RAILWAY LINK

New Zealand Herald, Volume LXIX, Issue 21328, 1 November 1932, Page 6

RAILWAY LINK

TARANAKI TO AUCKLAND DIFFICULTIES IN TUNNELS DISPOSAL OF A CREEK HEAVY FORMATION WORK. / No. VI. .lifter transport, tunnelling presented the erri'atest problem in the final link on the. Tavanaki-Main Trunk railway, It was this phase that-y governed the time, in which {hi- railway could he completed, since in railway construction once all the tunnelling work is opened up, it can proceed only at a certain pace. At Tangarakau tunnelwas started in 192& and has only ju;;t been completed. The first tunnel started, just beyond Tangarakau through the Mangatatoko ridge, did not present any serious difficulties'. Hard arid dry papa rock was experienced throughout. Nearly all the driving w;is done from the Tangarakau side, at winch end it is constructed on a 10-c'nain When completed the line was correct. Lining was carried out. in this and i/r, 3 other tunnels with mass concrete, usually 12in. thick. Straightforward going was also experienced in the second tunnel, 54-£ cl'iains leas;, but in No. 3, the longest in the section, totalling chains, very different ground was met. Heavy and sodden formation was experienced from the start, necessitating pumps and bailing to make work possible and safe. Extra strengthening was Necessary as well, and this also retarded progress. A Serious Problem The most awkward conditions, however, arose in No. 4. For nearly a year work wan held eip by difficulties at the entrance C 1 tiing and these were later added to by thi> presence of ft creek. Immediately a ciiii was made irito the ridge, which to ail appearances was solid, it began to Slip bad,jy, and repeatedly crushed in the abiding timbers. Once this difficulty was overcome and t'ifinelling had progressed about four chains, a creek came in several feet above t •){ tnrinel floor at a spot where the tunnel emerged briefly into a deep ravine in the liill. As it was intended to rnak* the tnnnel continuous, the water presented a serious Obstacle. The damming of the creek and piping it over the works temporarily solved the difficulty, but its permanent <:iifposal created for the engineers a prob- ]?,:?» of greater difficulty. Eventually fit was carried (iter the tunnel in a kind of spillD7iiy, the tnnnel, specially buttressed at this point, acting as the dam. Thus the line now passes beneath the creek. , Concreting a Tunnel The two tunnels on the Raekohua section' between Tahora and Tangarakau prevented little difficulty. The first is built em a 10-chain curve for six chains and both are.'short. This section, on which a I<i!nporary line had been built, was; completed sooner than intended owing to the commencement of the operations of a coal company at Tangarakau. By giving earlier access "the line was made remunerative as J-oon as possible. 'ln the concreting of No. 2 Raekohua tunnel, a method was adopted which the m'jn aptly named the "tin-hare." This was the bucket which carried the conrri'ste into the tunnel on a rnonoTfi.il and imS! endless ropo. By this means , the was run in and outt of the tunnel at » height of about 12ft. to the men n*.*,j;oi<)cretirig without interference with the deaning-up operations which were following in the rear of the tunnellers. On the other tunnels the concrete was trucked in: to th 6 face, but for thei short job this :nfi!?thod proved n. great success. Culverts and water drives by which the Bne is carried over small streams are in great abundance throughout the work. The precaution lias been taken to make them all large to guard against damage afl a result of cloudburst. Such an occurrence in/1924 so damaged the Whsingamomoria section that the line was blocked tor over a year, chiefly owing to the water drives becoming filled with debris. Heavy Formation Work He.nvy formation works were undertaken between tho tunnels. The scene in the Mangatatoko Valley as the line emerges from the first tunnel and passes across a huge filling and on a 12-chain curve through a big cutting battering out to 160 ft.; high, is an impressive one. The rails »re soon lost in the next tunnel, through which lies tho Mangaone Valley, Close to the tunnel entrance in this vallny is the notorious "pug" cutting. For nearly four years this cutting baffled the «!iigineerp. In tho end they were compelled to compromise by shifting the line. A huge pocket of "pug" formation—a mixture of earth and water resembling wet cement—was met in the ridge, which & (though possessing an appearance of solidarity was apparently a huge slip. Sluicing and trucking were of no avail. The ' pug" continued to slip over a length of seven chains from 150 ft. to 200 ft. tip on the face. The tunnel had been completed arid the formation was ready for plate-laying. Pushed to the last resource the engineers began to experiment i*rith the route. Only ft certain amount of deviation was possible through the limitations fixed by the tunnel entrance, but nfter n number of trials a feasible ronte Was "paper-located," which, with only ft slight deviation, left a 30ft. triangle of f Itolid formation between the line and the <'png." Difficulty of ISlips Further on in the Mangaone Valley is It "big bank," containing 75,000 yards of spoil. Almost Iwo vears were spent in filling with spoil I lie valley iri which the Maqgaone Stream originally flowed ffOft. below. After passing through this tunnel the formation in the Mangafete ,Valley changes to sandstone and hi places wet "pug." Wet slips caused a lot of tr"libJe in fhi=t vnllcy, tvliic li is a succession of cut-Mugs and fillings. Through (he last long tunnel (he line emerges in fl>e Tikhpuiti Valley where another "big Kink" built on n curve across a deep i'ft!ley contains 80.000 yards of spoil •whirli also look years In place. Apart from a minor deviation of Iho ' Ifaeo Stream/and several small! tunnels fhe remainder of the recent construction has lieen of a simpler nature. The whole of the Tahora-Ohura section illas been built in accordance with the jllest. trrink line standards. All I lie tunnels llave flatter grades in order to minimise the smoke trouble. The sharpest curves tire 7jr chains radius arid are used as infrequently as possible. Form a lion has i\ minimum width of 16ft. throughout and li'OJb. rails are laid. (To bn continued.)