Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IMPROVED SLOPE OF AN OPEN CUT MINE
This invention relates to an improved slope of an open cut mine and a method
of forming a reinforced rock slope in an open cut mine. The invention has been
developed for an application in an open cut mine and it will be convenient to
hereinafter describe the invention with reference to this particular
application. It
should be appreciated that the invention may have other applications in an
open
excavation environment, such as a quarry, and the invention is not to be
limited
to use only in a mine.
Mining companies use various methods of extracting ore from the ground, and
an open cut mine is one method that requires the removal of a substantial
volume of material. The excavation of the material develops a pit with a floor
and a slope extending up from the floor. The slope is formed by a series of
batters, with berms spacing the batters and providing rockfall catchment or a
ramp for mining traffic accessing the pit floor. It is generally desirable to
maximise the angle of the slope as it has the potential to minimise the
extraction
of waste material and maximise the extraction of ore.
A batter is generally excavated to an angle that, having regard to the
characteristics of the rock, is naturally stable. This is generally no greater
than
70 and normally more in the range of 50 to 70 . Where the berm at the top of
the batter is acting as a ramp, or accommodating other critical mine
infrastructure such as vent stacks or pumping stations, it is desirable to
decrease the angle to account for the additional load and risk.
It ought to be appreciated that any angle other than 90 will typically result
in
inefficiencies. Where the batter is formed in the ore being mined, a batter of
less
than 90 will result in ore being left behind to maintain the berm above.
Where
the batter is to be formed in waste material, a batter of less than 90 may
result
in waste material being unnecessarily removed, particularly when it is outside
the area of the ore deposit. Despite these inefficiencies open cut mines still
tend
to have batter faces at angles less than 70 to ensure natural stability.
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A safety issue with open cut mines relates to rocks falling from the face of
the
batter onto traffic or personal on the berm below. This is exacerbated where
the
batter is of less than 90 as the rocks bounce off the batter face and project
out
from the toe of the batter onto the berm, or past the berm and into lower
working areas of the mine. It has been found that with a batter angle of about
600, the rocks fall generally between 2m and 9m from the toe of the batter but
may well exceed this. This is a wide spread particularly given a berm may be
generally around 10m.
The above discussed is included in the specification solely for the purpose of
providing a context for the present invention. It is not suggested or
represented
that any of these matters form part of the prior art base or the common
general
knowledge in the field relevant to the present invention as it existed in
Australia
before the priority date of this invention. Throughout this specification the
terms
"open pit" and "open cut" may be used interchangeably.
According to one aspect of this invention there is provided an improved slope
of
an open cut mine including a plurality batters with a berm extending from a
toe
of each batter, each batter having an excavated batter face which is
substantially vertical and extends from the toe to a crest of the batter,
reinforcing means installed prior to the excavation of the face and spaced
from
the proposed crest, the reinforcing means including a plurality of reinforcing
members each located in a substantially vertical borehole formed at said
spacing from the proposed crest, each borehole extends to a depth of at least
the distance between the crest and the toe, each reinforcing member extends
substantially the depth of the each borehole
It is preferred that the depth of each borehole exceeds the distance between
the
crest and the toe by up to 15%. It is further preferred that each borehole is
spaced from an adjacent borehole by between 1000mm and 5000mm. It is still
further preferred that each reinforcing member is a bar or cable of a diameter
of
between 20mm and 100mm. It is preferred that the bar or cable is formed from
steel, or alternatively each reinforcing member is formed from fibreglass. It
is
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preferred that the reinforcing member is grouted in position in the borehole.
It is
preferred that the length of the reinforcing member exceeds the depth of the
borehole, and in particular the reinforcing member extends out from the
borehole to interact with a safety fence extending between the plurality of
boreholes. Alternatively the length of the reinforcing member is less than the
length of the borehole so that the reinforcing member is countersunk in the
borehole. It is preferred that the substantially vertical face is within the
range of
75 to 90 . It is further preferred that each borehole is spaced from the
crest by
no less than 800mm.
According to another aspect of this invention there is provided a method of
forming a reinforced rock slope in an open cut mine including drilling a
plurality
of vertical boreholes to a borehole depth, locating a reinforcing member in
each
borehole which is substantially the length of the borehole depth, grouting the
reinforcing bar in the hole, locating a crest of a batter which is spaced from
the
boreholes, excavating a face to the batter which is substantially parallel
with the
boreholes, the height of the batter face from the crest to a toe is no less
than
the depth of the borehole adjacent the face, wherein the excavation of the
face
is performed after the reinforcing member has been grouted in place in the
borehole.
It is preferred to drill each borehole so that it is no greater than 300 mm in
diameter. It is further preferred to space the crest of the batter from each
borehole so that the reinforcing member is behind the crest of the batter by
no
less than 800mm.lt is still further preferred to space each borehole from its
adjacent borehole at no less than 1000mm centres. It is still further
preferred to
provide a reinforcing member in the form of a reinforcing bar is no less than
40mm in diameter. It is still further preferred to drill the borehole depth to
exceed the height of the face of the batter by at least 15%. It is still
further
preferred to provide a length of the reinforcing member that is less than the
length of the borehole so that the reinforcing member is countersunk in the
borehole. It is still further preferred that the method of excavating the face
includes drilling and blasting to produce the substantially vertical face
within a
range of 75 to 90 .
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It will be convenient to hereinafter describe the invention in greater detail
by
reference to the accompanying drawings showing an example embodiment of
the invention. The particularity of the drawings and the related detailed
description is not to be understood as superseding the generality of the broad
definition of the invention as provided by the claims.
Figure 1 is a side cross-sectional view of a schematic illustration of an
improved
slope according to the invention.
Figure 2 illustrates a schematic view of an unimproved slope prior to
excavation
of the batters.
Figure 3 illustrates an improved slope with the batter excavated.
Referring firstly to Figure 1 that illustrates a natural surface level 1
having been
excavated to produce an improved slope 2 of an open cut pit. The improved
slope 2 includes a plurality of horizontal berms 3, each spaced by a vertical
batter 4. Each batter has a face 5 which extends from a toe 6 of the batter to
a
crest 7. The slope extends from the natural surface level 1 to the floor 8 of
the
pit.
Referring now to Figure 2 which illustrates a portion of the open cut mine
prior
to excavation. A crest 7 of the proposed batter 4 is selected, and a series of
boreholes 9 spaced from that crest 7 are drilled to a borehole depth. This
spacing is preferably no less than 800 mm, however this spacing x is merely
preferred. Spacings x of other distances are clearly possible, however it is
generally desirable that the spacing x be selected to be sufficient so as to
reduce the likelihood that the borehole 9 will be exposed when the batter is
excavated. The face 5 of the batter is exposed by a technique of drill and
blasting which can cause the crest 7 to crumble. Accordingly it is preferable
that
the borehole 9 be spaced from the crest by 1500mm to avoid the hole being
exposed when blasting.
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It is preferred that each borehole 9 have a diameter of no greater than 300
mm,
however this dimension is merely preferred. The function of the borehole 9 is
to
accommodate a reinforcing member 10 and therefore the diameter of the
borehole 9 is dictated to some extent by the characteristics of the
reinforcing
5 member 10. Furthermore, it is preferred that the reinforcing member 10 be
grouted in position in the borehole 9, and therefore the diameter of the
borehole
will need to be selected to satisfy minimum cover requirements, particularly
where the reinforcing member 10 is a steel bar. Accordingly, it is to be
appreciated that the diameter of the borehole 9 may vary, however it is
generally preferred that the diameter be less than 300 mm.
Each borehole 9 is preferably drilled behind the crest at a spacing y to space
it
from an adjacent borehole preferably by no less than 2000 mm, where the berm
is not supporting critical mine infrastructure. Where the berm is acting as a
ramp, or supporting critical mine infrastructure such as pumping stations or
ventilation shafts, the spacing of the boreholes may need to be less than
2000mm. It is more likely that each borehole be spaced y from an adjacent
borehole by between 2,000 and 5,000 mm. This spacing y will specifically be
dependent on the characteristics of the rock structure being reinforced by the
reinforcing member 10 and the desired level of reliability in the slope
stability. If
for example the rock structure comprises many closely spaced planes of
weakness such as a fault, joints or fracture, it may be appropriate to have
the
spacings y of each adjacent borehole relatively close. In contrast, if the
rock
structure comprises very few planes of weakness it may be appropriate to have
the spacings y relatively further apart. In either case, the spacings y can be
set
having regard to information revealed in a geological survey and variable
between adjacent boreholes 9.
The reinforcing member 10 located in each borehole 9 may take any
appropriate form. The preferred form of reinforcing member is a bar or cable
formed from steel, however as an alternative it may be formed as a fibreglass
rod. Where the reinforcing member 10 is a bar of steel, it is preferred that
the
diameter of the bar be between 20 mm and 100 mm. The diameter of the bar
will be selected according to the characteristics of the rock, depth of
borehole
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and/or the desired level of reliability in slope stability. The bar is placed
in situ
and grouted in position.
In one preferred embodiment it is preferred that the length of each
reinforcing
member 10 does not exceed the depth of the borehole 9. It is generally
desirable that the reinforcing member 10 and grout are countersunk so as to
reduce the likelihood that the top of the reinforcing member 10 is exposed.
The
top of the reinforcing member 10 might be exposed if the crest 7 crumbles, say
as a result of blasting the batter face 5. Accordingly countersinking to a
depth of
1000mm or greater is desirable. It may however be appropriate in certain
applications that the reinforcing member 10 does exceed the depth of the
borehole 9 as this enables the reinforcing member 10 to project from the
borehole 9. The projecting reinforcing member 10 may be used for any suitable
purpose, however it is preferred that it interacts with a safety fence 11
extending
between the plurality of adjacent boreholes 9. This alternate embodiment is
illustrated in figures 2 and 3.
The material 12 beyond the crest 7 that is to be excavated can be excavated by
any suitable means. The preferred means illustrated in Figure 2 involves
drilling
a plurality of additional boreholes 13 at the crest 7 of the batter 4, and
beyond
the crest 7. Charges can then be placed in the boreholes 13 and the material
be moved using drill and blast techniques understood and known by those
people operating in this industry.
Referring now to Figure 3 which illustrates the improved slope with the
exposed
batter face 5 extending at 90 to the berm 3 below. The batter face 5
according
to this invention is substantially vertical, and it is generally preferred
that the
substantially vertical face 5 be within the range of 750- 90 .
The boreholes 9 including the reinforcing member 10 according to the invention
extends to a depth at least equal to a distance between the crest 7 and toe 6
of
the batter face 5. It can be seen from Figure 3 that it is preferred that the
borehole depth extend beyond the toe, and it is particularly preferred that it
extend beyond the toe by up to 15%. It has been found that by extending the
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reinforced borehole 9 by this distance reduces the likelihood of shear failure
of
the batter at the toe of the batter.
It ought to be appreciated from the foregoing that an improved slope 2 as
hereinbefore described reduces the inefficiencies associated with a slope
having a batter angle of less than 700. Furthermore, it will reduce the
distance
from the toe within which rock fall occurs, providing a safer working
environment. Still furthermore, reinforcing the batter 4 before excavating the
batter face 5 is considered to be a relatively safe operation.
Various alterations, modifications and/or additions may be introduced into the
invention without departing from the spirit or ambit as defined in the
preceding
specification.
