Building of the Caissons.
In the building of the caissons, the general practice was to commence by laying down the bottom booms of the large lattice girders upon the timber trestles at the head of the launching ways, to bolt the 3-ft. cross-girders between them, and fix to both the floor-pistes. Outside the trestles, any projecting parts of the caissons were supported from the ground by timbers. The plates forming the inner shell were next put on, and also the sloping plates forming the shoe, the two forming one joint with the floor-plates. All joints were rivetted up as far as possible by ordinary hydraulic rivetters, and meanwhile the bracings of the big girders were put on and the top booms laid on these, and more plates were added to the inner and outer shells. Upon the girders the first platform was laid down, and a hand crane set up as well as forges, concrete mixer, and other gear. By the time the caisson was about 25 ft. high—much of the lower work being then done—it was got ready for lowering, as already described. It was even then still accessible in the lowest portions soon after half-tide for further work and for making preparations for launching. Previous to this, all joints in the plates forming the ceiling—the sloping face of the shoe and the outside shell of the caisson—were carefully caulked to make a good water-tight job.
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AIR–LOCKS.
In all cases, previous to launching a caisson, the shoe and the whole space over the ceiling of the air-chamber were filled up with concrete, the latter generally to a depth of from 4 ft. to 6 ft. This not only acted as ballast, but it gave much stiffness to all the lower portion of the caisson, and produced a draught of from 9 ft. to 101⁄2 ft. after it floated.
When finally the caisson had been built to the required height and weight, all the points of contact between cradle and caisson were carefully adjusted, and all the weight allowed to rest on the cradle, it being then ready to be launched. The cradle was loaded with pieces of iron to hold it down to the launching ways when the water lifted the caisson off its bearings and floated it away. The slope of the launching ways being about 1 in 11, it required a push from a 12-in. hydraulic jack, placed horizontally, to set the mass in motion, and this it did most effectually on all occasions.
The launching of these caissons took place generally at or near to the time of high water of spring tides, owing mainly to the shallowness of the shore in front of the launching ways—the caissons drawing from 9 ft. 6 in. to 10 ft. 6 in. of water at the time. As soon as afloat a tug-boat was attached and the floating monster at once towed to its final resting-place, or else to the end of the jetty, where convenience existed for charging concrete into it and placing all the necessary machinery on board, as also the temporary caisson on top. Should any tide not rise as much as was expected, it was the practice to hermetically seal the air-shafts and other outlets from the working chamber and force air into the latter, in order to increase the buoyancy.
The first caisson—that for the south-west pier, Queensferry—was launched on May 26, 1884, the ceremony being performed by the Countess of Aberdeen, the Earl being at the time Lord High Commissioner to the General Assembly in Edinburgh. The last caisson was launched on May 29, 1885—the south-west Inchgarvie.
The Temporary Caissons.—The temporary caissons, placed on the top of the permanent ones for the purpose of keeping the water out while the circular granite pier was being built, were of the same construction as already described in connection with the Inchgarvie north piers. (See Figs. 32 and 51.) The lower tier of caisson segments, in addition to the large number of small bolts which made the water-tight joint, had two long 2-in. bolts to each section of caisson, or twenty-eight bolts on the circumference. These bolts terminated at top in a screw thread with nuts, and at bottom in a long shackle, which grasped a lug rivetted to the outside of the permanent caisson. The temporary caisson was made large enough to allow space round the completed pier for the masons pointing the joints in the granite blocks. Two tiers of caisson total height, 20 ft. were generally held sufficient to keep out the water; but in places where rough seas and much spray were likely to occur, a third tier, or at any rate a portion of it, was put on. The south-east caisson on Inchgarvie had a special temporary caisson constructed for it, consisting of plates 22 ft. in length. (See Figs. 36 and 53.)
The four Queensferry caissons, after being launched, were at once towed out and placed in their proper positions, being secured to the jetty in such a manner as to allow their rising and falling freely with the tide. The Inchgarvie caissons, on the contrary, were merely placed in a temporary berth near the end of the jetty, and they were completed there, and had the temporary caissons put on and all necessary machinery placed inside, before they were removed to their permanent places. This was done, not only because it was essential that these caissons should touch ground as soon as possible after their arrival in their berths, but also because a large proportion of the machinery inside and of the temporary caissons, was taken from the Queensferry piers, then already built up to above high water, and put on board, and the transport and double handling thus saved. It may not be amiss to state, as an instance, that the last caisson—No. 6, or south-west Inchgarvie—was launched on May 29, 1885, weighing a little over 500 tons, with a draught of 10 it. 3 in., was towed to Inchgarvie on July 16, drawing 31 ft. of water, and weighing 2877 tons, which was made up as follows:
| Tons. | |
| Permanent caisson | 457 |
| Temporary caisson (two tiers) | 65 |
| Two air-locks, air-shafts, concrete mixer, boiler, steam crane, &c. | 35 |
| Timber floors and staging | 50 |
| Concrete | 1418 |
| Brickwork | 852 |
| Total | 2877 |
Before entering upon the description of the mode of sinking the caissons, some account is needed to be given of the apparatus and machinery in connection with that part of the work.