QUARRY BLASTING

Evening Post, Volume CX, Issue 148, 20 December 1930, Page 10

QUARRY BLASTING

BIG SHOT AT HOROKIWI

ACCURATE CALCULATION

ROCK EOE SEA WALL Spectators of big shots in quarries find it all very interesting, but few realise that the result was pretty much, what was anticipated. They feel the ground lift beneath their feet, and see the hillside rise, apparently gently, in tho air, and then commence •to slide, but though they can hear and feel the tremendous effect of the confined explosive, thoy have little idea of how tho charge was calculated to give the .result desired, where tho drive to insert it was put, and they know nothing of tho different methods used to secure different results. .'.ln the case of the recent big shot put in the hillside at Horokiwi by the ' Public Works Department, no less than . 40001b of explosive was used, and yet the area of rock ruptured was that which had been calculated within a foot or two. This was the last of a series of shots in this quarry, the object being to secure solid- blocks of hard rock for the Tawa Flat deviation sea wall, just north of Kaiwarra. The railway embankment, is also being protected by a heavy apron of largo stone, and an additional safeguard has been made by the placing of heavy concrete blocks to form more or less of a core in the rock apron. Tho Public Works ' Department did extensive exploration work in the hills for suitable rock for this purpose, and was lucky to obtain what was required in the Horokiwi valley, which runs between Pctone and .Tohnsonville. Tho Railway Department years ago found it necessary to obtain rock from Pencarrow Heads. The first shot of any size put in by tho Public Works Department was in Ngahauranga Gorge, but the rock proved to be of very poor quality, and it was necessary to look elsewhere. In tho quarry at present being worked, somo five shots have been put in up to date. The result of the last shot —and the biggest—is shown on the picture page. It will be seen that rock of large dimensions has been obtained. The blocks required are of five tons, more or less, but some of those which have boon got will take a good deal of breaking down to bring them to. that and the quality of tho rock is all that could be desired. Tho smaller stone will be used for packing. PREPARING FOR BLAST. In all such undertakings there is a good deal of thought and preparation before operations aro commenced. Ine first thing to be done is to make a very thorough survey of the quarry, and when the quantity of rock decided upon is arrived at, a driMo is put into the quarry face on the same level as the floor of the quarry, the length of drive being based upon tho quantities desired to shift. The quantity of explosive to be used is then calculated. Tho character of the explosive is the next consideration, and this is ' determined by the results it is desired to obtain. In cases where large, unbroken stone is required, as in this case, it is desirable to use an explosive which has a lifting effect rather than a shattering one. What is required is just to break the surface. In military terms, a "camouflet,'? not designed to scatter Tock all over tho surroundings, or make a crater. In this case gelignite was used. A local product "gentler" (if ' such things can bo described as gentle) ■ 'in its action would have been used but for the fact that preceding orders, one of 20001b, had exhausted the immediate supply. The order for something like two tons of the local product was more than the last straw, though the camel was willing. SIZE OP THE CHARGE. In determining the size of the charge to be used three factors enter. The first is tho soil factor, a variable one depending on the nature of tho material surrounding the charge. In the tables prepared to assist engineers in their calculations sand is given a factor of 1.0, while hard rock ranges from 3to 4, according to its density. The second factor depends on the nature of the explosive used. There is no short cut' to economy in this respect, as guncotton, for instance, with its shattering effect, would be quite useless if large fragments of rock were desired. It is merely necesary to lean a package o£ gun-cotton against an abutment or bridge pile to assure its complete disruption, but the effects of this explosive on the Horokiwi quarry, though it might have produced a quantity of chip metal for bitumen work, would have entirely j wrecked tho stone for the purpose for which it was needed. Ordinary blasting powder has a factor of 1.0. The local product that would have been used had it been obtainable in sufficient quantities . has a factor of 2.1. Gelignite, which was used, has a factor of 3.0. Tho third point to be taken into consideration is the line of least resistance, and this, in the present case, was the shortest distance from the charge to the surface of the ground. Combining these factors, the quantity of rock that will bo displaced ean be very accurately calculated. In tho case of the last shot, as will be, apparent from the footnote to the pictures elsewhere, the radius of the rupture of tho rock surrounding the charge was exactly calculated, the broken face of the bluff coinciding within a foot of the calculated spot. FIRING THE SHOT. In laying the explosive, a side gallery is always used, an elbow being formed with the main drive, to avoid the possibility of a blast backwards. The shot is generally concreted in, and behind that the side gallery is strengthened with sand bags. The shot is always fired electrically, and both tho operator and the spectators stand a long way off, especially in the case of a, big blast. Four or five olectric detonators are distributed amongst the charge. The testing of the insulation of the electric equipment is a rather serious matter, as any faults developing would mean tho picking out of tho charge from the rear, a risky and troublesomo proceeding. A portable instrument called an exploder is used to fire the charge, current being generated by pushing down a pair of handles. When ifc is realised that three tons of rock are dislodged by every lib of gelignite used in such charges as that described, it will be seen that an enormous quantity of the hillside, solid rock though it proved to be, was brought down.