NOVEL PROBLEMS

Poverty Bay Herald, Volume LXVI, Issue 19881, 7 March 1939, Page 4

NOVEL PROBLEMS

TUAI TUNNEL SYPHON NO. 3 POWER SCHEME

TAPPER’S ELAT STATION EXTENSIVE WORK IN HAND (Herald Special Reporter). Engineers of the Public Works Department responsible for the new power-production scheme in the neighbourhood of Tuai are facing problems not previously tackled in the history of public works development in New Zealand.

It is a truism that every major item in any works programme has its own particular problems, but in the inverted-syphon section of the tunnel to be excavated between Tuai and Tapper’s Flat, the department has projected a scheme not merely novel for New Zealand, but possibly without parallel anywhere in the Empire.

Inverted syphons are to be found in various parts of the world, -but nearly all of those constructed to date have been made on the “cut-and-cove.r” system, the concrete structure being laid in an open cut and then covered again with the spoil from the cut. At Tua.i, the syphon is to be made as a tunnel, the lining being kept as close as possible to the faces. Special concreting technique is being evolved to deal with this particular phase. •

It is not many months since the Minister of Public Works, the Hon. R. 'Semple, announced that the Government proposed to spend £1,000,000 in the addition of a new power station in the valley of the Waikaretaheke, to double the power output at present available from Waikaremoana. The speed with which preliminary work has been undertaken was one of the chief impressions left on members of the Poverty Bay Electric-Power Board who yesterday visited Tuai, and traversed the ground on which the scheme will -be laid out.

Access Roads to Tunnel Face

There are at present on the payroll over 150 men, engaged on different features of the scheme. Their activities are spread over two miles of country, and include road-mak-ing to the points at which different faces of the tunnel have been opened up, excavations at the tunnel faces themselves, and the preparation of the site for the new power-station at Tapper’s Flat. Of considerable interest in itself is the preparation for diverting the Waikaretaheke Stream across what is known as the football ground at Tappers Flat, to leave space for the new power-house over the boulders of the old bed.

The diversion of the stream is nearly a quarter of a mile in length,' and one of the integral jobs is the building of a bridge across the diversion channel.

Lower down the channel, a weir is being constructed to break the force of the current, which otherwise would achieve a rate capable of scouring the. banks of the cut.

Storage Lake at Tuai

The main features of the new scheme comprise a storage lake in front of Tuai township, covering an area of 80 acres, and with an altitude of ‘9o2£ft. above sea-level; a tunnel 10,000 ft. in length leading to a surge-chamber above Tapper’s Flat; twin steel pipelines comprising the pen-stocks which will lead the water down to the turbines 400 ft. below the surge-chamber, and an installation capable of producing 40,000 kilowatts.

By way of preparation for these works, the Public Works Department has established two main camps, the chief of which is at Piripaua, near the new power-house site, and laid two and a-half miles of metalled road serving the camps and main work localities, and 105 chains of subsidiary road giving access to tunnel faces and quarry-site.

The scheme of construction should be complete, it is estimated, late in 1940, when the 40,000 kilowatts from the new station will be used in meeting increases in the demand for , national power. Already the mainstations of the North Island are working to full capacity, and the extension of Waikaremoana output will not leave any margin, for further load-development toy the time the works are finished. Speed of construction is the essence of the scheme, therefore, and no time is being lost in the initial stages. Aqueduct Project Negatived

The visitors yesterday were shown the locality in which the syphon section of the conducting tunnel is to be constructed, this being a stretch of marshy ground in a gully which cuts through the subsidiary range of hills flanking the Waikaretaheke Stream. Borings in this locality have shown that the ground is .unstable to a depth Of between 80 and 100 ft., and the' level of the tunnel is to be lowered 153 ft. to ensure that it will rest in stable country. The side sections of the syphon will incline at approximately 45 degrees, and the bottom section will have a length of 185 ft., constructed horizontally. The diameter of the tunnel is tb he ,16ft., and in the 615 ft. length which inclines the syphon the construction will foe particularly heavy. The cost likewise will he substantial, but the choice of this method of construction was determined by the fact that to carry the water on an aqueduct would involve a considerably greater outlay, and also would render the work susceptible to serious damage by subsidence in the marshy ground. Another feature of the scheme to which attention has not been directed previously is the construction of a canal which will lead the waters of the Waikaretaheke into the storage lake at Tuai. This canal will not carry a heavy volume b£ water in normal times, when the greater part of the outflow trapped at Lake Kaitawa will be passing through the turbines at the Tuai power-house, and thence into the storage lake below it. If it should become necessary to shut dawn the Tuai power-house, however, water will not flow through the pipe-line above that station, and instead will escape through the bed c: 1 ’ the Waikaretaheke. The new canal will pick up this flow at a lower point, and divert it to the Tuai storage lake, so that the station at Tapper’s Flat will be able to work independently. The length of the 1 canal will toe substantial, and will include a section on which a syphoning system l prbbabjv will be used.