BLASTING OPERATIONS.

Poverty Bay Herald, Volume LII, Issue 16946, 30 January 1926, Page 7

BLASTING OPERATIONS.

SUCCESS AT WJIABGOXGAOXOA. .PRELIMINARY SHOTS FIRED. GOOD STONE ADJOINS JETTY. When the Gisborne Harbor Board officials fired the first shots at Whareongaonga yesterday the intention was that the charge at the northern end of the servant ridge should provide the material required for the construction of the protective breakAvater at tho quarry, while the charge at the other end of the ridge, nearly a mile from where the jetty is to be built, was for the purpose of obtaining n 1 better quality of of stony for the breakwater at Gisborne.

As it turned out, liotvever, the second charge wits really unnecessary, as the initial shot revealed the fact "that unexpected supplies of good stone existed on tho point immediately adjoining Die jetty site. Just how much this will mean to tho undertaking it is difficult to estimate, but it is clear that the obtaining of stone in such a handy locality will obviate a great deal of haulage, and, it is considered, must considerably reduce the cost. The operations were Avatched with a great deal of interest, it bqing recognised that the firing of tho charges would be the real test of Diet storo deposit, and Avould either prove, or disprove the accuracy of the geological surveys which were made nearly twelve months ago. The arrangements for the undertaking Avero admirably made and carried out. The Harbor Board launch carried , a large number of officials and members-of the Board to the site, and several motor-cars conveyed parties by the overland route. Tho visitors included Messrs R. D. Dymock, O. G. Holdsworth, T. Corson, and J. Tombleson, members of tho Board, the engineer, Mr. B, Campbell, and mombers of his staff, the actingharbormaster, Captain J. W. H. Holmes, the secretary, Mr. H. A. Barton, and representatives of a number of firms which are interested in the harbor works. THE OPERATIONS EXPLAINED. . Whareongaonga, which lies ten miles south of Gisborne, in a direct line beyond Young Nick’s Head, consists of a long narrow ridge running right along the coast, with a narrow valley a little above sea level separating tho coastal ridge from tho high hills inland. The principal stone deposit lies inland, hut the coastal ridge Avas known to contain large quantities of stone, the quality of Avhich was dubious.

Tho breakwater and jetty is to bo constructed at tho northernmost end of the coastal ridge, where a natural inlet provides protection from all southerly weather. To obtain stone for this breakwater, and, at the same time provide a roadway to the jetty, it was decided to remove the point of the ridge, although it was not thought that a good quality or stono Avould be obtained from this locality. At the far end of the ridge it was proposed to open the quarry proper, thus avoiding the necessity of hauling tno material from further inland, although the inland deposit Avould, of course, have been available as a reserve.

The operations yesterday have now shown that it will not. be necessary for some considerable time to come, at least, to do any quarrying beyond the immediate vicinity of the jetty, stone of unexpected quality having been revealed right on the point where it had been thought' that tho material was little better than mullock. It will not be possible, however, to work continuously at this spot, oeving to weather conditions, and to ensure that tho quarrying will be continuous it is probable that another face will be exposed a few hundred yards along the ridge from tho jetty. At the outset the estimate of the cost of stono .was based on obtaining supplies from the inland deposit, but by deciding to work the coastal ridgo first a considerable saving was effected. Noav it is likely that a further substantial reduction in the cost will result from the fact that stono will be secured from the nearest point of the ridge instead of from the most distant. : EFFECT OF BLASTING. With the object of removing tho point* from the ridgo trvo tunnels had been made 40ft under the cliff, each being filled with three-quarters of a ton of explosive. Believing that fairly soft material was bcingjpenetrated, it Avas hoped to completely remove tho point, and provide all tho material necessary to the preliminary operations. The two charges were exploded simultaneously, and a dull explosion was followed by a gradual movement of the Avholo of the cliff lace, thousands of tons of stone falling to the beach below Avith a reverberating crash.

Although the full effect of the charge was not immediately apparent, it was clear without investigation that success had attended the first effort. Tfye countour of the cliffs was completely changed, and the debris reached right to the water’s edge and was piled up to a height of nearly 200 ft. When a detailed inspection of the locality was made the were amazed at the effect of the fcharge. Where stone of poor quality had been expected there was rcvoalcd a huge quantity of excellent material, the whole point having been cut off as if with a huge knife, and a good proportion of the material coming down in large-sized blocks, the largest of which measured over 1000 tons each. Blocks weighing 20 tons and upwards were in abundance, although, of course, they were liberally interspersed with small material. It would appear that tho cliff Was crusted with material of a poorer class, but that beneath the surface really good stone existed in large seams. The effect of the explosion was lo throw the loose crust further away, leaving beneath tho heavier material, with the result that the best of the stone is on the surface. It is likely, however, that a satisfactory proportion of the 50,000 tons of material that was dislodged will be found suitable for breakwater purposes, while it is certain that the stone that, has been uncovered, but not affected by tho charge, will be of excellent quality. EFFECT OF SECOND SHOT. The second shot fired proved disappointing, to say the least of it, although the indications given by it can only be regarded as satisfactory. In this instance a tunnel 00ft in length was driven beneath the cliff, which, at this point, was somewhat loss than 200 ft wide, and over 300 ft in height. It had been hoped to cut clean through the cliff to the sea face and expose the strata throughout, but although a ton and a half of explosive was used in the one charge, tho material proved far more substantial than had been anticipated. When the charge was fired the whole of the cliff was lifted, foul the explosive cut right through to the sea face as had been arranged, but a small point at the end of tho ridge proved a vory ouoctiv.. barrier and instead of collapsing the

cliff settled back into position. At this point the material' lies in avoll defined layers, and the impression given Avas that the charge removed one layer, and that the cliff came to- rest on the ono immediately" bolowi The wholo area was, of course, greatly shattered, and a large portion of the towering cliff Avas left overhanging. Those who are experienced in the Avork are of the opinion that much of the material Avill yet be displaced, but no further movement tqok.place overnight, and this morning only a feAv thousand tons ot debris had come clown.

As it happened, the failure of this charge was not a serious matter as, in view of the stone revealed by the other shot, it will not be necessary to secure stone from this iocaliay for some time to come. In view of the exterior appearance at the other end of the range, however, and the unfavorable report of Professor Speight, it was never expected that stone of the required quality would be obtainable without opening up the second locality. Jf it lias done nothing else, the second shot lias shown conclusively that the stone at the southern end of the fidge is of an extremely hard nature, and should the supply at the jetty end become exhausted there will be no occasion for concern. COSTS MAY BE REDUCED.

Whatever fears may have been entertained regarding the quarry in the past now appear to be completely dissipated. The engineer, Mr Campbell, stated yesterday that- tiie success of the operations had exceeded all expectations, while Mr G. Lomas, the quarry foreman, under whose direction the whole of the work was carried out, said he would not have believed it possible to obtain such splendid results. The failure of the second shot was a disappointment to everyone concerned, but in tiie light of what was revealed by the other explosion is not. in any way, regarded as a catastrophe. No one seems to doubt the fact that the Harbor Board is assured of a good stone supply, and this, of course, has been the heaviest obstacle lo overcome in connection with the construction works. Some idea of the success of the first shot can be obtained from the fact that the one shot, provided five times as much stone as is required for the whole of the preliminary breakwater. This work, it was estimated, would require approximately' 10.000 lons of material, not necessary of good quality, and with the one charge at, least- 50,(XX) tons was dislodged, the quality varying considerably, but a large proportion being even better than was required. In addition, there is exposed huge quantities of stone suitable for the main breakwater at Gisborne at a spot immediately adjoining the jetty. Mow much the estimate will be reduced by reason of Hie stone being more accessible is difficult to compute. If a saving of only 6d a ton in haulage is effected, however, this would represent, a sum of 815.C00 on the stone required for the innitial stages Mi the work. The cost of the stone is the outstanding factor in the construction work, and each reduction in this item will mean a substantial saving in the cost of the whole scheme.