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Volume 19, 1886
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Art. LXX.Notes on Blasting at Ahuriri Bluff, Napier, in connection with the Construction of the Breakwater.

[Read before the Hawke's Bay Philosophical Institute, 19th April, 1886.]


The starting-point of the Napier Breakwater being from Ahuriri Bluff, where the sea at high-water washes the base of the cliffs, it was found necessary to make room for the erection of working plant, offices, block yards, and other purposes. To enable this to be done, and also to procure rubble stone for the works, it was decided to blow down the face of the cliff, immediately adjacent to the works. This cliff is over 300 feet in height, and is composed of alternating strata of limestones and sandstones. At a height of 50 feet from high-water, two drives were put into the hill, each 90 feet in length and quite straight, in different directions. These were turned at right angles, and driven 12 feet further, and turned again at right angles to the original direction and driven 8 feet, making double elbows. The mouths of the drives were 3 feet wide by 5 feet high. They were narrowed at the extremity as much as possible, so that a man could just work. The end of the drives led into chambers prepared for the explosives used. The material worked into was a bed of sandstone, moderately soft at first, but gradually becoming harder and more difficult to work, till at last three men in three shifts (a man to a shift) would extend the drive 2 feet only; while, at the start, the same complement of men in the same time could do 5 feet.

The first drive put in was for a charge of blasting-powder, consisting of eight tons (2,000lb. to the ton). The inside dimen-

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sions of the chamber were 6 feet 6 inches cube, containing about 275 cubic feet of space. It was carefully match-lined with well seasoned timber. The powder used consisted half of English make, and half Colonial, made at Dunedin. The filling in of the chamber was an anxious piece of work: the barrels and boxes in which the powder was contained were opened at the mouth of the mine, and the contents were emptied into specially-constructed wooden buckets, bound with brass. The men were not allowed to have matches in their possession, and had to go in the mine without boots; and every other precaution was taken, so that there was not the least chance that a spark could be generated anywhere in the mine. The only lights allowed were one in each elbow, consisting of bull's-eye lanterns in recesses let into the rock. When the chamber had been half-filled, the igniting charge, consisting of a pound canister of fine gunpowder, was placed in the centre of the chamber; and from it were led two lighting-trains—one of gunpowder, in a train for a flash, the other of Rickford's slow-fuse; both of these were encased in timber. To sit on four or five tons of loose powder, while fixing the igniting-charge and the lighting-trains, gives one a peculiar sensation, which is greatly enhanced by the fiery purplish-red glint from the facets of the powder, reflecting the meagre light of the bull's-eye lantern 10 feet off, giving it the appearance of being on the point of explosion. When the chamber had been filled, the opening was timbered up, and a dry stone wall was built against the timber, all interstices being filled with fine material well rammed. This was continued to the first elbow; the corner being very carefully built, as well as the next elbow. The space between the two elbows had an intermediate stone wall, the rest of the space being filled in with loose material, well rammed. The main drive was then filled in to about half-way to the mouth with loose material, and a wall of stone every 10 feet. The two trains of fuse and gunpowder were carefully adjusted before the filling in began: and, on reaching the end of the filling-in, were extended 24 feet further, both with fuses.

The mine was fired on 8th March. Both fuses were lighted, and in 12 minutes the explosion occurred. This showed that the flash, or powder-train, had fired the mine. The fuse of 24 feet would occupy about 12 minutes to reach the powder-train, which would connect with the powder-chamber almost instantaneously.

As observed from one side, it appeared as if the face and brow of the hill rose slightly, accompanied by a slight report; opened out, apparently in strips; stood still for a moment, as if undecided whether to fall back or over—then immediately it went over with an immense crash and rumble, with occasional other minor rumbles, caused by the fall of overhanging material,

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which could not be seen on account of the great cloud of dust that had arisen. The material that fell into the sea caused a small wave of about 5 feet high to roll off the shore.

The estimated amount of spoil thrown down was 52,000 cubic yards, equal to about 87,000 tons in weight; the work effected was therefore about 12,180 times the weight of powder used, the result being better than given in Professor Rankin's work on engineering, where the average effective work is set down at about 10,000 times.

The cost of this blast was:—

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Blasting powder £526
Mining and tamping 141
Timber and carpentry 13
Total £680

Each cubic yard displaced, therefore, cost rather over 3d., equal to each ton a little less than 2d.

The shock of the blast was felt nearly all over Napier, much more in some localities than in others, irrespective of distance. Where it was most felt it resembled a sharp earthquake shock.

The second mine was charged with two tons (4,000lbs.) of Nobel's dynamite, and was fired on the 2nd April. The chamber was 5 feet long, 3 feet 6 inches wide, and 5 feet high. No timber or other lining was used, and the dynamite was packed in its paper wrappers. Two trains of fuse were attached to detonators, embedded in dynamite cartridges, and a few detonators were placed in the adjacent cartridges. The fuses were led away from the mine in timber casing to near the mouth of the drive, which was tamped up in the manner described for the powder blast. Dynamite being a very safe explosive to handle, the precautions taken for the powder blast were not enforced, and the workmen were able to go about in their boots and to use naked lights, and no feeling of uneasiness prevailed as when charging the powder mine. The length of the fuses were 72 feet each; and the mine fired in a little over half an hour after they were lighted.

In both cases the explosive chambers were situated 85 feet from the face of the cliff.

The effect of this latter blast was wonderful—its action extended far away behind the blast: the hill opened obliquely from the blasting point; the face of the cliff rose, spread out like a fan opening, and without any hesitation came down with a thundering crash, followed by a low rumbling and a great cloud of dust. There was but a small report, and very little overhanging material left. Immediately above where the charge had been fired a regular funnel had been scooped up to the top, by the pent-up vapours seeking an outlet.

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The estimated amount of rock displaced was 151,000 cubic yards, equal to about 252,000 tons.

The cost of this blast was:—

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Dynamite £500
Mining and tamping 140

Each cubic yard displaced, therefore, cost a little over 1d., equal to each ton at .6 of a penny.

The effective work of the dynamite amounted to lifting over 140,000 times its own weight, and did proportionately twelve times the work that the powder did.

The shock of this blast did not appear to be so much felt as the other; and in many places it was not noticeable, where the shock of the powder blast had been felt.