ARAPUNI AGAIN CAUSING CONCERN RELINING DIVERSION TUNNEL AT ARAPUNI

Sun (Auckland), Volume III, Issue 746, 20 August 1929, Page 1

ARAPUNI AGAIN CAUSING CONCERN

RELINING DIVERSION TUNNEL AT ARAPUNI

MENACE TO SPILLWAY Gangs Work Day and Night EROSION ON NEW COURSE Diversion of Water Necessary CONTINUED erosion below the spillway at Arapuni has forced the Public Works Department staff to take unexpected steps. Two extra penstock tunnels are being driven, and the diversion tunnel lined with steel. Men are working night and day on these jobs, which will allow the pressure in the dam and headrace to be eased.

wiHE present trouble has nothing to L do with the stability of the dam, but is a development of the spectacular erosion which began when the river was diverted over the spillway at yew Year, 1928. At that time the river cut a remarkable new course before it joined its old bed. Racing for three hundred Yards below the spillway in a series of turbulent rapids, it then plunged orer a rock outcrop 70 feet high and eddied in a large basin in which the trees of an ancient buried forest were eiposed. , , Hopes that the new course would be permanent, and that no further erosion would occur, have not been fulfilled. In the past month or two the erosion of the bed immediately below the spillway has taken serious form. One result is a general lowering of the bed below the spillway. Were this process to continue, the rock formation on which the spillway is built might be undermined, and the stability of that important structure threatened.

Further erosion has also occurred it the famous “Arapuni fall,” where the waters hurtle into the basin 300 yards below the spillway. Erosion hern was long checked by the firm nature of the big rock outcrop. But the immense force of the river playing against this rock has recently cut at least half of it away. Should this rock prove to be an isolated boulder, further erosion might cause the brink of the fall to recede, and accelerate the erosion between this point and the spillway. To meet the position drastic steps are being taken. It is possible that when present preparations have been completed no more water will be allowed to flow over the spillway until the part now being eroded has been protected by heavy concrete flooring. The preparations now in hand constat of:— 1. Excavation of two penstock tunnels, numbers seven and eight, which will take water from the head-race direct into the gorge above the power-house. 2. Lining of the diversion tunnel under the main dam. This tunnel was formed as a temporary expedient to allow the foundations of the (lam to be excavated in the riverbed. It was never designed to stand repeated use. Consequently steel lining has been considered advisable before the river is again run through it. The diversion tunnel is at present fluaed by means of huge gates operated by hydraulic winding gear at the top of a shaft nearly 200 ft deep. When the gates were last closed it was hoped that never again would they have to be opened. The process of opening them takes only a few hours, but during that time the whole Pressure of the enormous head of water in the dam will surge against their lower edges.

NEW PENSTOCK TUNNELS In the Armstrong Whitworth contract three penstock tunnels had to be driven. These were duly completed, and one of them, No. 3, is now tarrying water to the one turbine which is so far operating. The Power house has only capacity for four turbines in all, the idea being t*-at the four extra turbines ■which represent full development of the scheme could be built In later years when the demand warranted it. Before those turbines can be installed an extension of the powerhouse will be necessary. This means that number 1 and 8 penstocks, now being driven a:i an emergency measure, will simply open into the wall of the gorge, and ““t into any prepared site, as with three so far driven. Two months ago the Public Works department decided that the growing ueniand for power in the Auckland irovince, warranted the installation of , e fourth turbine. Tenders were

therefore called for the necessary generating gear, and the tender of A. P Paterson and Company, represent--1:1* a Swedish firm, was accepted. Work was about to start on the Penstock tunnel through which water ! _°"ld be delivered to this turbine, j . ben the present emergency arose and ! s was decided that, though they will | r,f rV ° 00 t ur bi nfl for probably a period ( years, numbers seven and eight tun- ; ■-bis should have precedence. Three shift's a day are now being j worked in each of these tunnels. Work proceeds for the most part ,r om the down stream face, but owing to the restricted space, not jrany can be on the job at a time. * experience with the original three tunnels was any criterion, humbers seven and eight will have to Pierce solid rock. i'i the forebav, the portion of the •Mrace whence the water is fed into * tunnels, the original contract proided that apertures for tunnels five, r evpn and eight should be formed ■n the concrete walls, and everything bt the actual control gates placed in i°*ttion. Hence only the piercing of

the actual tunnels will be necessary. Each tunnel is lined with steel, and is circular, 12 feet in diameter. Below the penstock intakes is the spillway weir and its extension, which gives a total effective crest of 690 ft. Below the extension is a mat, or apron, of reinforced concrete designed to prevent erosion under the spillway. Over this apron the water rushes like a rapid—over the spillway weir it is a waterfall. It is stated that when the contractors started the construction of the spillway they were under the impression that the bed was of rock formation. Following upon the discovery of the real nature of the formation it was found necessary to extend the apron from the spillway down the bed of the river for a certain distance to prevent erosion under the spillway. An allowance for this extension was made to the contractors by the Government when a final settlement of the contract was made. To counteract this erosion of the river bed and ensure the safety of the spillway and maintenance of the power supply, it is considered imperative that this apron should be continued still further down the river. By putting ths old diversion tunnel into operation and diverting some of the water into the old bed of the river it will relieve the pressure over the spillway, or, in other words, dry the river below the spillway to enable the apron extension to be carried out. “GROWING SERIOUS?” QUESTION IN HOUSE NO DANGER TO VALLEY (The SUN'S Parliamentary Reporter) PARLIAMENT BLDGS., Today. Hearing of the seriousness of the developments at Arapuni, Mr. W. J. Jordan, member for Manukau, is to ask a Question in the House of Representatives this afternoon. The question is as follows: If there is any foundation for the report that the bed of the river below the spillway apron at Arapuni is being washed, so endangering the stability of the spillway, and whether the Minister of Public Works, the Hon. E. A. Ransom, will make a statement regarding the stability or otherwise of the Arapuni works. Approached by The Sun representative, Mr. Jordan said that he had heard in Auckland during the weekend, and subsequently, that there were unexpected developments at Arapuni, and that, although the Public Works officials knew of the erosion it had been rather more rapid than expected. The position Is stated to be growing serious. Since his return from Auckland Mr. Jordan has had more communications on the subject, showing that the position is growing serious.

Mr. Jordan has seen Mr. Ransom, Who said that there was no danger of the lower parts of the valley being flooded as a result of the developments. Mr. Furkert was up there, and he was investigating. The Minister said the matter was being attended to by his department, and admitted that there was erosion, Mr. Jordan said, which may have developed into serious proportions.

SIDELIGHTS ON SCHEME LINING FOR DIVERSION TUNNEL MR. RADONE EXPLAINS POSITION It is of interest to recall that before the contractors, Armstrong, Whitworth, Ltd., ceased their association with Arapuni, they expressed the view that the diversion tunnel should be steel lined in case it should again be needed. To such an extent did they stress this opinion that they actually secured quotes of the possible cost of j materia] required. However, the Department did not consider the work then necessary. The circumstances which have now i arisen to make protection for the tun- ! nel desirable have been predicted by I a number of competent observers, i Ever since the river commenced to run i over the spillway and into the soft j country immediately below, engineers and others have said it would be a : remarkable thing it erosion did not continue. When the spillway was built, a tremendous slab of concrete some feet in thickness was laid in the channel on the downstream side, so that the force of the waters flowing over the spillway, and breaking into a churning cauldron at its foot, would not eat into the bed at that point. This concrete has so far prevented erosion 1 immediately beneath the spillway, but

if the erosion lower down were permitted to continue, it would only be a matter of time, perhaps of months, perhaps of years, before the continued wear would eat back under the concrete crust. To make the protection effective, it will have to be extended in a solid mass down the channel. MISTIMED Had the completion of the powerhouse synchronised with the completion of the dam, there would not, of course, have been anything like the volume of water flowing over the spillway, and erosion would not have been so heavy. Great quantities of water would have been fed through Nos. 1, 2 and 3 tunnels to the turbines, and the pressure over the spillway and in the cham/l beneath would not have been so great. Before the protective works below

the spillway can be puf in hand, the channel will, of course, have to be

completely dried out. At the same time, a sufficient head will have to be kept in the forebay to keep number three turbine, the one now feeding to Auckland, in operation. Mr. T. Rabone, engineer in charge at Arapuni, said this morning that he anticipated-no serious difficulty in doing this. Mr. Rabone added that the works now in hand should not be interpreted as an indication that the success of the scheme was in any way threatened. The department was simply taking reasonable precautions in order that the station would yield faithful and uninterrupted service in years to come. In order to ease the pressure, it is possible that the diversion tunnel will be opened, or partly opened, as soon as the work of lining it is finished. The tunnel is on a curve, and when it is open the rush of water impinges on one side of it. When the tunnel was examined after the gates were last closed it was found that, as a result of this pressure, the concrete lining was showing signs of wear. To prevent such tendency when the tunnel is in use in future, steel is the only thing that would be effective. NO OVERFLOW With the diversion tunnel taking water through the dam and out into the old gorge, which except for a minor trickle has now been dry for many months, the pressure in the headrace and over the spillway will be lessened. When numbers seven and eight penstock tunnels are open and taking water, no water need flow over the spillway at all, as these and number three tunnel, plus the diversion tunnel outlet, should easily accommodate the full normal volume of the river. With numbers seven and eight tunnels open, and number four to be drilled as soon as possible in order to get the fourth turbine working, only numbers five and six will be left untouched. It is not likely that these will be wanted for many years. Before they can be utilised an extension of the power-house will be required.