CONCRETE RAMPARTS

Sun (Auckland), Volume III, Issue 769, 16 September 1929, Page 1

CONCRETE RAMPARTS

PROTECTION AT ARAPUNI ENGINEERING FEAT IN ITSELF SOME INSIDE HISTORY Concern over the recent erosion below the spillway at Arapuni has been followed by a recurrence of activity on the job. Once again the echoes of toil ring in the dark vault of the diversion tunnel, and at the power house site the work in progress on Nos. 7 and 8 penstock tunnels adds to the general bustle of structural work on the building. At the spillway overflow, on which so much anxiety has been centred, there is yet little activity. A gang of men is forming a road from the transmission station to the falls, and across the overflow channel another gang is preparing access to a sand pit. Both these jobs are minor preliminaries to the task of checking the erosion with ramparts of concrete. No definite scheme can be prepared until the channel below the spillway is dried out. The banks have, of course, been tested again and again, but beneath the seething currents with which visitors are familiar may lie the secret of further impending erosion. That which has already occurred is not only a general retreat of the lip of the main fall, but also a wholesale collapse of the rock structure in the neighbourhood. Ou the spot where excursionists formerly stood to watch the plunge of the waters there is now just a yawning gulf, surrounded by high bluffs. THE DOUBLE WATERFALL From Its present appearance the waterfall is unlikely to maintain its present form for long. Its second leap takes riotous cross-currents into a throat of rock. On either side the pillars appear to be of insecure formation. Obviously any scheme of protective work will involve concrete work ou a most massive scale—an engineering feat in itself. How such a mass will be shaped and anchored is a problem the engineers have yet to determine. Its design must provide the easiest possible slope for the escaping water, and with perhaps at the foot some large-scale “sump” arrangement for breaking the force of the currents. Above the fall the overflow channel may be paved in concrete all the way to the spillway weir. Though this part of the work will be comparatively simple, ; the total cost will undoubtedly be heavy. While confident of their ability to master their difficul ties, the engineers by no means make light of them. The public will appreciate the quiet confidence of the men directing operations at Arapuni, and in view of the success which has attended the powerhouse part of the project will be prepared to rely on their judgment. But there are other factors which cannot be overlooked. Perhaps they will be discussed when the inside history of Arapuni is written—if ever it is written. Many engineers—and they are not all outside the Public Works Department—consider that the original plan should have embodied some sort of protection for the spillway overflow channel, and that It was courting trouble to turn the river over the spillway into uncertain country. The folly of doing so was realised by Armstrong. Whitworth’s, Ltd., whose engineers advised the desirability of constructing extra penstock tunnels — such as those now being constructed —as escape channels in case of need. They also foresaw the difficulties that would arise in clearing spoil from the extra penstock tunnels when road access thereto was blocked by the power house and spoil could not be dumped into the riverbed. To overcome this difficulty the department is »w erecting an aerial tramway arrangement to carry spoil from the gorge to the top of the eastern bank. In the meantime the spoil Is going into the riverbed, but not for long. Armstrong, Whitworth’s also saw that if the diversion tunnel wah to be used again protection near the gates was desirable. They actually secured quotations of cost, and submitted them to the department, but their recommendations were ignored. Yet this is the very work the department is being forced to execute under difficult conditions today. A DIVER ON THE JOB The scenes of activity in the gorge contrast with the tranquility of the lake and headrace. Jußt now only No. 3 penstock tunnel is taking water full bore. Above its intake in the forebay there is an ominous swirl, the visible evidence of the strong descending current. No. 2 tunnel has been partly open—its corresponding turbine, almost ready for action, is running for drying out purposes. No. X tunnel was used to relieve pressure below the spillway for a time when it was feared that the serious erosion might continue without pause. Since the tailrace outside the foot of No. 1 tunnel has not been completed yet, the bank alongside the power house had to be protected while No. 1 was open. For this purpose the old sheet piling used by Armstrong, Whitworth’s to check flooding at the power house founds;iocs was utilised. The heavy sheets of metal were laid on the slope of the batik beneath the j water, and despite their weight were visibly agitated by .be force of the j current while the tunnel was open. Last week, having served their per- | pose, they were hoisted out b. a I crane, a diver going down to hitch i up the plates to the hook. | Good progress is being made wi‘h | No. 7 and 8 penstock tunnels, and the i ur.nellers are making up to £2 a day. i Entrance to the working faces is s»i cured from a narrow drive into the . face of the gorge just above the pewerj house. A branch drive leads off to i No. 8, and No, 7 tunnel opens out l ahead. The spaciousness of the tuni nel proper is in contrast to the aar- ; row apertures by which It is reached, j These works and the protection of the - diversion tunnel will be described ip. i a later article.