PROBLEMS OF ARAPUNI.

New Zealand Herald, Volume LXI, Issue 18773, 29 July 1924, Page 9

PROBLEMS OF ARAPUNI.

HARNESSING THE WAIKATO.

DISTRICT ENGINEER EXPLAINS

FEATURES OF THE CONTRACT. BEST WORKMANSHIP ESSENTIAL. Some problems concerning the development of hydro-electric power from the Waikato River at Arapuni were discussed informatively by Mr. A. J. Baker, District Public Works Engineer, in the course of a, lecture last evening at the School of Engineering, under the auspices of the Auckland University College Engineering Society. JTh-ere Was a crowded attendance of members and others, including several civil engineers, interested in progressive work. Mr. R. •M- Webber, student chairman, presided. Before the lecture was started tribute was paid to the memory of the late Mr. Lawrence Birks, chief electrical engineer of the Public Works Department, whose death evoked expressions of keen regret. The Waikato River stood out as the most interesting stream in the North Island, said Mr. Baker, because of the geological histqry of its basin, its potential value as a waterway, and especially because of its value as a source of hydroelectric power. Its capacity at the maximum development of the three bestknown schemes in the iWaika<to was 283,000 horse-power, made up of 13,000 horse-power from Horahora, 150,000 from Arapuni, and 120,000 from the Kuka Falls. This, of course, did not represent the full possible development, as there are various sources of supply between Lake Taupo and Horahora. Tha Provision of Transport. The Waikato owed its great value for hydro-electric purposes to the immense storage and regulating function of Lake Taupo, with an area of 238 square miles and a catchment basin above it of 995 square miles. The greater part of the Taupo basin was overlain with rhyolite and pumiceous siands, these absorbing most of the rainfall and forming a high underground, storage. In Consequence, the small streams feeding into the Waikato had a remarkably even flow and as a result the maximum and (minimum discharges of the Waikato at Arapuni displayed relatively small variation. This facilitated big development without artificial storage. The project of development, for which tenders had been accepted by the Government, provided for the ultimate installation of eight 15,000 k.w. units operating under a normal static head of 175 feet. The provision of necessary roads and bridging access, however, involved a great deal of preparatory work. It had been estimated, continued the lecturer, that the transport of material from the railhead at Putaruru to Arapuni, a distance of nine and threequarter miles, would be equivalent to 40 tons a ; day over a period of four years. It was 1 interesting to note, for example, that 30,000 tons of cement were to be transported to the works; and it was anticipated that fairly general use would be made of steam traction trains. Careful consideration had been given to the system of haulage. A choice had to be made between a road or a railway. Estimated costs were taken out, and an analysis showed that haulage by railway would have incurred a unit cost of 2s 6|d §er ton mile for the construction material uring the term of tho contract, a unit cost of a fraction over Is lid per ton mile by a concrete road, and nearly is 7d per ton mile by a macadam road. Features of the Efeadworks.' The estimated cost (£62,000) for a concrete road was high, and as a concrete road would be considerably in advance of the district's needs, once the contract was completed, it was decided to construct a macadam road, for which - a sum of £8000 only was written off, this allowing for a rather more elaborate specification in respect to widths than the district otherwise would need. The width of formation was 22ft., and that of tho metalling 16ft. by lOin. Reference was made in detail to access bridges, these being designed to carry; if necessary, a "W." class locomotive. The bridge over the Waikato had interesting features. It has a deck span of 120 ft. over a narrow gorge where the river is 40ft. deep, and running swiftly. At the head race bridge piers had had to be sunk at great depth. The ground was not good, being largely running sand. The foundation shafts had to be timbered all the way down. The construction of this work was practically completed. In discussing the features of the Arapuni headwords, 'Mr. Baker that the rock formation in which the Waikato River gorge at Arapuni was cut was a volcanic tuff consisting of fragmentary material ejected from the volcanic vents. A very elaborate series of test shafts and galleries were made at the dam site. The rock was not firstclass material for building a dam of the size projected. Engineering and geological investigations of the site, however, were most thorough, and it was decided to build an arched dam with gravity cross section. To ensure watertightness the upstream face of the dam was to be of a special grade of concrete with a thickness varying from 2ft. to sft., and the contractor had been given option of using waterproofing material if he so desired. The onus of getting a watertight job was, of course, on the contractor.

Control of the River. The specifications provided for testing the works by raising the lake level from time to time.

One of the most interesting, as it was the most difficult, parts of the development of hydro-electric power at Arapuni was the control of the Waikato River during the progress of the constructional works. The normal flow could be taken at 7000 cubic feet per second, and tho ordinary flood flow at about 14,000 cubic feet. Floods of greater volume were infrequent, perhaps once in every few years. The river's flow in the flood of 1910 was up to 30,000 cubic feet per second.

The contract provided for the construction of one or more diversion tunnels of a total cross sectional area of at least equivalent to a diameter of 25ft. Excavations for the dam were to be protected by auxiliary dams constructed just bolow the intake of the diversion tunnel and above its outlet. With the diversion tunnel on a grade of 1 in 100, its capacity would be approximately 14,000 cubic feet per second. Construction of the tunnel would not provide any special difficulty. The concrete lining was to be two feet thick. Special care would have to be taken with the grade of concrete as the lining had to resist the scouring effect of water at a great velocity. Special Gates to Segulate Flow. Special gates had to be provided fpfoi* testing the dam structure during the progress of the work and had to be capable, if necessary, of emptying the lake. They also would have to regulate the flow of water while the lake was filling in order to keep Horahora running, the lake taking from 14 to 20 days to fill. It was clear that workmanship in the tunnel would need to be extraordinarily good. As regards head race and powerhouse there was nothing possessing very great or unusual difficulty. Steam or electrically operated shovels would be utilised for excavating 106,000 yds. of earth and rock out of the old river channel. The lecturer showed several plan maps and lantern slides illustrative of the preliminary and projected works, including the establishment of a model village for the workmen.