CA1135731A - Method of mining heavy coal seams in two or more benches - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

In mining a heavy coal seam that must be excavated in several benches, the excavation of the uppermost bench is accompanied by the introduction of a cementitious slurry into the resulting stope for consolidating the waste rock present therein, preparatorily to the excavation of the next-lower bench, and proceeding in like manner with the second bench if a third one is to follow. The cementitious slurry comprises burnt or slaked lime, and/or portland cement, suspended in water in a quantity of about 10 to 60 % by weight and preferably together with a small percentage of chlorides of one or more alkali or alkaline--earth metals, to which ceramic aggregates of large specific surface such as mineral ashes, slag, sand or dolomite powder may be added in a quantity of up to about 30 % of the weight of the water. The slurry, on being admitted to the stope in an amount of at least 10 % of the volume thereof, causes the waste rock to swell and to form a solid layer of up to approximately a meter in thickness which allows the next-lower bench to be excavated after only about a month's delay.

Description

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MET~OD OF MINING HEAVY CO~ SEAMS IN ~WO
OR MORE BE~oHES

Field of the Invention Our present invent1on relates-to a method of minlng heavy ooal ~eam~ which require excavation in two or more benohe~ at di~erent level~.
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In ~uch multilevel exca~ations it i~ ~ece3~ary to delay the start of operations at a lower level until well after the exca~ation o~ an upper level has bee~
oomple~ed i~ order to~ e the caved-in overburden and the ~rag~ented roc~s ~ e stope o~ the upper leYel time to ~ub~ide or settle~1nto a 901~a ~tructure foxm1ng a :co~pete~t roor ~or t~e next-lower ~tope. Such a sub~id~c~
period orten~la~ts~about one to three years~ dependi~g o~ local oo~diti~s and of~icial ~a~ety requirement~.
~ he drawbacks inhere~t ~n thi~ convent1onal ~ode o~ operatlo~ are manifold. ~hu~, the rs~idual coal prese~t ln the loo~e rock ~tructure of the upper stope may ~po~ta~eously 1~nite, e~pecially i~ mine~ endangered by .
~iredamp. Con~iderable expenditures are involved in , .. : .
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measures de~igned to guard againqt ~erious catastrophes which, howeYer, cannot be definitely prevented. During this waiting period, furthermore, the ~ain gallerie~ of the mining area and other subterranean facilities such as pu~pint chamber~, transformer ~tation~, power ~upplie9 and ven-tilation sy~tems must be maintained, again at considerable cost and with ~uch effort. The delay, obviously, keèp~ the mlne output low; moreover, the theoretical e~cavation rate i~ reduced since work underneath a caved-in stope cannot be carried out as efficiently as under virgin overburden ~o that the rate of excavation at the lower le~çl is diminished by about 30 ~o 60 ~. Finally, additional ~afety mea~ures are generally needed even after a long sub~idence period to enYure a suf~icient firmne~s of the roof at the lower level.
According to a prior proposal, a cutting and loading machine working on ~ mine face of an upper bench entrain~ a mat of wire netting to intercept the fragmented rock. The mat and the rock fragmentg serve as a supplemental roof for the next~lower level, yet this technique i8 not free ~rom problem~ of operation and safety. Thus, the correct emplacemsnt of the mat in the wake of the excavating ~achi~e ia complicated; even with proper positioning~ the mat can only lesse~ the impact of dropping clumps o~ o~erburden upon the underlying rock structure forming the roof of the ~13~73 next-lower stope but cannot densify or consolidate the rubble in the upper ~tope and t~us does not significantly contribute to the ~tability of the structure. Experience ha~ al~o ~hown that the mat will withstant only limited impact a~d will be torn b~ bloek~ of ~e~eral tons of overburd~n falling upon it; this may result in serious diff~cultiea ~or the operatlo~s going on at the lower level.
Furthermore~ the meohanical ~tres~es and diQlocations cauaed by thi~ ~thod on the floor of the upper ~ope could promote spont3neous ignition and might re~lt in undetected firee smoldering under the loosely piled rock fragments.
~ he use of liquid bonding agents to help ~olidify the roof or the walls of an undereround vault i~ al80 known.
A compo~it~on of this t~pe, known as ~hotcrete~ consi~t~ -o~ a ~ixture o~ comminuted portland eement, ~and and wa~er a~d can be ~prayed onto a tunnel wall to fill smal~ vold~
between roek ~ragment~. Another hydraulic bonding agent, .
described in German printed specification No. ~,216,0399 comprise~ gr~nular natural anhydrite and gyp~um ~e~ihydrate in a certain qu~ntity o~ water, tD whish an acti~ator may b~ add~d. Gon~entional techniques for using these composition~ cannot be readily utili~ed for reinforcing a ~tope, formed duri~g excavation of a coal bench undcr a pre~iously axoavated and caved-in le~el, to prevent it8 premature collap~e.

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The general ob~ect of our present invention, therefor~ to provide an lmproved method of mlnlng hea~y .coal ~ea~ in two or ~ore benches with avoidance of the above-di~cussed drawback~.
A more particular ob~ect i~ to provide a method o~
thi~ charactsr which utili~eQ inexpensive and abundantly available ~ub~tance~ for its implementation.

on I~ the mlning of a coal 3eam pur3ua~t to our pre~ent i~rentlon, a con~ntional initial step of excavatlng a~ upper bench with ~ormation of a ~tope in the wake o~ the excavation i~ ~ollowed b~ the introduction of a cementitious slurry into that stope in an amount upward~ of ~ub~tantially 10 ~ of that ~olume~ thl8 ~lurry compri~ing an aqueous ~uapension of calcareous matter9 in a proportion of substantially 10 to 60 ~0 by weight, to flood and engulf fragmented wa~te rock accumulatlng at the botto~.of the stope, After the hardening of the slurry ana the e~ ~lfed waste rock into a ~olid layer, th~ next-lower bench u~d~r that layer i~ ezcaYated. A 3i~ilAr layer i8 formed in the wakQ of the latter excavation i~ thi~ ~tep3 in its turn, is to be followed by furth~r exca~ating on a 9till lower lev~i, and 80 on.

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In mo~t lnstancc~, an a~ount oî ~lurry rangi3lg between about 20 and 25 ~ of tlle stope Yolume will be highly satisfactory.

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The surpri~ing effect of con~olidation of the bottom of the upper stope, which generally allows the ~tart of operations at the next-lower level after a delay on the order of one month instead of one or more year~, i9 due to the fact that the fine fraction of the fragmented overburden /having a particle size o~ less th~n 1 mm/ acts as a hydraulic aggregate in the cementitiou~ slurry. Thi~ fraction generally accounts ~or about 5 to 10 ~0, by volume9 o~ the overall amount of waste rock collap~ing onto the stope bottom. The compo~ition of the orerburden or capping, of cour~e, playQ a part in the cohe~iveness of the resulting layer~ The u~ual constituents such as clay, sand and the variou~ typeQ of marl can all be consolidated when pre~e~t in the rock fragment~. Shell ~arl i8 particularly advantageous in thiæ respect ~ince the calcium carbonate ~f the fossll ~nail shell~ enhances the solidificationO We have found that the rock fragme~ts permeated by the cementitious slurry not onl~ cohere but are al~o inte~nally con~olidatsd. ~huæ, the larger ~ragments are initially plastified and be~in to well u~der the efrect of the liquid andg together with the inte~ve~i~g similarly expanding ri~er fraction~, ~or~ a ~early air-impermeable ~tratum which harden~ llke concreteO
; ~he pre~erce of this hardenéd layer, the moi~turi~ing and heat-absorbing e~ect of the treat~ent liquid, and the oeallng Or r1rtually all air pas~ages comblne to minimiæe the "' .. ,. . .: .
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~3~3 ri~k sf spontaneous ignition. This concrete-like layer; which may have a thicknes~ between about 10 cm and 1 m, is of great load-bearing capacity found to increase even further under extcrnal pre~ure as the overburden in the abandoned part of the ~tope caves in on it.
A preferred range of the proportion of calcareous matter in the water of the ~u~pen~ion 1~ between about 20 and 40 ~ by weight. With thi~ ~uspen~ion we may ad~ix a chloride or one or more alkali or alkali~e-earth metals in a proportion between ~ub~tantially 0.~ and 6 % by weight, again with re~erence to the water9 preferably with a lower limit of about 0.8 ~ and an upper limit of about 3 %, thi~ admixture not only acoelerate~ the hardening proce~
but iB also found to increa~e both the initial and the final compre3sion resistance of the layer.
I~ ~ome in~tance~, as where there i~ an lnsurficient-amount o~ shell marl in the rock or where lim~stone rock produ¢e~ o~ly a s~all amount o~ fine grain ~raotion, the su~pen~ion may be enriched with cera~ic ~ggr~gate~ o~ largo ~pecific sur~aGe ~uch a~ cinders or 81ag readil~ arailable ~rom th~ boilers of an associated power pl2nt. Other aggregates of thi~ type inolude ~a~d~ and dolomlte powd~r. '~he ~omminuted aggregates may be added in a proportio~ o~ about ~ to 30 ~, preferably 15 to 20 ~D by we~ght wlth reference to the water of ~he ~u~pension.
~ h2 cementitious slurr~, with the added -- 8 ~

aggregates /if any/, may be prepared on the surface or und~rground and can be ~ed in by gravity and/or by pumping.
~o promote densification, the rubble inundated by this slurry i~ ~ubJeoted to mechanical agitation, ~uch as vibration.
To test the e~fectivene3s of our improred method~
~amples of the consolidated roc~ fragment~ /impregnated with the alurry in an amount of 20 ~ by volume, referred to the ~olu~ of the ~tope/ were subjected to a load correspo~ding to that o~ a caved-in ~tope. A~ter a loading for 30 day~9 the .
~n~-way br~aking strength of these samples was measured.
A serie~ of such test~ was performed with ~umachelle-type capping, thi~ being a rock characterized by a bigh co~tent of CaC03 of nonuni~orm distribution. ~he calcium carbonate wa~ found to be particularly prevalent around the mother rock wherea~ clay or occl~sion~ of bitumi~ous coal predominated elsewhere.

tough calclum-rich ~ample/ :-CaCo3 86.5 ~
gC33 2.1 %
residual slurry 11 4 ~0 B, /les~ calcium rich ~ample/
aa~3~ ! 66.0 ~0 ' ' ~gC03 1.4 ,~0 Re~idual slurry 32.6 ~0 C. /arerage composition/
carbonate content 82.0 ~0 ~3~

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re~idual slurry 18.0 ~
The one-way breaking ~trength in kp/cm2 ~kp -o kilopond, or kilogram forco/ i8 about 50 for coal and abou~
1~0 ~or the ~nperturbed oYerburdenO Corre~ponding value3 found ~or ~ampleæ treated with di~ferent compo~itions of slurry in accordance with our inrention are given in the following table:

Sample No. Composition of Slurry in ~reakin~, -- Percent by Yolume of Water Stre~gth in .. ' , kp/cm2 Portland Slaked : -Cement Lime MgC12 , 1 . 2240 - - 0~75 116, 34.~ ~ ! 1.20 167 3 18.0 4.~ 0.~5 102 4 26.0 8.0 1.20 . 145 .- ~ .
It will thu~ be se~n that the b~eaking strength after a 30-day ~and under load come~ close to a~d i~ some in~ta~ce~ even ~urpa8~e~ that o~ the vir~in overburden besiaes me ting the rzquirements.of tee~nological ~ea~ibility and ? operational sa~ety.

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~he above ~nd other ~eature~ o~ our present . J

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~ 10 --invention ~ill now be described in detail with reference to the accompanying drawing in which:
~ r~. 1 i9 a plan view schematically illustrating a two~le~el excavation of a coal seam in accordance with our inrention;
~ IG. 2 is a similar plan view illustrating a somewhat di~ferent mode of operation;
FIG. 3 is a plan view showing excavation of large--size coal seam in four layers;
. FIG. 4 is a view similar to ~IG. 3 but illustrating a modiflcat10n similar to that of ~IG. 2; and - FIG~ 5 is a cross-sectionaliview of a coal ~eam being excavated in the manner illustrated in EIG. l.

Reference will first be made to FIGS~ l and 5 in which a coal ~eam l /FIG.5/, overlain by bedrock 2 9 iS to be ~ined on two levela by the ~o-called longwall ~ethod. At the upper level, two parallel galleries 12a and 12b are built t4 com~unicate via cross cuts 12a and 17b with a ~ain gallery 8 and a venti1a~ing duct 9, the latter being shown o~ly in -~IGS. 3 and 4. Excavation starts at a cros3-cut 17c, interconnecting the two galleries 12a and 12b, to produce a mlne face ll pro~re~ing in the direction of an arrow A. ~h~
excaYating and load~ng equipment working on that mine face 73~

ha~ been æchematically indicated at 19 in EIG. 5.
A pipeline 3 at the bottom of gallery 12a carrle~
slurry from a nonillustrated underground or ~qurface 80urc~.
As the face 11 progreqse~, the crew handling the equipment 19 connects perforated branch pipe~ 4 to line 3 at locations spaced about 20 to 50 ~eters apart in the ~tope lO being ~ormed, The roof of the stope is 3upported in the usua7 manner by temporary props, not shown, which are subse~ue~tl~
withdrawn to let the overburden cave in at a safe dist~nc0 from the mine face 11 as indicated at 14. Prior to this cave-in, however, slurry 5 exiting from the branch pipe3 4 has formed a pool at the bottom o~ the stope which engulf~
accumulating ~ragments of wa~te rock and consolidates the~
i~to a concrete-like layer 6 as described abore. As the exca~ation progre~ses, the flow o~ slurry is cut off ~u~t ahead of the ~ections of pipeIine 3 about to be buried by the cave in.
- Additional slurry may be fed into the ~tope 19, i~ de~ired, from ho~es carried by the exca~ating equip~e~t 19,:
After the hardening of layer 6 under the c~ed- .
o~erburden 14, a ~econd e~cavatio~ iæ commenoed at a lower level with lateral galleries 22a and 22b mo~ing forward from a cro~s-cut 27c which extends at that le~el ~rom the mai~ gallery 8; the ends 23a, 23b o~ the~e lower 3~

gallerie~ ar~ ~aparated fro~ upper face 11 by a distance 26, ~xca~ating and loading equip~ent 29, working on a face 21, thus produces a lower ~tope 20 under eath the consolidated layer 60 ~he excavation o~ the lower bench along face 21 lags that of the upper bench along face 11 with a delay o~
about one month. '~he advance of lower faoe 21, trailing tha gallery ende 23a, 23b, ha~ been indicated by an arrow ~.
If desired, the roof o~ ~tope 20 can be further con~olidated by the pumping of additional slurry into aperture~ drilled from the 3tope 20 into the overhangi~g coal and/or rock structure.
Galleries 22a and 22~ must, of cour3e, be kept open during the entire excavation o~ the lower benoh for -ventilation, hAulage, and traffic by ~en and machine~.
In ~IG, 2 we have ~chematically illustrated a ~ode o~ operation involving rearward excavation at both level~. ~hu~9 two deep drift~ 122a, 122b interconnected b~ a cro~-cut 127c are for~ed around the area to be mi~ed and commu~icate ~ia cro~s-cut~ 127a, 127b with ~he main gall~ry 8 and with the ventilating duct 9 /cf. ~IGS~ 3 and 4/. Upper and lower galleries are in~talled in the~e drifts9 the uppcr galleries being buried by the progre~ive cave~in of the upper 8tope. The~e galleries, therefore, are abando~ed upon the~excavation o~ the lower bench. ~he two face~ 11 and 21 again adv~n¢eg a~ indicated by arrows A and B, with a oeparation 26. ~he con301idating layer 6 /~IG. 5, advanclng ,.~ .

- 13 _ generally at the ~ame rate as the upper face 11, also e2tend~ above the lower galleries in drift~ 122a and 122b.
~he layout of FIG. 2 enable~ the mining o~ a ¢oal vein havi~g a depth of about 4.5 to 6 meters, possibly eren up.
to 7 meters with the use of digging e~uipment protected by a tall shlcld.
~ IG. 3 shows four ~ine faces 119 21, 31 and 41 advancing, a~ reæpectively indicated by arrows A, B9 C
and D, on progressi~ely lower levels for the mining o~ a very deep and long 3eam. In thi~ instance, after co~solidat1on of the caved-in part 14 o~ the uppermost stope~ cro~-out~ 27a, 27b are formed at a di~tance 25 from face 11 to mark the starting points 23a9 23b of the ~ext pair of parallel gallerie~ 22a, 22b to ba dug preparatorily to e~cavation of the ~econd bench. When 'che corresponding ~tope has caved in on a supporting cementitious layer over a~ area 24, further cross-outa 37a a~d 37b are made at a distance 35 ~rom face 21 to mark the Ytarting point~ 33a~ 3~b o~ the next lower p~ir o~
gallcrie~ 32a, 32bo After the thira stope ha~ caved in and consolidated in an area 34, additional cros~-cuts 47a 47b are maae at a distance 45 ~rom face 31 to ~ar~ the ~tarting point~ 43a, 43b of galleries 42a, 42b at the rourth level, preparatorily to the excavation of the.

~ ~ 3 - 14 _ lower~o~t bench.
FI~. 4 show~ a combination of the method~
rapresented by FIGS. 2 and 3, with formation of two pairs of deep drift~ 122a, 122b to accommodate the galleries of the two upper levels and a ~imilar pair o~ drifts 342a, 342b for the gallerie~ o~ the two lowex levels. ~he latter drifts communicate with main gallery 8 and ventilation duct 9 via cros~-cut~ 47a and 47b~ re3pectiYelyO Face~ 11 and ~1 of the two upper benche~ are ~eparated by a di~tance 26 whereas aces ~1 and 41 o~ the two lower benches are separated by a distance 36; the ~eparation of cro~-cuts 37a and 37b ~rom face Zl he~ been dc~igneted 35.
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Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of mining heavy coal seams to be excavated in a plurality of benches at different levels, comprising the steps of:
/a/ excavating an upper bench with formation of a stope in the wake of the excavation;
/b/ introducing a cementitious slurry into said stope in a minimum amount of substantially 10 % of the stope volume, said slurry comprising an aqueous suspension of calcareous matter in a proportion of substantially 10 to 60 % by weight, to engulf fragmented waste rock accumulating at the bottom of said stope;
/c/ allowing said slurry and said waste rock to harden into a solid layer;
/d/ excavating a next-lower bench under the solid layer thus formed; and /e/ repeating steps /b/ and /c/ with every stope to be undermined by a further excavation at a lower level.

2. A method as defined in claim 1 wherein said slurry is admixed with a chloride of at least one alkali or alkaline-earth metal in a proportion between substantially 0.3 % or 6 % by weight with reference to the water of said suspension.

3. A method as defined in claim 2 wherein the proportion of the admixed chloride ranges between substantially 0.8% and 3%, by weight, of the water.

4. A method as defined in claim 1, 2 or 3 wherein the proportion of said calcareous matter ranges between substantially 20% and 40%, by weight, of the water.

5. A method as defined in claim 1, 2 or 3 wherein said slurry is introduced in step (b) in an amount ranging between substantially 20% and 25% of the volume of the stope.

6. A method as defined in claim 1, wherein ceramic aggregates of large specific surface are added to said slurry in an amount between substantially 5% and 30% by weight.

7. A method as defined in claim 6 wherein the amount of said aggregates ranges between substantially 15% and 20%
of the weight of said slurry.

8. A method as defined in claim 1, 2 or 3 comprising the further step of mechanically agitating the fragmented waste rock engulfed by said slurry in step (b).

9. A method as defined in claim 1 wherein excavations at successively lower levels follow one another with delays corresponding to the hardening time in step (c).

10. A method as defined in claim 9 wherein said hardening time is on the order of one month,

11. A method as defined in claim 9 wherein step /a/ is preceded by the building of a gallery common to said upper bench and to said next-lower bench, the part of said gallery at the level of said upper bench being flooded with said slurry in step /b/ and being abandoned in step /d/.