Analysis of the Harmfulness of Water-Inrush from Coal Seam Floor Based on Seepage Instability Theory

Joumal of China University of Mining TecoL 17 No, 4Availableonlineatwww.sciencedirect.com8CIENCEDIRECT·J China Univ Mining technol 2007, 17 (4): 0453-0458Analysis of the Harmfulness of Water-Inrushfrom Coal Seam Floor Based on SeepageInstability TheoryKONG Hai-ling, MIAO Xie-xing WANG Lu-zhen', ZHANG Yu", CHEN Zhan-qing'School of Sciences, China University of Mining Technology, Xuzhou, Jiangsu 221008, ChinayAncheng Institute f Technology, Yancheng, Jiangsu 224003, ChinaXuhai College, China University of Mining Technology, Xuzhou, Jiangsu 221008, ChinaSchool of Resources Geoscience, China University of Mining Technology, Xuzhou, Jiangsu 221008, ChinaAbstract: A theory of seepage instability was used to estimate the harmfulness of water-inrush from a coal seam floor ina particular coal mine of the Mining Group, Xuzhou. Based on the stratum column chart in this coal mine, the distribution of stress in mining floors when the long-wall mining was respectively pushed along to 100 m and to 150 m wassimulated by using the numerical software(RFPA D). The permeability parameters of the coal seam floor are describedgiven the relationship between permeability parameters. Strain and the water-inrush-indices were calculated, The wa-ter-inrush-index was 67.2% when the working face was pushed to 100 m, showing that water-inrush is possible and itwas 1630% when the working face was pushed to 150 m, showing that water-inrush is quite probable. The results showthat as long-wall mining is pushed along, the failure zone is enlarged, the strain increased, and fissures developed cor-respondingly, resulting in the formation of water-inrush channels. Accompanied by the failure of the strata, the perme-ability increased exponentially. In contrast, the non-Darcy flow B factor and the acceleration coefficient decreased exhile the increasee in the water- inrush- index was nearly exponential and the harmfulness of water-inrush incoal seam floor; harm of water-inrush; water-inrush-index: seepage instability; rock strataCLC number: TD 1631 Introductionto the following factors: height of the water-resistingseam, mechanical properties, mining activitiesSeveral kinds of theories, models and methods groundwater pressure and geological structure. Therehave been proposed to explain water-inrush from the are three methods to evaluate the effectiveness of theoal seam floor, such as the"Down Three Zones" protection zone for water-resistance: the inrush coefhysical method, the model of ficient method, the water-resisting coefficient methodfloor damage, the GIS-based forecasting model, the and the inrush index method. The key stratum theorytheory of key stratum, the catastrophic model and the developed by Qian, et al holds that there exists amodel of seepage instability-14. Among these theo- stratum in the coal seam floor which plays a domi-ries and models, the"Down Three Zones"theory and nant role in water-resistance and once this key straheory of key stratum are used most oftentum collapses, water-inrush occurs. The theoriesFrom the"Down Three Zones"theory, three zones, and models mentioned above explain the mechanisme, the mine pressure wrecked zone, the effective of water-inrush from the viewpoint of bearing capac-protection zone and the primitive rising zone exist in ity. However, some authors hold that water-inrush isboth the floor and roof of the coal seam" The for- the result of the interaction of the seepage and strainmation of the"Down Three Zones"is closely related fields中国煤化工Received 11 February 2007: accepted 16 June 2007CNMHGProjects 50225414 supported byional Outstanding Youth Foundation, 50574090,atral Science FoundationCorrespondingauthorTel+86-516-83885757;E-mailaddresshailkong@126.comVol 17 No 4According to the non-linear dynamic bifurcation c,=c+c: cr the isothermal compression coefficienttheory, a tinyin one of the controlling pa- of the liquid and c, the compression coefficient oframeters mayan intrinsic change of stability the rock stratain a dynamicHence, no stratum under theEquation(1)clearly expresses a nonlinear dynacoal seam floor can be ignored. Given the structure ofstrata, a method of separation of variables was usedy Sun, et al to study the response and to analyze the 2.2 Relationship between permeability paramestability of seepage in the coal seam floor I51.A crite-rs and strainrion for judging if water-inrush has happened in aThe three permeability parameters (permeability,coal seam floor is expressed by thickness, pressure non-Darcy flow B factor and acceleration coefficientdistribution and permeability parameters within each are control parameters in system(1), which are de-stratum and the mechanical properties of water. The termined by the distribution of both porosity andwater-inrush-index,defined by Kong, et al, was used fracture in rock strata. There is, no doubt, an interac-as a criterion for seepage instability to estimate the tion between the permeability parameters and theharmfulness of water-inrush( 7biWe have adopted the viewpoint of seepage insta- ability parameters and strain can be determined byility. A coal seam floor model containing abundantexponential functions, which have been provided bygroundwater was calculated by the software package Sun, et al as follows/20).RFPAWe sampled a floor from a coal mineof the Mining Group, Xuzhou. The distribution ofk=kge"me, B=Bem, ca-e2permeability parameters was calculated based on thedistribution of strainwater-inrush-index wasThus the stress-strain state has a decisive effect oralculated and the possibility of water-inrush ana. the stability of a seepage system2.3 Stability criterion of seepage now in a single2 Brief Introduction to the Seepage Insta.stratumbility ModelThe stability criterion of the seepage in a singlestratum has been given by Miao, et al and is exWater-inrush is the instability of seepage of water pressed by the permeability parameters of the stratum,in strata, generally accompanied by the failure of the mechanical properties of the liquid and thestrata and marked changes of the permeability pa- boundary pressure, as followrameters in the strata, The effect of stress on the permeability parameters is considered in our current ar-4B kpoP,≤0ticle. Permeability parameters are calculated based onwhere p, is the pressure difference between the two2.1 Dynamic model of seepage in coal seam floor ends of the stratum and h the height of the stratuma dynamic model involving a set of controlrameters has been presented by Miao, et al"2.4 Stability criterion of seepage flow in stratamodel consists of a mass conservation equation, aThe stability criterion of seepage flow in strata hamomentum conservation equation, a porous compreseen presented by Sun, et al as follows 20)sion equation and a liquid compression equation. It isexpressed as:心n)-地]0dt pc, axw here h is the height of each stratum, A the dy-=-1(+2+Bof liquid, k,, B.andmeability, the non-Darcy flow B factor and the ac-celeration coefficient of the ith stratum, respectively,where p is the relative pressure of the fluid, v seepage the pressure on both ends of the ith stratum are pivelocity, P and u the mass density and the dy- and Pi, respectively, i=l, 2,.,nnamic viscosity of the liquid, respectively, k, p andca the permeability, the non-Darcy flow Bfactor中国煤化工rush-indexand the acceleration coefficient of the rock stratarespectively,B and Po the porosity and pressure strata ue waCNMH Of seepage flow incorresponding to the referenced pressure Po, respec- et al and is expressed as "yt was defined by Kong,tively, ct the unified compression coefficientKONG Hai-ling et alAnalysis of the Harmfulness of Water-Inrush from Coal Seam Floor Based on455problem in the modeL.4 Model of Finite Element CalculationThe boundary of the surrounding rock(includingFrom equation (5), it can be found that seepage in- the seam floor)became variable as the workface waspushed along, so the displacement and stress fieldsstability will occur when x >l, otherwise it will notIt should be pointed out that thousands of speci-should have been calculated and analyzed by usingmens are needed to work out the parameters of dis. system dynamics with variable boundaries.Unfor-tunately, no software based on the concept of variabletribution of every stratum. The test period is too long boundaries has been published. RF(Rock Failto meet the requirements of engineering projects. ure Process analysis ) developed by the Center ofBased on what has been mentioned above, a probabil- Rockburst and Instability Seismic Research,North-ity description was not used in our current articleeast University, has been broadly accepted as anplication software by investigators both at home and3 Mechanical ModelFloor of Coal abroad. a weakened element method was adopted toeansimulate the failure process of surrounding rock inthis software. Based on the distribution of strain ofA mechanical model of a coal seam floor was the coal seam floor, the distribution of permeabilitconstructed based on the column of rock strata inour parameters(permeability, the non-Darcy flow Bselected coal mine(Fig. 1). The object of our study in factor and acceleration coefficient) could be calcu-the model consisted of the following materiallated according to the relationships expressed in1)two coal seams: one was to be mined to a height equation(2). The water-inrush index was calculateof 1.57 m and the orther was not minedby equation(5)2)an overlying strata over the mined coal seam upBased on the column of rock mass in the coal mito 153 m thicka 196 m x 500 m model was simulated by RFPA3)the coal seam floor, beneath the mined coal Fig 2). The permeability parameters and mechanicalseam down to an aquifer with a height of 41.5 m.properties of strata under the mining floor can befound in Table 1. Permeability tests have been carriedout by using an instantaneous penetrating methodThe permeability parameters of each stratum wereacquired based on a single time series of porousEpressure. For theory and method we consulted Sun,etr(m)Fig 1 Mechanics model of the mining floorThe length of the model was 500 m, both the leftand right boundary were taken as immovable, that isthe displacements in the horizontal direction are zeroFig. 2 A calculating model of the mining flooro it followed thatCoal stratum #4 is the mined coal seam, which waspushed to 200 m by 20 steps. Stress and strain fieldsat the moments when coal stratum #4 was pushed toThe overlying strata, not involved in the model, 100 m and 150 m were calculated by RFPAwere regarded as a load acting on the upper boundaryThe permeability parameters of strata under coal inwith a magnitude of 5.0 MPa. It followed that stratum #4 were calculated by interpolation(see Table,tgg. The water pressure in the aquifer was 1). The lithology, thickness and permeability parame-3.0 MPa, hence a,ly-o--Peble 1. The permeability parameters of coal are notWe presumed the displacement and stress at the shown in Table 1. because the permeability of coal isinterface among each stratum were sequential and hi中国煤化工 es greater than thatthat the mechanical properties of each stratum were ofn millions of timesrandom variables, obeying a Weibul distribution.greatCN Gon the stability ofBased on what has been mentioned above, the me- seepage in coal seam floor was negligible.chanical analysis was simplified to a plane strain56oumal of China University of Mining& TechnologyVol 17 No, 4Table 1 Mechanical properties and permeability parameters of strata under mininLithologyPermeability parameterson021.157×10e4.795×102e-842753x10°e2475Siltstone20545210000251182x10-18es6-2542×1020e514372×102e-1012Siltstone125700000.252095×t0-1e4-1.586×1021e178504×10°e7MSandy mudstone43.11.157x10-19cSandy mudstone0.79031157×1010c4795×102e-5042753×101023yMediumsandstone 10.9580001:825×10-18173524×102e86507×10"e-145 Stability Analysis on Seepage Flow in the face is pushed to 100 m. As seen in this figure,landCoal Seam Floorslide and floor heave are not obvious in the coal seamroof and floor. But tiny fractures emerged in the zoneThe strain field of the coal seam floor was variable, close to the workface. The breakage depth of the coalso the permeability parameters were variable too The seam floor was about 12 m. The number of fissuresstability of seepage flow in the coal seam floor and pores in the surrounding strata became larger andchanged as the workface was pushed along. Water their permeability increased. Fig. 3b illustrates theinrush indices at the moments when the workface was change of stress of each stratum under the coal seampushed to 100 m and 150 m were calculated in this floor, where the y-axis denotes the distance between asection. We discovered the mechanism of wa- certain stratum and coal stratum #4. The distributionteT-inrushof the permeability k, the non-Darcy flow B factor5.1 Analysis on the stability of seepage in coal and the acceeleration coefficient of the coalfleeam foor when workface is pushed to 100 mwere calculated and are presented in Table I and iI-Fig 3a is the sketch of the model when the worklustrated in Figs. 3c-e468101214161820.豆二MPa)a) sketch(b) Distribution of stress(e) Distribution of permeability505254565860(d)Distribution of non-Darcy flow B factorDistribution of acceleration coefficientFig3 Distribution of strain and seepage properties for mining floor while working face was pushed to 1001Given that the density of water was p=1000kg/m and the dynamic viscosity A=1.01x10-2Pas=33×10the total pressure difference was P-P=5.0MPa.Based on the properties and parameters presented1-AhTable 1 and Fig. 2, the water-inrush index was calcu中国煤化工lated by equation( 5)as followsCNMHG4(P0-pn)=2×l=-29×10KONG Hai-ling et alof the Harmfulness of Water-Inrush from Coal Seam Floor Based on4×103×5×10°×3.3×10×8.57×0parameters in the surrounding strata when the work-=0.67229×10)face is pushed to 150 m. It can be seen from Fig, 4athat, at the moment, the roof collapsed markedly, itwas evident that separation appeared and the surfaceBased on the result calculated above, the wa- of the earth sunk slightly. The strata close to theter-inrush index was x=67.2% when the workface workface failed. Moreover, new fissures furtherwas pushed to 100 m. It means that water-inrush will enlarged and some of them became visible cracksnot occur in the coal seam floor because x 1. Be- The depth of the failure zone in the coal seam floor iscause of some uncertain factors in engineering prac- about 18 m. At the moment of collapsing, slippagetice, the strain and permeability parameters calculated between the left and right of the roof induced by theabove are not precise and the probability of wa- rupture took place and the permeability parameterster-inrush should be considered. Therefore, essential changed dramatically. a passage for water-inrush hadmeasure,such as grouting, should be planned to pre- formed and rather hazardous conditions prevailedvent and relieve water inrushThe distribution of strain and permeability parameters5.2 Stability analysis on seepage when workface n coal seam floor can be seen respectively in Figsis pushed to 1504b-esing equation(5)and data in Table 1 and Fig 4,Fig. 4 presents the distribution of the permeability the water-inrush index was calculated asg)Effect52( d) Non-Darcy flow B factor(e)Acceleration coefficientFig 4 Distribution of strain and seepage properties for mining floor while working face was pushed to 150 m42(P2-p)=2×10essential measures should be taken to protect theexample, grouting which is usedfill up macroscopic fractures3.9×105 Conclusions∑The state of stress-strain in surrounding stratachanges when the working face is pushed. The per-meability parameters change correspondingly. Wa-ter-inrush occurs accompanying instability of seepagea=302×0,in the coal seam flometric sIze, permeability parameters, liquid viscosity, density of each4×103×5×10°×39×1042×19l×10y3stratum of seam floor and boundary pressure are in-cluded in equation (1). Therefore, water-inrush is a(-3.02×10phe中国煤化工 ctural breakage.fObviously, when the workface was pushed on to abilitCNMHGoyed, their permeinrush. If the sur150 m, x >>1 and water-inrush occurred because the rounding strata are wrecked, the permeability pa-seepage in the seam floor was unstable. Therefore, rameters change sharply and the water-inrush indexJournal of China University of Mining TechnologVol 17 No 4ncreases markedly; in other word, water-inrush may up in the surrounding strata. Therefore, a passage forwater to flow was created and water inrush wasThrough our numerical simulation, the stress and brought about.train of a coal seam floor in the coal mine of the2)The depth of damaged region in the coal seamMining Group, Xuzhou, were obtained. The perme- floor increased as the workface was pushed alongability parameters of the coal seam floor were calcu- The depth was 12 m and 18 m when the workfacelated based on the relationship between strain and was pushed on to 100 m and 150 m respectivelypermeability parameters. The instability of seepage in3)The water-inrush index changed as the workfacehe coalam floor is considered an eventalentas pushed along. For our specific coal seam floor,to water-inrush. The probability of water-inrush in the index reached 67.2% when the workface wasthis coal mine was analyzed. Although liquid-solid pushed to 100 m. Water-inrush became possiblecoupling is not discussed by us, a direct relationship When the workface was pushed to 150 m, however,was established through the consideration of the ef- probability of a water-inrush increased sharpe nd thebetween the permeability parameters and deformation the water-inrush index increased to 1630%fect of deformation on the permeability parametersBased on what has been mentioned above techniSumming up the above arguments, we draw the fol- cal measures should be adopted both before and afterlowing conclusions:mining of coal to forecast, prevent and end water in-1)As the workface was pushed on, the damage rush by methods such as geophysical prospecting andinto larger cracks and slippage and separation tumed outingarea became extended, some new fissures developed grouting.References1] LiB Y. Down three zones"in the prediction of the water inrush from coalbed floor aquifer Joumal of shandong instinte ofMining and Technology(Natural Science), 1999, 18(4): 11-18. (In Chinese)2] LiX J, Zhan w Q- Initiatory attainment in the study on water inrush in coal seam by using multigeophysical method, HebeiCoal, 1995, 3: 29-33 (Ln Chinese)[3] Shi L Q, Yin Z D. An outbursting model of floor damage in coal mines. Journal of Jiaozuo institute of Technology, 1998,17(6):403-405.( In chinese)[4] Jiang D, Wang J H, Chen P P, et al. Establishment and application of the GiS-based forecasting model of floor water burstingin coal mines. The Chinese Journal of Geological Hazard and Control, 1999, 10(1):67-72 (In Chinese)[5] Qian M G Miao Xx, Xu J L, et al. Theory of Key Stranam in the Control of rock Strata Movemen. Xuzhou: China Univer-sity of Mining and Technology Press, 2003. (In Chinese)[6] Wang L G Song Y. A catastrophic model of water-inrush from coal floor. Journal of Engineering Geology, 2000, 802):160-163. (In Chinese)[7] Miao X X, Liu w Q. Chen Z Q. Dynamics of Systems of Seepage Flow in Surrounding Rock Afected by Mining. BeijingScience Press, 2004.(In Chinese)[8] Miao X X, Chen ZQ. Mao X B, et al. The bifurcation of non-darcy flow in post-failure rock. Acta Mechanica Sinica, 2003,35(6):66-667.( In Chinese)[9] Zhang J C. Investigations of water inrushes from aquifers under coal seams. Intemational Joumal of Rock Mechanics andMining Sciences, 2005, 42(3): 350-360[10] Liu C T, Zhao Q M, Zhang Z. Prediction and evaluation of floor water-irruption with expert grading-analytie hierarchmethod, Ground Pressure and Strata ControL, 2001, 18(04): 97-99. (In Chinese)[111 Zhang W Z, Li X G The mechanical model of water-inrush in the floor broken style. Ground Pressure and Strata ControL,2001,18(04):100-103.( In chinese)[12] Shi L Q, Qu Y G Xu wG Method to determine water inrush from a fault in floor Ground Pressure and Strata Control, 2000,17(02):49-51.( In Chinese)[13]Shi L Q, Han J. Theory and practice of dividing coal mining area floor into four-zone Joumal of China University of MiningTechnology, 2005, 34(01 ) 16-23 (In Chinese)[14] Shi L Q, Han J, Song Y, et al. Forecast of water inrush from mining floor with probability indexes. Journal of China Univer-siry of Mining Technology, 1999, 28(05): 442-444.( In Chinese)[15] Sun MGLiTZ. Huang X w, et al. Mechanism of water inrush based on the instability of the seepage flow in rock strataJournal of China University of Mining technology, 2005, 34(3): 284-288, 293. (In Chinese)[16] Huang W. Study on the Stability of Non-darcy Flow in Layered Rock Mass [Ph D. dissertation]. Xuzhou; China University ofMining& Technology, 2005. (In Chinese)[17] Kong H L, Chen ZQ. Water-inrush-factor and its application in the analysis on harmfulness of water-inrush in the longwallmining in Longgu Coal Mine. Journal of wuhan Universify of Technology, 2006, 28(9): 81-82. 93. (In Chinese[18] Tang C A, Wang S H, Fu Y F Numerical Testing of Rock Failure Process. Beijing: Science Press, 2003. (In Chinese)119] Yang TH, Tang C A, Liu H Y, et al. Numerical model of the instability-failure process of the coal-bed floor due to confinedvater inrush. Journal of Geomechanics, 2003, 9(3): 281-288.(In Ch[20] Sun M G Huang X W, Li, et al. Permeability parameters of non中国煤化工 ss of limestone, Chinese Journal of Rock Mechanics and Engineering, 2006, 25(3): 4844[21] Ma ZG Huang W, Guo G L et al. analysis on failure of coveringCN MH Gechanics of systemswith variable boundaries. Journal of Liaoning Technical University, 2006, 25(4): 515-517. (In Chines