PROGRESS AT TEKAPO

Otago Daily Times, Issue 26747, 16 April 1948, Page 4

PROGRESS AT TEKAPO

Unusual Tunnel Construction

Incorporating that have not >een attempted in New Zealand before, the Tekapo scheme is as interesting as the Pukaki project, and, in addition to providing a great storage basin for the Waitaki River stations, will actually generate 25,000 kilowatts at its own powerhouse. There will be a tunnel 5000 ft in length taking the waters of the lake to the power-house, and on the river will be a dam to build up the level of the lake for release as required in winter.

Three hundred and fifty men are employed at Tekapo under the direction of a Dunedin-born engineer, Mr R. F. Deans Ritchie. Their main energies at present are directed towards constructing the tunnel, the power-house and the tail-race, and the dam will follow when the waters of the lake are controlled through the tunnel. Four big camps house the workers on this scheme, which will not be completed for at least two years. It is hoped that the turbine will be turning by the winter of 1950, but numerous difficulties may yet be encountered on a job of this nature. Giant Pastry-cutter The most interesting feature of the Tekapo scheme is the construction of the tunnel, for an overseas method used for driving through soft materials is being employed to pierce a typical New Zealand hillside. Actually, two tunnels are being driven — one from the north or lake side and the other from the south. They are expected to meet about October. At present the southern tunnel has been pushed .through 1850 ft, and the northern one 1500 ft. The method of construction is as if a great pastry-cutter were being pushed through a hill by a giant hand. Actually, hydraulic jacks, electrically driven, are forcing forward inch by inch a great steel shield, enabling concrete, rings to be bolted into position behind. Four or five of these 3ft rings are being placed in position each day. When the tunnel is pierced the rings will be surfaced to an almost glass-like finish to prevent the water passing through the tunnel from causing any erosion. Two small locomotives and rakes of trucks cope with the spoil. At the outlet of the tunnel will be a surge tank to control the water passing through the penstock to the. power-house. At the intake will be control gates, but, before the water can pass through, sheet piling, backed by spoil, will be driven into the bed of the lake so that the entrance can be cleared. Erection of Power-house Now that the major difficulties encountered in the tunnelling work have been overcome, and a comparatively fast rate of progress has been attained, the erection of the power-house is being undertaken. At present the base of the power-house is being prepared on the floor of a great hole 60 feet deep. Two feet of concrete is being spread to provide a flat surface for the main foundation of heavily-rein-forced concrete. The area of the power-house will be 100 feet by 50 feet and the reason for the deep excavation of the site is to get the maximum head from the tunnel, which will take a flow of 4000 cusecs. A 20-foot steel pipe will- bring the water from the surge chamber to the power-house. A draught tube will control it after its

work is over, and it will then pass on to the river for further use at the Waitaki station. Seepage has presented some difficulties in preparing the power-house site. The floor of the hole is 40 feet below the river and a pump is used to remove the water at the rate of 500 gallons a minute. When the generating scheme is in operation and the water can be controlled through the tunnel, the construction of the dam near the outlet of the lake will he commenced. This dam will raise the level of the lake 15 feet and through the tunnel the level could be drawn down a further 10 feet. This will give a fluctuation of 25 feet in tiie control of the lake for the supply of water to the Waitaki scheme during winter months.