ON RIVER BED

Auckland Star, Volume LXXV, Issue 59, 10 March 1944, Page 4

ON RIVER BED

DAM FOUNDATIONS

BIG KARAPIRO WORKS

(0.C.) HAMILTON, this day. Not the least interesting part of the task of constructing the huge hydro-electric station at Karapiro is the preparation for the dam across the old bed of the Waikato River, the diversion tunnel having proved commodious enough to take the full volume of water.

Two tall pylons have been erected, one on each bank of the river, and approximately 1100 ft apart. Between the two is to be strung a stout cable, along which will be conveyed material and plant for the work in the bed of the river. The 90 sections of the huge penstocks, fabricated and prepared at the workshops on the eastern bank by a Christchurch firm, will oe swung out into space on the cable, lowered as near to the eventual site as possible, and then manhandled into the eventual resting place. The sections of penstocks are of 5-Bin steel, 4tt wide and 21ft diameter. Each of the three penstocks will be composed of 30 such bands, jointed and joined together on the job. Each penstock will resemble the funnel of a huge ocean liner. Each must be exact in proportion, fitting snugly, to take the huge weight of water that will pour through it.

Then there is the construction work on the dam itself. The overhead cable will play no small part in the distribution of the mixed concrete right across the width of the river bed and a considerable distance into the banks on both sides.

Cement Mixing Device The cement mixing device in use at Karapiro is the only one of its kind. An ingenious system of elevators, levers and scales operates the proportion of cement, sand, gravel and water used for each batch of prepared concrete, and sends it in a continuous stream to the exact spot far below for use in what will ultimately be a huge bow-shaped dam, with a wide and straight apron on either side, holding the water back for distribution into the penstock heads as required.

Everything possible has been done to devise labour-saving apparatus, and thus to use a greater proportion of semi-skilled and skilled labour on the job than was the case during the construction of other hydroelectric power stations.

The stratum at Karapiro is unlike that at Arapuni, and presents somewhat different problems. Engineers are agreed that it is more stable rock than that at Arapuni, but whatever the nature of the rock or mineral base it is a requisite of success that the foundations are so constructed that they will be immovable. Boring plants have been brought to the job, and bores put down more than 100 ft, most of the way through firm rock. In a few places soft patches were found, and these had to be scooped out, and filled with "cement grouting. Occasional breaks in the boring outfit caused delay and inconvenience, but all difficulties are being steadily overcome.

Separating Sand and Pumice Naturally on such a job improvisations become necessary. Perhaps the one of outstanding importance occurred when the problem of obtaining the huge tonnage of sand for the concrete work on the dam was tackled. Prospecting over a wide area failed to reveal sand of sufficient quality to ensure a firstgrade concrete. Inquiries from contractors and local body engineers were also fruitless.

It was decided, therefore, to use the sand nearby, for there were huge deposits in what was once a sweep of the Waikato River, but that sand was too liberally impregnated with pumice. The problem was to devise some means of separating the pumice from the sand. Collectively the engineers probed the problem, and evolved a novel solution. The result has far-reaching interest, and incidentally to add a feather to the caps of the engineers responsible.

The top hamper of soil was moved from an area of nearly an acre, and the sand-pumice conglomerate dragged by power-operated scoops to where it could be drenched with water pumped from the Waikato River. The mixture was then run over a long' riflle screen where the light and fine pumice was floated off and the more solid pumice and good sand diverted into a hopper so arranged that a lorry could run below it, trip a lever and have the vehicle filled in a few seconds with good quality sand-pumice for conveyance to the mixer near the site of the dam.

Another Success A good deal of ingenuity was shown when the sand washer was devised. A drag grab was not obtainable of sufficient size for the haulage of sand and pumice along the chutes, so one was constructed for trial ana test, and it proved so successful that it is now incorporated in the general scheme, doing good work. When a new one is needed it will be larger, te fit exactly the width of the sluicebox, but it will be on the same principle as the one in use.

Some "pea" gravel, very fine in size, between sand and light gravel, comes from the Mercer district, being brought up the Waikato River on barges to a wharf at Cambridge and thence by lorry to Karapiro. The "pea" gravel makes a good mixture with the sand and larger metal.

An indication of the value of the sand-washing device is the fact that first-class sand for concrete can be delivered on the job for 5/ a cubic yard,

Hugo Metal Dump There is a huge dump of metal, aggregating 40,000 to 50,000 cubic yards. An ingenious device for loading it into trucks for conveyance to the site of the dam consists of a timbered tunnel 50 yards in length. Trucks drive into the tunnel, the driver pulls a lever while still seated in his cab, the metal is delivered into the truck from an appropriate hopper, and the truck is then driven out of the tnnnel and on a down gradient all the way to the dam. Most of the metal comes from a privately-owned quarry at Whitehall, and nearly all the rest from the Waikato County quarry at Te Miro.

Other intriguing features at the works include an adaptation for bending steel rods. To deal with these expeditiously a long table is used to run on railway wheels. As one pile of rods is finished, the table is pushed along to the next pile. The actual bending of the rods is done by electric power, round three snubs.

The way one-inch piping is dealt with is a revelation in speed and ease. The rods are arranged in new rows so that any particular size or length can be obtained with a minimum of delay.

The whole work is under the charge of Mr. J. T. Gilkison, who has a keen and loyal staff.