application of geomechanics in longwall operation

7/29/2019 Application of Geomechanics in Longwall Operation

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APPLICATION OF GEOMECHANICS I N LONGWALL OPERATION

D. S. ChoiConoco, IncorporatedPonca City, Oklahoma

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Permission i s hereby given to p ubl is h with appropr iate acknowledgments,excerp ts or summaries not t o exceed one-fourth of the e ntire text of the paper.Permission to print i n more extended form subsequent to publication by the Institu temu st be obtained from the E xecut ive Director of the Society of Mining Engineersof AIME.If and when this paper is published by the Suc iety of Mining Engineers of AOME, i tmay embody certain changes made by agreement between the Technical PublicationsCom mittee and the author, so that the form in wh ich i t appears here is not necessarilythat inwhich it may b e pub lished later.These $reprints are available for sale. M ail orders to PREPRINTS, Society of MiningEngineers, Caller No. D, Litt leton, Colorado 80.929.

PREPRINT AVAILABILITY LIST IS PUBLISHED PERlODICAbbY IN


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INTRODUCTION

Coal i.s expected t o become an inc r eas i ngl y impor tan t source of ene rgy i nth e fu tu re . Coal produ ct ion has a l read y increased f rom about 650mi l l i on t ons i n 2975 t o about 875 mi l l i on t ons i n 1984 (Wi lk inson ,1985) . The de c l i n in g t r e nd of l a bo r p r odu c t i v i t y i n t he e a r l y s e ve n t i e swas a so ur ce of conce rn f o r Co ns ol id ati on Coal Company (Consol). Toimprove pr od uc ti vi ty , a grad ual change from room-and-pillar mining t olongwall mining has been t ak in g pla ce with in Consol . Consol produces 45m i l l i on t ons o f c oa l a nnua l l y w i th about 70 percent of t he product ioncoming from underground mines.

Consol introduced longwall mining. i n th e ea r ly seventTes and cur ren t ly

ope ra te s abcut 20 longwall f aces . The longwal l f aces a r e i n mines i nth e Appalachian reg ion i n the S t a t e s of West Vi rg i n ia , Pennsylvania , andOhio. The succ ess f u l ope ra t ion of longwall f aces has cont r ibuted t o ar e c ove r y o f p r odu c t i v i t y and t o a n improvement of mine safety.

I n t h i s pape r , sa fe ty and pro duc t iv i t y d a ta of underground coa l mices

from 1950 to 1982 ar e br i e f l y reviewed t o show th e impact of th e enact-ment of t h e Coal Mine Eea lt h and Safe ty Act of 1969 on th e co al indus-try. I n add i t io n, some r e s u l t s of geomechanics work i n longwall miningt o improve p r o duc t i v i t y a nd s a f e t y a r e p r e s en t e d .


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SAFETY PRODUCTIVITY

Hi s to r ic a l l y , the coa l indus t ry has improved i t s sa fe ty record throughinnovat ion and mechaniza t ion and t ra in ing . Tn the f i f t i e s a n d s i x t i e s ,sa fe ty improvement came from th e adopti on of roof b o l t su pp ort s, con tin-uous miners , s h u t t l e ca rs , and conveyor b e l t s i n room-and-pi llar mining.I n pl ac e of wood t im bers , roof b ol t i ng became th e pr imary supp ort systemi n co a l mines, f r ee ing up t h e nar row en t r i e s f o r con t inuous miningmachines. Roof b o l t i n g sup port was fu r th e r improved over th e expansions h e l l an ch or t y p e b o l t b y t h e i n t r o d u c t i o n of t h e r e s i n g ro ut ed b o l t .The r e s i n g r ou te d b o l t r e du ce s t h e h ig h s t r e s s c o n c e n tr a t io n a t t h e b o l ta nc ho r and e l i m i n a t e s t h e p o t e n t i a l f o r l o s s of t h e a pp l i ed b o l t t en -s ion . Under roof bo l t suppor ty l a rg e cont inuous miners wi th sh ut t l eca r s and be l t conveyors have s t e ad i l y improved t h e i n dus t ry s a f e t yr e co r d a s pr e se n te d i n F i g ur e 1. The f a t a l i n j u r y r a t e p e r m i l l i o n t o n sof p roduc t ion dec l i ned l i ne a r ly a t a r a t e o f a bo ut .035 p e r m i l l i o n t o n sper year between 1950 and 1970 (IR of MSHA and i t s predeces so r s ) .

The pr od uct iv i ty of underground co a l mines had a l s o inc reased a t a r a t eof 0 .42 t ons pe r man s h i f t pe r yea r du r ing t h i s pe r jod a s shown i nFigure 2 (Min eral s Year Books and Keystone Coal In du st ry Mantials), I tdecreased from 1970 t o 1978 a t a r a t e of 1.1 t ons pe r man s h i f t pe r yea rpa r t ly because of th e enac tment of mining ru le s and reg ula t io ns such a sth e 1969 Coal Mine He alth and Sa fe ty Act.


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To improve pro du ct iv i t y, Consol s t a r t e d t o employ the 1ongwal .l miningmethod. The method pe rm its an improvement i n sa f e ty and pr od uc t i vi tybec ause p r oduc t ion c a n be r e s t r i c t e d t o one f a c e w i t h c l o s e s upe r v i s i on

' and th e fa ce workers a r e p ro t ec t ed under t he s t e e l canopy formed by th ef a c e s u p po r ts . I n a d d i t i o n , t h e f a s t moving s h u t t l e c a r s a r e e l i m in a t e dfrom t he f a c e a r e a a nd c on t inuous t r a ns po r t a t i o n i s pr ov id ed . On t h eo t h e r ha nd , some e a r l y l o n gw a ll s i n s t a l l e d d u r i n g t h e f i f t i e s and e a r l ys i x t i e s , which were des igned wi thout p rope r cons ide ra t i o n of va r io usge o l og ic c c nd i t i ons d i d no t meet t h e i r po t e n t i a l (M oroni, 1973 ) . Thegeo tech nica l problems encountered d ur ing t he ado pt ion of longwal l miningi n t h e e a r l y s e v e n t i e s w i l l be d i s c uss e d .

STRENGTH OF COAL MEASURE ROCK

S i nc e no c l o s ed f o rm a n a l y t i c a l s o l u t i o n s f o r c a l c u l a t i n g s t r a t a move-ment and f a i l u r e a r e ava i l ab l e , ma themat i cal model ing by means ofd i g i t a l c o m p u t e r s i s a l o g i c a l c ho i c e f o r a n a na l y s i s o f complex mines t r a t a and opening geometry. Rowever, modeling i s of l . imited va lu ebe c aus e o f t h e I n e b i l i t y of a c c u r a t e l y de te r m in i ng t he p r op e r t j e s ofg e o lo g ic m a t e r i a l i n - s i t u .

The rock above and below a coal seam i s peneral ly weak and incompetents h a l e 3 r d c l ays ton e . Obta ining meaningfu l phy s i ca l p r op e r t i e s f roml ab o r at o ry t e s t s i s v er y d i f f i c u l t .


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A d e t a i l e d d e s c r i p t i o n o f r o c k c o re i s a good ya r d s t i c k f o r j udgi ng t h ephy s ica l condi t io n of the rock format ions . The re ce nt ly publ i shed guidebook t o co r ed r ock can be used t o i d en t i f y t he r ock t ypes a ccu r a t e l y andco ns is t en t ly (Ferm and Smith, 1980). I n a d d i t i o n t o t h e c o r r e c t i d en ti .-f i c a t i o n , r oc k q u a l i t y d e s i g n a t i o n h e l p s t o e s t i m a t e t h e c o n d i t i o n ofth e coa l measure rocks . F igure 3 i s a p l o t o f RQD a g a i n s t t h e d i a m e t r a lindex compress ive s t r e ng th obta ined from rock core s near a coal seam.Four groups a re f ound i n t he p l o t : c l ays t one , s ha l e , s ands t one , andl im e st on e . T h e re f o re , t h e c o r r e c t i d e n t i f i c a t i o n of t h e ro c k f or m at io ncan he l p i n f i n d i ng p robl em a r eas .

Geologic log analyses as w e l l a s l a b t e s t s c an pr o vi de u s e f u lcomparisons. Usu all y a small t e s t 9 pe cim en d oe s n o t t r u l y r e p r e s e n t t h e

,beh avio r of rock masses sur rounding mine openings . Thus, f i e l d t e s t sa r e neces s a r y t o de t e r mine t h e r e s pons e of t h e weak c oa l measu re rockf o r any des ig n purpose and a back c al cu la t i on method can be used f o rapp l i c a t i on t o s i m i l a r m in ing cond i t i ons . Thus, a fo ll ow-up s t udy i nt he f i e l d i s of te n r equi red f o r improvements i n 1 .ongwall des ign .

SIZE OF CFIAIN PILLAR

Most longwal l pan els a r e developed wi t h t h e use of mining equipmentdes igned f o r room-and-pillar mining. The headga t e f o r one l ongwal lp a ne l and t h e t a i l g a t e f o r t h e n ex t a r c d ev elo pe d a s a s e t of e n t r i e s a t


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the same t ime. Thus, th e ta i l g a te en t r y i s v u ln er a bl e t o roo f f a l . 1 ~because i t i s inf lue nce d by th e previ ous mined-out pa ne l and must bemain ta ined for a longer t i m e t han t he headga te en t ry . Proper s i z ing oft h e c o a l p i l l a r s i n t h e he ad ga te and t h e t a i l g a t e e n t r i e s i s importantto p ro tec t t h e t a i lg a te en t ry having weak roof rock even where th e roofi s supported by roof b o l t s and wooden cr i b se t s . A method f o r computingth e minimum si z e of co al pi l l a r s w a s pre sen ted i n "Design of LongwallSystems" (Choi and McCain, 1982).

The r e s u l t s of the ana ly s i s has been t es t ed a t var io us mfnes. Thefol lowing i s an example i n which the s t r e s s bu i ld up and re su l t i ng rooff a l l s w ere c l o se l y m oni to red. The lon gwa ll pan el was 183m (600 f e e t )v i d e w i t h 1.7n ( 6 6 inch) mining he ig ht and 213 t o 244m (700 t o 800. f e e t )

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overburden . Coal p i l l a r s were inst rumented wi th s t res smete rs to f indt h e i n c r e a s e i n s t r e s s a s t h e l on gw al l f a c e r e t r e a t e d . F ig u re 4 showsth e l ayou t o f the ins t rumented p i l l a rs . F igures 5 and 6 a r e p l o t s oft h e stress i ncre ase aga in s t t h e pos it f .on of the longwall face . Thev e r t i c a l s t r e s s c on ti nu ed t o i n c r e a s e a s t h e f a c e pa ss ed t h e i ns tr um en t-ed ar ea by 61 to 122m (200 t o 400 f e e t ) . The magnitude ~f the increasewas on th e orde r of 6.9 MPa (1,000 p s i) exce pt f o r one st r e s s meter (D) .The measured st r e s s i s wit hin the. the or et fc nl abutment pressur e of 8.6MPa ( I , 250 ps i ) . I t i s a l s o i n p o r t a n t t o obse rve t ha t t he i nc rea se i ns t r e s s was of th e order of 1.7 MPa (250 p s i ) when t h e longw all wasa l i gned w i t h t he s t r e s s m et er s . T ha t s t r e s s i nc rea se d i d no t i nduce anyro of f a i l u r e i n t h e t a i l g a t e e n tr y . A s t he longwall re t r ea ted by , t he


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roo f o f t he c ro s scu t s betw een t he t r ack and t he b e l t en t r i e s f a i l e dwi thout except ion as shown i n Fi gu re 7 . The roof f a i l u r e grows from theb e l t en t ry s i d e t oward t he t r ac k en t ry a s expect ed f rom t he cond i t i on ofthe s ide-~butrnent pres sur e build-up. The exa ct r a t e of progre ss of thef a i l u r e v a r ie d c o ns i de r ab l y b u t w a s accompanied by an ample prewarningbefore fa i lu re . The most ex tens ive fa i l u re was in a four-way in t e r -s e c t i o n of t h e t r a c k e n t ry . I t f a i l ed midway between t he t r ac k a n d t h eescape e n t r i e s th rough th e four-way in t e rs ec t io n i n one day .

The s t r e s s j n cr e a s e and t h e r oo f f a i l u r e a n a l y s i s i s u s e f u l i n t hedes ign of any o th er longwal l fac es under s i mi l a r condi t ion s where thes ide-abutment p res sur e causes the fa i l u r e of t h e roof fo rmat ion cons i s -t e n t l y . T h a t i s , t h e measured s t r e s s l e a d s t o a way t o determine t hein-s i tu p roper ty of th e roof fo rmat ion by back ca l cu la t ion . In addi-t i on , t he obse rva t ions i nd i ca t e t h a t a row of coa l h l ocks of 25.6 by25.6m ( 8 4 by 84 f e e t ) i s s u f f i c i e n t t o s u pp o rt t h e a bu tm en t p r e s s u r e~ s s o c i a t e d i th a n a d j ac e n t p an el . T h is a s p e c t i s di scussed in conjunc-tion with ground movement.

WIDTH OF FACE

According t o a rec en t survey (Peake, 1984) , t he average wid th of1.ongwall fa ce s inc re as ed from 159 . l m (522 f ee t ) t o 167.3111 548 .9 f e e t )from 1979 t o 1982. It i s e xp ec te d t h a t t h i s t r e n d w i l l cont inue because


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high load ca r ry ing supp or t s and l a rge f ace conveyors a r e ava i l a b le andbecause the wider f a ces permi t inc reased coa l r ecovery . I n a d d i t i o n t h edr iv ing o f deve lopment e n t r i es f o r longwal l pane l s t ends t o l a g behindth e pane l min ing fu r t he r promoting th e use o f wider f aces .

To inv es t ig a t e t he in f luence o f t he f a.ce wid th on th e f a ce cond i t ion , i ti s d es i r ab l e t o m easu re t h e f ro nt -abu tm en t p r e s su r e a t v a r i o u s l o c a t i o n sa l o n g t h e f ace . A s a pr ac t i ca l mat ter , however , th e measurement i sq u i t e d i f f i c u l t be ca us e t h e i n s t a l l a t i o n o f s t r e ss m e t e rs a t t h e widef a c e r e q u i r e s a l ong con t ro l l ed boreho le i n a weak coa l under h ighp r e s su r e . I f t h e f ro n t abu tment p ressu re i s assumed e qua l t o t heside-abutment pres sur e , th e s t r e s s measurements presen ted previou slyi n d i c a t e t h a t t h e i n f l u e nc e of t h e c h a i n p i l l a r s on t h e l o ng w al l f a c e

.4

would be l i mi te d t o approximatel-y 61 t o 122m (200 to 400 fe e t ) . Thati s , ar.y f ac e wider th an 122 t o 264m (400 t o 800 f e e t ) under t h e samec o n d i ti o n s may n o t h a ve a n i n c r e a s i n g l y h i g h e r s t r e s s a t t h e c e n t e r o ft h e face. A s t h e f ac e becomes w i d er , t h e i n f l u en ce o f t h e ch a i n p i l l a ron t h e c e n t r a l p o r t i o n of t h e f a c e d i mi ni sh e s.

I n a d d i t i o n , t h e h y d ra u l i c p r e s s ur e i n t h e l e g s of f a c e s h i e l d s u p p o r t swas measured i n an a t t empt t o def in e the e f f ec t o f t he f ront -abu tmentp r e s su r e . A hi gh front-abutment pr es su re may induce high convergence a tt h e f a c e a nd t h e r e f o r e d e t e r i o r a t i n g f a c e c o n d i ti o n s e v id en ce d byi n c r e a s i n g l e g p r e s s ur e s . F i g u re 8 i s a p l o t o f t h e h y d r a u l i c p r e s s u r e sof t h e f r o n t l e g s of 122 fa c e su pp or ts of a 183m (600-foot) fac e.


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I t shows tha t t he pres sure in crea ses f rom sh ie l ds 1 t o 4 0 , becomes moreo r l e s s cons t an t between s h i e ld s 41 and 8 0 , and decreases f rom sh ie lds81 t o 122. I t appear s t h a t t h e cen t r a l zone w i th a cons t an t l e g p re s -su re requ i res a c ons tan t suppor t load because the zone i s independent oft h e s u pp o rt i n f l u e n c e o f t h e c h a i n p i l l a r s . T h is o b s e rv a t i on a l s o l e a d st o t h e c o nc l us i on , t h a t t h e c e n t r a l p o r t i o n of t h e f a c e i s n o t i n f l u -enced when th e fa ce width becomes wider than 122m (400 f e e t ) i n t h i scase.

ROOF ROCK BREAKUP AND GROUND MOVDCNT

S i nc e t h e c o a l i s removed c l ean ly wi th no p i l l a r s tumps bel .ng l e f t2behind the longwal l face , s t r on g format ions i n th e roof may cr ea t e a

l a r g e o pe ni ng b eh in d t h e r e t r e a t i n g s u pp o rt l i n e . A s t he open ingbecomes la rg er , a la rg e amount of s t r a i n energy i s s t o r e d i n t h e r o ofwhich can be re le ased i n a fash ion s im i l a r t o co a l bumps. S ince mos t o fConsol t s mines have severa l s t ro ng fo rna t io ns such a s l imes t one ands an ds to ne i n t h e r o o f , it i s e s s e n t i a l t o mo ni to r t h e br ea ku p o f t h e

roof formations. A method of p ro je ct in g th e roof breakup with t he useof a computer model was deve loped (Choi and PcCain, 1983).

Dur ing the breakup of t he s t ro ng fo rma t ions i n t he r oo f , t he l e gs o ffa ce support experienc e high pre 'ssure. Most of th e unsu cces sful a t -t empts a t l ongwal l min ing i n t he ear l y s i x t i e s a r e a t t r i h u t e d t o t h e


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high pressure as soc ia t ed wi th the roof breakup. Cur rent ly the f acesuppor t i s des igned t o be more rugged and s t ron ger i n ord er t o avoidf a i l u r e .

Consol rou ti ne ly mo nitors t h e ground movement t o de te ct th e roof breakupwhen a longwall i s intgoduced t o a new area . Figure 9 i s t h e p l o t ofground movement along a pa nel ce nt er l i n e from i t s s t a r t up p o s it i o n .Un t i l t h e pane l r e t r ea t e d 48.7m (160 f e e t ) , t h e r e w a s no ground movementa t a l l ( cepe a ) . Dur ing one mine s h i f t , t he l egs of t h e f ace s upport sy ie lded and s ur fa ce subs idence occur red r apid ly a s shown by b i n th ef ig ur e. Af t er the brea k of th e roof format ion, t he ground movementbecomes gentler and slower as i nd i ca t ed by c . The major movement occursa f t e r t h e f a c e r e t r e a t s a d i s t a n c e a pp ro xi ma te ly e q u a l t o t h e o v erb urd en

>

th ickne ss , ind ica t in g th a t th e h igher roof format ions have broken andal lowed the sur fa ce t o subs ide a s shown i n F igure 10. The surfacemovement th en becomes grad ual wi th no no t ic ea bl e ov ers t re ss i ng of th ef a c e s u p p o r t s a s t h e l on g wa ll f a c e r e t r e a t s . I t i s po i n t ed ou t ,however, t h a t t h e cas es of a , b , c , and d had almost t h e same ar ea ofsur fac e subs idence . During th a t per iod , the overburden rock formations

were breaking up making a higher f r ac tured zone above the coa l seam.The maximum sub side nce th en o cc urr ed immediately a s t h e fa c e moved( cas e e ) and cont inued a s the panel was re t r ea te d. With theaccumulat ion of th es e da ta it i s now rout ine t o in t rodu ce the longwal ls yste m a t o t h e r l o c a t i o n s i n t h e a r e a.


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A s mentioned be fo re , t h e measurement of ground movement was t o det erm ine

the support c apac i ty o f coa l p i l l a r s . F igure 11 shows the ve r t i c a lmovement of t he s ur fa ce ove r a 213m (700-foot) overburden p an el . Eventhough the roo f i n t he c ros scu t s f a i l e d , t he subsidence p r o f i l e i nd i -ca t e s t ha t t he coa l p i l l a r s a r e suppor t i ng the s ide-abutment p r e s su re .Fur thermore , t he t a i l g a t e e n t ry was no t sub jec t ed t o add i t i o na l p r e s su recoming from t he pre vi ou sl y mined-out pan el.

CONCLUSIOPJS

The paper summarizes some r e s u l t s of ground c on tr ol work ca rr ie d out i np lanning f o r longwal l min ing. The , resu l t s cont r ibu ted t o th e succ ess f u lopera t ions of longwal l s i n Consol 's mines . I t i s expec ted t h a t longwal lmining w i l l be adep ted t o va r io us min ing cond i t i ons and w i l l con tinue t ogain an increa sed por t io n of underground c oa l product ion.

The work described i s a p a r t o f t h e e f f o r t i n pro mot in g s a f e t y andproduc t iv i t y i n undergtound co a l mines . S ince t he e a r l y s even t i e s , suchda ta have been co l l e c t ed t o he lp t he l ongwa l l s ope ra t e s a f e ly andef f i c i en t l y . Most of da t a g iven in th e paper were observed repea ted lyover long mining cycles . We ar e very ap pr ec ia t i ve t o Consol managementf o r t h e i r pa t i ence and encouragemen t i n co l l e c t i ng th e base da t a .


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REFERENCES

Anon., 1970-1981, "I nj ur y Exper ien ce i n Coal Mining," IC8613, IR1062,IR1074, IR1075, IR1076, IR1097, IR1108, IR1108, IR1112, IB1122, IR1133,IR1134, Mine S afe ty and Health Administ rat ion end predec essor agencies:MESA and USBM.

Anon., 1950-1976, Mi ne ra l Year Book, Bureau of Mines, Washington, D.C.,Government Printing O f f i c e .

Anon., 1978-1982, Keystone Coa l In d u s t r y Manual, EIcGraw-IIi11, New York.

Choi, D. S . and McCain, D. L., 1982, "Design of Longwall Systems,"2

Trans. SME-AIEE, Vol . 268, pp 1761-1764.

Choi, D. S. and McCain, D . L., 1983, "Ground C on tr ol Asp ect s of Longval lCoa l Minin g," Proc . of RETC, Vol. 1, Chicago, pp 178-190.

F e r n , J. C. and Smith, G. C ., 1980, "A Guide t o Cored Rocks i n t hePi t t sb ur gh Basin," Report of t he BuHines Con trac t No. 50188115,

Moroni, E. T., 1973, "Lonp ial l Experiences i n the I l l i n o i s No. 6 Seam"I l l . Mining I n s t i t u t e , S p r i n g f i e l d , Ill . . , October, pp 28-34.


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Peake, C. V . , 1984, "Longwall Productivity Shows Solid Growth," Coal-~ g e , ugust, pp 6 1 .

Pi lkinson, J . F . , 1 9 8 5 , "0utlook 1985: Heavy St oc kp ile s Squeeze Pr ic es ,"Coal Age, January, pp 50-60.


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Source: Injury Experience in Coal Mining. InformationalReports of MSHA, or Its Predecessors.

Figure I. Fatal Injury Rates BituminousCoal Industry Underground Mines1950- 1982


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F igu re 2. Productivity Bituminous Coal IndustryUnderground M nes 1950-1982


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Diarnetral Index Strength, MPa ( psi )

Figure 3. Rock Qual i ty Designation vs.Diametral Compressive Strengtho f Coal Measure Rock


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Face Position, m ( f t )

Figure 5 . Stressmeters A 8 B


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Face Position, m ( f t1

Figure 6. Stressmeters C 81 D


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loo


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Shield Number

F igu re 8. Front Leg Pressure


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- 5.2 0 15.2 30.5 47.7 6 .O 76.2(- 50) (50) ( 1 00) (150) (200) (250)Distance From Start-up Position, m ( f t 1

Figure 9. Ground Movement Along PanelCenter with 37 m Overburden


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Figure 10. Ground Movement Along PanelCenter with 213 m Overburden

0-e'8V .30( 1 )a

Q)0LZ8 )w.-u, -61-5 (2)a

.91-(3) a b c d e1.22 I1 I(4)-76.2 0 76.2 152.4 228.6 3048(-250) (250) (500) (750) (1000)

Distance Fr om Start-u p Position, m (fi)

I


24/24

--Panel Center

I I I I

Distance From Panel Center, m ( f t )

Figure I I. Ground Movement AcrossLongwall Panel with 213m

application of geomechanics in longwall operation

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