THE RANGITATA BRIDGE.

Dunstan Times, Issue 573, 11 April 1873, Page 3

THE RANGITATA BRIDGE.

—o — . - This structure, which may ho said to have been formally opened when Mr. Diair. C.E., and his party drove across it in August last, is said by the Timaru Horakl to be the first structure of this kind in New Zealand. It was designed by the late Mr. Paterson, and the supervision of its construction, after his death, devolved upon Mr, Bla : r. The following particulars of the Rangitata at the site of the river, are from a report by Mr. Paterson : -When at its lowest, and- in the smoothest place, the current runs six hub s an hour, and in heavy iPods about fifteen. The fall iivtho surface of the water is almost 40 feet a mile, equal to a gradient of 1 in 132 feet. It gets steeper than this towards the mountains, and flatter towards the sea. The maximum rise in the river is five feet, and the lower side of the girder is six feet clear of the highest floods- The bridge is 1238 feet long between the ontsMe of the abutments, and 20 feet inside. There are eighteen spans of C 6 feet each to centres, all of iron, and two end spars of timber, 33 feet to centres. Each pier consists of two cast iron columns or cylinders, 38 feet in heighth and 4 feet in diameter, openin'* out to five feet at the bottom, The columns are placed 9 feet apart, and arc unconnected except at the top, where there is a beam across. The metal in the columns is one inch, and one and one-eighth inch thick, strengthened with ribs and flanges. The various rings and segments are bolted together, the joints being perfectly air and water-tight. The columns are sunk 25 to 30 feet in the ground, and filled with cement concrete. An idea of the magnitude of the bridge may he gathered from the following schedule, stated in tons, of the materials used in itg construction.-—Masonry, WO ; concrete, 1300 ; cast iron, 410 ; malleable iron, 280 ; timber. 283 ; total. 22G5 tons. The bridge, althcndi intended only for dray traffic, is sufficiently strong for a railway. The total cost of the bridge amounted to 19,642/., or an overage of 13'. Bs. per running foot, which, co n sidevingthe nature of the situation and the character of the structure, makes it decidedly the cheapest m New Zealand, and one of the cheapest in the whole world, A description of the manner in which the cylinders were sunk will prove interesting : —The method was the pneumatic process : this consists in the application of compressed air in resisting water pressure. Assuming that one of the columns of the bridge, which is open at the bottom, but perfectly air-tight at the sides and top, is placed in posiiion, the water stands in it at the same level as in the river outside, and,there is a pressure at the bottom duo to the head of water. Now, if air is pumpe l into the cylinder at a higher pressure than this, the [ water retires before it, leaving the bottom I quite dry for the men to work on, It may safely be, said that without the pneumatic apparatus, abridge would never bo built at this part of the Rangitata river. From the size of the boulders it would be perfectly impossible to drive or screw in piles of any description whatever, and for the same reason a coffer-dam could not bo constructed. About four feet below water level, a bed of holders 10 feet thick was mot with. They ranged from Ito 8 cubic feet; the largest which was found in the 12th pier measured from 3 to 0 inches long, by 2 tcet broad and 16 inches thich ; and so open was the strata through which the cylinder passed that the water came in freer at the bottom than the top. The air-lock or chamber was a cast iron cylinder, 7 feet 9 inches high, and 5 feet G inches in diameter, fixed on the top of the piers to enable workwmen to go out and in without allowing the compressed air to escape. Usually the lock is fixed inside the cylinders, hut in this case they were too small, so it was placed outside, and was supplied direct from the receiver—an arrangement which did not disturb the equilibrium or the akin the shafts. There wore three men constantly in the cylinder, and one at the bottom excavating the material and filling it into buckets, and two in the lock hoisting up the buckets and throwing their contents into the river. The sinking went on day and night, the men working in four hornshifts. The air machinery was of tin host description, and worked most satisfactorily ; it could have clone tw : co the work required of it. When at its full depth the press of air in the column was about 13 pounds on the square inch, sufficient to drive a steam-engine, still the men worked under it with case for hours at a time. Indeed, it was not ad visa’le to go out too often, as they were in danger of catching cold. Although the sensation is rather a peculiar one, the effect of the air on oven the uninitiated is not very serious. For a minute or two on going in thero is a rush of air through the ears and nostrils, but it ceases whenever the air m the body roaches tin. same pressure as that in the cylinder. Independently, however, of this, and the feeling that one is twenty-five feet under water, ther» are peculiarities i-i the situation worth experiencing. The cylinder is usually filled with afrnrious mist, the murkiness of which is intensified by the glare of the lights, and there are heard at intervals strange sonndsof rushing air and water, mingled with the steady beat of the machinery. The progress made with the first two or three piers wps slow and discouraging to all concerned ; the boulders were very large, and had to ho drilled and epbt before removal, hut after this, although the sinking did not get any easier, the workmen got more expert, and the work proceeded faster. The total time occupied in sinking under pressure was 3773 hours, which gives an average of 99 hours for each column, or three 'and'a quarter inches in an hour. The fastest sinking was four feet in four horns, and the slowest when the cylinder scarcely moved for a whole day*.