A Treatise On The Principles And Practice Of Harbour Engineering

176 HARBOUR ENGINEERING. This land traveller was a kind of movable bridge running on a double track. Its extreme span was 220 feet, the length comprised between the supports being 66 feet, and the two cantilever arms 66 feet and 88 feet respectively. It served five rows of caissons. The caissons were put together on wood blocks or packings, but, as soon as the rivetting was finished, these were removed and the caissons lowered to the ground. At this point, concreting work was put in hand, commencing with the fioor. The concrète was composed of 3 parts of broken stone, 3 parts of sand, and 12j Ibs. of Portland cement to the cubic foot. It was mixed by an electric motor mixer, served by a crane which lifted a box coiitaining the dry ingrédients and tipped them into the hopper of the mixer. Hence there was a graduai sliding progression into and through a cylinder working with a rotary movement and slightly inclined. The materials were turned over and mixed—dry throughout one-third of the length and wet throughout the remaining two-thirds, the water being administered through a central perforated tube. The output could be regulated at will. By the time the concrete had reached the extremity of the cylinder, it was thoroughly incorporated and all the stone well bedded in mortar. It was then allowed to fall into compartments on small wagons, and conveyed along double tracks on to a traveller of the same form and range as that already described. The wagon boxes were tipped into shoots attached to the traveller, and set over the caissons. While the concrete work of the floor of a new block was in hand, a large framework “ mould-stripper,” consisting of a stage resting 011 strong cross- beams supported by two upright frames the same distance apart as the frames of the concrete mixer and running over the same track, removed the moulds from the blocks previously finished, with the assistance of two electrioally- worked derrick cranes. The concrete mixer next turned back and concreted the partitions, and, this being done, both it and the mould stripper were free for another block'. The blocks, as was remarked, were constructed in the inner harbour and a branch dock, which was emptied for the purpose of these extension works. As soon as the new lock and its entrance channel had been completed, water was again admitted, and the blocks, being finished, were ready to be towed to their allotted positions. The first block was set in place on 20th May 1900, and two others succeeded it before the end of the year. The work was then interrupted by a severe storm on 27th January 1901, and was not resumed until 20th October following, in conséquence of the damage which accrued to a framework jetty connecting the mole with the shore. The operation of setting the blocks in position was as follows :_____ In the first instance the blocks, as stored in the inner harbour, were allowed to fill with water to keep them stationary. Their sides projected from 15 to