A Treatise On The Principles And Practice Of Harbour Engineering

STONE: NATURAL AND ARTIFICIAL. &7 desirable for maritime undertakings is of varying size, and, in fact, is such as results more or less naturally from the simple blasting of rock. Except for copings and string courses, no dressing is required, and the main bulk of the work is executed in blocks of irregular size and shape. In order to obtain these blocks to fairly large dimensions, some discrimination has to be exercised both in regard to the manner of boring the holes for blasting purposes and the nature and amount of the charges employed. Much, of course, depends upon the disposition of the working face of the quarry and its relationship to the strike and dip of the strata. Natural joints and beds should obviously be taken advantage of to the fullest extent. These features are most irregular and uncertain in the igneous rocks, and therefore call for the aid of some skill and experience in their utilisation. When blasting operations are projected on a large scale, the system of mine firing ’ is adopted, and headings are driven in from a vertical face, or shafts are sunk from the top—the relative economy of these methods being dépendent ou the height of the quarry escarpment. Drainage and ventilation are more readily assured by the use of headings. In this case galleries are fornied of the smallest possible sectional area consistent with the working space required for a man in each ; they are arranged zigzag in direction or with one or more abrupt turns, and they terminate in chambers which are filled with explosives. Shafts, on the other hand, are straight and vertical. Mine firing, which produces buge downfalls of stoue—ranging, in many in- stances, from 100,000 tons to 500,000 tons—results in the dislodgment of so many and such enormous masses of rock that these last have to be again broken up into serviceable sizes by nieans of smaller charges. The method, there- fore, does not altogether obviate the alternative system of small-charge firing, which, in less extensive operations, is more generally adopted. Drilling - Operations.—For the purpose of boring the necessary holes, either to receive the blasting charge proper or as a preliminary in the forma- tion of a shaft or heading, various kinds of drilling instruments are employed, including the jumper, the haud-drill, and the machine drill. Of these, the two former involve manual labour; the last is mechanical and automatic. Where the work is sufficiently extensive to justify the initial cost of installation, there can be little doubt as to the superior economy and efficiency of machine drills. They can be worked much more accurately and with greater ease and convenience, there being situations where the application of hand-drilling would prove awkward, tedious, and slow. The Jumper is an Implement worked by one or several men. It consiste of a long heavy bar of steel, sometimes circular or cruciform in section, but generally octagonal. The length varies from 6 to 8 feet, and, thougli not commonly the case, the bar is sometimes thickened in the middle in oidei to give increased momeiitum to the blow. In drilling a vertical hole, the jumper is lifted and allowed to fall, its uprightness being maintained throughout. It is caught at each rebound and raised again, being given, at the same time, a slight turn. For horizontal work, the drill is swayed backwards and forwards