9781439834220.ch13

Following a burn or cylindrical cut pattern five to

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Unformatted text preview: ilar to the longhole raising (fig. 13.9(b)) but in place of drilling holes of 50–60 mm dia. following a burn or cylindrical cut pattern, five to six holes (one or two in center and rest at all the corners), as shown in figure 13.9(a), of dia. 100 mm or more are drilled. An extra hole in the center is purposely drilled to take care of any abnormal hole deviation and damage of the central hole during raise blasting. However, on completion of drilling holes’ survey for their deviation is undertaken. Raises of longer lengths upto 150 m can be drilled with the application of the drills used for this method. Blocking the blastholes: As in the conventional crater formation the charge covers the bottom of hole for some length and rest of the hole length remains empty. Now, think of inverting this figure or the scenario, it will reveal that the charge should be placed at a certain height (which can yield the desirable results) from the free face, and hence, blocking the hole at a certain height above the free face is essential. This process involves securing two wedges at the desired location near the bottom of the blasthole i.e. from the free face. Explosive is charged on top of the blocking. Angles of holes determine where the hole to be blocked. Given below are, generally accepted values, for blocking heights:3 Hole angle 80–90°: 1.2 m; 57–79°: 1.5 m; 50–56°: 1.8 m, less than 50°: 2.1 m. The process of blocking the holes has been shown in figure 13.9(c).8 For the purpose of blocking the holes, conical or rectangular wooden blocks (fig. 13.9(c)), which can be suspended from the top sill by 5–6 mm dia. polypropylene rope, are usually used. In the recent past at some mines, the rope has also been replaced by the primacord (40 grain). This process could reduce the hole loading time by 20% or so. Hole blocking begins by tying one wedge block onto the 40-grain primacord and lowering the block up to the pre-determined blocking height. After this the second wedge blocks is lowered down so that it hits the first one. The primacord is tugged to ensure that the blocks wedge together against the wall of the hole. About one foot of small rock cuttings is poured into the hole to ensure proper blocking of the hole. The holes can be blocked even following different techniques. Figure 13.9(c) illustrates some of these practices. This may be noted that any extra hole which is not charged, should be filled with some stemming material to avoid its damage and jamming at the time of blasting the raise round. Blasting: The hole is then loaded with the explosive. Its amount depends upon its density and ratio of hole dia. to length so that a spherical charge can be obtained. For example for a 165 mm dia. hole, the charge amount of an explosive of 1.40 gm/cm3 density works out to be 27.2 kg; Hence, as per this calculation half the weight of the explosive is first dropped or charged, then booster primer with proper delay is lowered down. The rest of the explosive...
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