The Forth Bridge/Inchgarvie Caissons

by Wilhelm Westhofen

Inchgarvie Caissons.

With regard to the Inchgarvie south caissons, the mode of working varied somewhat owing to the nature of the rock bottom. The preparations which had been made for the reception of these caissons, and the levelling by means of concrete piers and sandbags, has already been described. Previous to launching the two Inchgarvie caissons, a strong timber shoe was fixed in each of the two places which would come to lie immediately upon the two piers (Figs. 30 and 31), built up of concrete bags on the opposite side to where the caisson was likely to first touch the rock. The timber frame was brought down flush with the cutting edge, so as to give the caisson a solid base to rest on.

STRUTS AND WEDGES IN AIR CHAMBER

When floated across from the Queensferry jetty the caissons were at once attached to the mooring ferry chains provided for them, and, in addition, had stout wire ropes passed round them, the other ends of which were passed round the north piers already built. Careful watch was kept day and night to guard against accidents, and further loading with concrete was at once proceeded with, while the remaining machinery, air-locks, &c., were placed inside, and the flexible tubes for the supply of compressed air and water, attached. As soon as the caisson commenced to touch ground at low water, the loading was suspended and a descent made into the air-chamber to examine the points where the cutting edge was touching and to put in any further support that might be considered advisable. In this way two piers of sandbags were built up to the ceiling as an additional safeguard against slipping, and these piers were near the centre of the caisson, but rather on the side of the concrete piers. Work was now commenced in short shifts, both before and after low water, in order to remove as much rock as possible under the cutting edge on the high side, as it was deemed advisable that at least one-fourth to one-fifth of the circumference should rest on the solid rock and in a sort of chase before the caisson was loaded to such an extent as to prevent floating even at high water. It was thus fully a fortnight after the caisson was moored in its place before it was allowed to settle down, and to rise no more. Excavation then commenced in earnest, although the rate of progress was slow at first owing to the comparatively small area which could then be attacked. The mode of working was very simple. A number of holes were drilled at first by ordinary hand jumpers, one man holding the drill and two striking; later on by the pneumatic rock drills driven by compressed air at 70 lb. to the square inch. The holes were drilled under, and some 12 in. beyond or outside the cutting edge, and when carried to their full depth were plugged and others proceeded with, until a sufficient number were done. They were then charged with explosive and the charges fired. In these caissons also blocks of rock were left standing for the cutting edge to rest upon, while between them the rock was removed. The rock-drills were also worked within the area of the chamber so far as the bottom was uncovered by water, and where possible ordinary quarrying with crowbars and heavy hammers was carried on. At first the charges were fired by time-fuzes, but, as the number of holes increased, insulated wires were laid to the boreholes, the ends being gathered together and brought up into the air-shaft, where they were fired by an electric battery simultaneously. A wooden trap-door was provided to close the bottom of the air-shaft and prevent the fumes from passing upwards, all the men being withdrawn during that time. After the charges were fired the air compressors were worked at somewhat greater speed to increase the pressure, and the chamber was soon clear again. The firing was done as often as possible at the end of a shift to cause the least delay and loss of time. The firing of the blocks of rock upon which the caisson rested, was generally done at low water, the caisson crushing into the debris with irresistible force. A number of men then descended and cleared away the debris, which was sent up in the skips, passed through the lock, and discharged over the side as described in the case of the Queensferry piers. Another lot of men cleared away some of the sandbags and cut under the edge into the concrete piers, while the remainder, were at once set to to drill fresh holes where isolated points remained standing.

Fig. 53. Pier with permanent caisson.

As mentioned in connection with the Queensferry caisson, careful examination was daily made, or even oftener, as to the position of the centre of the caisson, and if it had shifted it was at once tilted to one side and speedily brought back to its place.

In these caissons, of course, the air pressure became greater with every descent; for the water had full access to them at all times. On the other hand, the atmosphere was singularly sweet and fresh, as a large quantity of air was forced past the cutting edge, and ascended in a mass of bubbles by the side of the caisson, looking exactly as if it were boiling all round. The effect of this is shown on Plate VI., where the south-east caisson is nearly down to its final depth, while the south-west is floating still. The caissons were lighted by arc lamps and incandescent lamps, and so large was the air-chamber that it required five or six lights of 2000 candle-power each, to give a tolerable light; it took much care and trouble to protect the lamps and cables against the effects of blasting.

The rock-drills were of an ordinary type, mounted on three-legged stools or stands for vertical or steep-inclined drilling, and on timber frames for horizontal or slightly-inclined working. The arrangement through which they were made to discharge the compressed air by which they were driven into the general atmosphere, did great service in keeping the air pure just in the places where most of the men congregated, and when in good working order, they drilled a large number of holes in the course of a day.

MODE OF FIXING UNDER BEDPLATES ON PIERS.

In these two caissons the tendency to go down hill manifested itself strongly, and powerful timber struts and hardwood wedges covered with steel plates had to be employed to keep the caisson in its position, or to force it back when it had moved. These were set on prepared faces on the rock, as shown in the sketch, Fig. 52, and their action upon the caisson will be readily understood.

The actual blasting of the rock under the cutting edge does not appear to have injured the steel edge in any way; but large chips of rock lodging between the outside face and the iron plating caused some indentations, which were not, however, of any consequence.

On several occasions photographs were taken in the air-chamber, some of them requiring an exposure of fifteen to twenty minutes; but they were not very successful, owing to the changes in the atmosphere and the uncertain light of the arc lamps. Whenever the air pressure increased to a slight extent the atmosphere became quite clear and transparent; then the air would rush out at some point under the caisson edge with a noise like distant thunder, and a great wave of cold water came rushing back. This caused a dense white fog to suddenly rise in the air-chamber, which obscured everything for a few moments and then gradually disappeared again.

Through the gaps left in the heaps of sandbags a number of strange visitors used to make their appearance, attracted, no doubt by the glare of the lighted chamber, which at night could be distinctly seen from above—such as salmon, dogfish, octopus, many other fish, crabs, and a large number of lobsters. One of the latter a—large specimen—got very excited in the chase after him, and leaped up nearly the full height of the chamber in his frantic endeavours to escape—finally jumping into an empty skip, whence he was promptly transferred to the boiling-pot.

With regard to the working hours of the men employed after the caisson had once settled down on the rock, work was carried on day and night, the only stoppage being from 6 P.M. on Saturday till midnight on Sunday; but the air compressors had to be kept going all the time, and the full pressure of air maintained. Watchmen were also kept constantly on duty in the chamber when work was not carried on. At first the men worked in eight-hour shifts, with eight hours off, later on six-hour shifts with six hours off, and finally, in the higher pressures, with four-hour shifts with eight hours off.

The total excavation for the four piers on Inchgarvie, and the four piers on Queensferry, and the four piers on Fife, are given in a tabular statement further down, together with other quantities.

The total of the wages paid by the sub-contractors to their employés, including managers', engineers', and time-keepers' salaries, amounted to 1l. 15s. per cubic yard of rock excavated in the Inchgarvie south-east caisson, and to 21. per cubic yard in the Inchgarvie south-west pier.

Neither of the Inchgarvie caissons was carried down to the full depth contemplated (see Fig. 53), as it was found that only a very small area remained to be filled up when the caissons had got to the depth where they were left. These places were carefully levelled and stepped, and iron plates were fitted in to close the gap between the caisson edge and the rock, and the spaces were then carefully built up with concrete bags and the whole grouted with cement at slack water. Upon this foundation concreting was commenced, and the whole chamber gradually filled with concrete in the manner described for the Queensferry caissons.

In Table No. VII. are a few data of interest in connection with these two caissons.

Table No. VII.—Progress of Work on Inchgarvie Caissons.

	South East Caisson.	South-West Caisson.
Launched	March 30, 1885	May 29, 1885
Towed to its berth, Inchgarvie	May 15, 1885	July 16, 1885
Caisson and shield ready for lowering	February 29, 1884	September 10, 1884
Touched ground at low water	May 25, 1885	July 29, 1885
First descent to chamber	May 26, 1885	August 5, 1885
Commencement of sinking	May 27, 1885	August 6, 1885
Level below high water at commencement	51 ft.	64 ft.
In final position	August 15, 1885	October 1, 1885
Level of same below high water	63 ft. 9 in.	72 ft. 1 in.
Commenced concreting chamber	August 19, 1885	October 2, 1885
Finished concreting chamber	September 1, 1885	October 10, 1885
Caisson filled to granite level	September 15, 1885	October 30, 1885
First granite laid	September 19, 1885	October 30, 1885
Pier completed	November 20, 1885	February 22, 1886