The Forth Bridge/Inchgarvie North Circular Piers
Inchgarvie North Circular Piers.
On Inchgarvie the site of the two north or shallow piers being wholly submerged at high water, and about one-half in the case of the north-east, and three-fourths in the case of the north-west pier, submerged also at low water, the preliminary work was tidal, and between spring tides no work could be carried on at all at this place. When it is considered how exposed the position was there—the work having to be carried on upon a narrow ledge of rock attacked by wind and waves from all sides—it will be understood that the progress could not be very rapid. The conditions of the contract here required that the rock should be excavated in steps, and that the rubble masonry comprising the foundation of the circular granite piers should be bound by an iron belt 60 ft. in diameter and 3 ft. deep; the highest portion of the rock upon which this belt rested to be 2 ft. below low water; the belt, or at any rate a part of it, to be brought down to form a protection for the foundation rubble masonry upon the lower steps.
It was therefore decided to cut a chase 8 ft. wide (3 ft. to the inside and 5 ft. to the outside of the 60-ft. circle) out of the rock where it was higher than 2 ft. below low water, to make the 60-ft. belt of three thicknesses of 1⁄2in. plate, and to carry the centre plate downwards, after it had been cut, in such manner as to fit as nearly as possible the natural contour of the rock. (Fig. 21.) A light staging was, therefore, erected above high water, the correct centre of the pier placed upon it, and by means of a trammel-rod 30 ft. in length, from the end of which a pointed sounding-rod was suspended, a correct reading was taken every 6 in. on the circumference of the 60-ft. circle, after a diver had been round to clear out any loose stones lying in the line, or picking off any sharp points projecting. These readings were plotted and the centre plates cut to it. In the mean time work had been done upon the chase, and, when nearly cut down to the right level, the belt was put together on the staging exactly above the site of the pier. The plates projecting downward, and forming the shield, were stiffened by T bars vertically over the butts, and, where required to be carried down to a considerable depth, as in the case of the north-west pier, they were further stiffened by horizontal circular girders and stayed to the rock by bars of angle-iron. The whole belt was now rivetted up, and, when ready, received two coats of red-lead paint, and was lowered down by means of hydraulic jacks into position. (Fig. 22.)
The top edge of the 3-ft. belt was then levelled all round, and corrected where necessary. A heavy angle iron, 6 in. by 6 in. by 7⁄8in., ran round the inside of the 3-ft. belt, and upon this was now set a single tier of temporary caisson, 10 ft. in height, and consisting of fourteen segments of about 30 cwt. each in weight. This helped to keep the belt down to the rock, and a number of heavy blocks of stone were placed on the top of the caisson for the same purpose. A sluice door in the lower part of the caisson was kept open to admit of the tide flowing in and out.
Steps were now taken to make good the joint between the 3 ft. belt and the shield and the bedrock. This was done in the following manner: A number of concrete bags, about 14 in. by 30 in., and 8 in. to 9 in. thick, were prepared and passed down to a diver, who laid them round the outside of the belt at a distance of about 4 in. A second row was next laid round the outside of the first row, and tolerably close up, the space between the two being made up by clay puddle well stamped down. Any split, or hole, or crevice in the rock was also filled with clay. Upon these two lower rows, other bags were now laid crosswise; upon these, two rows lengthwise, and a fourth row crosswise on the top, which was laid close up to the belt. This was done in sections of about 15 ft. to 16 ft. length all along the shield, but round the outside of the treble belt only two bags deep were laid. On the inside also a single row of clay bags, backed by a row of concrete bags and loaded with stones, was laid round the complete circle. Cement grout, without intermixture with sand, was now prepared and passed down to the diver—but only at slack tide, high water, or low water—who lifted off one or more of the top bags and poured the grout into the narrow space left, until it overflowed. He then replaced the bag and proceeded to the next division until all was done. Forty-eight hours were allowed to elapse for the setting of the cement; the sluice valve was then closed, and the caisson pumped out gradually. When leaks were discovered, the diver descended to examine the outside, and, where necessary, cut out some of the grouting and replaced it by new.
As it was not considered that this cement joint would be able to stand the full pressure of the tide rise, the caisson or cofferdam was worked as a half-tide one, it having to be pumped out every tide as soon as the water had fallen below the top edge of the temporary caisson. In addition to the hydrostatic water pressure, the caisson had to stand the heavy seas thrown against it, whether coming from west or east. Under these circumstances, it was often considered advisable not to pump out the cofferdam, but leave the sluices open and allow the tidal flow free access. Under such conditions, it will be easy to see that, during a season of bad weather, much delay could not be avoided, and though the work of excavation had been commenced in the summer of 1883, it was not till the middle of April of the following year that the first rubble masonry could be laid in this pier. In working the excavation, no blasting was done within 11⁄2ft. of the iron belt, but the rock was quarried up to within 6 in. and rubble then built in at once. Any steps cut in the deeper portion were invariably at least twice as broad as they were deep. The deepest point to which excavation had to be carried in this pier was at 8 ft. below low water.
The cofferdam or caisson for the north-west pier, Inchgarvie, was done in the same way precisely as described for the north-east, only that, owing to the experience gained by the divers and other men engaged upon the work, the progress was much more rapid.
In the north-west pier the depth of the shield was 15 ft. below low water, and extended to nearly one-half of the circumference. There were, therefore, in addition to the vertical T bars which covered the butt-joints of the shield-plates, three horizontal circular girders, carried at a distance of 4 ft. 6 in. from each other, and from these a number of horizontal tie-bars with cross-bars at the ends were carried radially and level to the rock opposite and pinned to it, and afterwards built into the solid rubble masonry. (See Fig. 22.)
This mode of making the joint between the rock and the iron belt was simple and quite effective. Most of the leaks were due to natural crevices in the rock, running from the inside to the outside at a considerable depth. These were circumvented by building small clay dams round, and leading the water by a shoot to a pump. Leaks were also caused by the action of heavy waves running tip to the temporary caisson at low water with great violence, and shaking the whole fabric.
The whole of the north-east pier was built in a half-tide caisson, as the work was not pressing; but in the case of the north-west pier, so soon as the rubble masonry inside had been brought up to low-water level, a second tier of temporary caisson was added, and the work could then be carried on at all states of the tide. While tidal work was carried on in these two cofferdams, the amount of water which had to be pumped out every tide was 250,000 gallons in the one case, and 340,000 in the other. The time occupied was 50 to 55 minutes, but work was, of course, commenced so soon as the higher parts were laid dry. For pumping out smaller quantities of water collected through leaks, pulsometers or small centrifugal pumps were used. The temporary caisson is shown in Plate VII.
A statement of the time occupied in building the foundations of these two piers is given in Table No. II.
| North-East Caisson. | North-West Caisson. | |
| Rock excavation commenced | June 22, 1883 | January 13, 1884 |
| Lowest point of foundations | 26 ft. below high water | 33 ft. below high water |
| Caisson and shield ready for lowering | February 29, 1884 | September 10, 1884 |
| Joint between caisson and rock made good | April 2, 1884 | October 17, 1884 |
| Caisson pumped out first time | April 5, 1884 | October 20, 1884 |
| Rubble masonry in foundation commenced | April 16, 1884 | October 24, 1884 |
| Foundation finished to low water | June 1, 1884 | December 20, 1884 |
| First granite laid | June 2, 1884 | December 23, 1884 |
| Pier completed | November 17, 1884 | March 18, 1885 |