POWER SUPPLY

Otago Daily Times, Issue 22628, 20 July 1935, Page 14

POWER SUPPLY

NEW TUNNEL AT WAIPORI REPLACEMENT OF PIPE LINE The undertaking upon which the City Council has embarked at Waipori, in following out its scheme of replacing the existing pipeline with a tunnel over 2700 feet in length, is one of greater magnitude than the casual reading of technical reports may have suggested to the consumers of the city’s supply of electric power. Although the contractors to whom the work has been entrusted began operations in May of last year, it will still be more than six months before they are able to announce the completion of their task.

At Lake Mahinerangi, which conserves the vast quantity of water required to keep the power-producing plant operating at full strength, one of the' stations is situated, but the water which supplies the lower powerhouse, which is the main source of the city’s electric energy, has to be conveyed a considerable distance

Illustrations showing the progress of the work on the new tunnel at Waipori will be found on page 5.

before it reaches the point at which it is used. At present it is picked up from the Waipori River below the dam and travels through a tunnel 4700 feet long before it reaches a surge tank, where it joins the pipelines that carry it the remainder of ths distance to the turbines. DANGEROUS EARTH MOVEMENT So vital a part of the power scheme do the pipes form that, were they to become inoperative, Dunedin’s main source of power would be cut off, and some alarm was caused when five years ago, owing to an earth movement, all of the pipes slipped nearly six inches down the steep hillside on which they are laid. As the occurrence constituted a serious threat to the safety of the existing scheme, the City Council decided that the pipes should be replaced by a pressure tunnel through the hill, driven at so great a depth that it would be in no danger of being affected by the earth movement above. ’ It is upon this undertaking that workmen have been engaged for several months past, and they have progressed so far with their task that, of the total length of over 2700 ft of tunnel which has to be driven, only 300 ft remains to be completed. The contract, however, also includes the laying in the tunnel of pipes 6ft 9in in diameter and the surrounding of them with a layer of concrete Ift thick. The driving of the remainder of the tunnel, it is expected, will be completed about the middle of September, and a further six months will be required for the carrying out of the remainder of the contract. In the new scheme it has been possible to use a portion of the existing tunnel, and at a point in its course it will be joined by a new tunnel 650 ft in length, providing a connection with a new surge chamber, which has already been constructed. At the surge chamber the main tunnel, which will be 2100 ft in length, begins its descent to the power station. This tunnel, which is at three different levels, has a total fall of 750 ft. The surge chamber, which is an important central point of the new scheme, is also one of its most interesting features. Shaped like a wine glass and 65ft in depth, it has a dual purpose, for, not only will it provide a safety valve for the backward pressure of water when the supply is turned off at the power house, but it will also provide a temporary source of water to meet any sudden increase which may be desired at the station. DIFFICULTY ENCOUNTERED The council has had the misfortune in carrying out the scheme to encounter an obstacle which has figured largely in iss recent discussions. The early investigations indicated that the whole of the tunnel would be driven through solid rock, but the actual experience of the contractors has been that soft material has been encountered over a considerable portion of the course of the tunnel. While timbering has been necessary, inconvenience has also been caused by a seepage of water from above. The council has been assured, however, that the difficulties which have been encountered will in no way impair the ultimate stability of the scheme, the completion of which should create in the minds of those controlling the destiny of Waipori a feeling of greater security as well as confidence in the efficiency of the enterprise.