A Treatise On The Principles And Practice Of Dock Engineering

204 DOCK ENGINEERING. The total weight of a caisson is, on an average, nearly 410 tons, including 350 tons of kentledge. This weight is reduced to about 290 tons when the caisson is immersed, and to 30 tons when it is sunk and the working chamber full of air. Having dredged and prepared the site, as before deseribed, the caisson is conducted to its place between two barges connected by a framing which forms a deck above the caisson. The latter is then lowered into position and detached from its supports. The working chamber having been filled with compressed air, the surface of the ground uncovered is cleared and levelled, and a block of masonry built upon it about 4 feet in height, its other dimensions corresponding to the size of the chamber and the width of the wall, with a slight clearance in the former case. This completed, the caisson is removed to an adjoining site by a reversal and renewal of the process, the blocks being constructed as closely together as possible and leaving only an interval of about 3 feet between them. A second caisson following the first builds the second course, and at the same time by sitting over the joints between the blocks fills up the vacant spaces with the aid of a diver, who rapidly constructs a brick wall, back and front of the spaces, which are then pumped dry and filled with masonry. The fifth caisson is self-acting ; it can sink or float by its own appliances. It is larger and heavier than the others, and is intended to be worked at variable depths, being used principally for constructing the bottom course of blocks. Its functions generally, however, are the same as those of the other four chambers. Ordinarily, no excavations were made within the caissons except such as were necessary to prépare a level seat for the wall; in certain cases, however, it was possible to descend about 6 feet below the initial position of the chamber, but there would have been risk in sinking lower, on account of the possibility of not being able to liberate the caisson. Where firm earth is met with above the floor of the basin, which is the case along and in the neighbourhood of the landward side, the wall is only built to its full width above that level. The lower portion of the wall is simply constructed as a retaining wall or revetment of about 5 feet in thick- ness, as shown in fig. 139. In this case the caisson is sunk to rest upon the higher level, and the firm stratum below being practically impermeable, the revetment is put in by means of a trench, and the upper portion of the wall is proceeded with as usual. The rate of working has depended on the nature of the foundation; in the case of very hard ground requiring the pick, the rate of descent of the caisson did not exceed 6 inches per day of 24 hours. In the building of the wall each mason executes about 88 cubic feet of masonry in a shift of eight hours. The construction of one block of masonry absorbed three days, including the manipulation of the caisson and the making good of the joint in the course below. The cost of the masonry, exclusive of the