The Forth Bridge/Fife South Circular Piers
Fife South Circular Piers.
As the two south piers on Fife were also situated on the sloping face of the rock, and required excavation at different depths, a somewhat similar course was followed in the construction of the cofferdams or caissons, but with one important difference. In the case of Inchgarvie the caissons were set on the rock, and secured to it, as far as possible, by weighting them, and the joint was made good previous to any blasting of rock near the 60-ft. circle being done, except so far as the excavation of the circular chase was concerned; while in the case of the Fife south piers, diamond-drilling plant under a subcontract was employed to drill all over the area of the pier and blast the rock before any cofferdam or caisson had been put down. For this work an iron girder staging was erected over the piers, on which a traveller with running gear from end to end was placed. The depths to which the different steps in the excavation had to be carried was given, and holes drilled to these depths. When a number of holes were ready, they were charged with explosive and fired; but the rock was not removed at the time, and lay there in great shattered masses.
The consequence was that when it was attempted to place caissons with projecting shields to fit the contour of the rock, the leaks were of such a nature as to defy any pumping power then at hand.
The Fife south-west pier, in which the deepest part was 7 ft. below low water, and in which only a very small portion lay below the level of the circular chase 2 ft. below low water, could be managed with some little difficulty; but in the south-east pier, where the shield had to be carried to 19 ft. below low water, it became necessary to construct a puddle clay dam round the outside of the caisson with piles heavily shod and driven into the debris of broken rock. Even then the difficulty could only be overcome by collecting together the water from a number of leaks, and by means of powerful pumping machinery to get the upper hand of it. Rubble masonry was then built in all places where running water could be kept off, and the leaks were thus gradually hemmed in, and finally stopped by pouring cement grout into these places at slack water and with the sluices open, in order that no water might be forced through the grouting before it had set.
A large diving bell, with air-lock and the necessary machinery to move the bell all over the area of these piers, had been constructed, as it had been intended to form the foundations of concrete deposited through the water by hopper-bottomed skips; but as it was subsequently decided to have the foundations of rubble masonry instead, it became necessary to lay the bottom dry, and the diving bell was never made use of. The north piers on Inchgarvie and the south piers on Fife are founded on the solid whinstone rock.
Table III., given below, shows the time occupied in building the foundations of the four circular piers on Fife, the depth to which the foundations were carried, and other particulars.
| Fife Circular Granite Piers. | ||||
| North-West. | North-East. | South-West. | South-East. | |
| Excavation commenced | August, 1883 | Feb., 1884 | August, 1883 | Nov., 1883 |
| Lowest point of foundations | 7 ft. below h.w. | 7 ft. below h.w. | 25 ft. below h.w. | 37 ft. below h.w. |
Rubble masonry in foundation commenced
|
Nov., 1883 | March, 1884 | June, 1884 | March., 1885 |
| Foundation finished to low water | Nov., 1883 | April, 1884 | June, 1884 | May, 1885 |
| First granite laid | Dec., 1883 | April, 1884 | June, 1884 | June, 1885 |
| Pier completed | Sept., 1884 | Sept., 1884 | Jan., 1885 | August, 1885 |