ARAPUNI WORKS.

Press, Volume LXIII, Issue 18911, 28 January 1927, Page 12

ARAPUNI WORKS.

ENGINEERS FACE HARD PROBLEM. WEAK COUNTRY BENEATH COFFER DAM. .( [THE PRESS Special Service]" AUCKLAND, January 27. Arapuni has known many difficulties and discouragements. Determination and engineering skill have rendered none of them insuperable, but at the power-house site unforeseen hindrances have arisen to disperse which neither human grit nor mechanical ingenuity has proved effectual.' Last October tho excavations which had been made below tho river level for tho foundations of tho power-house were flooded, and except for a brief interval they have been under water ever since. The excavations have still to go down another. 10 feet or so before concreting can commence. . < .

Serious Setback. No secret is made of the seriousness of the setback. For nearly four months construction work atthe powerhouse site has been at a standstill, all efforts being concentrated upon the 6eemihgly 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 optimis'tio of the chances, but even trieci engineers admit that the outcome of the experiment is problematic.

Exceptional Floods. The trouble arose during the great flood on October 10th to 12th, the worst experienced 6inoe the- contract was started in August,' 1924. According to the specification's and conditions drawn up by the New Zealand Government, the contractors aro called upon to provide against floods of 14.000 cusecs, but ion October 12th 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 keep put the water from the excavations. The "foundations which had been sunk were submerged and'the river flooded the outlets from the Penstock tunnels.

Water Breaks Through. Every effort: was devoted, when the river subsided, to the task of pumping the water out, and by November the centrifugal sinking pumps had so effectually accomplished their parpo.se that the work of excavation was at last renewed. Then came, the crowning disappointment of a long continued 6peil of bad luck. On December loth the "country", beneath the coffer dam, which • had'-nevete 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 iivtwo hours the excavations were completely flooded. An attempt to block the opening and pump out the< water succeeded ;in low-ei-ing the level by. five: feet,, but. the breach, was too large and the water rose again. , .

■"•■ Not Solid Bock; The problem faced was disheartening. It'was evident that' the plan so far adopted of. building the of sacks of concrete placed in position under the .water by a diver was ' useless as long as the water could penetrate under it. According to tests made;by 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 interstices of which the water would find entrance.

A Steel wall. During the past few days a small' gang of picked men has been sinking piles of sheet steel, furnished with an interlocking device that renders them very. nearly. watertight. These steel plates, about 12in wide and' varying from 18ft to 22ft long, are being driven vertically, into the shingle and boulders by a piling hammer operated by compressed air. This machine, which has been lent to the contractors by the Public Works Department, performs its work efficiently and speedily, so that 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.

Extremely Bad Weather. That the setback at the power-houso site will mean a delay in the, completion of the main scheme thore 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 a. greater capacity than the' 14,000 cuse'ea which had been allowed for,. arid no fewer than five of those occasions occurred during tho past year, an extremely bad winter.

, .Extension of Contract. The ; three-year contract expire?, next August, but it is clear that an extension of time -will have to be asked for, a matter, perhaps, of six months. Even then there would be a clear margin to April Ist, 1928,' the date on which the Government has agreed to supply the Auckland Power' Board at Penrose substation.

Spillway Weir Completed. The stoppage of work at the' powerhouse site has, however, resulted in the speeding up of the work in progress at the spillway 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 thei down-stream side.