NEW TUNNELLING METHOD

Press, Volume LXXVI, Issue 23053, 22 June 1940, Page 10

NEW TUNNELLING METHOD

STEEL SHIELDS INSTEAD OF TIMBER TEKAPO HYDRO-ELECTRIC SCHEME [From Our Parliamentary Reporter.] WELLINGTON. June 21. A method of tunnelling new to New Zealand but yvhich has been successfully used in England and the United States will' be adopted for the driving of the 6000 ft tunnel through the glacial moraine to the hydro-electric station to be built at Tekapo. Instead of timbering, steel shields will be used Describing the method, the Minister for Public Vvorks (the Hon. R. Semple), in an interview to-day,’ said that because of the ■' nature of the ground, the amount of timber required to maintain the weight, and the danger to the tunnellers, it had been decided to use hydraulically-operated steel shields. One will be used at each end of the tunnel. They will be moved by 25 100-ton capacity rams in each snield to -overcome friction and other forces. The material will be excavated in the shield from steel safety platforms and elevated by belt conveyors to a central conveyor which would discharge from overhead into trucks. A steady stream of material will be leaving the shields. The nature of the material was such that it could be broken away very fast by pneumatic picks and similar equipment. Very little explosive will be required and the tunnellers will be working at all times in perfect safety, protected in all directions by the heavy steel shields. The lining of the tunnel, the Minister said, will be ‘ effected by assembling precast reinforced concrete blocks, curved to fit the tunnel, inside the end of the shield, so that when the shield advanced away from the blocks the tunnel would be left completely lined with concrete and safe from any falls of rock. The finished lining of reinforced concrete will be applied to the inside of the blocks to make a smooth surface for the water to pass over. “This will be one of the few tunnels in the world to be driven through glacial moraine, and this is an excellent opportunity to utilise this greatlyimproved method of tunnelling,” said Mr Semple. “The shields will be made in the Temuka workshops of the Public Works Department, most of the material being available already, and the rams? will be made in the Dominion by private firms.” The adoption of this method made not only for safety and speed, but a lower cost of tunnel a foot. The shield method was first used in the construction of the London railway tubes and in recent times had been used in all classes of ground, including broken rock. The application of the shield to the Tekapo tunnel will follow the lines employed in the driving of the tunnel under the Hudson river at New York, where rock and coarse gravel, as well as soft mud and ooze, were encountered.