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9781439834220.ch13

Depending upon the stability of the rock the material

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Unformatted text preview: m of the mass, a stressed zone of elliptical shape is formed above this opening. Depending upon the stability of the rock the material within the stress zone cave in sooner or later, if not supported by some artificial means. Depending upon the rock properties and structural geology the total height of cavity may exceed the optimum distance of spherical charge from the back many times. Thus, cratering characteristics of the rock mass to be blasted are studied, and use of this concept is made to carry out the blasting operations in the stopes or raises by blasting vertical or steeply inclined holes of large diameter (165 mm) in the upwards direction and retreating towards the top cut or drilled horizon. The blasting operation when carried out in this manner, the method is known as Vertical Crater Retreat (VCR). Formulae Used:10 A crater is consists of 5 holes, one at the center and rest four at the corners of a raise. Crater theory is valid for the central hole only, and for the rest of the holes, the charge depth increases from 10–20 cm between each hole, as shown in figure 13.9(b).10 The charge depth can be determined by using following formulae. (13.1) © 2005 by Taylor & Francis Group, LLC EXCAVATIONS IN UPWARD DIRECTION – RAISING 345 Water steaming Sand Wooden plug 5 5 9 9 1 1 3 3 7 7 (b) Conventional cylindrical cut for large-hole raising H2O (a) VCR raising – some details (c) Techniques to plug bottoms of big-blast holes Figure 13.9 Drop raising – some details. (13.2) (13.3) S is the Strain Energy Factor usually 1.5 but depends upon explosive and type of rock. (13.4) (13.5) Drilling: The term down-the-hole (DTH) is used as a generic name given to all those drills which are referred as or known by their trade names such as down-hole drills or in-the-hole (ITH) drills. These drills differ from the conventional drills by virtue of placement of the drill itself in the drill string. The DTH drill is placed immediately after the bit, which always remains in the hole. Thus, no energy is dissipated through © 2005 by Taylor & Francis Group, LLC 346 SURFACE AND UNDERGROUND EXCAVATIONS the steel or coupling and penetration rate is almost constant regardless of depth of hole. It is pneumatically operated and flushing is done by the compressed air with water mist injection. In some cases dry drilling with dust collectors becomes essential to avoid use of water as this drill can tolerate very small amount of water for the purpose of flushing. Most of DTH drills operate up to air pressure of 250 p.s.i. (1724 kPa). Mission Megamatic is one of such units,9 which have been tried in some of the mines in Canada, India and few others. In this technique use of these drills is made to drill the parallel holes in the intended direction of the raise to be driven. All holes are drilled to get through into the next lower level up to which the raise is to be driven. With careful drilling hole deviation should not exceed 1–2%. Thus, this technique is sim...
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