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These factors determine a type of drill required to

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Unformatted text preview: By the manner in which the drilling tool i.e. the drill acts upon the hole bottom (percussive, rotary or percussive rotary) The forces and the rate with which the drilling tools act upon the hole bottom Hole diameter and its depth The method and speed with which the drilling cuttings are removed from the hole. These factors determine a type of drill required to suit a particular type of rock, as drillabilty of rocks differs widely. This factor can be determined in the following manner. When using a percussive drill, the compressive forces prevail and shearing forces when rotary drilling. The magnitude of these forces w.r.t. drillability in a given rock is considered to be almost equal. Thus, the compressive strain c and shearing strain sh are of decisive importance. Since breaking of rock is possible only when the cuttings of rock are removed from bottom of the hole, the bulk density of rock, , must also be therefore accounted. The drillabity index can be assessed by the formula (4.1).6 © 2005 by Taylor & Francis Group, LLC 74 SURFACE AND UNDERGROUND EXCAVATIONS (4.1) Whereas: Compressive strain c in kg/cm2; Shear strain sh in kg/cm2; Bulk density in kg/dm3. If value of Id is 1–5 it is easily drilled, if from 5.1 to 10 then medium drilled, difficultly drilled when Id ranges from 10.1–15, extreme difficultly drilled if Id ranges between 15.1 to 24. 4.10 DRILLING POSTURES In order to obtain tunnels or the mine openings of different size, shapes, orientation and slope/gradients, the holes are drilled in different directions. In the horizontal drivage work holes drilled are almost horizontal or slightly inclined upward or downward to it. This is known as breasting. During stoping operations also drilling is carried out in this posture for slashing the ground, driving rooms and galleries. Pusher-leg mounted jackhammers or drifting jumbos fitted with drifters are used for drilling in this posture. To drive an opening upward, such as raises, or during vertical development in the upward direction; vertical or steeply inclined holes are drilled upward, this is known as over-hand drilling. During some of the stoping operations such as cut & fill, shrinkage etc. drilling in this posture is carried out. Drills such as stopers or drifters are used for this purpose. In order to drive an opening in the downward direction such as shafts, winzes etc. vertical or steeply inclined holes are drilled in the downward direction. This is termed as under-hand drilling. During some of the stoping operations such as DTH, VCR etc. drilling in this posture is undertaken. Drills such as sinkers, DTH machines, drifters are used. Apart from drilling in horizontal, vertical up or down directions, during stoping operations drilling in all directions may be required. This is achieved by the use of fan and ring drilling jumbos, specially designed for this purpose. Suitable drifters are mounted on these jumbos. REFERENCES 1. 2. 3. 4. 5. 6. 7. Atlas Copco manual, 4th edition, 1982 and Atlas Copco Leaflets. Hartman, H.L.: Introductory Mining Engineering. John Wiley & Sons, New York, 1987, pp. 117–18. Henry, H.R.: Percussive Drill Jumbos. In: W.A. Hustrulid (ed.): Underground Mining Method Handbook. SME (AIMMPE), New York, 1982, pp. 1034–40. Jimeno, C.L; Jimenó, E.L. and Carcedo, F.J.A.: Drilling and Blasting of Rocks. A.A. Balkema, Netherlands, 1997, pp. 8–47. Kurt, E.H.: Primary Breaking. In: W.A. Hustrulid (ed.): Underground Mining Method Handbook. SME (AIMMPE), New York, 1982, pp. 999–1005. Rzhevsky, V .: Opencast Mining Unit Operations. Mir Publishers, Moscow, 1985, pp. 117. .V Sandvik – Tamock: Drills and drilling accessories – Leaflets and literature. © 2005 by Taylor & Francis Group, LLC...
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