ARAPUNI POWER-HOUSE.

Otago Witness, Issue 3803, 1 February 1927, Page 6

ARAPUNI POWER-HOUSE.

ENCROACHMENT OE RIVER.

AUCKLAND, January 27. Though Arapuni has known many difficulties and discouragements, determination and engineering skill have rendered none of them insuperable, but at the powerhouse site unforeseen, hindrances have arisen, to disperse which neither human grit nor mechanical ingenuity has proved effectual. Last October the excavations which had been made below the river'-level for the foundations of the powerhouse were flooded and except for a brief interval they have been under water ever since. The excavations have still to go down another 10ft or so before concreting can commence, and no secret is made of the seriousness of the setback. For nearly four months construction work at the powerhouse site has been, at a standstill, all efforts being concentrated upon the seemingly hopeless task of keeping out the river. Whether a fresh attack now being made on the problem will meet with success is a matter of speculation. Those on the ground are optimistic of the chances, but even trie dengineers admit that the outcome of the experiment is problematic. The trouble arose during the great flood from October 10 to 12, the worst experienced since the contract was started in August, 1924. According to specifications and conditions drawn up by the New Zealand Government the contractors are called upon to provide against floods of 14,000 cusecs, but on October 12 the river was carrying 24,000 cusecs, or nearly twice that volume of water, and the flow , did not recede below the 14.500 cusecs mark until nine days later. The result was that the river completely overflowed the coffer dam, which had been built to keen out the water from the excavations. The foundations which had been sunk were submerged, and the river flooded the outlets from the penstock funnels. When the river subsided every effort was devoted to the task of pumping the water out, and by November the centrifugal sinking pumps had so effectually accomplished their purpose that the work of excavation was at last renewed. Then came the crowning disappointment of a long-continued spell of bad luck. On December 15 the “country” beneath the coffer dam, which had never proved entirely efficacious owing to the weakness of the subterranean strata, developed a fault. In five minutes the water had torn a large opening and in two hours the excavations were completely flooded. An attempt to block the opening and pump out the water succeeded in lowering the level by sft, but the breach was too large and the water rose again. The problem faced was disheartening and it was evident that the plan so far adopted of building the coffer dam of sacks and concrete placed in position under the water by a diver was. useless as long as the water could penetrate under it.

According to test made bv the Public Works Department and Sir W. G. Armstrong Whitworth and company’s engineers before the .work was started ' the “country” at this point was composed of solid rock, which would act effectively as a barrier against the pressure of the river. This estimate unfortunately has never been realised, and it is perfectly clear that while the initial bores sunk at this spot accurately registered rock they failed to indicate that it consisted only of boulders, through the intersectices of which the wate- would find an entrance. During the past few days a small gang of picked m<,n has been sinking piles of sheet steel furnished with an interlocking device that renders them very nearly watertight. These steel plates, about 12 inches wide and varying from 18ft to 22ft long, are being driven verticallv into shingle and boulders by a piling hammer operated by comnressed air. This machine, which has been lent to the contractors by the Public Works Department. performs its work efficiently and speedily, so that the completion of the steel wall which is to keep back the water is now in sight. When ready, the pumps will be started once more, and the efficacy . of the new plan will be put to the test. That the setback at the powerhouse site will mean delay in the completion of the main scheme there can be no question. Weak “country” and floods have brought their attendant delays and losses ever since the work started. On eight separate occasions the river has been flooded to greater capacity than the 14,000 cusecs which had been allowed for, and no fewer than five of those occasions occurred during the past year. ’ The three-year contract expires next August, but it is clear that an extension of time will have to he asked for a matter perhaps of six months. Even then there would be a clear margin to April 1, 1928, the date on which the Government has agreed to supply, the Auckland Power Board at Penrose substation. The stoppage of work at the power house site has, however, resulted in the speedingup .of work in progress .at Spilway weir, which is completed with the exception of one section which has been left temporarily to provide an opening for spoil trucks. A heavy iron rail is to run round the crest and provision will, be made for stop logs to be fitted, closing any section of the weir at will, should it be necessary to effect repairs. To prevent damage by suction a huge concrete slab is being laid on the sluice bed on the down-stream side. a