The Forth Bridge/Queensferry Caissons

by Wilhelm Westhofen

Queensferry Caissons.

The north-west caisson is the one which met with the accident we have described. Hence the time elapsed between launching and sinking Table No. VI. shows the levels of strata below high water through which these caissons were passed during sinking. As all strata were more or less on the slope, the mean height is here indicated, except with regard to extreme depth of excavation, which is, of course, level. In the case of the north-west or tilted caisson, a considerable quantity of mud and silt had been removed previous to its being raised; hence the greater depth at commencement of sinking.

When working in the hard boulder clay, for twenty-four hours with the full complement of twenty-seven men below in the air-chamber, and with four hydraulic spades going, a bucket was sent up every five minutes, or 288 buckets in the twenty-four hours, which was equal to a little over 5 cubic yards per man per twelve-hour shift. The total to 145 cubic yards for each foot in depth. Even this rate of progress was frequently exceeded under favourable circumstances, but was of course largely above the average daily work.

The different strata through which the caissons passed, were — water, deposited mud, stiff mud, silt, a layer of pebbles and stones, soft clay and boulder clay. In the last strata were found large rocks well rounded off, of granite, limestone, freestone and other kinds, many of which showed upon their flat faces, the distinct grooving or scoring due to glacier action. Amethysts and pebbles of all sorts, and large round boulders of conglomerate were also found, but no traces of fossils or of animal life, not even a live toad.

TABLE No. IV.—Nature and Depth of Strata through which Caissons were Sunk.

All below High Water. $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Level of	mud or silt	27 ft.	27½ ft.	40 ft.	52 ft.
	"	clay and boulder clay	48 ft.	52½ ft.	75 ft.	62 ft.
	"	cutting edge at finish	48 ft.	52½ ft.	75 ft.	62 ft.
	Depth through hard ground		23 ft.	20½ ft.	14 ft.	23 ft.
Total excavation in cubic yards			6372	6651	6827	6271

The excavation finished, the chamber was cleared of all material used during sinking, and preparations made for filling the whole space with concrete. The bottom ends of the air-shafts were closed by plates previously prepared, to which was attached a hinged door opening downwards. The large air locks were then removed from the tops of the shafts, and small tubes 18 in. in diameter fixed inside and carried up some distance to a platform, where the concrete mixer was stationed. A valve or small airlock was set on the top of the 18 in. tube. Outside the latter was of course ordinary atmospheric pressure. Similar arrangements were made for the other small shafts placed in the caissons from the beginning. Concrete was now deposited close to the lock, a signal was given, and the lower door in the air-chamber was hermetically sealed, the air let out of the shaft and the upper valve opened. Concrete was shovelled in till the pipe was nearly filled, the door then closed, and a signal given. Those below then opened a small valve to let compressed air into the shaft, the lower hinged door was opened and the mass of concrete fell on the floor and was taken up into barrows and carried all round the edge. There it was firmly rammed in and pounded with wooden rammers into every hole and corner, and thus gradually laid up the sloping face of the shoe. Against the ceiling also the concrete was carefully rammed, and thus by degrees the whole chamber built in, leaving only passages between the places where the concrete way passed down. These also were by degrees filled up and the last of the concrete had to be passed through the men's lock and down the air-shaft in buckets until this also had to be filled. The air pressure was of course kept on during all this time to keep the water from washing in and out. Finally, all the shafts were filled up, and cement grout was run into these until it stood up to the level of the water outside, the pressure being kept on these shafts for about thirty-six hours longer. The remaining space of the caisson above the air-chamber was now filled up with concrete to the low-water level at which the granite courses commenced.

The concrete in the air chamber was of the following proportions: Round the cutting edge and under the shoe, and for about 4 ft. all round within this space, 27 cubic feet of stone, 6+¹⁄₂ cubic feet of cement, and 6+¹⁄₂ cubic feet of sand. Inside this space and inside the caisson above the air-chamber, the proportions were 27 cubic feet of stone to 4+¹⁄₂ cubic feet of cement, and 4+¹⁄₂ cubic feet of sand. The grout used was pure cement and water.