BELOW THE ROAD.

Auckland Star, Volume LXVII, Issue 141, 16 June 1936, Page 9

BELOW THE ROAD.

TUNNELLING FEAT. ENGINEER'S INVENTION. WAITAKERE PIPELINE JOB. Picture a tunnelling job, in the way it used to be done. . . . Piles of timber, piles of earth, mud and slush, and men working laboriously by the aid of hurricane lamps while they fitted timbering and wooden arches, or put in the concrete mixture that was to form the final walls and arch. Then see it done as it is being carried out by a waterworks department team at the junction or Point Chevalier and Great North Roads. No timber is being used apart from runways and boardwalks, and the job is being carried forward cleanly, steadily and safely with a minimum of effort and a minimum of expense.

It is a new idea in tunnel building, the invention of the waterworks department field engineer, Mr. 13. L. Hamilton, and it is likely that the new method will be used extensively in the future in places where the soil will stay unsupported for a few hours. The job is part of the Waitakere pipe line reconstruction and deep tunnelling is required to pass under the tramlines at the Point Chevalier Road corner. The top of the tunnel is lift Gin below the level of the roadway, passing through sandy clay that under ordinary engineering construction methods would require to be heavily timbered. The tunnel is being carried forward under the road a distance of 150 feet, and then back a further distance of 210 feet under the* pathway of the Great North Road to connect with the new pipeline which is now being extended towards that point. Costs Reduced. The timber used in tunnelling has to be of the best and as it has to be left in the tunnel after it is completed it forms a heavy item of expenditure. It was this feature that turned Mr. Hamilton's attention towards tlie present scheme, which not only reduces the cost by about fifty per cent, but.quickens the

job and gives a better and cleaner result. As usually is the case, it is an exceedingly simple idea. The arches, the major feature of tunnelling work, are made 011 the ground level. Wooden arch patterns are used for this, the concrete being mixed and filled 011 the epot. It takes some days for each arch to harden properly, but they are then ready for insertion in the tunnel. The new idea is largely contained in the platform on which the arches are conveyed to the tunnel. It. is a wheeled truck with a bottle-jack set in the centre of it. The arch, with its wooden support is attached on top of this, and does not require to be touched again until it is set in position in the tuniiel. The truck is then run along a set of rails to the hoist, let down to the tunnel and wheeled along to the excavation. There it is hoisted into position with the aid of the jack, swung round and laid in position 011 the already formed concrete walls. The arches are 2ft Gin wide, and weigh approximately half a ton. and as placing them in position is the easiest part of the job normally, it can be understood what an advance the new method is.. Four arches are placed in position each day, and the excavation is unsupported for only 10 hours.

The irn-n In the gang work in three shifts, each of eight hours, and the tunnel is extended at the rate of 10ft a day. During the night the excavation work is done, the earth, as it is broken, being pnt in light trucks \vhich arc then run to the hoist, carried up, and deposited on a raised platform above, the hoist shaft. The earth can then be cleared easily into a truck which pulls in under the platform. The morning shift carries on with the concreting of the floor and 2ft walls of tlie new excavation. In the afternoon the arches are carried down to the excavation, and are placed in position in a period of about five minutes. The space above each arch is then packed with concrete. At the end of each day the 10ft section is completed. And .so the work is carried forward. For the men down below in the confined, hot area of the tunnel, above which the noise of passing tram oars and motor cars is heard as from another world, it is not easy work—as anyone who has worked in a stooping posture for any length of time will understand, but it is considerably easier and simpler than it was under the old process.