REPAIRING ARAPUNI.

New Zealand Herald, Volume LXVIII, Issue 20882, 26 May 1931, Page 10

REPAIRING ARAPUNI.

TIRST OFFICIAL DETAILS.

;)■ LINING THE HEADRACE. FALLS STRUCTURE DESCRIBED. WORK AHEAD OF SCHEDULE. // ' .. 'An inspection of the Arapuni remedial Works was made yesterday by members of the Auckland Electric-Power Board. J Tho Minister of Public Works, the Hon. W. B. Taverner, had been unable, on account of indisposition, to leave Dunedin, his deputy being the Hon. A. J. Mur- / doch, Minister of Agriculture. Mr. F. W. Furkert, engineer-in-chief of the Public Works Department, and Mr. IT. M. Kissel, chief electrical engineer, wero also present, and with Mr. Murdoch )'] they had somu discussion with Mr. W. J. Iloldsworth, chairman of the board, and its officers in regard to the terms of the agreement under which the board is to carry the load during the period Arapuni will be idle. Messrs. J. A. Young, C. 11. Clinkard and W. J. Broad foot, M.r.'s, also made the tour of inspection. Tho activities of tho moment were recently described in tho Heuald, and the chief interest of tho day, therefore, lay m details, given for tho first time officially;" of a number of the features of the work.

Lining the Headrace. Tito manner in which tho headrace is to bo lined was explained. On top of tho filter-bed, which consists of a layer of rubble and then layers of shinglo and sand, \vi 11 l>o placed metal or a coaling of bitumen and sand. Upon this surface there will be a layer of steel sheets, coated •with bitumen on eacli side. Robertson protected steel was mentioned by tho Swedish expert, Professor Hornell. If this is not used steel that is coated with bitumen on tho spot may be used., Tenders to bo called shortly will probably supply 'definite information on tho point.

Whatever bo the typo of steel sheets they will bo laid to overlap, but there will bo no riveting, as tho aim is to mako an impervious lining which will allow some movement in the event of any further rock fault developing. Professor Hornell was against too rigid treatment. On top of the steel another coating of bitumen will be applied, and then the /final surfaco of concrete paving blocks cast in a shape to enable angles to fit into ono another. The joints so created will be sealed with bitumen.

As already explained, the portion of the channel involved, over 1000 ft. in length, is being given regular banks, which will bo surfaced in the same way, except that , the first course will be of concreto paving: blocks, as bitumen could not bo easily applied on the comparatively soft soil. On top of these the courses will be bitumen, steel-plate, bitumen, and concrete paving / blocks. In this confined portion of the headrace the water will bo some 20ft. deep, but there seems to be little doubt that such an elaborate system of paving will r be adequate to prevent leakage.

Purpose of Filter-Bed. The cut-off concrete wall at the head of this section is now being raised in tho deep trerich dug for it. This is intended to prevent any water finding entrance from upstream beneath it. The only

water which the filter-bed is expected I to receive, is surface seepage from the banks. This will speedily find its way into the main drain that runs right down the centre of the channel bed to a shaft, which will deliver it to the concreted gallery that runs into the gorge near tho north corner of the power house. This, has a concrete invert and will be fitted with means of recording in the power

bouse control room the flow of water. Thus the engineers will be immediately informed of any untoward development. The only critical comment from members of the Tower Board in regard to the surfacing of the head-race was that, it did not go all the way. The reply was that'above the cut-off wall the rock was not of columnar formation and was, therefore, safe against serious penetration by water, and that whatever water it did absorb would be caught and delivered into the gorge by a series of three deep galleries, one above the other, a few yards below the cut-ofi wall. Had it been

necessary to confine and pave the whole of the head-race the cost would have been very greatly increased. Surfacing the Falls.

Without access to the drawings it is impossible to describe fully the method of concreting the face of the falls. The general plan, as already indicated in the Herald, is to retain the midway bench and give to each of the two drops the parabolic curve which water takes. At tho bottom of the first drop there will bo no pool to dissipate the energy. Instead, two rows of " teeth " will bo sei, ■ up, each " tooth " consisting of a block of concrete eight feet high and eight feei; wide. A working model proved that these obstructions would extract the force of the current, which will move on, tamed and docile, to tho next drop, at the bottom of which the pool, 50 feet deep, will absorb tho energy. Tho pool will be concreted, tho concrete extending 200 or 300 feet from tho base of the falls. Great Hole Eroded.

One reason why " teeth " will tic used at tlio bottom of the first leap is that the excavation of a pool of the necessary depth would extend so deep into the rhvolite that the block would bo seriously weakened. It was ascertained that the hole eroded on the bench went to fi depth of 47 feet, which carried it through tho hard rhyolite into a much softer stratum. This hole was being filled yesterday with concrete from a chute, into tho mass of which broken rock was being thrown. It will ho remembered thai: Professor Hornell did not express any definite opinion as to the concreting of the falls. JI o said ho required more data. Ihe most important picco of knowledge) that was likely to influence him toward advising concrete protection was the fact of this deep hole, but no one knew it existed. Water covered it and sounding disclosed no depth because it was full oil eroded rock. Ihe bench block had another weakness. When the base pool was emptied some horizontal holes 35 feet, deep were found to have been eaten out under it.

Another piece of official information is that tho spillway channel will bo concreted in the ordinary manner. Replying to a question from a mem ber -of tho Power Board, Mr. Kurker said tlio department expected to complete the remedial work at Arapuni "some time next year." Tho work was ahead of programme, but lie would not care to be too precise in predicting the day of completion. Mr. Furkert was also asked if tho cause of the vibration of No. 3 unit had been determined. II C said that it had nothjng to do with the earth fault. A modification of the draught tube-would be employed to endeavour to correct it.