A Treatise On The Principles And Practice Of Harbour Engineering

HARBOUR ENGINEERING. 178 When a block had reached the extremity of the finished portion of the jetty, one end of it was moored up thereto by cables which led back to winches on the jetty, and the flood-tide commencing to slacken, the block was gradually swung round by the tugs and brought into proper alignment. Its position was accurately determined by two guiding elements: one, on the right hand, the long beam already alluded to, and arranged so as to bear against the jetty structure ; the other, on the left, at the outer extremity, an enormous truncated cylinder of concrete 13 feet high, weighing 55 tons and resting on the sea bottom. As soon as the slack of the tide arrived—it only lasts a little more than a quarter of an hour,—the plugs which closed three orifices in the sides of the block were removed, and water flowed into the interior of the caisson, which gradually foundered. The process of sinking was often hastened by the water pouring over the side walls until the caisson finally disappeared from view amid a swirling sea. The sight was remarkably impressive. The caisson reappeared above the surface at low water, when 3 feet or so of it became visible. It was freed from its temporary strutting, guide beam, and moorings. The filling in of the compartments with concrete commenced immediately, and was continued to conclusion without cessation, except during the period of strongest tidal run towards high water. The concrete was manufactured in a yard situated at the juuction of the jetty with the shore. At this point were assembled immense banks of gravel from the Rhine, and quantities of Portland cement from Cronfestu, Niel, Tournai, and Les Laumes. The stores for the cement consisted of three galvanised iron buildings, covering a superficial area of nearly 1100 square yards. Four electric concrete mixers, each manufacturing 260 cubic yards of concrete per day, were located at each side of the stores ; they were fed by an electric crane, and water was laid on to each from a water tower. Suitable sidings permitted waggons carrying skips to present them- selves under the shoots of the concrete mixers. The gravel and cement duly measured were deposited in layers in these skips; the crane lifted the skips, swung them, and emptied their contents into the hopper of the mixer. The completed concrete was discharged into skips holding 12 cubic yards each. These were carried on special trucks drawn by ropes from electric capstans. When four skips had been marshalled, a locomotive took them in charge and conveyed them to the extremity of the jetty. Here a Titan crane or a pair of sheer legs lifted them from the trucks, carried them over the caisson, and lowered them to the bottom, where they were automatically discharged without loss or disturbance. After each discharge the skip was withdrawn and replaced on the waggon.