BLUFF HARBOUR DEEPENING.

Otago Witness, Issue 2850, 28 October 1908, Page 9

BLUFF HARBOUR DEEPENING.

NEW ROOK-BREAKER. Our Bluff correspondent writes: — "In dealing with the removal of the subaqueous rock outside the wharf, the Bluff Harbour Board officers are about to put forward an entirely new effort. Hitherto they have been digging into it by means of drilling and blasting, which i 3 at best a laborious and costly process. What they now purpose doing is to attack the work by means of the Lobnitz rock-breaker. So far as I havo a-^certained, tho process is quite a novelty, and in no shapo whatever has it been adopted in thcee colonies. Its effectiveness or otherwise is being looked forward to with spccultivo interest, and, as is common in such circumstances, public opinion is at variance on the subject. The principle of the machine is that a ram or smashing g«n* weighing 10 tons is lifted by a wire- rope wound round a loose drum, drnen by a friction clutch. It is allowed a fall of Bft, and, on an average, eight or 10 blows penetrate the rock to a depth of 3ft, which is sufficient to allow of its being dredged a depth of 2ft 6in. The average quantity of reck broken by one machine per week, working night and day, and allowing for all stoppages, is 908 cubic jard-, at the cost of B.Bd per yard. The average quantity of rook drilled and! bitted per week is 488 cubic yards, and The average cost of drilling and blasting 3» per yard. As compared with blasting, the rock-breaker disintegrates and break* the rock into smaller pieces, so that the quantity lifted by the dredge in a given time is about 15 per cent, more than that of blasted rock. The average quantity of blasted rock removed by one dredge per day of 24 hours, and allowing for aH stoppages, is 158 cubio yards, and of that broken by the rock-breaker, 182 cubic yarda. The cost per yard of ddredging blasted rock is 2s 6d. The cost of dredging rock broken by the rook-cutter ie 2s 2d per yard. The difference in cost per cubio yard in favour of the removal of rock by means of the rock-breaker is set down ac

2s 2.6 d, or on 500,000 oubic yards, say r £54-,166. The above is based on a ram or breaking-down gear of 15 tons, whereat the one being erected at tHto Bluff dees not exceed 10. — The Rxissien Empire comprises 8,379,044 square miles, of which 2,052,490 are in Europe. *