Three Possible Explanations Of Arapuni Trouble Given.

Star (Christchurch), Issue 19107, 27 June 1930, Page 4

Three Possible Explanations Of Arapuni Trouble Given.

REPORT FROM THE GEOLOGISTS IS BROUGHT BEFORE PARLIAMENT.

Per Press Association. WELLINGTON, June 26. In a statement in the House on Arapuni, the Hon W. B. Taverner, Minister of Public Works, traced the history of the trouble from the time cracks first developed, and added that it was apparent almost from the first that extensive w*ork would be necessary to make gcfod the position, and that the plant would of necessity have to be shut down for a considerable period. Geological Investigation. The Director of the Geological Survey branch of the Scientific and Industrial Research Department proceeded to Arapuni immediately after the trouble occurred, and after further examination when the head race was laid bare he consulted with the head of his Department, and the Minister in charge of that Department agreed that a technical committee consisting ot the head of the Department (Dr Marsden), Dr Henderson (Director of the Geological Survey branch). Dr Marshall (a member of the staff and at one time professor of geology at Otago University), and Professor Bartrum (Professor of Geology at Auckland University ) should further examine the position and make a joint report. This they have done. A summary of it is as follows: “A block of country near the penstock tunnels between the spillway and power-house broke away at 1.30 am. on the /th instant, developing a crack extending along the front of the penstock tunnels, and spillway. Water poured into this crack, developing hydros! a tic pressure which bent the block, including the power-house, progressively over towards the river. This bending extended over two days, while the base of the cracks filled up with grit, allowing fuller development of pressure. It is evident that the block moved about a base veil below the river level, i.e., some 300 feet below the penstock head. After the diversion tunnel at the dam was opened and the level of the water in the penstock head began to recede, the block moved slowly back by about three-quarters of its original displacement, and the leaks were reduced in greater proportion. The Origin of the Trouble. “It appears evident that the movement of the crack after its original formation can be accounted for by the hydrostatic pressure of the water entering the cracks, and the Question naturally arises as to the origin of the original crack and whether or not further disturbances are likely to occur. Several alternative explanations may be advanced to account for the original rupture. The whole block of country may have been in tension from natural causes and the extra excavation for the new power house, together with hydrostatic thrusts in the penstock head, may have just caused the limit of resistance to be overcome. This seems the more likely explanation. Alternatively it is possible that water leaked into deep vertical joints below the penstock head developing sufficient pressure to cause rupture. No evidence, however, can be found for such joints in the absence of deep borings. A third possible explanation is

stress caused bv a possible earthquake wave, which, owing to the fact that the power house and penstock head were rigidly tied, might oossibly cause such a rupture Against this is lack of observation of any such tremor. Tt is true that the initial movement of the block caused tremors and vibrations of the block, which were observed by a man living in a hut near the penstock head.

“ These cracks in the power house, however, do not necessarily, indicate disturbance of the power house foundations, since they consist of a transverse crack of the whole building at a point where its width changes suddenly, and cracks round the supports of the heavy generators and turbines (350 tons). The rock surfaces visible indicate no movement of the rock masses relative to one another. The whole occurrence might well be unforeseen and due to an unexpected weakness ol the support of the whole block, which has moved. This supporting material is only just strong enough to withstand the weight of the overlying rock without failure.

“ Further buttressing of the whole block would appear necessary to prevent recurrence of the movement and leakages of water from the forebay.” The geologists are of opinion that there was no undue water percolation from the spillway into the block of rock between the spillway and the gorge and power house previous to the formation of the crack. Thus the initial movement is unlikely to be due to this cause. “ Meantime I wish to make the following observations. The amount already invested in the Arapuni scheme, including substations and transmission lines and works in progress, is £2,500,000. In the previous year, before Arapuni came into operation, working charges for the Waikato system amounted to £76,231 and standing charges for interest and depreciation and renewals were £74,571, or a total of £150,802. For the year ended March 31 last the respective figures were £62,702 and £195,752, altogether £258,454, so that the difference, amounting to J £107,652, represented the additional total operation costs of Arapuni for the year. Units sold for the year from the whole Waikato system were 191,000,000, and the net revenue, after paying working expenses of £72,702, was £162,981. The average operating capital for the year was £2,649.000, so that the net revenue over and above working expenses was 6.15 per cent on operating capital, notwithstanding that the plant installed was insufficient to take the whole available load during the earlier months of the year. The present installed capacity of Arapuni station is 45,000 k.w., and consists of three units A fourth unit is now in process of shipment from England, and, had the course of events not been interrupted, would have been installed on arrival, thus bringing the total capacity of the Arapuni plant to 60,000 k.w. In a subsequent. statement details will be given showing the cost involved in calling into operation standby plants and other stations which were required to take the load when the -supply ceased from Arapuni, but it may be remarked that the estimate at present for generating costs at those stations r is at the rate of £85,000 per annum, as against £12,000 for generating costs had Arapuni continued in commission.”