TAWA FLAT

Evening Post, Volume CIX, Issue 15, 18 January 1930, Page 10

TAWA FLAT

WORK GOES WELL

THE PIONEER TUNNEL

WHERE SfPOIL GOES

The big job ofiputting through the Tawa Flat, tunnel., and nobody m'io has visited the wo/rks in progress there knows how big a. job it is ; is now going well. The shorter tunnel gave some trouble with looao ground and water, and. the extensiye timbering uecded, and this took longer thau was anticipated, but in tlie second tunnel progress so far is according to schedule, and though the ground is not as good as it might bo i:n parts, on the whole it is much better than was the case in the first tunnel." The work is going ahead so well, in fact, that a spurt will be necessary in, the case of the goods yard to bo built on the reclamation (when that is finished) and the new railway station plans cannot afford further modifications, otherwise the tunnels may be re'ndy before them. If this does occur, however, there need be no waste of time, from a layman's point.of view at least, as the expresses and- through traffic could be diverted by switches to the new line, enabling trains to leave Thorndon, even though the new yard and station are not ready. J WWLL IN HAND. Tho start* of the work was made in July, 1927,, when buildings were put up, but tunnelling, was started late that year. Labour troubles caused a cessation .of the work until Easter, 1928. It is now considered quite possible that -both tunnels will be completed, concrete lined, and- ready for the rails*1 by the middle of 1932. Another yesir will be needed to lay the rails, put up the station buildings, etc. The preliminary headings should meet in tho lionger tunnel in the winter of. 1931. • ■

The first tunuol heading was completed in August" last year. Already the tunnel has been widened-and the concrete lining put in for 27 chains, 16 chains at the Hntt Road end, and 11 charms at the other. Not long since, the headings being driven west from tha shaft at the viaduct and east from No. 4 portal (the exit on the Tawa Flat side) met, making 53 chains of heading completed between the shaft and 'No. 4 portal. The heading has been driven 30 chains eastwards from the shaft. At the other end of the long tuunel, in Ngahauranga Gorge, 25 chaj ns of heading have been driven westwards. ' Six chains of concrete are finished at No. 3 portal, and eight at Noi. 4 portal. 'There remain a mile and twenty-five chains of heading to drive hi the long tunnel, and two and a-half males of concreting to finish. ; The big muckers, their smaller brothers, the cement gun and steel profile for the concrete lining, and in fact all the modern plant put in has proved '.justified by results, though firmer i ground would make it even more useful. Timbering still remains a problem, as the best of the ground cannot be trusted to stand without it, and some of the loose, broken stuff requires spocial attention to timbering every yard driven.. Tour hundred feet below the surface, a seam of almost pure clay was met in the rock. This is almost directly beneath Johnsonville. Fortunately this was comparatively dry, and so gave little trouble, but this is not the sort of country which will stand up long enough for the "muckers" to clear the spoil ahead of the. timber. A TIME-SAVING HEADING. The "pioneer tunnel;" used for the first time in New Zealand, is now driven to within a chain or two of the face in the lower main tunnel, which it parallels at the distance of a chain. It may seem strange to drive two tunnels where one might be supposed to serve, but time will be saved nevertheless. From the pioneer tunnel slanting openings are cut into the main tunnel, and provide access for materials and -a ■free run for trucks; whatever is going on in the main tunnel, thus obviating stoppages when the latter is obstructed. Better ventilation, the laying of air pipes for working machines, and the getting rid of water, are also made possible. There is another advantage, in the case of the concreting work. The steol profile, which is wheeled along the tunnel on rails, on a gantry, and behind which the liquid concrete mixture is blown by the concrete gun, can be dismantled, and the segments taken through the pioneer tunnel to one of the openings to the main tunnel, and thus it will be possible for the work of concreting to go on at more than the two external faces in each tunnel where it is being carried on at present.

Probably four faces will be opened up by the pioneer tunnel for concreting, from which work will proceed in both directions, which will' mean ten steel profiles at work at once, instead of two, a great time saver. While a great deal of the rough finishing of the arch in the widened heading in the main tunnel can be done by the pneumatic picks, preparatory to the permanent timbering which is left in, occasional hard benches are encountered which necessitate blasting, and the pioneer tunnel leaves the way open to other parts of the main tunnel. The level of the heading in the main tunnel is considerably above that of the finished tunnel, while the pioneer tunnel, on the level of the finished main tunnel, gives an oven grade for getting spoil out and bringing in materials. It has already speeded up tho work. THE TAWA FLAT SIDE. One of tho most interesting development!! on the Tawa Flat Bide of the big "hole in the hill" ia the line of rails laid for the disposal of the spoil. Owing to tho difference in the levels of heading and finished tunnel^ spoil from the hoading being driven eastwards frbm tho shaft is still being hoisted at the shaft, and dumped in the adjacent gully, but tho spoil from the widening of the heading ahead of the steel profile at work at No. 4 portal is brought out at the Tawa Plat end, the electric locomotives haul half a dozen or more four-yard trucks of spoil into a shunt siding, whence the locomotives push the trucks instead of hauling them up a graded line of rails nearly to the level of the existing railway along the edge of a gully, which will hold 140,000 cubic yards of spoil, and more by putting in a further switch siding and short climb. Some of the spoil will bo needed for the fillings between No. 4 portal and tho Tajva Flat station, an average of four feet of filling over the wholo distance. This dump will also serve as a magazine of good ballast for any part of the line in furture. Originally it was proposed to haul tho spoil practically straight up tho ' hill by cable trucks, but the new idea saves tho employment of twelve men, three men and the locomotives now doing tho work of fifteen, \rtjo would have been required in the case of the cable proposition. This spur lino for spoil disposal greatly intrigues motorists, some of. whom are under the impression that it will form a connecting link with tho present line. DISCIPLINING A STREAM. Tlic stream that the line skirts after Icaring No. 4 portal has been put in n straight jacket in places, diverted in others, and generally transmogrified. It is cheaper to divert a small stream like this than to build bridges, but eight of the.latter have beer, necessary, de-

spite all the landscape gardening that has been done with the stream. The Takapu Koad overbridge has been finished, a fine structure that giveß a suburban look to the surroundings, arid probably two more ovorbridges will bo needed before Porirua is reached. Only one small railway bridge remains to be built to finish the series. KAIVVAERA RAMP. At first an attempt was made to secure sufficient hard rock in big enough pieces to form the basis of the ramp connecting the first portal with the Kaiwarra station yard, but the paucity of good stone has resulted in tho laying of five-ton blocks of concrete instead. These will be heavy enough to withstand tho smack of the waves, which i sometimes is severe, and on them will be laid the spoil from Nos. 3 and 3 portals, covered by the softer material from No. 1 portal. The movable trestle, an ingenious fixture which a"ows of the spoil being laid to tho right height and at tho same time filling a section of the ramp at the same batter automatically, will be used, shortened as it proceeds towards Kaiwarra, until the decreasing height of the ramp renders its use unnecessary.