proceed with the building of the inner shell either, but to substitute for both a stout lining of clinker bricks built in cement. The shape of this and its strength are shown in Fig. 47.

The three other Queensferry and the two south Inchgarvie caissons had in the mean time been sunk to their full depth, and in fact had nearly disappeared out of sight, and all machinery and apparatus were available for dealing with this the last.

Before the middle of February, 1886, this prodigal among caissons had joined his brethren far down in the prehistoric clay, and there is no reason to think that the masonary pile which stands on it is less able to carry its due share of the enormous load laid on them than either of the others.

The foundations of the four circular piers on Fife, and of the two circular north piers on Inchgarvie, have been fully dealt with, and it remains to describe the mode of founding the two south piers on Inchgarvie and the four piers at Queensferry. The two former are founded on the solid rock, sloping here at an angle of about one in five to the south-west; while the four piers at Queensferry are placed on the boulder clay which is found there in a very hard and solid state at a minimum depth of 48 ft. below high water. The boulder clay here is sloping to north-north-east, also about one in five. These caissons were all sunk by means of the pneumatic process at considerable depth under water, under some difficulty, and under great pressure of air. To manipulate these cylinders of immense size, weight, and height, in the face of the uncertain conditions of wind and weather, and under the influence of a strong tidal current; to put them weighing between 3000 and 4000 tons into their places, and to hold them there and pass them down through material ranging from soft mud to hard rock, was certainly a most formidable task. Yet, with one exception, just above mentioned, namely, the tilting of the Queensferry north-west caisson, these huge cylinders caused proportionately less trouble and anxiety than the shallow cofferdams of the other foundations. This was, no doubt, owing in a great measure to the practical knowledge and experience of the men in charge of this portion of the work. The sinking of these six caissons was let as a sub-contract to M. Coiseau, of Paris and Antwerp, whose large staff of men been engaged for several years in the construction of the great quays and harbour walls and docks at the latter place by the pneumatic process. It would be as well to take this opportunity of dispelling the notion which has somehow gained credence, that the foreign workmen—mostly North Italians, with a sprinkling of French, Belgians, Austrians, and Germans—were better able to work in and resist the high atmospheric pressures necessary to this kind of work, than the British workmen. That this is not so is proved by the fact that large numbers of the Forth Bridge men—carpenters, fitters, and ordinary labourers—were frequently employed below, and many of their number for the first time, when the pressure was already considerable, without experiencing any great inconvenience or harm. It is, in the first instance, a matter of habit, although it certainly requires good health, freedom from pulmonary or gastric weakness, and abstemiousness, or, any rate, moderation in taking strong spirituous liquors. Some of the most experienced hands of M. Coiseau suffered when they had been making too free with the whisky overnight, and a good dual of the disorders that ensued were traceable to the same source; though, on the other hand, wet feet, or incautious and sudden change from a heated atmosphere into a cold and biting east wind, insufficiency of clothing, and want of proper nourishment, had their influence in causing illness among the workers. Although these six caissons were founded at depths varying from 63 ft. to 89 ft. below high water, with an average time of seventy-eight days for each, not one death can be properly and justly attributed to working in high pressure. Two men died during the time, but both were already consumptive when they commenced working here, and the rigour of a Scotch winter had, at any rate, as much to do with their death as the air pressure. Another man became insane, and had to be sent back to his own country.

The principal bad effect produced by the air pressure appears to be that of severe pains in the joint⁸ and muscles of the arms and legs. As these have been, in most cases, traced to hard work and consequent copious perspiration, and also to too long a stay under pressure, it has been suggested as a probable cause that small globules of air make their way through the skin, or between the skins, where they remain, and on the workman returning to ordinary atmospheric pressure, expand, and thereby cause the most agonising pains in the joints, the elbows, shoulders, knee-caps, and other places. In seeming confirmation of this, the sufferers got instant relief on returning into the high pressure. Thus it happened that many of those afflicted with this disorder spent the greater part of Saturday afternoon and Sunday under air pressure, and only came out when absolutely obliged to do so. Various researches wore made by members of the medical staff in the endeavour to give relief or obtain a cure, but, so far, not with any degree of success.