THE GRAYING DOCK AT PORT CHALMERS.

Evening Star, Volume VII, Issue 1959, 16 August 1869, Page 2

THE GRAYING DOCK AT PORT CHALMERS.

It is not often that opportunity is afforded in the Colonies for practically seeing the various mechanical labor-saving processes adopted in the construction of extensive engineering works. In this respect the. young colonist is at great, disadvantage, compared with youths in Europe and America. In Great' Britain and the United States it is barely possible to go into even the smallest country town without witnessing the progress of some great public work ; and to those who may be called upon at a futu e time to undertake, or superintend or decide upon the carrying out of undertakings involving similar processes, every step in their construction not only tends to satisfy a laudable curiosity, hut to impart practical instruction.

The Graving Dock at Port Chalmers is therefore an object of interest not only on account of the weighty commercial and maritime advantages expected from it, but as affording a practical illustration of the means by which, in modem times, works are constructed in a few years, that required the lifetime of a generation to comp etc, in a far less efficient manner, in olden time.

We suppose that every body knows what a Graving Dock is, although there are thousands in Otago who have never seen one. The end proposed, and the necessary means to it are thus briefly described in Knight’s Encyclopedia. A dry or graving d"ck is “ used for receiving ships in order to their being inspected and repaired. For this purpose the dock must be so contrived that the wa ! er may be admitted or excluded at pleasure, so 'hat a vessel can be floated in when the tide is high, and that the water may run out with the fall of the tide, or be pumped out, the closing of the gates preventing its return.” This, then, is the end and purpose of the work going on. The rise and all of the tide being only a few feet at Port Chabners, made it necessary to carry the wo-k into deep water, and when the dock is fiui-hed the water will have to be pumped out. To eifect this, two embankments hav# had to be made in Sawyer’s Bay at such a distance apart, as to leave room for the largest ship to lie in the enclosed space. The bottom and sides of the dock will have to be coated with masonry of the firmest and best description, and in order to do this work, it is just as necessary to exclude the water as it would be were a vessel under repair lying there. A stranger visiting the place, unacquainted with what was being done, would, at the first glance, imagine that he was on some piece of waste ground on which quarrying was carried on. But an analytic survey would show him that instead of the confusion that appeared at first sight, everything was arranged in a strictly preserved order, with a view to some prescribed pu' pose. The huge piles of masses of basalt show that each one has been squared and dressed to fit a given place determined beforehand. Its destination is known by its number, and that there may bo n > mistake, that number is marked on different blocks in different colored paint, so that should there be similar numbers, the one is easily distinguished from the other. That these, heavy blocks of stone have been placed in stacks by the readiest and easiest means is plain when the rails, trucks, and do ricks are noticed, by whic'i this distribution has been effecte . The hillside too has its inclined planes by which the loaded trucks descend on one and drag emp'y trucks up another. There is a patent stonebreaker in the bush that reduces all the pieces of basalt unfit for mason work to metal for ballasting and for road making, and thus as soon as the water can be pumped out of the dock every-

thing is ready for the work then to be done. To enable the contractor to do this it was necessary to form what is called a “ coffer-dam ” at the entrance of the dock, and it need hardly he said that as it must eventually be removed its pl-m must be such as to give strength to resist the column of water pressing upon it when the dock is emptied, and at the same time to be easily withdrawn when the purpose for which it was made is answered. The strength required may be guessed at, when it is considered as an approximate estimate, a C"lumn of water two feet deep presses with a force of one pound upon every square inch of surfa e against which it rests. At Port Chalmers the coffer-dam is of piles of wood, driven close together, so as to form a segment of a circle, curving outwards to the sea, and extending from side to side of the dock. These, pi es are firmly tied and bolted together by strong beams. In most similar cases, two rows of piles parallel to each other are driven into the ground, and the intervening space is filled with earth and clay well rammed. If the second row can he * dispensed with, there is a clear gain in the labor both in constructing and removing the dam. This is done at the Port, and the dam is expected to prove watertight through its being caulked with straw between each pile. The efficacy of the plan wi‘l shortly be tested, as in a very few days two centrifugal pumps, worked liy a portable engine of sixteen horse power, will be at work emptying the dock. These pumps are expected to ra?se 4,000 to 5,000 gallons per minute, so that in a very few days, should the cofferdam prove tight, the water will he pumped out, the farther excavation made, and the masonry work with all possible speed proceeded with.

W.* shall watch with interest the progress of the work, which promises fair to be completed within the contract time.