THE NEW BRIDGE.

Dominion, Volume 7, Issue 1933, 16 December 1913, Page 12

THE NEW BRIDGE.

* —— TO WANGANUI EAST.

A STRUCTURE TO ENDURE,

Wanganui is within three months of having a new bridge across the river. This lias long since been considered an urgent necessity on account of tlio rapid growth of residential settlement in H'angamii East, a splendid expanse of undulating and Hat country tluit lies across the river immediately to tlio east ot' the most thickly-populated part of Wanganui town, 'to reacn this district (formerly a separate borough, but now included in nnganui Borough) it is necessary to cross the old bririge at the loot of Victoria Avenue, and then follow tho river road past Shakespeare Cliif for about threo miles, wlien one reaches an extensive area of the best class of icsideiuial land that is roaded and subdivided into building allotments with a promising colony ol cottage homes already in occupation. . The new bridge, which takes oil at Dublin Street, should briii;; residents of Wanganui East within from twenty to thirty minute:;' walk of the town, and halt that time if the tramcani (that arc to travel the new'bridge) are used, besides which it will provide another convenient outlet for WanicaViiii's growing population. The old ntu. - about 40 years of age, lias served its turn wonderfully well, and with lair usage should last lor many years tn come, but it- wai? considered a -matter of expediency to havo a second way out, and so —the East Bridge! Unforsecn DifJicuK.'cs. The design for tho new bridge was prepared for the City Counicl by tho the l'ublic. Works Dopainneut, wriicli originally stipulated lor a sub- I structure of terro-concretc .piles, and! a Pratt truss superstructure'DtiOit;'between the banks of the river in nine spans of 100 ft. llin. each. Tenders wero called, and in October, 1011, that of ilr. Robert Saunders, of Wellington, was accepted, tile contract price being £26,8/-l. The contractor lost no timein commencing the work, but soon found that' there wero unexpected diiliculiics" to contend with .in building the bridge as specified. When attempting to drive the piles (1G inches square by ol) feci in length and weighing 7 tens' each) with a 2i ton monkey, he found a resistance ill the river bottom that was- nut anticipated. It was found impossible to penetrate the consolidated gravel of the river-bed. The oxidisation of the gravel presented a material that declined to loosen or give under impact, and after making exhaustive tests, tlio contractor had to report that it war. impossible to construct the bridge as spcciiied. This led to all sorts of complications in which the contractor, the Council, the Public Works Department, and several lawyers became concerned, with the result that the work was hung up for nearly twelvo months whilst, everyone thought hard. It was another Bridge of Sighs. It was then suggested that the bridge, could bo constructed with concrete cylinders instead of piles, but some doubt was expressed as' to whether concrete cylinders (minus the usual , cost-iron casing) would servo the purpose, the mode being new to New Zealand. The contractor (Mr. Saunders) was confident that it would, and in the. end, he was permitted to go on with tho work and was allowed an additional £5250 on his contract price. Since then the work has gone on merrily, and now there are six of the nine spans in position, and the seventh is being constructed. The wholo of tho piers (a pair of concrete-filled cylinders in each ease joined by a solid wall of concrete) are already ill position, and Mr. Sanders hopes to have tho bridge completed byMarch next. How It Is Done. The construction and sinking of tho cylinders is a most ingenious and fascinating phase of modern engineering processes. Tho cylinders are. of rein-

forced concrete, east on the spot, and | when of sufficient length are lowered ■' into the water, not, however, before they have been allowed a month's time . to set hard. They are then cleaned; .: out, and tho sinking process begin;;.: " As each one has to be sunk some thirty' '■' feet below the level of the river,, tile, task would appear to be one fraught 1 with the gravest risk to those employed in the work. This, however, is not the i, case. '.tiie. cylinders are made air-'' >; tight, and a gang- of men are set io'. burrow a hole in the bed of the river] i wide enough to take the cylinder, the,, pressure of air maintained' (by Llie' ■ Triger Plenum process) being quite sufficient to prevent the cylinder from moving. : Tho spoil is passed out through an air-lock, and when a depth of .about three feet lias been 'excavated, tho men i con:c out through the air-lock, the- airpressuro is released, .and the sli-ton cylinder sinks by its own weight.- Then the air-pressure is again applied, the men re-enter the cylinder, and continue | their excavating until it is time for another fall, so on until the cutting edge is 30ft. below the bed of the river. A pressure of i'i'lb. of air obtains within the cylinder, under which condition the men are able to work long ' hours., without feeling any discomfort.' (ircat I logs (.sometimes as much ■■ as -lit. in thickness, have been di'-nnvml in the earth beneath the river be:!, and these liaxe had to be cut through before lhe cylinder could be sunk. Quicksand-and artesian water spouts have also been'a source of ineonvonienee and del.iv. ■ Some idea can he gained of the solidity j

awl strength of Oie piers of "the new bridge when it is stated that they are, when completed, 80ft.. in height—3oft. lielmv the rivt-r bed, L'Oft. in tho water,', and 30ft.-out of it at mean tide. 'L'iio I'ratt truss superstructure is of enormous strength, wondorfuliy braced, and on the deck level is joined together from side to side with trenched- steel plates. These aro to he' subsequently' Silled in and levelled off with concrete, and on that will he' laid a flooring of jarrah blocks. Mr. Saunders informed a Dominion reporter that by the time -She work was completed lie. will lmvo used over 1000 tons of steel, 30 tons of rivets, and 3800 yards of concrete. Tho bridge would be a very strong one. but,, seeing that it was for a tramway as well as ordinarytraffic, there was nothing like being on the safe side. A-five-foot footway would he ereeted on cantilevers outside the superstructure on the south side of the: bridge. . '"%■ <'-V Wanganui is wretchedly equipped for street name plates. The .Jlorough Council should bear in mind that. now and again .AVangnimi is ''visited by strangers wlio do not know the names of the street;; and who want 'to. Wa-l nganui lias long since passed.the conn- 1 try town stage, and street' names on plates are now a real necessity.-