The Forth Bridge/Preliminary Work in Connection with the Four Queensferry Caissons

by Wilhelm Westhofen

Preliminary Work in Connection with the Four Queensferry Caissons.

Over the area to be occupied by these four circular piers, and to some extent outside and round it, a number of soundings were taken to ascertain the depth of water, of mud, of silt, of soft clay, and of hard clay. These data are given in another place. The T head of the jetty, which had in the mean time been completed, left four corner spaces free for the reception of the caissons, and strong timber dolphins of treble and quadruple piles were placed on three sides of the space left. As the tide currents are not nearly so strong here as on Inchgarvie, and as the caissons would almost immediately touch the soft ground during low water, the danger of their getting away was much less here, and no outside moorings were therefore provided, but strong chains fixed to the piles, and ready to be attached to lugs rivetted to the caissons, and stout ropes and wire hawsers to pass round them, were kept in readiness.

Air compressors, electric light machinery, overhead water tanks, pumps to supply the same, steam cranes, and other plant, were got ready, and large quantities of broken stone, cement, and sand were stored close by for use. Water pressure at 1000 Ib. per square inch was brought down the incline from the accumulator on the top, and conducted to the end of the jetty.

The Pneumatic Caissons.—The six caissons which were sunk by the pneumatic process were mainly built of wrought iron, and were in the first instance constructed and put together by Messrs. Arrol Brothers, of Glasgow namesakes, but otherwise in no way connected with Messrs. W. Arrol and Co., of Dalmarnock Iron Works, Glasgow. These caissons were taken to pieces again and sent to Queensferry to be built up and rivetted, as hereafter described.

Description of the Caissons.—The four caissons of the Queensferry group are all of one design, differing only in height. There are two shells, carried up at distances varying from 7 ft. at bottom to 4 ft. 6 in. at top from each other. (See Figs. 32, 33, 34, 35.)

The two Inchgarvie caissons, although externally the same in appearance, differ in so far as the inner shell is only carried up as far as the top of the cylindrical portion, being replaced above that by a backing of Staffordshire blue bricks built in cement, which was carried up in proportion as the caisson was sunk below low water. (See Fig. 36.)

Generally speaking, the caissons are made up of two parts the cylindrical and the taper part. The cylindrical portion is of varying height, with a slight taper upwards to facilitate the sinking. This portion is 70 ft. in diameter, and above it is the taper part, which is 24 ft. high in all cases, and terminates with a diameter of 60 ft. at top.

The lower, or so-called cutting edge of the caisson, is stiffened by a stout double angle and by a broad steel belt, 18 in. wide, and 1 in. in thickness.

At a height of 7 ft. above the cutting edge an air-tight floor is formed which extended over the whole area up to the outer skin or shell. From this floor upwards, started the inner shell of the caisson, about 56 ft. in diameter; and here also, but downwards, started the sloping plates, which terminated at the cutting edge, and which were backed and held by a number of triangular brackets set against the floor to one side and against the outer shell to the other. A circular space of triangular section was thereby formed, access to which was had by manholes cut into the floor between every two brackets. Under the floor a space was thus left of the form of a truncated cone 70 ft. at bottom, 56 ft. at top, and 7 ft. in height, which was called the air-chamber or working chamber. Its function was to act as a diving-bell for the purpose of gaining access to the bottom of the river and carrying on the necessary excavation over the area thus made accessible. The triangular space surrounding the air-chamber was called the shoe, of which the lowest part, as already said, formed the cutting edge.

Fig. 36. Section of Caisson at Inchgarvie.

Fig. 37. Caisson and Cradle on Launching Ways.

The roof of the air-chamber was supported by four strong lattice girders 18 ft. in height, and reaching from side to side of the inner shell, but being carried through to the outer shell by means of bulkheads or plate diaphragms set between the two shells, and rivetted through with the end posts of the large girders. At right angles to these latter were carried plate girders 3 ft. deep, and spaced 4 ft. apart, centre to centre, all over the ceiling of the air-chamber. The floor plates were so arranged that their joints, where it could be done, were butted immediately under the centres of these girders, the bottom flanges of which acted as covers on one side, while there were covering flats on the other side.

Between the shells, the stiffening was obtained by circular lattice girders, which consisted of angle rings attached to both the inner and outer shell at every plate joint, and of angle bars set radially and attached to the angle rings by gusset plates, and also by inclined angle struts which passed from the angle ring attached to the inner shell to the next higher ring attached to the outer shell. The top of the caisson was formed of a horizontal circular plate girder upon which the temporary caisson was set and bolted down, the joint being made by india-rubber cording about 1 in. in diameter.

As already stated, the inner shell in the Inchgarvie caissons was not carried up to the top, but terminated with the cylindrical portion. To make up for the loss of strength a circular plate girder about 2 ft. 8 in. deep is carried right round at this place, similar girders being placed at the second plate joint below, and at every second plate joint above, all these circular girders being combined by vertical angles about every 6 ft. on the circumference. (See Fig. 36.)

Three holes about 3 ft. 7 in. in diameter were cut into the ceiling of the air-chamber at convenient distances, and here were inserted three air-shafts, the lower edge projecting into the air-chamber about 6 in. They were made up of lengths of 8ft. each, with an angle hoop rivetted on at top bottom, forming the flanges. Two of these shafts were used for the removal of the debris of excavation, the other for the ingress and egress of the workmen. In the Queensferry caissons the shafts were pretty evenly distributed over the area of the working chamber, the reason being that in these caissons, as soon as they touched ground, work could be carried on all over the area at once. In the case of the Inchgarvie caissons, only two shafts—one for men and one for material—were used, and these shafts were close together at one side, because most of the work there had, owing to the sloping face of the rock, to be carried on near the point where the cutting edge first touched the rock. In addition to the air-shafts, a number of cast-iron pipes were fixed, reaching from the air-chamber ceiling to various parts of the outer shell, and these were used for the removal of silt and soft material, all of which, in combination with water, were ejected by means of a small quantity of compressed air being allowed to escape.

In the air-chamber itself, the main difference between the Queensferry and Inchgarvie caissons consisted in a different arrangement of the shoe.

In the former, the sloping plates were carried right down to the cutting edge, while in the latter they only went half-way down the slope, being then bent in a horizontal direction, and brought to the outside shell and then rivetted on. For the support of the lower remaining portion of the outer shell, strong brackets were fixed about 3 ft. 6 in. apart on the circumference, and these terminated at the lowest angle, which, with the 18-in. steel belt, formed the cutting edge of the caisson. This was done in order to allow the sinkers the fullest access to the rock immediately under the cutting edge, it being of the greatest importance that it should be undercut to the extent of at least 6 in. all round. The section of the Inchgarvie shoe is shown in Fig. 52.

It has already been stated that launching ways were laid down on the south shore to the east of the jetty. These timbers (Fig. 37) were laid on the natural slope of the ground, holes being dug at intervals which were filled up with concrete, and thus provided strong piers to carry the weight of the caissons. At the higher end of the timbers several trestles were set transversely, of such height as to keep the work out of the reach of the water. The tops of the trestles formed the platform upon which the construction of the caisson was commenced and carried on until about 25 ft. or 30 ft. of it, counting from the lower edge upwards, had been built. The weight was then about 300 tons. By means of hydraulic jacks the caisson was then slightly raised and packed, and the trestles withdrawn. It was then lowered down to within half a foot of the top of the launching cradle. This was made up of transverse girders laid on longitudinal timbers, which in their turn were laid on the timbers forming the launching ways. The top of the cradle was, of course, horizontal, and it was essential to support on it as large a portion of the ceiling of the air-chamber as could be reached, and also as much as possible of the lower or cutting edge of the caisson, which latter was also supported for the time being by a number of timber blocks placed on the ground.