THE RANGIGATA BRIDGE.

Tuapeka Times, Volume V, Issue 242, 19 September 1872, Page 6

THE RANGIGATA BRIDGE.

This structure, which may be said to have been formally opened when Mr Blair, C.E., and his party drove across it last week, ia said by the Tfrnant, Herald to be the first structure of its kind in the Colony. It was designed by the late Mr Paterson, and the supervision of its construction, after his death, devolved upon Mr Blair. The following particulars of the Rangitata at the site of tke river, are from a report by Mr Paterson : — When at its lowest, and in the smoothest place, the current runs six miles an hour, and in heavy floods about fifteen. The fall in the surface of the water is almost 40 feet a mile, equal to a gradient of 1 in 132 feet. It gats 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 6 feet clear of the highest floods. The bridge is 1258 feet long between the outside of the abutments, and 20 feet wide. There are eighteen spans of 66 feet each to centres, all of iron, and two end spans of timber, 33 feet to centres. Each pier consists of two cast iron columns or cylinders, 38 feet high and 4 feet diameter, opening out to 5 feet at the bottom. The columns are placed 9 feet apart, and are 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 pei fectly air awl 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 be gathered from the following schedule, stated in tons, of the materia's used in its construction : — Masonry, 300 ; concrete, 1800 ; cast iron, 410; malleable iron, 280; timber, 285 j total, 2265 tons. The bridge, although intended only for dray traffic, is sufficiently strong for a railway. Tho total cost of the bridge amounted t0L19,642, or an average of Lls 8s per running foot, which, considering the nature of the situation and the character of the structure, makes it decidedly the cheapest in New Zealand, and one of tho cheapest in tbe 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 airtight at the sides and top, is placed in position, the water stands in it at the same level as in the river outside, and there is a pressure at the bottom due to the head of water. Now, if air is pumped into the cylinder at a higher pressure than this, the water retires before it, leaving the bottom quite dry for the men to work on. It may safely be said that without the pneumatic apparatus, a bridge would never be built at this part of the Rangitata river. From the size of the boulders it would bs perfectly impossible to drive or screw in piles of any description .whatever, and for the sarae reason a water-tight coffer-dam could not bo constructed. About four feet below water level a bed of boulders 10 feet thick wai met with. They ranged from 1 to 8 cubic feet ; the largest which was found in the 12th pier measured 3 feet 6 inches long by 2 feet broad and IG inches thick. And so open was tho 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 6 inches in diameter, fixed on the top of the piers to enable workmen to go out and in without allowing the compressed air to escape. Usually the lock is fixed inside the cylinders, but 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 air in the shafts. There were three men constantly in the cylinder, one at the bottom excavating the material and filling it into backets, 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 hour shifts.

The air machinery was of the best description, and worked most satisfactorily ; it could have done twice 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 steamengine, still the men worked under it with ease for hours at a time. Indeed, it was not advisably 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 pressure on even the uninitiated is not very serious. For a minute or two on going in there is a rush of air through the ears and nostrils, bat it ceases whenever the air in the