THE WAITAKI DAM.

Otago Daily Times, Issue 20832, 26 September 1929, Page 6

THE WAITAKI DAM.

EXCAVATIONS BELOW RIVER LEVEL. WORK REACHES INTERESTING STAGE. {Special to Daily Times.) OAMARU, September 25. During the last couple of months conditions have been favourable for pushing ahead the construction of the dam across the Waitaki River above Kurow, and progress is being kept up to schedule. The difficulties of sinking the huge cofferdam in the middle of the river for the central span of the dam have been overcome, and have proved to be less serious than were anticipated. With the removal of the gravel from the cofferdam, the work has reached an interesting stage, and with shifts operating day and night to prepare the rock bottom to receive the first foundations, those in charge are constantly on the alert to provide against all contingencies that may arise, and to ensure that everything is in readiness for the laying down within the next two months of the huge mass of concrete that will constitute the central span of the dam. Although various phases of the construction work are being steadily prosecuted on the land span of the* site of the dani, and at the power house site, chief interest naturally centres upon the cofferdam, where men are tolling m the bed of the river, 20 feet below the present water level, removing the heavy gravel and boulders from rock bottom. Generally speaking the 40-foot steel sheeting -piles .which comprise the walls of the cofferdam, have been driven from one foot to three feet into the rock, in the process of which a few of the steel piles were driven clean through large rocks. As the excavation work proceeded the tongued and grooved joints of the steel piles were rendered comparatively watertight through caulking with oakum, while the deposit of fine sand in the seams through pressure from the outside has rendered the position even more satisfactory. Thus it is that despite the large surface area, and the enormous pressure from gravel and water on all sides, two small electric pumps working continuously, . and a large electric pump brought into action at required intervals, have proved more than adequate to deal effectively with the water leaking into the cofferdam. In this connection it is interesting to that to provide against such a contingency as the electric power failing at any time, a large petrol pump has been installed to deal with the situation should the necessity arise. The Waitaki River is at present a little above its lowest winter level, but the weight of water and gravel exerts a pressure of 800 tons on each of the four walls of the cofferdam, which have been designed to withstand a pressure of 3000 tons. One can readily imagine that without adequate staying and bracing, the cofferdam would collapse like a house of cards under the crushing weight to which it is subjected; but a glance into the interior discloses an elaborate system of heavy wooden beams and stout tramrails fixed as shored' or stays transversely from wall to wall, which appear to be staunch enough to withstand enormous lateral pressure. As the excavation of gravel proceeded the shores were lowered into place until the bottom of the cofferdam was reached, and as the concrete wall rises, so will they be removed.

Rock bottom has now been reached, though in one corner there is still a depth of 14 feet of gravel to be remov6d owing to a depression in the rock formation. During the last six weeks no less than 2500 cubic yards of gravel have been removed: This material is .being hauled up an incline to the surface by an electric winch. Here the large boulders are rejected, and the lemamder are loaded by gravitation into railway trucks, which are hauled across the trestle bridge over the river, to a huge bin, where in due course the gravel will be put through the concrete mixers, to he returned whence it came as a component part of the concrete dam. Deep excavations into the . solid rock are being made for the foundations of tlie power house, and the work is being pushed ahead methodically in everv direction by day and night, and is rapidly reaching a stage when the actual construction of the first span of the dam will be commenced. A comparison that is not without interest has been made between the Waitaki and Niagara. In flood time the maximum discharge of water in the Waitaki at this point is 160,000 cubic feet per second, whereas at Niagara the discharge under normal conditions is 200,000 cubic feet per second, so that under the conditions stated the flow in the Waitaki is almost two-thirds that ot iMngarA,