A Treatise On The Principles And Practice Of Harbour Engineering

96 HARBOÜR ENGINEERING. with powder, lightly tamped and exploded several times, till a crack was m*de 1°inglt"d‘na11^ 5 then> int° the crack, large grained powder was poured and exploded, brmging down a considérable mass of rock. By this means the stone was not too violently shaken, and good face stones were obtained. Many of these were 9 feet long by 3 feet 3 inches thick and 15 feet on the bed. The cost of dressing them into shape came to 6d. per cubic foot. The stones selected for rubble hearting weighed from 3 to 15 tons. The greatest quantity of material conveyed to the bank in one day was 3000 tons. Seventy-four tons of powder were consumed per annum. Quarrying for Holyhead Breakwater?-The stone-a quartz rock was obtained from an adjoining hill known as Holyhead Mountain, and he quarries were distant rather less than a mile from the commencement oi the work. At the outset, quarrying operations were carried on by a system of single- hole firing, but, although many hands were employed, the output proved insufficient for requiremeuts. Blasting on a much larger scale was then resorted to, by sinking shafts and driving headings or driftways to receive large quantities of powder. The first large mines were in shafts about 6 feet by 4 feet, sunk from the top and of varying depths, according to the height of the face; but when the quarries had been more opened and the face got very high, sometimes the top only was prepared for blasting by shafts, and the bottom by headings of the same size, or somewhat less. Ultimately, headings were preferred to shafts and adopted whenever practicable. They proved more convenient, as the men could work in front instead of at their feet ; the men did not get wet from ram and the ventilation was better. Headings were also less dangerous, as tie tamping was less hable to be blown out. On the other hand, shafts were more easily taniped and required smaller charges of powder, the rock being already weakened by the excavation. On an average, 4 tons of rock were blasted per Ib. of powder, the extremes ranging from 5 to 2 tons. Generally the charges varied from 600 Ibs to 21,000 Ibs. Experience determined the following coefficients for the formula given on page 90 :— For ordinary shafts, coefficient =T\ to ^=-066 to '05; For ordinary headings, coefficient =^ = '083. In the exceptionally difficult case of a mine called a ‘'rooter-out,” the coefficient became -1. This was a mine in which there was a natural joint on one side only, so that the rock had principally to be torn away from the solid mass. In such cases the lowest results were achieved, and, further, the stone displaced was usually in large masses requiring further breaking up, while in more favourable cases the stone resulting from a blast was suitable for immédiate use. Figs. 86 and 87 are types of the best and worst kinds of mine respectively. 1 Hayter on Holyhead New Harbour, Afin, Proc. Inst., C.U., vol. xliv.