BUILDING GIANT COFFERDAM

Timaru Herald, Volume CXXXVIII, Issue 19941, 27 October 1934, Page 7

BUILDING GIANT COFFERDAM

BATTLE WITH A TURBULENT RIVER Studying its Varying Moods Following the construction of the trestle bridges, a start was made with the first cofferdam, which was built in the middle of the river, the huge steel sheet-piling being driven through

With the heavy gear that had to be transported, it was found necessary to extend the railway from Kurow, and the track was laid, in many parts over difficult country, along the south branch of the river. It has been es- j timated that the co-operation of the Railways saved over £IOO,OOO, so the railway more than paid for itself by a i very long way. In addition to the carriage of heavy goods, such as the machinery, steel and cement and timber, the railway was most useful for passenger transport particularly in the carriage of children from the Hydro to school at Kurow. Following the establishment of a model camp, the main work was rapidly pushed forward. On the south bank of the river huge concrete mixers were erected, over which were great hoppers, from which the gravel for the concrete was delivered to the mixers. Leading up to the hoppers were railways built on the hump system. A similar railway led from the mixers over the railway bridge to the north bank. Thus the tip trucks full of concrete travelled without human aid and then dumped their supplies into long iron chutes, which conveyed the mixed concrete to the spot where it was being laid. Night and Day Practically from the very beginning of the construction work the day at Waitaki Hydro was one of 24 hours. Men worked under blazing electric light at night and in sunshine by day fighting their endless battles with the river and time. Some excavated; some tended pumps; some shovelled concrete into position; some worked in stuffy tunnels 10 feet below the river’s bottom; some worked in offices; some fitted the huge electrical portions of the generators and turbines; above them was the engineering staff; and above the engineering staff, Mr Packwood, seeing to it that the plans were carried out in meticulous detail.

“The development and operation of the Dominion’s hydro-electric resources has continued to become one of the most important activities of the Department during the year,” declared the Minister of Public Works (the Hon. John Bitchener), in presenting the Public Works Statement in the House of Representatives recently.

many feet of gravel and huge boulders into the solid rock beneath. Constructed on the tongue and groove principle, the piling was substantially watertight, and as the water level inside the cofferdam was lowered, the pressure from the outside forced sand into the interstices, thus sealing the walls of the cofferdam and enabling the seepage or inflow to be coped with by the pumps. This first cofferdam, shored on the inside with a network of large beams of timber and rail and tram lines to withstand the tremendous pressure of the water and gravel on the outside, presented, perhaps, the most difficult problem with which the engineers had to contend. There were many hours of anxiety on account of the danger of a

sudden inrush of water flooding the cofferdam in which the men were working. While being driven through solid boulders on the river bed, the ends of some of the stout steel piling were curled up and rendered ineffective for some feet, and these breaks in the armour of the piling required special attention, sometimes under hazardous conditions, to render them watertight and make the cofferdam safe. Second Darn The following season, the second cof-fer-dam was constructed, occupying the northern or Canterbury half of the original waterway, and confining the river entirely to the southern half. This coffer-dam, 15 acres in area, contained about 250 feet of the length of the permanent dam, the base of which was founded at a depth of over 40 feet below water level. In this section of the dam eleven sluiceways or waterpassages each 13 feet deep and 10 feet wide were provided to carry the river flow during the next stage of construction. The concrete was temporarily finished at water level, so that floods of greater volume than the capacity of the sluices could pass over the top at the then stage of construction. Big Flood While operations were in progress in the second coffer-dam, there were two trestle bridges, one at the upstream and the other at the downstream end of the coffer. In January 1929 came the biggest flood experienced during the whole period of construction. It was by no means an “old man” Waitaki flood, but it was sufficient to carry away several spans of the trestle bridges, the water very nearly reaching the top of the coffer-dam. It might be suitable to mention here that the engineers have been extremely fortunate in having completed their task without a serious wild mood on the part of the river. Many oldtimers are amazed that the Waitaki should have behaved itself for so long a period as six years. Still, on occasions this unruly river did register some emphatic kicks and the flood damage caused temporary setbacks. River Diverted The next step was the dismantling of the second cofferdam, and rebuilding its steel walls in the southern half of the river channel. This forced the entire flow of the river over to the north side through the sluices above mentioned. The most critical stage was entered upon in June last year when the third cofferdam confined the river to less than half of its original waterway. About 50,000 cubic yards of gravel and rock had to be removed to reach the desired foundation level, and concreting could not begin until September. Between then and last Christmas, however, concrete was placed at the rate of 30,000 tons per month, considerably exceeding anything previously accomplished in New Zealand, and the permanent structure was brought to a safe height above flood level. After the completion of the third section of the dam in the southern cofferdam, the latter was removed. The , removal of the cofferdam was a big feat in itself. Fifteen hundred steel piles, which were interlocking, had to be drawn like huge teeth. This was l done by a great reversible steam tripi hammer, each mighty wallop of its I huge weight lifting a pile about an f inch or so at a time. It was slow . work, but sure, and the power exerted could be imagined on an examination of the tom and twisted bottom ends of the piles when drawn out of the rock ; in which they had been embedded, s The last cofferdam removed, the > river continued to flow through eleven t sluiceways in the northern section. The , weather conditions, however, were not t so favourable, for heavy falls of snow, 1 followed by very severe frosts—on one f occasion 48 degrees of frost were regis- % tered—made matters exceedingly un-

pleasant for the workmen in carrying out their daily tasks.

Final Operations

Observations and recordings made by the department showed that the month of August was the most suitable for the final operations, the river reaching its lowest level in this month, and so a beginning was made towards the end of July with the permanent sealing of the temporary sluiceways. A steel gate was lowered in steel runners on the side of the wall at the head of the first sluiceway, the weight of water on the upstream pressing the gate firmly against an edging of resilient belting, which made the gate watertight. The sluiceway was cut off from the river flow, pumped empty and filled with concrete, and the process was continued until six sluiceways were sealed. The gates proved entirely satisfactory, as the leakage could have been taken away in an ordinary glass. This was very necessary as the placing of the concrete could only be carried out successfully under dry conditions. Towards the middle of August the river level remaining low, three more sluiceways were cut off by the steel gates, and sealed, leaving two remaining to cope with the flow of the river. Confined to these two small openings, the river began to form a lake on the upstream side, the height of the water being several feet above that on the downtream side of the dam, but as the water rose the pressure increased and the discharge of water through the sluiceways gained in velocity. All work was synchronised to permit of the filling in of the small gaps in the crest, the removal of machinery from the upstream side of the dam, and the closing of the last sluiceways to be finished at the same time. On September 25 the last gates were lowered into position, and two days later the cavities were sealed with concrete, and this huge undertaking was completed.