The Forth Bridge/Preliminary Work in Connection with the Inchgarvie South Piers

by Wilhelm Westhofen

Preliminary Work in Connection with the Inchgarvie South Piers.

It has already been mentioned in the description of the Inchgarvie staging, that places were prepared for the reception of the two south caissons presently to be brought across from the Queensferry jetty.

In order to obtain at an early date a correct idea of the contour of the rock upon the sites to be occupied by the two south piers on Inchgarvie, a circular raft was constructed of timber balks decked for about 10 ft. round the outer circumference, with 3 in. planking. The raft is shown in Fig. 31. It was made sufficiently strong to resist the action of the waves in an ordinary breeze of wind, or to bear, if necessary, the strain of being beached on shore in case stormy weather should set in. It was a little under 70 ft. in diameter, and the surface of the planked space was 6 in. above water. Large mooring blocks were laid at some distance on three sides, the fourth being attached by cable chains to some of the iron columns of the staging. Upon the raft four crab winches were placed, by means of which either of the mooring chains could 36 hauled in or slackened. The raft could thus be easily placed in any desired position within tolerably wide limits. A central staff was fixed upon the raft, and about a foot from the outer edge a ring of gas-pipe was laid down upon which the grooved wheels of a carriage ran true to the centre of the raft. A drum was placed upon the carriage, somewhat overhanging the edge of the raft, and upon this drum the sounding line was coiled. This consisted of a fine steel wire and a long weight weighing about 60 lb., with a point at bottom, feet and inches being marked upon the wire by copper tags attached.

Fig. 32. Cutting Edge of Caisson

Two theodolites—one in the centre line of the piers longitudinally on the iron staging, the other on a masonry pier at right angles to the centre line, sat in line with the centres of the two south piers—were stationed to cheek the position of the centre of the raft every few minutes, while alterations in the tide level were observed on two tide gauges, and recorded. Owing to the heavy current running at this point both during ebb and flood the raft could not be held in position with any degree of accuracy, nor could the sounding line be kept plumb in spite of the heavy weight attached. Soundings were therefore only taken during slack water—that is, for about an hour before and an hour after high water or low water.

After the actual contour of the rock, on and within the two 70-ft. circles, had been taken, the raft was shifted about in various directions and further soundings recorded, the exact position of the centre being always fixed by the two theodolites reading angles from fixed stations. The whole area for about 40 ft. or 50 ft. round the two caissons was thus sounded, and a very fair idea of the contour of the bottom obtained. About 3000 soundings were thus taken, representing a large amount of work and trouble with the sounding gear, the mooring chains, the winches, and last, though not least, with rough weather, and distortions thereby produced in the raft itself.

THE PNEUMATIC CAISSONS; QUEENSFERRY.

The raft was also used for fixing the guide piles or columns against which the caissons would come to lie, and, as a precautionary measure, the centre was set 1 ft. up the slope of the rock, as it was tolerably certain that during the sinking operations the caisson would slide away from the newly cut face.

As soon as the soundings had been taken, and the guide columns fixed for the reception of the caissons, heavy mooring blocks were laid down to the south-east, south, and south-west, with heavy cable chains attached, the ends of which were temporarily secured to the staging. Two stout wire hawsers were also prepared to pass round the caissons as soon as they would be placed in position.

To hold the caissons with some amount of security when once they would take the ground, and give them a good bearing all round the edge, it had been decided to place a large number of sandbags upon the rock, and bring these up to the same level and somewhat above even as the highest point of rock the cutting edge of the caisson would be likely to touch. In order to still more securely fix this bed of sandbags, two rectangular piers were built up first of bags filled with concrete, and these were placed (see Figs. 29 & 30) opposite the highest point of the rock on the circumference of the 70-ft. circle. The concrete was of good strength, there being 27 cubic feet of stone to 4¹⁄₂ cubic feet of cement and 4¹⁄₂ cubic feet of sand. It was mixed dry, slightly wetted, and at once filled into bags. About twenty-six of these went to a yard, and they were put into boxes with hinged bottom and lowered down by a steam crane and emptied. They were put together by a diver, and, when a couple of feet of pier had been built, a number of sandbags of much larger size were put all round it to keep it in place and prevent the tide from washing out the cement. When the concrete piers had been completed, the remaining space under the cutting edge was laid with sandbags, except in two or three places where channels were left through which the air could escape, and through which also the sandbags and other debris could be pushed down the rock during the sinking of the caissons. In two places well within the air-chamber, and near where the centre of the caisson would come to be, two piers of sandbags were built up upon which the ceiling of the air-chamber would be supported. It remained now to lay down the pipes for the supply of air and water, and the electric light cables for immediate connection with the caissons as soon as they should be moored in place.

THE PNEUMATIC CAISSONS; QUEENSFERRY.