GREAT TASK ENDING

Dominion, Volume 27, Issue 33, 2 November 1933, Page 12

GREAT TASK ENDING

Long Tawa Flat Tunnels

WEEK’S WORK REMAINS

Magnitude of Undertaking

By about Wednesday next the second and*longer Tawa Flat tunnel will have been completed after over five years work. Then for the first time will it be possible to obtain a totally uninterrupted view through this 2 ~-3 miles long man and machine-made hole through the hills beyond Ngahauranga. At the present time, only about one chain near the middle of the tunnel remains to be moulded and concreted, and when that is finished next week the Public' Works Department will have completed its huge undertaking. Machines will stop operating, the men will mostly disperse to other jobs, and the Railway Department will at last take over. To it will fall the task of turning what is now only a pair'of tunnels impressive though they are in themselves—into an all-important part of New Zealand’s railway system which is expected to bring about great savings in the future through increased speed, efficiency, cleanliness, and convenience. At the present time it seems unlikely that the Tawa Flat deviation line will be opened"'for railway transport before the new central Wellington station and yards are completed. In the meantime, the line from the beginning of the deviation on the Wellington side of Kaiwarra to where it joins the present Main Trunk line slightly to the north of the Tawa 'Flat station will have to be ballasted and railed, and the electric locomotives to be used between the city and Paekakariki will have to be got ready. Obvious Advantages. When the service is begun, however, the benefits will be obvious. The distance will be shortened by two and a half miles, and the'time by about half an hour. In contrast to the existing line through Ngaio, Khandallab, and Johnsonville, which runs to 51S feet above sea level, the highest point of the deviation will be only 195 feet, and the steepest grade will 'be 1 in 110, as against the present grade of 1 in 36. The electric traction to lie used also will mean complete cleanliness through the tunnels. The magnitude of the work necessary for the present completion of the two tunnels, the one nearer the city 61 chains long and the other 2 miles 54 chains, and second longest in New Zealand, is difficult to imagine. In piercing them, about as much rock had to be excavated as from the great seven-mile Otira Tunnel, the reason for this being the difference in width necessitated by the double railway track. Eight- hundred and twenty thousand tons of rock have been blasted out with 160 tons of gelignite. This represents over two pounds of explosive to the ton, which is a big ratio, and indicates the fairly hard nature of the formation that was pierced. No signs of valuable metals, by the way, were discovered inside the hills. | Heavy Walls and Ceiling. The, total length of tunnelling is 18,170 feet. The cement used in lining the walls with a two to three feet thickness lias been 26,000 tons, together with 35,000 cubic yards of sand. Heavy beams of pinus insignis, lOin. square, have been used to brace the walls and ceiling, and, as a departure from the usual practice in New Zealand, have been left in position and covered by the concrete, which has been pumpforced into place behind huge seventon steel profilings, or moulds. This has resulted in an unusual thickness and strength of tunnel lining. The two tunnels themselves represent by no means nearly all of the work that has had to be coped with. The great ramp on the city side, by which 15 acres of the harbour has been reclaimed, has taken considerable time to construct. Also 1314 feet of double track bridges have been built, as well as a 25-foot railway overbridge at Takapu Road. A tributary creek to the Porirua River has had to be diverted and straightened out, cuttings and embankments have been made, and many miles of culverts and spring and stormwater drains have been built. Progress of Work.. Work on the deviation was started in July, 1927. The preparations for tunnelling were begun soon after, but it was not until'July, 1928, that the Public Works Department began on the actual piercing. From then on to the present time work has gone on in three shifts through the 24 hours. Two strikes by the men on the job caused interruptions, and the men have for some time been working for only five days a week in preference, to some of their number being, for economy reasons, dismissed during the job. When construction was at its height about 450 men were employed; as the tunnels neared completion this number has been decreasing, and now there are only about 110 men altogether, most of whom will leave the works in the course of a few days. The changes that will be brought about by the cessation of the work are an impressive indication of the general magnitude of the undertaking. Two very large Public Works settlements, one at Khandallah and the other at Glenside. between Johnsonville and Tawa Flat, will gradually be abandoned. “Lucky Job All Through.” A “Dominion” representative was conducted through the, nearly completed large tunnel by one of the two works overseers, Mr. J. Cameron, who has possibly the longest record in tunnelling of anyone in New Zealand, as he has been engaged in it off and on for the last 30 years. In that time he has worked in 26 tunnels, including the Otira tunnel, where he was an overseer. He pointed out the great difference made to modern tunnelling by the use of powerful electric and compressed air machines, saying that in the old days at least ,15 years would be required to build a tunnel such as the present longer one of the two. “This is a big difference from the horses we used to use,” he remarked, pointing to a squat, heavy little “bull,” or battery-driven traction engine, 18 of which have been used during construction. Unfortunately large undertakings such as this can rarely be completed without some loss of life. “This has been a lucky job all through,” said another workman laconically; “only two have been killed. In the Otira tunnel 15 were killed, I believe, and in one of the big Italian jobs once there were supposed to be 700. We have been lucky.”