Principles of mining : valuation, organization and administration; copper, gold, lead, silver, tin and zinc . (4^ O 9 U U ^ i3 ij ti Fig. 29. — Longitudinal section showing square-set timbering. of ore, but generally are applied only in deposits too wide, or to, rock too heavy, for stulls. Such trusses are usually constructed on METHODS OF SUPPORTING EXCAVATION. 105 vertical and horizontal lines, and while during actual ore-breakingthe strams are partially vertical, ultimately, however, when theweight of the walls begins to be felt, these strains, except invertical deposits, come at an angle t
Principles of mining : valuation, organization and administration; copper, gold, lead, silver, tin and zinc . (4^ O 9 U U ^ i3 ij ti Fig. 29. — Longitudinal section showing square-set timbering. of ore, but generally are applied only in deposits too wide, or to, rock too heavy, for stulls. Such trusses are usually constructed on METHODS OF SUPPORTING EXCAVATION. 105 vertical and horizontal lines, and while during actual ore-breakingthe strams are partially vertical, ultimately, however, when theweight of the walls begins to be felt, these strains, except invertical deposits, come at an angle to lines of strength in thetrusses, and therefore timber constmctions of this type present. Fig. 30. —Square-set timbering on inclined ore-body. Showing ultimatestrain on timbers. little ultimate resistance (Fig. 30). Square-set timbers aresometimes set to present the maximum resistance to the directionof strain, but the difficulties of placing them in position andvariations in the direction of strain on various parts of the stopedo not often commend the method. As a general rule square-setson horizontal Imes answer well enough for the period of actualore-breaking. The crushing or creeps is usually some time later; 106 PRINCIPLES OF MIXIXG. and if the crushing may damage the whole mine, their use isfraught with danger. Reenforcement by building in waste isoften resorted to. AVhen done fully, it is difficult to see the util-ity of the enclosed timber, for entire waste-filling would in mostcases be cheaper and equally efficient. There is always, with wood constructions, as said before, thevery pertinent danger of subsequent crushing and of subsidencein after years, and the g
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