TAWA FLAT TUNNELS

Evening Post, Volume CXVI, Issue 106, 1 November 1933, Page 12

TAWA FLAT TUNNELS

FINISH NEXT WEEK

WHEN PROFILES MEET

WHAT HAS BEEN DONE

The end of ono of the biggest engineering jobs in New Zealand will come early next week, when the second and longer Tawa Flat tunnel will have been moulded throughout with its two feet of concrete lining, and, except for the cleaning up and levelling of the floor, will be ready for the laying of the rails. Today it was possible .to see through it, though the men at work obstructed a clear view, and the pinpoint of light in the distance is, after all, a very tiny indication of the sunshine beyond. Such a stupendous task as the piercing of the hills to the west was undertaken for the best of reasons frqm the railway point ■of view. The new line will lie only 195 feet above sea level, whereas the existing line is 518 feet above sea- level, and the steepest grade will be one of 1 in 110, as against 1 in 36 at the steepest part, of the present wearisome climb out of the city. Two and a half miles are cut out of the distance,' and some half-hour in time. Moreover, the electrification of the line as far as Paekakariki will do away with much.of the soot that collected in the carriages when impatient persons opened and shut windows at the wrong time amongst the short tunnels. Apart from the convenience to passengers, freight haulage will show a big saving. HOW WORK WAS DONE. The system followed in these tunnels necessitated the employment of much modern machinery used for the first time in tunnelling in New Zealand. A compressor house at the intersection of the line with the Ngahaurai-^a Gorge supplied the air for those machines which were not electrically operated. Following up the tunnnllers, with their pneumatic drills, came the "muckers," small ones in the case of the preliminary headings, and larger ones while the tunnel was being opened up to its full size. The inside measurements of the finished tunnel are 24 feet at the bottom, 28 feet 6 inches at the widest part of the arch, and 20 feet high. As /the concrete filling is two feet thick throughout, parts of the tunnel in tho rough were over 30 feet wide and 22 feet high. The concrete sides were sunk three feet below the I permanent level of tho floor. The i principle was to follow up the 10 feet by 8 feet headings with widening operations, and as sufficient portions of the tunnel became available, the steel profiles, jointed shells the shape of the finished inner surface, were run into place on their rails, and the concrete was blown in behind and above them by means of "concrete guns." Electric locomotives aided rapidity of transport, the small "muckers," capable of working in a space of 10 feet by 9 feet, tore into the loosened rock, and loaded it into the^ trucks. Gravity aided the evacuation *of much of the spoil. Much of the spoil was crushed in a plant at the gorge, and was mixed there with cement and sand to form the_ concrete for the moulding. Some of it,'as the work progressed, was put into the long ramp that leads upwards from Kaiwarra Station to the Hutt Boad overbridge. VARYING METHODS. There are two tunnels, making about three and a half miles in all. In the shorter one, between the Hutt Boad and the Ngahauranga Gorge, th. only special difficulty which was encountered was loose wet ground. So persistent was this trouble that it became necessary, as the tunnel was being worked from both ends and the pumps had difficulty in keeping down the water on the side where the grade ran into the tunnel instead of out of it, to drive an adit with an exit above the meat works to run off the surplus water. This occasioned some delays and a vast amount of timbering at the bad places. In the longer tunnel, though the processes of piercing, widening, and concreting were the same, other features of -attack were used to speed up the work. A shaft was sunk near the Belmont Viaduct, so that the work could proceed on four faces at once, that is from both ends, and also in opposite directions from the foot of the shaft. In addition a pioneer tunnel was run in off the main tunnel at the Ngahauranga Gorge end, paralleling it at about a chain distance. This speeded up transport problems, and assisted drainage, and at the same time, by piercing between the pioneer tunnel and the permanent one, still more faces could be opened up once the main heading had been pierced. ' This pioneer tunnel will be closed up now that the job is finished. PROGRESS OF WORK. Preliminary work was started in July, 1927, and in October, 1927, a contract was let for driving tho heading in tho small, tunnel. ' Work was started in November, 1927, and was carried on slowly. A strike occurred on December 16, 1927/ and lasted some time, ending, in a commission. Tenders for the big tunnel closed on February 22, 1928, but no tender was accepted, and it was decided that the Public Works

Department should carry out the work. Before it was possible to get on with the job, the plant to carry out the work had to be ordered. This did not arrive until early in 1929. The shaft mentioned above was started in August, 1 1928. It was not until July ; 1929, that a full start was made with1 the new plant. Fifteen chains of concreting were then done in tho first tunnel, and six chains in the second tunnel, the big plant being in full swing. Since then the remainder of the work has been carried out, for the last two and a half years on a reduced schedule. Forty-five chains of the first tunnel were concreted in' July, 1931. The second tunnel'will bo completed early next week. BIG FIGURES. Statistics stimulate the imagination of many people more than do simple facts. The following figures are interesting. The total length of tho two tunnels is 18,170 feet. Tho amount of rock excavated is 460,000 cubic yards, or 820,000 tons. To break out this rock there have been used 160 tons, or roughly 350,000 pounds of gelignite. Timbers ■ used for holding back the ground totalled 2,500,000 super feot of 10 inches by 10 inches sawn pinus insignia, and another 2,500,000 super, feet of laths 6 feet long by 6 inches by 2 inches, besides which there have beeu used 182,000 lineal feet of round, timber averaging 9 inches in diameter. Of the 460,000 cubic yards of rock taken out, 120,000 cubic yards have been crushed and used for concrete aggregate, as well as -35,000 cubic yards of sand, and 26,000 tons of cement. The spoil has been dumped in three places. Besides the contents of the ramp, fifteen acres of water have been reclaimed; There is a smaller dump.at tho head of the shaft at the viaduct. Just beyond No. 4 portal (Tawa Flat end) there is a dump in a gully, the spoil in which will later bo crushed and used for ballasting tho tracks. Much of tho rock was first-rate for this purpose. Besides the actual making of the tunnel, quite a lot of work has been carried out, 1314 feet of double track bridges having been built, besides a railway overbridgo at Takapu Boad 25 feet wide. The creek eventually forming the Porirua Biver has cost a good deal in diversions and straightening, these diversions, etc. necessitating the removal of 100,000 cubic yards of spoil, which was used in the formation of the railway track. Besides this there were 240,000 cubic yards of excavation in cutting along the track. There Are miles of culverts and drains to keep the track dry, or to carry storm water under it from small creeks. Obstacles and deterrents notwithstanding, the cost of the job is pretty well within the estimate of the Public Works Department, and is held to be cheaper than it would have been if carried out by contract. When the Public Works Department began the work, Mr. A. F. Downer was in charge, but for the last four years Mr. C. Langbein has had control. On its conclusion lie will be transferred to the Masterton office of the Department, and will there supervise highways as the representative of j the Wellington District Engineer.