Note: Descriptions are shown in the official language in which they were submitted.
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CONCRETE BLOCK MINING METHOD
The present invention relates to a method of mining a
deposit in unstable subsurface conditions, such as caused by
clay or an aquifer.
An ore deposit located in an unstable subsurface may pose
a serious challenge to efforts to mine the deposit,
particularly where the unstable subsurface surrounds the
deposit. For example, the surrounding unstable conditions
typically prevent or hinder drifting of stable working
galleries above, to a side, or below the deposit. If the
deposit is hazardous, such as in the case of a deposit of high
grade uranium, there is the additional risk of water borne
contamination coming from the ore source through unstable
subsurface and penetrating any development drift in the
vicinity of the ore body.
Problems from unstable subsurface encountered in the
context of applying various known mining methods have prompted
the development of specific solutions. For example, U.S.
Patent No. 4,431,341, issued to Nilberg, discloses a method of
constructing a concrete-lined, subsurface chamber along a bore
hole. The chamber may be used for conducting mining operations
such as horizontal drilling. In this method a concrete-lined
bell-shaped chamber is made by steps which include excavating
concrete from within side-walls of an initially poured and
hardened bell-shaped concrete mass. Thus, the remaining
concrete acts as a support liner for the excavated space. The
method is stated to be particularly useful in unstable
subsurfaces such as tar sands and oil sands. We note that the
method does not involve any transient stabilizing of the
subsurface by the use of freezing. It may be that compressed
air would be used to keep the excavation from collapsing during
the construction phase compression chambers created would
prevent the flooding of the excavation while it was being mined
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and before the concrete lining was completed. The method is
also for use in vertical bore hole applications and is neither
taught nor appears to be workable in connection with a
horizontal excavation of a mine gallery through unstable
subsurface.
United States Patent No. 4,516,878 issued to Rebhan
describes horizontal tunnel building, rather than a mining
method, using the concept of freezing soil. In this method,
a soil freezing work pipe is inserted into a hole bored along
the axis of a planned tunnel and the soil is cooled until a
frozen soil mass is formed around the work pipe. When the
frozen mass has a diametre greater than that of the planned
tunnel, frozen soil is excavated while leaving a frozen soil
lining. Such lining is then immediately supported by a primary
or permanent support lining. The primary support lining may
be blown or injected concrete, and the permanent lining may be
extruded or reinforced concrete. The establishment of the
primary and permanent linings is conventional but
unsatisfactory in application to mining unstable subsurface
where massive support is required for an excavated area.
Canadian Patent No. 210,592 issued to Stuckey discloses
a method of mining shafts, stopes or drives which consists of
drilling a hole to intersect water bearing or conveying
crevices, closing the mouth of the hole, forcing cement or
other solidifying material into the hole whereby the bore and
surrounding crevices are filled with solid material so as to
prevent the flow of water in the crevices, and subsequently
removing the rock or matrix in the hole by ordinary mining
operations. This method is principally used to gout or fill
cracks in rock and not to stabilize poor or unstable ground.
There is no indication how this method may work in particularly
unstable subsurface conditions where the initial bore hole
cannot remain intact until the subsequent steps are performed.
It is not possible to inject cement into massive clays or
similar unstable ground as any cracks that may develop in clays
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are usually filled by that material because of its elastic
properties. This method therefore also appears to be entirely
unsuitable to producing a horizontal working gallery in poor
subsurface conditions.
Finally, Canadian Patent No. 21, 176, which issued in 1885,
discloses a method of sinking shafts through unstable
subsurfaces, which method includes freezing a portion of the
subsurface. Freeze pipes are used to freeze the relevant area
whereupon excavation of the resulting frozen block may be
performed. The freezing is maintained until the excavated
shaft is lined entirely and finished in masonry or other
permanent material. This patent relates to vertical shaft
construction and does not teach any application to horizontal
gallery excavation through surrounding unstable subsurface.
The above sampling of known methods fails to reveal any
method which would be useful in excavating horizontal mine
galleries through surrounding unstable subsurface in a manner
so as to provide a secure, potentially permanent gallery, in
particular a working gallery for an ore deposit.
SUMMARY OF THE INVENTION
The invention therefore provides, in one aspect, a method
of constructing a concrete-lined horizontal gallery in an
unstable subsurface. Firstly, from an established gallery or
shaft, a zone of unstable subsurface is frozen extending in a
selected horizontal direction. The zone has preselected
dimensions for eventually accommodating the horizontal gallery
in accordance with the following described method steps. Next,
a preliminary chamber is excavated, so as to have a non-
collapsing frozen periphery, from within a frozen perimeter of
the zone. The preliminary chamber has dimensions selected to
correspond to an outer perimeter of a non-collapsing concrete
lining of the concrete-lined horizontal gallery to be
subsequently formed in the preliminary chamber. The
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preliminary chamber is then completely filled with settable
concrete. After the concrete has set, a final chamber is
excavated from within an outer perimeter of the concrete so as
to have a non-collapsing concrete periphery as well as gallery
dimensions, thereby forming a non-collapsing concrete-lined
horizontal gallery. It may be preferred, before mining the
final chamber, that a series of preliminary chambers are mined
and filled with suitable concrete in order to establish the
massive concrete block from within which the final chamber is
established.
In another aspect of the invention, there is provided a
method of mining a deposit located in unstable subsurface,
which method comprises, firstly, sinking a shaft in
substantially stable subsurface at least to a depth from which
a horizontal gallery may be extended partly through stable
subsurface towards a preselected location in unstable
subsurface for constructing a working gallery for the deposit.
Preferably, there are two such horizontal galleries, one for
extending above and the other below the deposit. Secondly, the
method comprises extending a horizontal gallery from the shaft
towards the preselected location through substantially stable
subsurface until intervening unstable subsurface is
encountered. Next, a zone of unstable subsurface is frozen
extending, from where the intervening unstable subsurface is
encountered, in a horizontal direction towards said preselected
location, the zone having preselected dimensions for
accommodating an extension of the horizontal gallery in
accordance with construction steps described subsequently. A
preliminary chamber is then excavated which has a non-
collapsing frozen periphery from within a frozen perimeter of
the zone, the preliminary chamber having dimensions
corresponding to an outer perimeter of a non-collapsing
concrete lining of a gallery selected to be formed in the
preliminary chamber. The preliminary chamber is then
completely filled with settable concrete, the concrete is
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allowed to set, and a final chamber having a non-collapsing
concrete periphery and gallery dimensions is excavated from
within an outer perimeter of the concrete, thereby forming a
non-collapsing concrete-lined gallery. The steps of freezing
a zone through to excavation of a concrete lined gallery may
be repeated sequentially to advance construction of the
horizontal gallery until a working gallery is provided at the
preselected location. Once the working gallery has been
formed, mining of the deposit occurs from the working gallery.
It is an advantage of the present methods of the invention
that there would be a substantial contribution to the sealing
of all fissures through which water might percolate, and
thereby prevent deteriorating ground conditions from being
established if any conventional mining might be established.
In addition, such methods would also serve as a shield from
certain kinds of radiation that emanate from a high grade
uranium deposit, and would serve as a base from which other
protective methods for other forms of radiation could be
controlled.
BRIEF DESCRIPTION OF THE DRAWINGS
In drawings representing a preferred embodiment of the
present invention,
Figure 1 illustrates a vertical section through subsurface in
which the early stages of freezing and initial subsurface
excavation according to the present method have been performed;
and
Figure 2 illustrates a vertical section through subsurface in
which the later stages of concrete filling and excavation of
the concrete according to the present method have been
performed.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
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The present method uses the principle of freezing to
support unstable ground while an excavation is being
established. Following this, a permanent, massive support
system is installed and then the temporary control, the
freezing, is turned off once the permanent support is in place.
The present method is for use in gaining access to any
part of an ore body which is not normally accessible using
conventional horizontal mining methods. It is particularly
applicable in some of the high grade uranium deposits in
Northern Saskatchewan where the ore is surrounded by a halo of
weak clay material which, once mined, does not have the
intrinsic strength to stay open under normal conditions and
allow permanent access for the removal of the ore. For the
purposes of the Figures, access to a position above the ore
body is described but this can apply to below or to any
position where access is required.
The present method uses the freezing technique to gain
access to the desired stoping position and establish a
permanent access. Figure 1 shows an ore body and an access
drift "A" at some distance above the ore body. Normally, the
access drift cannot be mined beyond "A" to a position above the
ore body due to instability of the surrounding subsurface, e.g.
clay. Preferably, a series of horizontally extending freeze-
holes "B" are drilled around the position or access that is
ideal for stoping the ore body, and a frozen ring is thus
created in the clay. Once the unstable area is protected, i.e.
rendered stable by the freezing, an excavation "C" is
established. Note that a temporary support perimeter of
frozen clay is left around the excavation. The entire
excavation "C" is then backfilled with competent concrete.
Once the concrete has set, a smaller excavation "D" (referring
now to Figure 2) is made within the concrete to form an upper
chamber for removal of the ore using, for example, raise bore
stoping methods. It may be necessary to undertake a similar
procedure to establish an access below the ore body by carrying
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out the same procedure at point "E". The thickness of the
concrete around "D" will have to be designed to withstand the
pressure that the clay or an aquifer exerts on the excavation.
Geoscientific calculations preferably are used the determine
the desired thickness. This final excavation preferably should
be done by using a Roadheading machine to establish "D" so that
there is no weakening of the protective concrete such as may
occur by using drill and blast methods.
The principal access established from "A" over the top of
the ore body within a protective concrete shell provides the
basic primary access for stoping. Preparation for stoping
could proceed as follows: any preparation work for the stoping
freeze hole drilling could be established. Raise boring could
then be undertaken of frozen block below. The concrete block
could be advanced laterally in both directions from within the
block by drilling freeze holes, excavation of the original clay
material, installing concrete and then excavating a mining
chamber. The second chamber would be a continuation and joined
to the initial concrete block excavation. As the chamber gets
bigger, it will have to be supported internally to prevent
collapse. Progressively the entire ore body can be accessed
using lateral extension of the Raise Boring chamber, and mined.
Should cribbing be used as a support, then the excavation can
effectively be kept open for the life of the mine.
The present method is innovative in that freezing is used
to establish an access to the ore body and a concrete block is
used to provide a competent shell from within which a permanent
access for stoping is available. The freezing that is applied
to gain access initially can be shut off once the concrete
block has been established. The excavation within the concrete
block can be maintained indefinitely using timber cribs and
does not require backfilling as stoping advances and excavation
within the prepared block extends in outward directions. The
method will afford a minimum disruption to the surrounding clay
area and, assuming the concrete block is adequately designed
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and supported, could be kept open indefinitely.
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