Page 12 - Minutes of Proceedings of Civil Engineers Vol CXCV 1913-14 Part 1
P. 12
52 wENTwoRTH-sEErLDs oN TIIE coNsTRUcrIDN oF [Minutes of were then run up to their highest position, and fresh eye-bars were inserted. This process was repeated until the cutbing edge of the caisson rested on the bottom of the trench. Sinking the Caiesons.-The caisson was then sunk by grabbing inside its two pockets and by weighting it with rails 24 feet long, standing upright on its walls, and supported by a framework of light steel bolted to tbe caisson. Fino sand of the kind occurring in this trench is by no means ideal material to sink through, as the grab seizes it with difficulty, and, being very firm when undisturbed by running water, it will not permit the cutting edge of the caisson to penetrate it. To overcome these difficulties, divers were sent down into the pockets, and, armed with high-pressure water-jets, they blew the sand from all around the cutting edge into the centre, where the grab could lift it more easily, The caissons were sunk in pairs, and in sinking the first pair it was found that a quantity of sand ran into the caisson from under the cutting edge, causing settlement of the ground out- side the trench. Theresult was that a large hole suddenlyappeared on top, which had to be filled with good material. Fortunately the timbering of the trench was practically undisturbed. After this the weight on the caisson was increased to 76 tons, or 138 tons in all, including the weight of the caisson itself. With this load-which is equivalent to 3| cwt. per square foot of skin-surface-the caissons were sunk without difficulty. As each pair of caissons were sunk to the requirecl depth they were cleaned out and filled with mass concrete, which was lowered through the water in boxes with bot'tom doors. After three pairs of caissons had thus been sunk close to each other, the remainder were sunk in alternate pockets of the trenoh, leaving a space of about 12 feet between each pair of caissons, which was ex- crrvated by grab and by divers armed with water-jets. The face of sand at bnck and front of this space was supported by vertical timber runners resting against horizontal walings, which were stretched frorn one caisson to the next and fixed by divers. The pit thus sunk was filled with mass concrete up to 1 foot below dock- bottom, i.e., to the same level as the top of the caissons. Then a layer of muss concrete 12 inches thick was laid all over the trench, to briug tho foundation up to dock-bottom level. The front part of this lrryer wls finished off accurately by divers, and on it was placed n rorv of 6-ton concrete face-blocks, each 8 feet long, 4r feet wide, antl lJ foct 9 inches high. These blocks were backed with mass concrettr, and in front of them, over that part of the ca,issons which formorl the wall-too, good clay puddle was deposited
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