TAWA FLAT LINE

Evening Post, Volume CXII, Issue 86, 8 October 1931, Page 11

TAWA FLAT LINE

READY IN TWO YEARS

WORK IN THE TUNNELS

RAPID PROGRESS

One of the biggest tunnelling jobs in New Zealand has now entered on the final fast stages of excavation rendered possible by the adoption for the first time in New Zealand, of the "pioneer" heading1. Prom now on spoil will pour out of the tunnel, largelj' by gravity, at an increased rate. Hitherto much of the work lias been preliminary. Now efforts are being concentrated on the enlargement of the main heading and the concreting by the use of the steel profile and cement gun, and there will be more to show for the work done henceforward, and the special machinery will come into its own. 810 JOB HALF BONE. If the work i« carried on at the present rate, the tunnels should be complete in eighteen months, and the track and all that is necessary to enable trains to run through the tunnels should bo ready by the middle of 1933. The first tunnel of 63 chains is complete.except for the track laying, and has most of the ballast put in. Over it a temporary track for.the electric locomotives has been laid to carry the spoil from, the second tunnel. In the second tunnel, which is 214 chains long, 103 chains, or nearly half, has been widen-, od to the full width of 29ft by 22ft, and of this length 85 chains have been moulded with the two-foot thickness of concrete lining, so that more than half of the total length of the two tunnels is now concreted. .Though it has been decided to electrify this section of the lino, this work will wait until the closing stages of the track laying, if it is not indefinitely postponed for financial reasons. DIAGRAM EXPLAINED. With the view of explaining the progress made and the principle of the pioneer heading, or.pilot heading, which will cut the time taken over this big work in half, the accompanying diagram of the longer tunnel was prepared by the. Public Works Department at the request of "The Post." At the left band is No. 3 portal in Ngahauranga Gorge, and at the right hand is No. 4 portal at the Tawa Flat end of the tunnel. The lower diagram represents the tunnel and pioneer heading, and the upper represents the rising grade of the line towards No. 4 portal, and above it the elevation of the hills nnder which the line runs, somewhat more jagged looking than in reality owing to their condensed outline. The plans were originally drawn to the scale of ten chains to the inch horizontally, and 50 feet to the inch vertically as regards the elevation of the hills,' but the reproduction is much reduced, and.for the sake of clearness the pioneer heading is shown further away from, the. main, tunnel than it really is. WHAT HAS BEEN DONE. The sinking of the 125 ft shaft at the j viaduct allowed drilling to proceed at , .four*places'at, the same time, and this speeded up the work. The diagram ; shows that 51 chains have been opened .'full width at the Ngahauranga Gorge .end. There is a two-chain gap of preliminary heading, and then a/further ■seven chains excavated to the full width. Then follow 73 chains of preliminary heading to "C," where work has • been commenced on the excavation of -what ■will be made' a full-width chamber of considerable extent, which will later be used for concrete mixing. At present the concrete comeß down the shaft. Between "0" and the shaft there are.27 chains of heading still to be widened, and beyond the shaft there are fourteen chains more which require widening. From that point to the No. 4 portal the tunnel is widened to its full width, and * good deal of it is concreted. TOES OF PIONEER HEADING. Turning off to the left, and then paralleling the main tunnel a chain away, the pioneer heading, 10ft by Bft, is connected with the main tunnel' at varying intervals by eleven communication headings. The whole of the pioneer heading and all the communication headings are laid in track for the electric locomotives, which means that work of any kind may be done in the main heading while traffic -to other parts of the main heading can go on uninterruptedly through the pioneer heading.' 'At a slightly lower levelj the pioneer heading also serves to drain the main heading. There is always a small river running down it, while the floor of the main heading remains fairly dry. There is a further advantage that the compressed air, water, and ventilation pipes all run along the pioneer heading, being led into the mam heading at whatever point they may be required. This leaves more space in the main heading for the special machinery, which would otherwise be serioiißly cramped in action. One of the main advantages secured by this modern method is seen in the case of the seven-chain block exeavateel to full width between two.sections of preliminary heading. Spoil from this has. frfeo exit, while operations there do not prevent spoil from further in passing it in the pioneer tunnel This principle is applicable at any point in the main heading paralleled by the pioneer heading, and will speed Up the widening.. : SPOIL TOR RECLAMATION. As the grade of the tunnel runs dowm toward* the harbour from the Tawa Flat end, spoil can now be mn out to the Kaiwarra ramp from the second tunnel by gravity, the only upgrade haulage done by the electric locomotives 'being in the case of drills, material, and timber. The Tate of progress should be considerably increased henceforward, as attention is being fully concentrated on the widening. Until the u»e of the Johnsonville shaft 8s cut out, spoil will still be delivered there and at the ramp, but after that the spoil from all but one of the cnlargings will all go to the ramp. It will take some time to finish the ramp to its full width, and this will not end the use for the spoil there, because the reclamation of the ground between the ramp and the Hutt road will take nearly all the spoil available. The rails of the Wairarapa lino are to be shifted parallel with the ramp tracks, and between this and the Hutt road there will be built up some four acres of land which should be sufficiently valuable to help to pay for the cost of the deviation.