THE WAITAKI BRIDGE.

Globe, Volume I, Issue 16, 18 June 1874, Page 3

THE WAITAKI BRIDGE.

A correspondent of the “ Otago Guardian” gives the following account of the progress made with this bridge : —“ Not the least important link in the chain of works now in progress on the main trunk Middle Island railway line is that which connects the two provinces of Canterbury and Otago, namely, the large iron cylinder bridge over the Waitaki. This formidable river has hitherto divided the provinces by a more than insular severance, the passage between being tedious, expensive, and not a little dangerous, and the consequent intercourse between the two provinces, at least by land, anything but free and intimate, as it must have been but for this serious obstacle. Its removal therefore cannot do otherwise than benefit both provinces, and the progress of the work which is to effect this desirable end cannot but be viewed with considerable interest. A large and valuable tract of fine laud in South Canterbury has till now been cut off practically from a port of shipment, and cultivation was, therefore, out of the question. Although this land is held in comparatively few hands, it’will very soon, without doubt, bo made to produce something better than tussock, and the natural outlet for the produce is through Otago. On the other hand a good many of our Northern farmers, being unable to obtain land on lease or by purchase, have gone over to Canterbury to find it there, so that the more intimate connection now about to be inaugurated must, as we have said, prove mutually advantageous. Since it never rains but it pours, we are not, it appears, only to have one bridge, but two bridges arc actually in contemplation ; the second and less important one being some twenty-four miles higher up the river, and bringing a considerable tract of land about Elephant Hill into contact with the Awamoko railway, on this side the river, besides being of considerable use to several important stations. What we have now to describe is, however, the lower bridge, about three miles from the mouth of the river, which is to serve both for the railway and for general traffic, and we think we arc correct in saying it is the largest and most expensive bridge in New Zealand : not perhaps the largest, for the Rakaia bridge is, we believe, longer, but that is a pile bridge, of far less imposing character and far less permanent construction. It was intended that the Waitaki bridge should be erected on iron piles, and .orders were set home accordingly ; but the Government Consulting Engineer in England took upon himself to alter the designs to that of a cylinder bridge, and the necessary material which is said to have been intended for a bridge in India, was sent out accordingly. This raised the cost from £35,000 to over £OO,OOO, and caused considerable delay. However, into the engineering merits of the case it is not our province to enter ; it is some consolation to think that at least we shall have for the money a noble structure of an entirely permanent character, and of immense strength. A recent visit to the works by our correspondent enables us to speak very favorably of the progress made by the contractors, Messrs Hunter and M'Gavin, who arc evidently determined to benefit, if possible, by the wise provision in the contract which gives them a premium of £lod for every week they save on the contract time of eighteen mouths. Wc believe the actual time of commencement dates from the middle of April, some delay having arisen in supplying the machinery, which it is the part of the Government to furnish. The immense quantity of material required cylinders, girders,

&c.—had been carted to the river for some time, and now spreads over a surprisingly large area of the river bed on this side, and the contract was only taken after the Government had sunk two or three cylinders to show (he way. The amount of Messrs Hunter and ?»I 'Gavin’s tender was, in round numbers, £31,000, and it was the second lowest, the highest being no less than £50,000. They are evidently not afraid of tackling their work, however, and our correspondent was agreeably surprised to find, when he visited the works on the Gth inst., that 50 men were at work, and every preparation being made to put other 40 or 50 on, besides two steamengines, two sets of pumps, and four‘airlocks,’ and 12 sets of the patent excavator — a sort of screw backet, which alternately loosens and brings up the stuff. Mo loss than 152 cylinders had been got into line, each being sunk from 3ft (o Oft. The whole number of cylinders is only 210. and a considerable portion of the remainder will bo sunk in the bed of the main stream, which, at its deepest, is 14ft. At present the work is being carried on almost entirely on a dry river bed, the winter season being the most favorable in this respect. Every effort is being made by the contractors, therefore, to make good their footing on this part of the work during the favorable season ; and having got their work now well laid out, it will be comparatively easy to sink the cylinders to their full depth as soon as all the air locks and excavators, now on the ground, are at work. Only one of the former, and seven of the latter, were actually at work when our correspondent visited the works, but the others were in a forward state. The result of nine day’s work with one air-lock was the sinking of three cylinders to their full depth, after they had been well entered by the excavators. At this rate, with four air-locks at work, the whole 210 cylinders would be sunk in 158 working days, and it must be understood that work is carried on nigbtand day with the air-locks, thought not with the excavators. Provided all the other work can bo kept abreast of the sinking, there, seems no reason to doubt, therefore, that the coutractcau and will be completed well within the contract time, surprising as this seems, and highly creditable to the energy of the contractors. Some difficulty will no doubt be experienced in the main stream itself, which has now been reached, and it will be necessary to drive piles for the scaffolding with Ihe aid of a punt; but at present the stream is only about 120 yards wide, and it is not likely to increase during the winter and early spring. We append some details kindly furnished by the contractors ; The total length of the bridge is nearly three-quarters of a mile. There will be 105 40ft spans, two cylinders in width to each span —the total width, 18ft Gin, The cylinders are in length as follow, counting from the bottom : —One cylinder, Oft; one do, 3ft; three do (each 6ft), 18ft; two do, (each 3ft), Gft; one do, Ift Gin ; total, 37ft Gin. The lowest cylinder of all is 3ft wide, that being the least width a man can well work in ; the shut cylinder next above tapers up to 2ft Gin, whmh is the size of all the rest. The top of the cylinders will be Bft 2in above highest flood level ; on this will come a 2ft Gin strong iron girder, and the superstructure of beams and planking will raise the roadway 2ft sin above the top of the girders. The cylinders will be filled with concrete. Nine men are employed on each air-lock —one in the cylinder alone, two in the chamber over it, and the remainder outside —the chief difficulty being to keep the cylinder perfectly upright as it goes down. Each is weighed to the extent of 25 tons on the top, the heavy cylinder covers coming in very handy for the purpose. The material is a hard, compact shingle, with very few large boulders. The pressure of air on the one at work on the Gth inst. was 91b per inch, but is increased at the rate of .Jib to each foot of water. A reservoir of compressed air, in the shape of a small Cornish boiler, is kept supplied in case of any sudden stoppage to the engine. We need not minutely describe the process, which is the same as that used at the Rangitata bridge, the machinery being identically the same, having been procured by the General Government from the Provincial Government of Canterbury. The railway line between the bridge and Oamaru is all but complete, with the exception of some timber bridges, ready for the permanent way, and we hope within eighteen months to chronicle the crossing of the first locomotive from Otago to Canterbury, both of which important provinces will, we trust, henceforward advance hand-in-hand.