Mechanism of mine water-inrush through a fault from the floor

Available online at www.sciencedirect.comMININGScienceDirectSCIENCE ANDTECHNOLOGYEL SEVIER，Mining Science and Technology 19 (2009) 0276- 0281www.clsevier.com/locate/jcumtMechanism of mine water -inrush through a fault from the floorHAN Jin', SHI Long-qing', YU Xiao-ge', WEI Jiu-chuan', LI Shu-cai2'College of Information Science and Engineering, Shandong University of Science and Technology, Qingdao,Shandong 266510, ChinaGeotechrical & Structural Engineering Technique Center, Ji 'nan, Shandong 250061, ChinaAbstract: The mechanism of mine water inrushes in coal mines in China differs considerably from that in other countries. In China,most water inrushes occur from floor strata, where the water-inrush sources are karstic limestone aquifers. Our study describes themechanism of mine water inrushes through a fault in the mine floor using principles of strata mechanics and the path of water in-rush from an aquifer to the working face. A criterion to judge whether a ground water inrush will occur through a fault or not is alsodescribed, together with a case history of water inflow in the Feicheng coalfield, China.Keywords: fault; water-nrush; mechanism; Feicheng coalfield1 Introductioncontributes 4.9%, flooding caused by alluvial wateraccounts for 1.4%, and inflows caused by sandstoneThere are very few cases of water inrushes fromaquifers account for 1.4%. The principal limestonethe floor in major mining countries such as the Unitedaquifer is Ordovician and is distributed extensively inStates, Russia, Poland, Canada, Australia, Germany,northern China. About 75% of the coal production inIndia and the United Kingdom". In these countries,China comes from that region, where about 50% ofthe incidence of mine water inflow can generally bethe coal reserves are mined from coal seams of theatributed to a large number of various cause, such asPermian and Carboniferous Systems. These strata liea sudden inflow of surface water accidental contactsuncomfortably on the Ordovician limestone, which iswith unconsolidated surface deposits during workabout 800 m thick; there are no Silurian and Devo-development, strata water entering working faces, anian strata in northern China. After nearly a centuryshaft encountering a confined aquifer during sinkingf mining, most Chinese mines have been extractingoperations, clearing of old shafts, contacting aban-deeper coal seams, especially those coal seams lo-doned mine workings, failure of underground watercated in the Permian and Carboniferous Systems.dams and seals or leakage of water from an aquiferBecause the distance between the current mining ho-through a bore hole'4. The main method for the con-rizons and Ordovician limestone is only 20~40 m,trol of mine water inflow in these countries is the ap-geological structures such as faults play an importantplication of various mine dewatering techniques.role in controlling the inflow of karst water into theIn contrast, more than 90% of water inrushes inmine workings.Chinese coal mines are due to water inflow fromkarst aquifers through coal seam floors. The main2 Distribution of strata pressure on a coaltechnique for controlling water inrush through theseam as a consequence of miningfloor is to leave a sufficiently thick barrier against theaquifer and/or grouting of the floor". Coal mine in-There are two patterns of vertical strata pressureundations in China can be divided into four categories,distribution on a coal seam, corresponding to differ-according to the source of the mine water inrush. In-ent mining depths and coal seam strengths. One of theundations caused by limestone aquifer account forpattermns is the single elastic distribution shown in Fig.92.3% of the cases, flooding caused by surface waterlal4 o!. The main characteristic of this stress distribu-Received 19 August 2008; accepted 22 January 2009Projects Y2007F46 supported by the Natural Science Foundaion of Shandong Province中国煤化工Scsn see RecFoundation of the Ministry of Education, 108158 by the Key Project of the Ministry CCNMHGhe National Natural ScienceFoundation of China:MYHCorresponding author. Tel: +86 532 80691759; E-mail adress: shilq@ sdust.edu.cnHAN Jinet alMechanism of mine water inrush through a fault from the floortion is that peak vertical strata pressure is centralizednot break the coal seam and associated floor strata.on the fringes of the coal face. The pressure decreasesTherefore, the floor strata remain intact and wa-in accordance with a negative exponential law to-ter-resistant. The second pattern of vertical stratawards the solid coal; the coal seam is in a state ofstress distribution on a coal seam is based on bothelastic compression and the stress on the coal seam iselastic and plastic stress distribution and the forma-proportional to its elastic deformation. Because thetion of a yield zone in the coal seam ahead of the face,vertical strata pressure does not exceed the compres-as shown in Fig. 1b7.sive strength of the coal seam, the strata pressure doesVertical strata pessureVertical strata pressureFaceGoafw.w Face.. Goa......Coal seamElasticCoal scam(a. Elastic zone-. Elastic zone -Fig.1 Distribution of vertical strata pressure on coal seamIn the elastic zone, the coal seam undergoes elasticσ=σ +k,σ3(1deformation and the peak vertical strata pressure OC-where k。 =1+sin中σ is the ultimate strength ofcurs on the interface of the elastic and plastic zones.1 - sinpThe stress gradually decreases towards the solid coalthe coal seam, o。the minimum principal stress, φuntil it attains the original virgin state of verticalstress. The stress in this zone is also proportional tothe internal friction angle of the coal seam obtainedthe elastic deformation of the coal seam. Therefore,from laboratory tests and σ。the uniaxial compres-the floor beneath this stressed zone does not undergosion strength of coal as obtained from laboratorypermanent deformation. In the plastic zone, in con-tests.trast, the coal seam suffers permanent deformationdue to strain softening and/or plastic flow. In the ab-O↑Coal seam| Strinplastic yield, sofing ; Elastic zonesence of high horizontal stress, the coal seam wouldzone人20nCdeform continuously. However, the horizontal stressincreases towards the solid coal in advance of theworking face and the force to prevent the plastic de-Eformation also increases. Thus, the plastic deforma-tion in the coal seam is limited. The stress distributiondue to vertical pressure in the plastic zone is similarto that of the horizontal stress, i.e., the pressure pre-sents a gradually rising curve from the coal face to-F_ I Face_ Goafward the interface of the plastic and elastic zones.Obviously, only the floor strata corresponding to theplastic zone is likely to be broken by the verticalstrata pressure, creating a pathway of water inrushfrom the floor into the mine workings. The width ofFig. 2 Ideal softening model of elastic-plastic strain andthe plastic zone, as shown in Fig. 2, can be calculateddeformation areas of coal seamin terms of the ideal elastic and plastic strain soften-(F=plastic yield zone; S=strain softening zone; E=elastic zone)ing model of the coal seamlo. The deformation of thecoal seam can be divided into three stages: elasticA large number of triaxial tests in the laboratorydeformation, a plastic strain softening stage and thehave shown that during the strain softening stage, theflow stage. The degree of strain softening can becohesive strength of a coal seam decreases more rap-quantified by the softening angle 6o or softeningidly than the internal angle of friction of the coal.modulus Mo， where Mo =tanOo. For an idealThus, the main contributing factor to the deformationelasto-plastic body, θo=0, and for an ideal elas-of coal is the loss of cohesive strength of the coal astic-brittle body, 0o =909, Mo-→∞. During the stage of中国煤化工nship:σ= f(os,e).elastic deformation, when the deformation ap-expr(proaches the yield point of the coal seam, the Mohr-ThusYHCNMHGCoulomb criterion should be satisfied" as follows:=σc(E)+k,σ5(2)278Mining Science and TechnologyVol.19 No.3where ε is the plastic strain value andence of friction. Therefore, the in-situ stress state ofthe unit element is basically the same as that obtaineddσc =-M.de?(3)in the laboratory and Eq.(4) can be substituted by theDuring the plastic stage of deformation, the coalfollowing equation:seam strength decreases to its residual strength, de-σ, =σ。+k,σ, .(7)noted by:σ=σ +k,σ,(4)Applying boundary conditions and solving Eqs.(6)and (7), the following solution is obtained:where σ is the residual strength of the coal seam inuniaxial compression.0= k= C<(eD*-1)The state of stress in a coal seam ahead of a long-wall face can be shown in Fig. 3 by considering theσ, = o°eDx(8)coal seam as a homogeneous continuous body, in2k。plane strain condition while ignoring the effect ofD, = tanqgravitylo'. A section of the coal seam at a distance xMahead of a coal face is considered to be a plastic zoneSimilarly, in the plastic softening zone, Eqs.(3) andof width dx and height M. It is subject to the vertical(4) can be approximated as:stress o, and the horizontal stress σx. The slidingσ,=σ。+k。σx -MoEP(9)resistance force (T) on the interface between coalseam and roof or floor is given by:Applying the continuity conditions to the interfaceT=σ, tanφ=σy,f(5)of the plastic strain-softening zone and the plasticyield zones and solving Eqs.(6) and (9), the followingwhere 9 is the friction angle between coal seamsolutions are obtained:and roof or floor and fi the coefficient of friction be-tween coal seam and roof or floor.-(e.x -1)+ D2[101)- D,(x-x,)门]k。“{σ, =σ°lDx + D,0e01-1)-1](10)ace__ GoafD2 =;2k。tan.where ε is the strain gradient in the plastic zone and10,1xj the width of the yield (plastic broken) zone.At the interface of the elastic and plastic zones,x=xp andσ, = KyH . Then according to6!:x=一ln{乱[KyH-D2-]}(1)where K is the coefficient of stress centralization atig.3 Stress state in the plastic zonethe interface of the elastic and plastic zones, y vol-ume weight of the overlying rock and H the miningThe stress on a unit element in the y-direction is indepth.a state of equilibrium while the stress on a unit ele-ment in the x -direction is in limited equilibrium as3 Distribution of ground pressure on floorexpressed by the following equation:Mσ, + 2σ tanqpdx- M(σ, +dσ,)=0The stress distribution characteristics on the floor(6l Mdσ, = 2σ, tanqpdxof an excavation can be simulated by finite elementanalysis or obtained by back analysis using measuredBased on the idealized strain softening elasto-plas-displacement as an input parameter.tic model, the strength in the plastic broken zone canFigs.. 4 shows the results of the finite elementbe described in terms of Eq.(4). It may be noted thatanalysis8in Eq.(6)， σ and σ, are not principal stressesFig. 4a shows a typical example of the stress disbecause there is a force of friction acting on the in-tribution 9n the floor of a coal seam in the immediate中国煤化I jain observations areterface of the coal seam and adjoining rock. Similarly,even when the strength test of coal is performed inas fcCNMHGngonthecoalseam.the laboratory, there is also a friction force on the endcontrols the vertical stress on the floor of the coalplane, σ。and φ are actually obtained in the pres-HAN Jinet alMechanism of mine water inrush through a fault from the floor279seam in the vicinity of a longwall face. Thus, the ver-stresses are relatively low in the depressed area un-tical stress distribution on the floor is relatively highderneath the goaf.in the area of the abutment zone while the floorCoal seamFaceGoafx (m)耳(m)403020100-10-20-30-404030.9°-10-200 -30-40: -1.0/ 00-./0310-0.0520%113官2030||1.大起0 0.)耳200! 20304olPeak ires IheoL40(a) Distribution of vertical stress(b) Distribution of horizontal stress(C) Distribution of share stresson the foor(0,1YH )on the floor(0,/iyH,h=1/3)on the floor(rx1yH )ig. 4 Distribution of components of stress on the floor in the vicinity of a longwall faceIt can also be seen that the magnitude of the verti-ment zone, the horizontal stress is concentrated in acal stress on the floor is not proportional to the verti-shallow zone on the floor and is relatively distressedcal strata pressure on the coal seam. Therefore, thein the deeper zone. However, in the goaf area thecontour of the vertical strata pressure is not symmet-horizontal stress on the floor is relatively low in therical with the vertical stress on the coal seam. Theshallow area in comparison to the deep area (Fig. 4b).vertical stress on the floor decreases with the floorThe distribution of shear stress on the floor shows andepth following a negative exponential law.area of high shear stress concentration as a conse-The peak stress line, which is constructed by join-quence of mining (Fig. 4c).ing the peak flexure points of vertical stress on theFig.5 shows the practical examples showing thefloor, is inclined in the direction of the working face,calculated stress obtained from the back analysisat an angle (0) between 20° and 25° to the verticalmethod for the floor of the No.2701 longwall face inline.the Fengfeng Colliery, China!. The examples showThe distribution of the horizontal stress on the floorthat the stress distribution features on the floor ob-appears to increase or decrease due to the effect of thetained by the back analysis are the same as those ob-vertical stress concentration or release of the coaltained by the finite element analysis.seam and the floor due to mining. Below the abut-20 -404-20 -40 -606040200-20-40-6iaa.orisof Com scam0rCoal ca20- 2.00f20'.s/s.0 sg全30宜30三40A.二402.040s500，604.03.0 OTensile sessras50一l.0(a) Vertical stress contours(b) Horizontal stress contours(C) Shear stress contoursFig.5 Calculated stress obtained from the back analysis method4 Condition for water-inrush from thetent of the coal seam in the plastic zone. The thick-ness of broken strata on the floor is represented by h;floor through a faultor h2, CD is the horizontal projection correspondingto h, EF is that corresponding to h and AB is theFig. 6 shows the mechanism for water inrush fromhorizontal projection corresponding to point A.the floor through a fault!'19-10]The distance betweenthe coalface and the fault surface is w, which is also4.1 State of floor stratahe thickness of the barrier against water. The dipConsidering the state of the floor strata, the peakangle of the fault is a. Point A is the intersection ofstres中国煤化iline; all the floorthe fault surface with the peak stress line, z is the ver-stratare unbroken whiletical distance from the bottom of the coal seam to atheYHCNMHGofthelinemaybepoint A and xe is the extant of the coal seam in thebroken by the surata pressure. Thus, only the floorelastic zone. It should also be noted that x is the ex-stratum below the plastic zone of the coal seam or on280Mining Science and TechnologyVoL.19 No.3the right side of the peak stress line has a possibilityposite to the direction of mining advance. When theof being broken by the ground pressure, provided thatdip direction of the fault is the same as that of miningthe compressive stress on the floor strata exceeds theadvance, the equation is given below:compressive strength. It may be noted that the weaker若sinacosθthe floor strata, the larger the thickness of the barriercos(a+日)required to prevent water inflow.sin acos θ4.2 Pathway of water inrush from the floor(w-xp)(14)The pathway of water inrush from the aquifer tothe working face should be from the aquifer through a5 Case history: water inrush through afault and the broken strata on the floor into the work-fault in the floor of ] Daifeng coal mineing face. The water will follow this path since there isno broken stratum on the floor on the left side of theThe Daifeng colliery is situated in Shandong Prov-peak stress line and the water in the floor aquifer can-ince, China. In this mine, three steeply dipping coalnot flow through the strata on the left side.seams, i.e., coal seams 8, 9 and 10, are being minedby the longwall method of mining. The coal seamsFaultare in the Taiyuan group, which belongs to the UpperCoal seam wilm.m....Face GoafCarboniferous System. The main underlying aquiferis an Ordovician limestone formation. The distancefrom the bottom of number 9 coal seam to the top ofthe Ordovician limestone is 45 m.In June 1970, face No.9204 in the No.9 coal seamwas being mined by the longwall mining method. Theface length was 215 m, the extracted seam height1.45 m, the dip angle of the coal seam 10° and themining depth 100 m. Water inrush occured on 29"/Peak stress line. qJune 1970 when the longwall face had advanced 93 mfrom the starting line of the face .The maximum rateFig. 6 Mechanism of water inrush from the floorof inflow of water was 1628 m' per hour, measuredthrough a faultby neighboring hydrologic observation boreholes. AsAs well, if the thickness (h) of broken strata in thea consequence, the face was completely flooded. Thefloor h=hi on the right side of the peak stress line, themain cause of this inundation was later found to bewater inrush cannot occur because there are intactthe presence of a normal fault (designated as faultstrata between CD and AB and the intact strata inter-number F206) in advance of the face line. The dipcept the pathway between the fault and the brokendirection of the fault was against that of the face ad-strata in the floor. But if h=h2z, water inrush must oC-vance and the dip angle (a) of the fault was 65°. Al-cur since the pathway for water inflow has continuity.though the existence of the fault was known in ad-Therefore, the condition for mine water inrushvance, the technical personnel did not pay attention tothrough the floor from a fault is met when the thick-it, because the throw of the fault was only 3.2 m andness (h) of broken strata in the floor is not less thanit was thought to be‘a minor fault' that could not leadthe thickness (z) of the intersection point between theto a water inrush.peak stress line and the fault surface, ie.,Actually, the fault turned out to be a major conduit)f water, irrespective of the fact that the fault had ah≥z(12)small throw. When the extent of the face advance4.3 Calculation of distance zreached 93 m, the distance from the coal face to theIt can be seen that the area (S) of the triangle .fault plane was only 12 m (viz. w=12 m). The surveyof the site of the inundation revealed that the thick-formed by point A and x can be determined as fol-ness of broken strata on the floor in-situ was 17.8 mlows:(i.e., h=17.8 m). The dip angle of the peak stress lines=xz=x好sinarsin(90* -日)was 20°. The ratio of the plastic zone (xp) of the coal2sin(a+90* -0)seam to the mining thickness (M) was 2.15, i.e.,since x =w-x, then上=2.15_x。 sinacos日_ sin acos日(w-x)(13)then中国煤化工12 m,cos(a-日)Eq.(13) describes the situation where the dip directionsubstMYHCNMHGof the fault is against that of the peak stress line, op-w=12, x。=3.12m, θ=20°, ax= 65°HAN Jinet alMechanism of mine water-inrush through a fault from the floor281into Eq.(13), we have:the broken strata on the floor and then into the work-x。sinacosθsin acos θing face. Mine water inrush can occur from the floor(w-x,)through a fault when the thickness of broken strata oncos(a-日)the floor is not less than the extent of the intersectionsin 659 cos 20°between the peak stress line and the fault surface. Ifcos(65° - 20°)(12- 3.12)=10.7 m.this condition is followed, an adequately thick barrierSince h=17.8 m>z=10.7 m, the water inrush shouldagainst water inrush can be designed effectively.take place in terms of Eq.(12).On the other hand, from Eq.(13), we obtain the fol-Acknowledgementslowing equation:Financial support for this study was obtained fromw=zcos(a -日)(15)the Natural Science Foundation of Shandong Prov-ince (No. Y2007F46)，the Ministry of EducationTaking z=h=17.8 m into Eq.(15) and calculatingDoctor Discipline Special Scientific Research Foun-Wo=W,dation (No.20070424005), the Key Project of thew=w=zcos(a- 0)Ministry of Education of China (No. 108158) and theNational Natural Science Key Foundation (No.sin acosθ50539080) and is gratefully acknowledged.17.8cos(65° - 20°)sin 65° cos 20°2 +3.12=17.9 m. .ReferencesTherefore, a barrier of at least 17.9 m should havebeen retained to prevent water inrush from the floor[1] Singh R N. Mine inundations. Interational Journal ofthrough the fault. When the extent of the face ad-Mine Water, 1986, 5(2): 1-28.vance was 93 m, the distance from the coal face to the[2] Vutukuri V s, Singh R N. Mine inundation-case histories.fault plane was only 12 m. In order to retain a 17.9 mMine Water and the Environment, 1995(14): 107- -109.thick barrier against the water, the face should have[3] Gao Y F, Shi L Q. Water Inrush from Floor Law andbeen stopped at the distance (93+12)-17.9=87.1 mWater Inrush Superior Plane. Xuzhou: China Universityof Mining and Technology Press, 1999. (In Chinese)from the fault. Thus, when the extent of the face ad-[4] Song Z Q. Theory of Applied Ground Pressure Control.vance reached 87.1 m, the mining activities at theXuzhou: China University of Mining and Technologyworking face No.9204 should have been stopped inPress, 1989. (In Chinese)order to avoid the incident.[$] Jing Z G Study of the relation between mining activityand water inrush from floor in Fengfeng coal mine. Coal6 ConclusionsSociety, 1984, 5(4): 45- 49. (In Chinese)[6] Jiang J Q Stope Rock Stress and Movement. Bejing:China Coal Industrial Press, 1993. (In Chinese)From the above study the following main conclu-[7] Wilson S H. The stability of underground workings insions are reached:the soft rock of the coal measures. International JournalWhen the vertical strata pressure exceeds the com-of Mining Engineering, 1983, 2(1): 91-187.pressive strength of the coal seam, a plastic zone[8] Song Z Q. Basic law of stope over rock movement. Jour-nal of Shandong Institute of Mining and Technology,known as the yield zone is formed ahead of the long-1979, 5(1): 87- 91. (In Chinese)wall face in a coal steam. The peak stress line of[9] Shi L Q. The forming mechanism of induction height instress distribution on floor strata is inclined at an an-Feicheng coal field. Journal of Geoscientific Research ingle of 20°- -25° to the vertical ahead of the workingNortheast Asia, 1998, 1(1): 150-152.face. Only the floor strata in the rear area of the peak[10] Shi L Q, Han J. Floor Water-inrush Mechanism andstress line have a possibility of being broken byPrediction. Xuzhou: China University of Mining andTechnology Press, 2004. (In Chinese)ground pressure. The pathway for mine water inushfrom the floor is from the aquifer through a fault to中国煤化工MYHCNMHG