Critical Break Length of Floor Aquiclude During Longwall Mining Above Confined Aquifer

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1 Eart Sciences 07; 6(6): ttp:// doi: 0.648/j.eart ISSN: (Print); ISSN: (Onine) Critica Break Lengt of Foor Auicude During Longwa Mining Above Confined Auifer Baoua Guo,,, *, Feng Jiao, Xiaoyu Yang 4 Coaborative Innovation Center of Coa Work Safety, Jiaozuo, Cina Scoo of Energy Science and Engineering, Henan Poytecnic University, Jiaozuo, Cina Key Laboratory of Safety and Hig-Efficiency Coa Mining, Ministry of Education (Anui University of Science and Tecnoogy), Huainan, Cina 4 Sendong Coa Group Company, Cina Senua Energy Company Limited, Erdos, Cina Emai address: guobaoua@9.com (Baoua Guo) * Corresponding autor To cite tis artice: Baoua Guo, Feng Jiao, Xiaoyu Yang. Critica Break Lengt of Foor Auicude During Longwa Mining Above Confined Auifer. Eart Sciences. Vo. 6, No. 6, 07, pp doi: 0.648/j.eart Received: September 5, 07; Accepted: September, 07; Pubised: November, 07 Abstract: Foor water inruses not ony cause eavy casuaties and property osses, but seriousy restrict te production of coa mines. Te auicude was simpified as te fixed - fixed beam under te uniform oad, and critica break engt formua of foor auicude in tree faiure modes incuding bending faiure, sear faiure and bucking of pressure bar were estabised. Ten te critica engts of auicude in tree faiure modes were cacuated according to te geoogica and mining conditions of 50 pane in Jiuisan coamine. Te resuts sow tat te critica break engt in bending faiure is te smaest, wie te critica engt in sear faiure mode is te biggest. And faiures of fixed fixed beam in tree faiure modes a occur at two ends so tat water inrus from foor may occur at open-off cut or working face. Additionay, tree critica break engts increase wit auicude tickness but decrease wit confined auifer pressure. According to te cacuated resuts, water inrus may occur at te working face or open-off cut wen mining advance reaces.47 m, and tus tree contro measures of owering auifer pressure, increasing auicude tickness and sortening te distances of first and interva weigting of main roof were discussed. Te concusions can provide references to contro water inrus from foor auifer in coa mines. Keywords: Coa Mine, Water Inrus, Foor Auicude, Fixed - fixed Beam, Critica Break Lengt Introduction Cina is one of te most serious countries suffering from water inrus disasters, wic not ony caused eavy casuaties and property osses, but aso seriousy restricted te production of coa mines. According to statistics, 60% of coa mines in Cina are treatened by te confined auifer more or ess. For exampe, more tan 0 coa mines are treatened by te confined auifer ony in nortern Cina, wic resuts in about 40% of coa resources can not be expoited. Te mecanism of mine water inruses in coa mines in Cina differs consideraby from tat in oter countries. In Cina, most water inruses occur from foor strata, were te water inrus sources are karstic imestone auifers []. Tere are two types of water inrus from foor tat one is caused by geoogica structures suc as fauts and karst coapsed coumn wic are freuent to occur in Cina and te oter is caused by faiures of foor auicude during te mining process. Te atter type of water inrus presents an ascending tendency in tese years as te confined auifer pressure becomes iger wit te decrease of saow coa resource. For exampe, te pressure of confined auifer is generay - 6 MPa in Jiaozuo coa fied in Henan Province []. Tus, auicude is te key barrier to prevent mining excavation from water inrus during coa mining above confined auifer. Te formation of water inrus cannes dominates weter inrus appens wen tere are no geoogica structures suc as fauts and karst coapsed

2 Eart Sciences 07; 6(6): coumn existing in te foor strata, so te study on faiure caracteristics of auicude is uite crucia to inuiry water inrus mecanism of intact foor. Many scoars carried out a ot of researces on water inrus mecanism, and put forward various teories and metods. Wang et a [] presented a micromecanics-based couped damage and fow modeing approac to simuate te progressive deveopment of fractures and te associated water fow in te foor strata during mining above a confined auifer. Zang [4] appied te finite eement metod couped wit stress-dependent permeabiity to anayze ydrauic conductivity enancement due to coa extraction. Yang et a [5] presented a fuy couped fow-stress-damage mode to simuate te progressive deveopment of fractures and te associated groundwater fow under incrementa oading conditions resuted from mining processes. Kong et a [6] used a teory of seepage instabiity to estimate te armfuness of water-inrus from a coa seam foor in a particuar coa mine of Xuzou Mining Group. Zu et a [7] deveoped a numerica mode of te key strata by using Diian Mecsoft's Reaistic Faiure Process Anaysis (RFPAD) program to predict faiure modes and to estabis rues for safe mining above te auifer. Jiang [8] estabised a numerica mode of water-resisting strata using te anaytica modue of fuid structure interaction of FLAC to anayze te destruction of water-resisting strata in foor of coa seams being mined. In te aspect of on site monitoring, Jian et a [9] used a ig-precision micro-seismic monitoring tecniue to monitor te continuous, dynamic faiure of an incined coa seam foor. Huang et a [0] caracterized te ydrauic conductivity of rock formations between deep coa and auifers using injection tests. Li et a [] used Direct Current (DC) resistivity metod for routine detection, advanced detection and rea-time monitoring of water-bearing structures to avoid water inrus azard in underground engineering. In summary, scoars ave aready carried out toroug studies on water inrus mecanism by simuating or monitoring te faiure process of coa seam foor. However, te study on break mecanism of te auicude as a bearing beam is rare. Conseuenty, te mecanica mode of taking foor auicude as fixed - fixed beam was estabised, and ten te critica break engt formuas of foor auicude in tree faiure modes incuding bending faiure, searing faiure and bucking of pressure bar were given. At ast, te above formuas were appied to cacuate te critica break engts of foor auicude in Jiuisan coamine, Jiaozuo group.. Mecanica Mode of te Foor Auicude Strata section of ongwa mining pane is sown in Figure, from wic we can know tat surrounding rock strata incude roof, coa seam and foor, wie foor consists of confined auifer, auicude and oter rock ayers. Wen a coa seam is near orizonta or genty incined, te foor auicude can be regarded as a fixed - fixed rectanguar beam as sown in Figure, were te tota tickness of auicude and oter rock ayers between coa seam and auicude is ; te uniform confined auifer pressure is. Tus te gravity of auicude is γ, and te euivaent water pressure of te fixed - fixed beam is: σi σi Roof Coa seam Foor = γ () Figure. Strata Section of Long Wa Mining Pane. Figure. Fixed - fixed Beam Mode of Foor Auicude.. Critica Break Lengt of Foor Auicude.. Bending Faiure / Mining excavation Oter rock ayers Auicude... Singe Layered Auicude Te bending moment M of fixed - fixed beam under te uniform oad is expressed as M = x + () 4 Were is te engt of te beam. From formua () we can know tat te bending moment of fixed - fixed beam gets te maximum wen x = and M max is M Confined auifer y max = () Wen straigt beam is in pure bending state, te norma stress of te point on cross-section is M σ max = y max (4) I z Coa seam Were I z is inertia moment of te cross-section to te neutra axis. For a rectanguar cross-section beam I z is / x σi y σi / x /

3 5 Baoua Guo et a.: Critica Break Lengt of Foor Auicude During Longwa Mining Above Confined Auifer defined as foows: I Z = (5) Formua (4) sows te maxima tension of beam ocates at te fartest point on cross-section from te neutra axis; because te neutra axis is te symmetry axis of cross-section, te maximum norma stress is at te cross ine between te cross-section and te ower surface ( ymax max = ), wic is σ = (6) Tus, bending faiure wi occur at two ends of te beam firsty. But once te ends are broken, te fixed - fixed beam mode wi become te freey supported beam mode. For a freey supported beam subjected to uniform oads, te maxima bending moment ocates at te midde of te beam and te maximum is /8. But te beam may not continue to break as te decrease of euivaent water pressure after eiter end is broken. Because tensie strengt of strata is reativey sma, te foor auicude wi be easy to break wen foowing condition is met, σ max σ (7) t Were σ t is te tensie strengt of foor auicude, MPa. Bring Formua (6) into Formua (7), tat is σ (8) Were is te critica break engt in bending faiure mode, m. Terefore, t σ t (9)... Muti-ayered Auicude Wen tere are oter rock ayers between auicude and te coa seam, water inrus wi occur once break of auicude appens. Provided te number of ayers is n, te bending moment of eac ayer is te same, and tere is no orizonta disocation between te adjacent ayers, te bending break engt of te combined beam coud be expressed as foows:.. Sear Faiure n σt i (0) i=... Singe Layered Auicude Not ony bending faiure mode, but aso sear faiure mode soud be considered wen te critica break engt of foor auicude is anayzed. As sown in Figure, te sear stress distribution is uite different aong te ong axia of te beam, and te maximum of sear oad ocates at bot ends of te beam, wic is T smax = () Te tensie stress σ t on every cross section of te beam as sown in Figure is M σ t = y () I Substituting Formua () and Formua (5) into Formua (), we get tensie stress on te cross section as foows: z x + σt = y () Wie considering mor-couomb strengt criterion for rockmass, sear stress τ on cross section is written as foow: τ = ( σ σ )tanφ + (4) t c Were σ is orizonta stress after excavation, MPa; ϕ is friction ange of te auicude; c is coesion of te auicude, MPa. So, te sear resistance of te overa cross section is r t T = [( σ σ )tan φ + c] dy = ( σ tan φ + c) (5) From Formua (5), we know te sear resistance of every cross section aong te ong axis direction is a te same and tus weter te beam is broken or not depends on sear oad distribution. Sear faiures wi appear at te ends of te beam as te maximum sear oad ocates tere. Provided T > T, we get smax r Furter, ( σ tanφ ) + c (6) ( σtan φ + c) (7) Were is te critica break engt in sear faiure mode.... Muti-ayered Auicude Wen tere are oter rock ayers between te auifer and te coa seam, provided te number of ayers is n, te bending moment of eac ayer is te same, and tere is no orizonta disocation between te adjacent ayers, te critica break engt of te combined beam in sear faiure mode coud be derived as foows.

4 Eart Sciences 07; 6(6): Te sear resistance exerting on cross section of te beam end is n n j T = + C ( σ tanφ ) tanφ (8) r j j j i i j j j = i= j+ i= and Provided n ( σtanφj Cj ) j (9) j= A = + B = n n j tan i i j φj j = i= j+ i= (0) Formua (8) can be written as foows: Tr = A + B () Te critica break engt in sear faiure is ± 6AB = () 4B Based on formua (), we can get te furter answers: () Wen φ is a constant, te foor above confined auifer is regarded as one type of rock, so we can get B = 0 A = σ tanφ + C. and ( ) x () Wen φ is a variabe vaue, and B 0, critica break engt is cacuated as foows because it cannot be zero or negative.: Wen A > 0, Wen A < 0 and B > 0, Wen A = 0, 6AB = () 4B + 6AB = (4) 4B = (5) B... Comparison Between Critica Break Lengt in Bending and Sear Faiure Modes To compare critica break engt of foor auicude witout oter rock ayers between coa seam and auicude in bending faiure mode and in sear faiure mode, Formua (9) and Formua (7) can be expressed as foows: If et σt = (6) i >, 4( σ tan φ + c) = (7) ( σitan φ + c) σt (8) Wen Formua (8) ods, a sear faiure tends to occur, oterwise, a bending faiure tends to occur on te foor auicude... Bucking of Pressure Bar... Singe Layered Auicude Provided te engt of fixed - fixed beam is muc onger tan its tickness, te bucking faiure of pressure bar may occur. Terefore, te stabiity of te fixed - fixed beam under te euivaent water pressure and te orizonta stress σ i need to be anayzed. Cacuation sketc of bucking of pressure bar is aso sown in Figure. Te moment of fixed fixed beam considering te orizonta stress σ i is expressed as foow: M( x) = Pw( x) x + (9) 4 Were P is orizonta oad, P = σi; w is te defection of pressure bar. Te differentia euation of te defection curve is '' EIw ( ) = M x (0) Were E is te eastic moduus of pressure bar. Putting Formua (9) into Formua (0), we can obtain Were w'' + K w = x x EI 4EI () K P = () EI Te genera soution of te above differentia euation can be expressed as foow: EI w( x) = A coskx + A sinkx + x () P 4P P By appying te boundary conditions dw w ( ± ) = 0, ( ) x= 0 0 dx = (4)

5 5 Baoua Guo et a.: Critica Break Lengt of Foor Auicude During Longwa Mining Above Confined Auifer We can get EI K A = ( )sec P P, A = 0 (5) So te defection differentia euation is expressed as foow: K Kx EI x EI w( x) = ( )sec cos + (6) P P P 4P P From Formua (4), te maximum defection occurs at te midpoint of te pressure bar, wic is ω K max ( x) ω ( 0 ) ( )sec EI EI = = (7) P P 4P P Ten putting Formua (6) into Formua (9), we can get K EI EI M( x) = ( )sec coskx P P (8) Te maximum moment occurs at te midpoint or two ends of te pressure bar, wic is and Tus, K max (0) ( EI )sec EI M = P P (9) max ( ) M ± = (40) K ( EI P )(sec ) M(0) M( ± ) = (4) P Wen M(0) M( ± ) > 0, te break wi occur at te midde of pressure bar, oterwise, te break wi occur at two ends of te pressure bar. Te maximum norma stress on te cross-section is σ P Mymax max = A I (4) If M(0) M( ± ) 0, tus, y max = and K ( EI P )sec = P( P + σt) + EI (4) Oterwise, y max = and ( σ + σ ) σ σ σ = = + = (44) i t i t t... Muti-ayered Auicude Wen tere are oter rock strata between te auifer and te coa seam, te euivaent eastic moduus E eu in Formua (4) can be derived based on an euivaent bending stiffness, and te inertia moment of te cross-section to te neutra axis I eu in Formua (4) is cacuated by substituting te sum of eac ayer tickness. 4. Case Study 4.. Geoogica and Mining Conditions Permian System Sanxi formation (Ps) II coa seam was expoited in 50 pane in Jiuisan Coa mine, Jiaozuo Group, Cina. Te foor strata consist of fine grained sandstone, imestone witout water and mudstone (auicude) from top to bottom. Buried dept of te coa seam H is 400 m, approved te average specific weigt of overying strata γ is.5 MN/m, ì=0.5 and orizonta stress concentration factor k is, so te initia orizonta stress σ =0 MPa based on te foowing euations. µ σ = kσi = k rh µ (45) Main auifers incude L 9 and L 8 Ordovician imestone and te water pressure is 4 MPa. Tus =.69 MPa approved te average specific weigt of rock ayers between coa seam and confined auifer is aso.5 MN/m. Tensie strengt of auicude is.4 MPa, and oter mecanica parameters of foor strata were sown in Tabe. Based on tese mecanica properties of strata in foor, te critica break engt of auicude coud be discussed. Were σmax = σt, A is te cross section area, A = =. Tabe. Mecanica Parameters of Foor Strata. Rock type Tickness(m) Coesion(MPa) Friction( ) Inertia moment Eastic modue(gpa) Mudstone Limestone Sitstone

6 Eart Sciences 07; 6(6): Critica Break Lengt in Tree Faiure Modes () Te critica break engt in bending faiure is cacuated using Formua (9) as foow: and n σ t.4 i =.5 = 4.7m.69 i= () For te critica break engt in sear faiure, A = ( σ tan φ + c ) = 5.9 > 0 j= j j j j B = tan.46 0 i i j φj = j= i= j+ i= And tus, te critica break engt in sear faiure is cacuated using Formua () as foow: 6AB = = = B () Assumed te break occurs at te midde point of te pressure bar, te critica break engt in defection faiure is cacuated as foow: I eu + + ( ) = = 6.5m Based on an euivaent bending stiffness, eastic modue can be cacuated in foowing way, E eu = EiIi =.8 GPa I i= Based on Formua (4), =4.48 m (4) Assumed a break occurs at eiter end of te pressure bar, te critica break engt in defection faiure is cacuated using Formua (44) as: ( σi + σt) (0 +.4) = =.5 =.4m.69 Because <, is te more possibe break engt in bucking of pressure bar mode, wi be used for comparison wit oter critica break engts in bending faiure mode and sear faiure mode. From above cacuation resuts, te break engt in bending faiure mode is smaest wie te break engt in sear faiure mode is biggest. Terefore, it is more possibe for water inrus from foor caused by bending break of auicude. m 4.. Infuencing Factors of,, 4... Auicude Tickness According to te geoogica and mining conditions of Jiuisan coa mine, te reationsips between,, and auicude tickness are sown in Figure. From Figure we can see tat tese tree critica break engts increase and te increasing rate of critica break engt in sear faiure mode aso increase wit te auicude tickness. Terefore, it becomes difficut for water inrus from foor wit te increase of te auicude tickness. Additionay, te critica sear break engt is te biggest and te critica bending break engt is te smaest under te same auicude tickness; te critica break engt in bucking of pressure bar mode is bigger tan te critica break engt in bending faiure mode for te function of orizonta stress, but it is smaer tan te critica break engt in sear faiure mode. Critica breaking engt/m Auifuge tickness/m Bending faiure Searfaiure Defection faiure Figure. Reationsips Between,, and Auicude Tickness Confined Auifer Pressure Te reationsips between,, and confined auifer pressure are sown in Figure 4. From Figure 4 we can see tat tese tree critica break engts decrease and te decreasing rate of tese tree critica break engts aso decrease wit te confined auifer pressure. Terefore, it becomes easy for water inrus from foor occurs as increase of confined auifer pressure. Te critica sear break engt in sear faiure mode is te biggest and te critica break engt in bending faiure mode is te smaest under a same auicude tickness wen confined auifer pressure is ower tan.46 MPa; te critica break engt in bucking of pressure bar mode is bigger tan te critica break engt in bending faiure mode for te function of orizonta stress, but it is smaer tan te critica break engt in sear faiure mode unti confined auifer pressure is more tan.46 MPa. But based on te geoogica conditions, water inrus from foor is more possibe to occur because of te bending break of auicude and terefore, te water inrus type doesn t

7 55 Baoua Guo et a.: Critica Break Lengt of Foor Auicude During Longwa Mining Above Confined Auifer cange wit te variation of confined auifer pressure. Critica break engt/m Bending faiure Sear faiure Defection faiure Confined water pressure/mpa Figure 4. Reationsips Between,, and Confined Auifer Pressure Contro Tecniue of Avoiding Water Inrus from Foor Auifer Te caving roof woud exert norma oad on te foor after first weigting of te main roof, and tus te euivaent water pressure and auicude tickness acieve teir critica vaues before te first weigting of te main roof. According to above resuts, water inruses caused by break of auicude in tree faiure modes occur easiy at two ends of beam before te first weigting of te main roof. Tat is to say, water inrus from intact foor usuay appens at an open-off cut and a working face. Te first weigting distance of te main roof in 50 pane is about 4 m, and a water inrus wi occur for te bending break of foor auicude wen te ongwa mining distance reaces 4.7 m if tere is no corresponding contro tecniue. Terefore contro measures of owering confined auifer pressure and/or increasing auicude tickness soud be conducted to avoid a water inrus into mining excavation from foor auifer. Wen drainage is appied to decrease confined auifer pressure, te confined auifer pressure needs to be reduced to under 0.96 MPa ( = 0.65 MPa). But because Ordovician imestone water ere is ric and as a wide range of ydrauic contact wit te surrounding auifer, it is ard to work wen te contro measure of drainage is used aone. Wen grouting is used to increase te auicude tickness, te euivaent auicude tickness needs to be increased to above 9. m. Te sketc map of arrangement of grouting oes is sown in Figure 5. Te basic principes of grouting are as foows: before mining advance, severa driing fieds aving a certain interva from eac oter are arranged in auage gateway, ten ~5 grouting oes are dried in te foor of entire pane from eac driing fied. Te karst fissure and water conducting fracture in confined auifer wi be fied by injected cement grout troug grouting oes to greaty weaken te water ric property of te auifer and to cut off most water suppy cannes wit surrounding auifer. Tus te ric auifer can be transformed into an auicude or a weak auifer, wic eps improve te ect of owering te pressure of confined auifer by drainage, and utimatey benefits coa mining witout water inrus. Grouting oes Ventiating gateway Working face # # # 4# 5# 6# 7# Driing fieds Hauage gateway Figure 5. Sketc Map of Arrangement of Grouting Hoes. To be sure, if an artificia caving tecniue is used to sorten te first weigting distance of te main roof to beow 4.7 m, no oter measures may need to be taken in teory to avoid water inrus from foor in mining process. But periodic weigting distance of te main roof (te average vaue is.5 m) is near 4.7 m, so tat artificia caving tecniue is needed to ensure eac periodic weigting distance of te main roof beow 4.7 m and tus it can avoid water inrus from foor auifer occurring in te woe mining process. 5. Concusions A mecanica mode is estabised based on te auicude being simpified as te fixed - fixed beam under te uniform oad. Te critica break engts of auicude in tree faiure modes incuding bending faiure, sear faiure and bucking

8 Eart Sciences 07; 6(6): of pressure bar are derived. Tese tree critica break engts increase and te increasing rate of te critica break engt in sear faiure mode aso increases wit te auicude tickness; tese tree critica break engts decrease and te decreasing rates of tese tree critica break engts aso decrease wit te confined auifer pressure. According to te geoogica and mining conditions of Jiuisan coamine, Jiaozuo group, critica break engt of auicude in tree faiure modes incuding bending faiure, sear faiure and ducking faiure of pressure bar are.47 m, 4.76 m,.4 m respectivey. Tus, water inrus may occur at te working face or open-off cut wen te mining advance reaces.47 m. Besides, tree contro measures are approved incuding owering confined auifer pressure by drainage, increasing auicude tickness by grouting and sortening te first weigting distance of te main roof by artificia caving. Sometimes it is difficut to work by using a singe metod, so it may be better to co-ordinate te use of above metods. Acknowedgements Tis researc is supported by te Cina Nationa Natura Science Foundation (Project Nos ) and te key scientific and tecnoogica project of Henan Province (5006). Conficts of Interest Te autors decare tat tere is no confict of interest regarding te pubication of tis paper. References [] HAN, J., et a., Mecanism of mine water-inrus troug a faut from te foor. Mining Science and Tecnoogy (Cina), (): p [] Li, H. M. and Fu, K., Some major tecnica probems and countermeasures for deep mining. Journa of Mining & Safety Engineering, 006(04): p (in cinese). [] Wang, J. A. and Park, H. D. Coa mining above a confined auifer. Internationa Journa of Rock Mecanics and Mining Sciences, (4): p [4] Zang, J., Investigations of water inruses from auifers under coa seams. Internationa Journa of Rock Mecanics and Mining Sciences, (): p [5] Yang, T. H., et a., A couped fow-stress-damage mode for groundwater outbursts from an underying auifer into mining excavations. Internationa Journa of Rock Mecanics and Mining Sciences, (): p [6] KONG, H., et a., Anaysis of te Harmfuness of Water-Inrus from Coa Seam Foor Based on Seepage Instabiity Teory. Journa of Cina University of Mining and Tecnoogy, (4): p [7] Zu, Q., et a., Numerica anaysis of water inrus from working-face foor during mining. Journa of Cina University of Mining and Tecnoogy, (): p [8] Jiang, Z. H., Numerica anaysis of te destruction of water-resisting strata in a coa seam foor in mining above auifers. Mining Science and Tecnoogy (Cina), 0. (4): p [9] Jian, S., et a., Determining areas in an incined coa seam foor prone to water-inrus by micro-seismic monitoring. Mining Science and Tecnoogy (Cina), 0. (): p [0] Huang, Z., et a., Caracterizing te ydrauic conductivity of rock formations between deep coa and auifers using injection tests. Internationa Journa of Rock Mecanics and Mining Sciences, 04. 7: p. -8. [] Li, S., et a., Detecting and monitoring of water inrus in tunnes and coa mines using direct current resistivity metod: A review. Journa of Rock Mecanics and Geotecnica Engineering, 05. 7(4): p