Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF T~E INVENTION
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This invention relates generally to longwall mining
methods which are used in coal and the like underground rnines
and more particularly to those designed to alleviate the earth
pressure developing around the working face.
In longwall mining, an excessive earth pressure lS often
met in the vicinity of the workinq face particularly in cases
where the depth of mining is substantially increased and tends
to cause bulging collapse of the working face, thus impairing
the mining efficiency. In coping with such situations, it has
been usual in practice to take measures such as of increasing
the pressure-hearing capacity of the support structure, adding
protecting devices thereto, or restricting the entrance of
mine workers to the working area for safety of mining operation.
SUMMARY OF THE INVENTION
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Under the circumstances, the present invention is
intended to make any measures conventionally taken, in the
longwall mining, for prevention of face bulging collapse
practically needless or to minimize the need for taking such
conventional safety measures.
According to the present invention, there is provided
a new longwall mining method designed to alleviate the earth
pressure developing around the working face and characterized
i in that it comprises the steps of forming in the unmined ore bed
lying ahead of the longwall working place in the direction of
excavation at the level of the working place a tunnel which
extends over the whole width of the working face and between
two face gateways formed on the opposite sides of the unmined
ore bed in order to disperse the stress envelope initially formed
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around the working place and then excavatiny that region o~
the ore bed which lies between the working face and the tunnel.
The above and other ~eatures and advantages of the
present invention will become apparent froJn the following
description when taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
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In the drawings:
Fig. 1 is a diagrammatic plan view, in longitudinal
cross-section, illustrating a longwall mining face;
; Fiq. 2 is a cross-sectional side elevation taken along
the line II-II in Fig. l;
Fig. 3 is a view similar to Fig. 1, illustrating an
embodiment of the present invention as applied to the mine face
of Fig. l;
Fig. 4 is a view similar to Fig. 2, ta~en along the
line IV-IV in Fig. 3;
Fig. 5 is a view similar to Figs. 1 and 3, illustrating
another embodiment of the invention which includes a different
formation of tunnels in the unmined ore bed;
Fig. 6 is a view similar to Figs. 1, 3 and 5, illustrat-
ing a further embodiment of the invention which includes yet
' another tunnel formation in the unmined ore bed;
Figs. 7 and 8 are views similar to Figs. 2 and 4,
i illustrating further embodiments of the present invention which
include tunnels formed in the ground overlying the ore bed to
be excavated and the ground underlying the latter, respectively;
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Fig. g is a view similar to Figs. 1, 3, 5 and 6, illustrat
ing the working face of Fig. 1 further embodying the second
aspect of the mining method of the present invention; and
Fig. 10 is a longitudinal cross section taken along ~he
line X - X in Fig. 9.
Description of the Preferred Embodiments
.
Referring first to Figs. 1 and 2, which diagrammatically
illustrate a longwall mining area, reference numeral 2 indicates
an ore bed such as a coal or rock salt seam; and 1 indicates the
longwall mining face of the ore bed 2 being excavated in the
direction of arrows P. Reference numeral 3 indicates face gate-
ways formed on the opposite sides of the ore bed 2 being ex-
cavated. In such underground working area, a stress envelope
develops as indicated by the chain-dotted lines Al because of
the presence of the mined-out opening or void 4. As is known,
the stress envelope Al represents the zone of heavy stress
concentration formed around the void under ground pressure and
is a factor which can cause bulging collapse at the working face
la while on the other hand exerting unwanted forces upon mining
machines in use with the result that the mining efficiency is
reduced and the machine loads heavily increased. In the example
illustrated, the mined-out opening ~ is left unfilled allowing
- free rock fall and, as with this case, even in cases where the
mined-out opening is filled as excavation proceeds, the state of
balance of earth pressure must be more or less broken compared to
its state before excavation and there exists at all times an open
space extending along the working face over the whole width
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thereof that is required for mining operation. Because of
these facts, the stress envelope developing around the work
ing face is substantially limited in radius of curvature ir~e-
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spective of whether the mined-out area is filled immediately
or left unfilled.
The mining method of the present invention is advantageously
applicable to a longwall mining face which is subject to an
excessive ground pressure as described above. S~ecifically,
as seen in Figs. 3 and 4, there is formed in the unmined ore
bed 2 lying ahead of the working area 1 in the direction of
excavation P at least one elongate void or tunnel 5 which extends
over the whole width w of the working face. Such pressure-
releasing tunnel or tunnels 5 are ordinarily formed substantiallyin parallel to the working face la and extend between the face
gateways 3, formed on the opposite sides of the unmined ore bed
2. The formation of pressure-leasins tunnel or tunnels, however,
may be determined as desired in accordance with the surround-
ing conditions, including the ground condition around the work-
ing area and the state of stress envelope Al, the ore bed 2
being excavated or the over- and underlying grounds 6 and 7.
Various tunnel formations other than that shown in Figs. 3 and
- 4 are illustrated in Figs. 5 to ~. Shown in Fig. 5 are voids
or tunnels 5 which are more or less inclined to the working
face la instead of being parallel thereto and extend through
the unmined ore bed 2 as with the case of Figs. 3 and 4.
Fig. 6 illustrates a tunnel formation similar to that of Fig. 5
except that the tunnels 5 in this formation is inclined in a
direction opposite to those in the formation of Fig. 5.
Fig. 7 illustrates another tunnel formation in which tunnels 5
are formed in the overlying ground 6 at a level slightly above
the ore bed region 2 to be excavated. Reference numeral 3a
; indicates raises driven upwardly from each of the face gateways
3 (not shown in this fiyure) into the overlying ground 6 for
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tunnel driving. A further tunnel formation is shown in Fig. 8
~hich includes tunnels 5 driven in the underlying ground 7 at -~
a level slightly below the ore bed 2 by way of winzes 3b driven
downwardly from the face gateways.
As the result of formation of such voids or pressure-
releasing tunnels 5, the stress envelope Al initially formed
around the working area l is dispersed as indicated by the
chain-dotted lines A2 in Fig. 4. In this manner, the earth
pressure in the vicinity of the working face la is widely reduced
and the danger of bulging collapse eliminated. Subsequently,
the ore bed 2 is ~excavated until the working face la reaches
an appropriate position advanced beyond the first tunnel 5.
Such mining operation is repeated progressively so that at
all times ore excavation is effected efficiently under reduced
ground pressure.
According to the second aspect of the present invention,
in cases where no satisfactory reduction in earth pressure is
obtainable at the working face la just by digging tunnels 5 in
the manner described above, a multitude of njection holes are
drilled in that region of unmined ore bed 2 which lies between
the working face la and the first tunnel 5 and, it desired,
also in the next ore-bed region extending between the first
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and second tunnels, as indicated at B in Figs. 9 and 10, in
advance of actual excavation, and high-pressure liquid of low
cost such as high-pressure water i5 injected into such injection
holes 8. The drilling of injection holes 8 may be advanced
from the working place 1, either one or both of the gateways 3
and/or the neighboring tunnel or tunnels 5. Further, not only
the number and length of injection holes 8 but their angle of
inclination to the horizontal or to the direction of the widt~
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of working face la or to the length of face gateways 3 or that
of tunnels S can be determined as desired in accordance with
the conditions of ore bed 2 and over- and underlying grounds 6
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and 7. In this mining method, the injection of high-pressure
liquid is intended to convert the energy of ground compression
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into that of plastic strain to further reduce the earth pressure
in the ore bed 2 being excavated by utilizing the characteristic
of an ore bed that its ultimate breaking strength is generally
reduced with rise of its water content. Within the ore bed 2
injected with high~pressure liquid, there arise plastic and
flow deformations and the energy held in the bed 2 is converted
from its initial elastic form into an inert one so that the
danger of bulging collapse at the working face la is effectively
eliminated. It will be appreciated, therefore, that the safety
in underground minins operations can be materially increased
by repeating the driving of tunnel 5, injection of high-pressure
liquid into ore bed 2 and its excavation successively in that
order.
As will be apparent from the foregoing description, the
longwall mining method of the present invention has among others
the advantages that the earth pressure at the working face la
can be fully reduced to enable safe excavation involving no
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bulging collapse and that the ground resistance to machine
excavation is widely decreased to enable mining operation with
substantially improved efficiency.
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