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Unformatted text preview: also at depths where excessive pressure and temperature prevails.
Better raise configuration: A smoother raise configuration is obtained which helps in
equipping it in a proper manner. For driving the service and slot raises this technique
finds wide application.
Flexibility and simplicity: Method does not require any elaborate arrangement to accommodate the equipment as it is required in case of raise borers and Alimak raise climbers
(e.g. raise access of 9 m 4 m 3 m). The man, machine and equipment that are meant
for blasthole drilling in the stoping operations can be utilized to undertake this operation. © 2005 by Taylor & Francis Group, LLC 344 SURFACE AND UNDERGROUND EXCAVATIONS Economical: For the raises of small lengths this is the most economical method as better productivity can be achieved by utilizing the same resources meant for stoping
operations in a mine. In addition, to fit this equipment no extra preparation and excavation of any kind are required. It allows the use of cheaper explosive like ANFO, contrary to the costly conventional high-density explosives used in other raising methods.
For the raises upto 25 m lengths this method is almost a mandatory particularly when
two levels are available, and blasthole drills are used in the stoping operations.
13.10 BLASTHOLE RAISING METHOD: DROP RAISING
The advanced version of the long-hole raising technique is the “Drop Raising” in
which large dia. and longer holes are used to drive the raises (figs 13.4(e) and 13.9(a)).
This technique is basically based on the vertical crater retreat (VCR) concept, discussed below:
VCR concept: The term ‘crater’ in blasting terminology is applied for creation of a surface cavity in a rock mass as result of detonating an explosive charge into it. This
blasting concept was initially used as a tool to evaluate the capability of an explosive.
It gained importance in surface blasting operations, and in the recent past, in underground blasting operations too.
Based on the research work carried out, the explosive charges used in crater theory
are spherical or its geometric equivalent. In blasting practice the spherical charge is
defined as the one which is having a length to diameter (L: D) ratio to 1:4 or less, and
up to, but not exceeding a L:D 6:1. Thus, for holes of 165 mm dia. a charge of
990 mm length would constitute a spherical charge.
Crater theory when used for research purposes, the charge is fired in the upward
direction, enabling crater to form towards a horizontal free face. But in an underground
situation when a spherical charge is blasted in the down ward direction towards a free
face, which could be back of an opening or ceiling of an excavation, an entirely new
concept of crater formation has emerged out. In this case crater is formed in downward
direction. Adverse effects of gravity and friction do not affect results. To the contrary,
the gravity enlarges the crater dimension by removing the entire ruptured zone, as shown
in figure 16.14(c).
Once the excavation of an underground opening disturbs equilibriu...
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