Alluvial Gold

By E. T. Spidy .

A description of the world's most modern gold dredging methods, and the construction of these monster machines in New Zealand.

(Superintendent of Workshops, N.Z.R.)

A modern gold dredge In operation in California, U.S.A.

Gold! The allure of gold! Its fascination! Its value! The harder it is to get—the more intense are the efforts of engineers, scientists and others to wrest it from old mother earth.

And they succeed! Yes, they do— despite all difficulties. The old golddigger, of fifty years ago employed ways and means that were the methods of his day. Maybe he did just “scratch the surface” when judged on the basis of present methods, but for those days he did a very thorough job, and he pioneered the development of to-day's monster mechanical diggers. But today's problem is different, inasmuch as deep digging is the way to success, and in New Zealand at the present time we are constructing gold digging machines that dig 110 feet deep.

One must give figures if any appreciation of the magnitude of such machines is to be conveyed. Think of 110 feet in terms of height. Well, the modern gold digging machine, or dredge, as it is called, with its huge buckets, the whole structure floating in a pond of water about 500 feet long, operates below the surface of the ground to an equivalent distance in terms of depth. The pond usually has its water level 25 feet below the ordinary ground level, and the dredge digs out from under the water to a depth of 85. feet, thus making a total of 110 feet of earth, sand, stone, etc., all of which it carries on board, extracts the gold from it, and then dumps the waste material behind.

Now, the illustration at the top of this page gives one an idea of what a modern dredge is like. It shows a gold dredge in operation in California. This dredge has a line of digging buckets each of which carries up to 16 cubic feet of spoil. In general characteristics, it is very similar to the machines the Government have undertaken to fabricate in its Railway Workshops and erect on the sites selected on the West Coast of the South ‘Island, near Hokitika. All the other illustrations are photographs of the actual work of erecting, in progress, at Kanieri and Arahura, where the first two dredges are located.

The buckets of these two dredges are each of 18 cubic feet capacity, and are therefore larger than the first mentioned dredge. What this means will be appreciated by those who have had to shift a couple of yards of sand, or something similar, at one time or another. In the case of the dredges the buckets dump this spoil into the top hopper at the rate of twenty-one buckets per minute, and since each dredge is scheduled to work 550 hours per month, you can readily calculate the amount of earth lifted per year.

Each steel bucket alone weighs 21/2 tons empty. There are 107 buckets in the complete string, so you can readily calculate what they weigh altogether. Then, of course, there is the bucket ladder beam itself which weighs 240 tons. This beam carries the rollers for the buckets to travel on, and it is 161 feet long from the bottom pin to the top pin, and the total weight of steel hanging on the top pin is 480 tons.

It will interest those of a mechanical turn of mind to know that the “pin” referred to is 16 inches in diameter, 11 feet long, and weighs 61/2 tons.

If you should get tired of your mechanically minded young son's questioning, you could set him the following practice lesson. The digging ladders of a dredge of the dimensions (already given) requires to be installed on the dredge, and the pin of the dimensions (already given) placed in a hole located 42 feet above the deck of the dredge. How would you get it all in place—the items being on the bank and the structure floating in the pond. The maximum your crane can lift is only 12 tons. Figure it out! In the present case it has been figured out and the work is being done.

On board the dredge at Kanieri—left. Mr. E. T. Spidy, and right, Mr. Colquhoun, Dredge Construction Engineer.

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A view of the Kanieri Gold Dredge under construction.

Now, just a few other dimensions of the dredges the Railway Shops are building. From the tip of the digging bucket at the bow to the tip of the stacker ladder at the stern, measures 460 feet. The total weight of the Kanieri dredge in working trim will be 3,443 tons.

This new era of gold digging on a modern scale is different in many ways from all that has gone before. A gold dredge of the size of that at Kanieri, has over fifteen years solid work ahead of it, on six hundred acres of land, if it digs an average depth of only 78 feet.

Prospecting, as one generally apprehends it, is a thing of the past on jobs like this. It is to-day made successful by scientific methods.

The pontoon, or hull, of the dredge at Kanieri, is 186 feet long and 72 feet wide. The two main girders (inside the hull) that form the backbone of the whole construction, are 12 feet deep and 186 feet long.

A visitor to the dredge site at Kanieri seeing this huge structure floating in a small pond of water, about 250 feet square, would naturally wonder how it came to be there. When the dredge is working, of course, it is easy to comprehend that it digs its way forward and sideways, and then tips all the spoil behind it, that is, after it has taken the gold out, and the pond moves on with the dredge. The pond does not get any bigger—it simply follows wherever the dredge digs.

But in the first place, there is only bare ground, or more often, ground filled with innumerable tree stumps, bits of bush and so on. In our case, the Kanieri Dredge Company's Engineer took us to the burnt out ground, and showed us four pegs with bits of rag on them, and said: “This is where she has to be built.” In due course this 250 feet square section was dug out level, to a depth of 12 feet all over. It was then ready for us to erect the dredge on.

As the steel plates and girder sections from the mills arrived at Addington Workshops they were marked off, drilled, some unit sections were put together, and then they were shipped to the site. Erection was commenced on the bottom of this “pit” as it is

A view showing the massive steel superstructure of the Kanieri Gold Dredge

“I see from the papers,” remarked a veteran clubman, “that they're smoking more cigars in England now than for years past.” “They're welcome,” said somebody else. “I'd sooner have my old briar than the best cigar going.” “Same here,” chipped in the chap with the smart smoking cap, “nothing like a pipe—especially if it's crammed with ‘toasted’ — preferably Navy Cut No. 3” “I've never sampled ‘toasted',” said the veteran, “some thing extra special?” “Yes. In fact it's ‘toasted’ first and other brands amongst the ‘also ran'! Give it a go, Major, and as an old smoker, try Cut Plug No. 10 (Bullshead). It's full strength. Oh, and there's another toasted brand for the pipe—Cavendish—the sporting mixture, and two cigarette blends, Riverhead Gold and Desert Gold. You'd say they were imported if you didn't know better.” “Well,” said the veteran, “since they're all N.Z. grown and manufactured, you say, I suppose I can get them anywhere?” “Of course you can, old man, at any tobacconist's shop.” And they adjourned to the card-room for bridge.^*

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Placing bottom screen bearings in position on the Kanieri Gold Dredge.

called, the two main girders being built first. Then, piece by piece, the structure took shape until the hull was ready to float—not launch, mind you, but float. This was done by diverting natural streams in the neighbourhood into the pit, the water rising in the pit until it lifted the whole pontoon, or hull, off its “pig-stys.” All other erection work is then done afloat.

In the fabrication of this dredge we must give credit where it is due—in this case to those concerned in the original conception of the scheme, to those who did the detail work at the main shops, right down to the boys from our own workshops who are doing the actual work of construction. It is a tribute to the accuracy of modern methods that the various pieces fitted together without a hitch.

It is one of the noteworthy achievements of a modern up-to-date dredge, that in dumping its spoil it puts the large stones on the bottom, the medium sized gravel next, and the fine soil on the top. Yes, I know that is a new one, and it is accomplished really quite simply. Just a question of sorting the tailings and having different dumping stations for the stones of different size. The same principle is used in sorting eggs. You see, the smallest gravel travels the farthest, and so lands on the top of all the large stuff. And all is done automatically.

Electric power — yes, the whole dredge is operated by electric power from a land line connected to the dredge. All the dredge machinery consumes power, and so boosts up another of our main New Zealand industries. For the connected load on the Kanieri dredge no less than 1530 h.p. is used. No comment is necessary in connection with that, is there, for one dredge?

Looking along the deck towards the stern of the Kanieri Gold Dredge.