Coupling mechanism between mining-induced deformation and permeability of coal

International Journal of Mining Science and Technology 23(2013)783-787Contents lists available at Science DirectInternational Journal of Mining Science and TechnologyELSEVIERurnalhomepagewww.elsevier.com/locate/ijmstCoupling mechanism between mining-induced deformationand permeability of coalXue Dongjie . D,, Zhou Hongwei Wang Chaosheng, Li dongpingSchool of Mechanics 8 Civil Engineering, China University of Mining 8 Technology, Beijing 100083, Chinatate Key laboratory of Coalrces and Safe Mining, China University of Mining S Technology, Beijing 100083, ChinaARTICLE INFOA BSTRACTArticle historThe coupling mechanism between mining-induced mechanical behavias permeability of coal isReceived 24 February 2013Received in revised form 9 March 2013effectively obtained in laboratory This study means significant undeng of the prevention ofAccepted 10 April 2013coal-gas outburst. The testing samples of coal were drilled from the 1ining face at the depth of690 m Based on the redistribution of stress during the excavation, the coupling test between mechanicalstate and seepage has been designed using the triaxial servo-controlled seepage equipment for thermofluid-solid coupling of coal containing methane. It is the result that there are two main factors influenc-Mining-induced meal behaviorthe mining-induced mechanical behavior of coal, such as the change ofe change of o1-o3and△a1-△G3,TheCoal deformationfailure mode mainly depends on the value of 01-03, and the peak strength value mainly depends onthe value of A1-A3. The difference of mechanical response between geostress and mining-inducedCoupling tesstress has been obtained, which can be a theoretical support for safe mining such as reasonable gas drain-age prevention of coal-gas outburst and gas over-limito 2013 Published by Elsevier B V on behalf of China University of Mining technology.1 Introductionnumerical simulation and practical investigation. all the researchindicates that there is relationship between stress or strain andFrom stress-based view, there are usually zones of released permeability of coal [4-6]. The determined relationship is not clearabutment pressure, increasing and in-situ stress before working just only from qualitative analysis of abutment pressure and monface of coal. Corresponding to the abutment pressure, the horizon- itoring results in local zone before mining face So the quantitativetal stress changes from uni-axial stress state to in-situ stress state. analysis of stress state and volumetric deformation under variousFrom strain-based view, there is deformation distribution from mining layouts can be beneficial to the determined relationshilelastic to plastic zone. According to the mechanical principal of Especially horizontal stress also mainly influences coal deformafailure, gas can completely escape from coal in the damage zone. tion. Indeed many triaxial tests have been done to discuss mechanHowever due to the formation of self-shielding mechanism of coal, ical properties of coal or rock by different loading and unloadingthis zone is not enough in length to avoid coal-gas outburst in the paths to simulate the evolution of stress in deep [ 7-14 This canfront of coal mining face. This mechanism means that in the zone not illustrate the principal of path without considering quantitaclose to peak value of abutment pressure, the increasing expansion tive characteristics of mining-induced evolution. Based on theof volume induced by the failure of structure in coal contributes to coupling mechanism between mining-induced mechanical behav-gas desorption, but there is another volumetric equilibrium caus- ior and permeability of coal from testing results, this article mainlying cracks to be closed. Various mining layouts can cause different and effectively makes an explanation on permeability evolution ofstress environment of coal in the front of mining face [1-2]. There- coal considering characteristics of stress redistribution in the frontfore, the investigation on evolution, generation and distribution of of mining facecracks can improve borehole arrangement for gas drainage. Thecoupling mechanism between mining-induced mechanical behav-ior and gas permeability under various mining layouts can be a2. Coupling test between mining-induced stress andheoretical support for improving safe mining 3permeability of coalIn recent years, more and more studies on the abutmentpressure have been done with consistent view based on theory,2.1. Sample preparation中国煤化工e14120 miningCorresponding author. Tel. +86 15101127335face in No. 8Group in HenanE-mailaddressxuedongjie@163.com(d.xue).s HCNMHG-656 to-510 mh t: /d. do, oree o m imes.213.1. D1 hed Dy Elsevier Bv, on ehalt of China University ot Mining technolOgyD. Xue et aL/Intenational Journal of Mining Science and Technology 23(2013)783-782Non-pillar mining Top-coal caving Protective coal-seam mining2. 2. Experimental principle and programThe sample was put on the triaxial pressure chamber and fixedThen loading-unloading program based on ratio of abutment pres-sure and horizontal stress was carried out. The process included255three stages based on change of horizontal stress, such as hydrostatic pressure, first and second stage. The specific testing programwas designed as followsFig. 1. Top view of samples for coupling test.(1)Hydrostatic pressure stateand ground is from+120 to +150 m The depth is about from 630 to806 m with average dip angle of 22 descending from west to east(2)Mining-induced evolution of abutment pressureThe gas pressure is 1. 8 MPa, and content in coal is 23.0 m/tAccording to dangerous grade of outburst from national regulations about coal mining, the mining face is defined as dangerous where a is the peak coefficient of 3.0, 2.5, and 2.0, respectively, forface. A total number of 110 samples were collected from the non-pillar mining, top-coal caving and protective coal seam miningface with th30 x 40 x 40 cm. Then the y the bulk density, kN/m; and H the depth, m.samples are processed into standard cylindrical specimen(φ50mm×H100mm)to(3)Evolution of confining pressure corresponding to a,from,HayH including two stagese proposed three various mining layouts including non-pillarmining, top-coal caving mining and protective coal seam mining as1st unloading stage: 02=03=50the typical cases corresponding to high-efficient, green and safe2nd unloading stage: 02=03=201mining [1]. Based on statistics, the peak coefficient of abutmentpressure is 2.5-3.0, 2.0-2.5, and 1.5-2.0, respectively. Three samThe above testing program can effectively characterize the dis(Fig. 1). According to the real distribution of abutment pless, sts tribution evolution of mining-induced stress in the front of miningface. The influence of mining layouts on mining-induced mechanand horizontal stress under the three various mining layouts, con- ical behavior is illustrated by o. Due to the maximum value oftrol in laboratory of abutment pressure and horizontal stress canquipment parameters, the mining depth was designed to bebe instead of axial and confining pressure, respectively. The experimental program of triaxial test was made for simulation on60 m with the hydrostatic pressure state of 01=02=0increasing vertical stress and decreasing confining pressure of coal / H=9 MPa. Therefore loading-unloading program of non-pillarmining can be designed as followssimultaneously So the mining-induced evolution and distributionJ 1st stage: 01=1.5 H=13.5 MPa(loading)02=03=301=5.4 MPa(unloading)[ 2nd stage: 01=ayH=3 x9=27 MPa(loading)a2=03=52a1=1.8 MPa(unloading)of stress change were reproduced in the laboratory. The triaxialLoading-unloading program of top-coal caving mining can beservo-controlled seepage equipment( Fig. 2)for thermo-fluid-solid designed as follows:j1st stage: 01=1.5yH=13.5 MPa(loading)02=03=301=5.4 MPa(unloading)1 2nd stage: 01=ayH=2.5 x9=22.5 MPa(loading)02=03=52 01=1.8 MPa(unloading)coupling of coal containing methane in Chongqing Universityd, which is composed of servo-controlled loading system,Loading-unloading program of protective coal seam mining cancompression chamber, water bath with thermostatic control be designed as follows1st stage: 01=1.5,H=13.5 MPa(loading)o2=03=301=5. 4 MPa(unloading)2nd stage : 01=a,H=2x9=18 MPa(loading)02=03=501=1.8 MPa(unloading)V凵中国煤化工ystem, pore pressure control system, data collecting system andother supporting system with the maximum axial pressure ofThe coupling gasCNMHGSimultaneously100 MPa, confining pressure of 10 MPa, and heating temperature according to the principle of steady-state method from the darcyof100°CD, Xue et aL. /International Journal of Mining Science and Technology 23(2013)783-787ateralVolumetric strain Axial strainFig. 2. Triaxial servo-controlled seepage equipment for thermo-fluid-solid cou-pling of coal containing methane0.30.1500.15030.450.6Fig. 5. Conventional stress-strain curves of coal samples without mining-inducedo. First unloading stageA:Protective coal-seam mininat second unleillustrated as follows: (1)The stage of hydrostatic pressure(OA)including axial stress and confining pressure loading up to 9 MPaning at secondat the velocity of 0.027 MPas with the ratio of 1.(2)The firstunloading stage including axial stress up to 1.5 times of hydrostatica=ayHpressure with the value of 13.5 MPa at the velocity of 0.05 kN/s,and confining pressure unloading to 5.4 MPa at the velocity of0.012 MPa/s controlled by(01-03): 03=3.25: 1. There is no dif-ference in this stage of non-pillar mining, top-coal caving mining,and protective coal seam mining. 3)The second unloading stageincluding three different paths of BE, Bd and BC, respectively, fornon-pillar mining, top-coal caving mining, and protective coalseam mining. BE stage means the axial pressure of compressionloading up to 27 MPa at the velocity of 0.05 kN/s, and confiningpressure unloading to 1. 2 MPa at the velocity of 0.006 MPa/s con-axial load was applied, then displacement-controlling load at the trolled by(01-03): 03=3.75: 1. BD stage means axial pressurevelocity of 0.1 mm/min In the procedure gas pressure in air inlet of compression loading up to 23.5 MPa at the velocity of 0.05 kN/and outlet of chamber was kept stable. Then gas flow is continu- s and confining axial pressure unloading to 1. 2 MPa at the velocityously recorded within a certain time. The permeability of coal of 0.008 MPa/s controlled by (01-03):03=3.5:1.BC stagecan be calculated by the formula:means axial pressure of compression loading up to 18 MPa at the2qpouLPi-P5(7) velocity of 0.05 kN/s, and confining axial pressure unloading to(G1-03):3=3,25:1where K is the gas permeability, um q the flow velocity, m/s: pothe standard atmospheric pressure; A the cross-sectional area of 3. Mining-induced mechanical behavior of coaloal sample, cm u the viscosity of gas, MPa s: L the length of sam-ple, cm; and p1, p2 the absolute pressure of air inlet and outlet.ased on the above program, loading and unloading path in pro-respectively, 0.1 MPacess is shown in Fig. 4. After loading up to theoretical peak value ofabutment pressure there is no any increment of axial stress due to2.3. Testing processcontinuous unloading confining pressure to sample failure. Pressure difference still increases before failure. however to the samesed on the above principle, an experiment was carried out by group of coal sample according to different loading paths the valueol-loading velocity(Fig. 3). The specific procedure of test is of a1-o3 is different at the peak value of failure, which indicatesAxialstress② Top-coal cavingProtective coal-scam miningI Non-pillar miningcOnfining 2 Top-coal caving② Top-coal(3 Protective coal-seam mining3 Protective coal seam mining中国煤化工00CNMHG(a) History of pressureb) History of pressure differenceFig 4. Loading and unloading path in process of test.D. Xue et aL/Intermational Journal of Mining Science and Technology 23(2013)783-787-Lateral stresslu m030.0200100.010020030.0404-0.030.02-0.0100.010020030.040.010.0(a)Non-pillar mining(b) Top-coal caving mining(c) Protective coal seam miningFig. 6. Relationship between axial stress and strain, confining pressure and lateral strain.20r Axial stressstrain curvecurveAxial stressstrain curve400080。al strain000axial strain00.005001000150.0200.02500300050.0100.0150.0200025003000.00500100.0150.0200.0250.030(a) No 26 sample for non-pillar mining(b)No 16 sample for top-coal mining(c)No 28 sample for protectivFig. 7. Coupling characteristics of axial strain and permeability.that (1-O3 is one of the main factors influencing the failureFig. 7(b)shows the stress-strain curve of top-coal caving miAo1- AO3 mainly controls difference of loading-unloading pro- ing including five stages such as increasing stage before peak, steacess in the test, which indicates that the evolution of Ao -Ao3 dy pressure stage around peak, slowly decreasing stage, rapidlis the main factor influencing the failure strength of the coal sam- decreasing stage and residual stress stage. permeability increaseple. Therefore it is obtained that the failure mode mainly depends little before failure. When the axial strain changes from 0.0175on the change of o1-o3 leading to shear-predominant failure with 0.022, there is a rapid incrementtensile-complementary failure. This is not definite and absoluteFig. 7 (c)shows the testing curve of No. 28 sample for protectivebecause the value of o, and o3 also influence mechanical behavior. coal mining. Permeability has the same changing process with theIndeed, G1 and O3 are mainly related to in-situ stress, tectonic above two othersstress and others. The evolution of o1-03 and Ao1- Ao3 is mainlyAfter failure there is a rapid increment of permeability with theproduced by stress redistribution due to human disturbance. By three various mining layouts, while each has its own characterisomparing the traditional test results(Fig. 5), the relationship tics. The mining face is free with serious damage and many cracksbetween axial stress and strain, confining pressure and lateral leading to full desorption of gas. So the zone near face is dangerousstrain is shown in Fig. 6. There are three obvious features in the with over-limit gas, and enhanced ventilation is indispensable tomining-induced test. There is obvious phenomenon of plastic reduce gas concentration in mining Gas in inner coal flows towardflow when unloading confining pressure, which can only appear free face, and is blocked in increasing pressure zone, so drillingunder high confining pressure in the traditional test ( 2) When holes should be through such zone to release gas and reduce danunloading the same value of confining pressure, the lager value ger of coal-gas outburst.of Ao1-Ag3 comes with the higher failure strength of peak value,which illustrates that the coal failure strength is more related tothe loading-unloading process in the mining-induced mechanics 5 Conclusionstest. 3 Strain softening obviously appears in the lateral strain coresponding to stage of unloading confining pressure as shownThe mining-induced mechanical behavior of coal samples isinvestigated by comparison with the traditional results. The aimis to demonstrate the behavior mainly influencing the risk4. Mining-induced coupling characteristics betweencoal-gas outburst. We approach this in laboratory bypermeability and mining-induced stresscoupling mechanism on volumetric deformation and permeabilityOur conclusions can be summarized as follows:The axial stress-strain curve and permeability-strain curve of(1)Based on redistribution of stress, the mining-induced testpillar mining are drawn in Fig. 7(a)together. The initial axial can quantitatively characterize the evolution of stress in the frontpressure is 9 MPa, the peak value is 27.01 MPa and the peak coef- of mining face. Thefluence of vari-ficient is 3 according with theoretical design of loading-unloading ous mining layouts.中国煤化工program. There is a very complicate process of decreasing stress(2)The failure mCN MH Gange of a1-O3after failure. There is an obvious phenomenon of plastic flow when leading to shear-predimplementaryunloading confining pressure. In the whole process, permeability failure. This is not definite and absolute because the value of a1 andincreases suddenly and sharply when axial strain changes to 0. 175. 3 also influence mechanical behavior. Indeed o and o3 are mainlyD, Xue et aL. /International Journal of Mining Science and Technology 23(2013)783-787related to in-situ stress, tectonic stress and others the evolution of [2] Xue D. Zhou Hw. Tang XL Zhao YE. Mechanism of deformation-induced01-O3 and Ao1- Ao3 is mainly produced by stress redistributiondamage and gas perme(3) There are three obvious features in mining-induced test. [31 Yyouts Chin j Geotech Eng 2013: 35(2): 328-36due to human disturbance[3 Yuan L. Theory of pressure-relieved gas extraction and technique system ofintegrated coal production and gas extraction. J China Coal Soc 2009: 34(1)There is obvious phenomenon of plastic flow when unloading confining pressure, which only appears under high confining pressure [4] Xue DL. Zhou HW, Kong L, Zhao T, Yi HY, Tang XL. Mechanism of unloading-in the traditional test @2 When unloading the same value of confinGeotech Eng2012;34(10):1910-6ing pressure the larger value of Ao1-Ao3 comes with the higher 5] Zhou HW, Zhang T, Xue DJ Xue JH. Evolmining-induced crack networkfailure strength of peak value, which illustrates that the coal failurein overburden strata of longwall face. J China Coal Soc 2011: 36(12): 1957-6[6 Wang GR, Xue D), Gao HL Zhou HW. Study on permeability characteristics ofrength is more related to the loading-unloading process in themplete stress-strain processmining-induced mechanics test. (3) Strain softening obviously ap7-12,pears in the lateral strain corresponding to the stage of unloading 7 Xp lc Pteg s t ax a se Jaocontrollend eew an e,t fer element ndconfining pressure.solid coupling of coal containing methane. Chin J Rock Mech Eng 2010; 29(5)(4)Generally speaking, it is a complicate coupling process be-tween coal deformation and permeability. Permeability evolution 8 in GZ, Li GZ. Zhao HB, Li Xs, Jing XF, Jiang CB. Experimental research on gasabout three various layouts are consistent, but each hasRock Mech Eng 2010: 29(1 ): 170-5Decimens in complete stress-strain process. Chin Jcharacteristics. Permeability increases little in stage from initial9 Wang LG, Miao XX Cusp catastrophe model of relations among permeability,tress and strain of rocks. Chin roalue to peak stress, while there is a dramatic increment when (10) Jiang CB,Yin GZ, Huang QX,Si HR Expele failspressure unloading process.China Coal Soc 2011: 36(5): 802Acknowledgments[111 Sun PD, Ling ZY. Experimental study of the law for permeabilityon of 3-triaxial compression J Chongqing Univ 2000: 23(S1)Thanks for the funds supported by the State Key Basic Research [121 Yang Y] Song Y, Chen S). Test study on permeability properties of cocimen in complete stress-strain process. Rock Soil Mech 20Project of China(No. 2011CB201201)[13 Xie HP, Zhao XP, Liu JF, Zhang R, Xue D. Influence of different mining layoutsReferen[14 Xue D]. Zhou HW, Kong L, Tang XL Yi HY, Zhao YF Deformation analysis oftransversely isotropic coal-rock mass with porous and cracks. Int J Min Sci[1]Xie HP, Zhou HW, Liu JF, Gao F, Zhang R, Xue D, Zhang Y Mining-inducedTechnol2012;22(6):809-15中国煤化工CNMHG