Research on evolution of mining pressure fieldand fracture field and gas emission featuresLi Huamin, Xiong Zuqiang, Li Dongyin, Yuan Ruifu, Wang WenSchool of Energy Science and Engineering, Henan Polytechnic UniversityJiaozuo, Henan 454000, China)Abstract: The relation between mining pressure field-fracture field and gas emission of working face is analyzed, andthe concept that there is a stress point ( or strain point)among permeability of coal is presented. It is believed thatmutation of coal permeability caused by the sudden loading or unloading of working face roof as periodic weighting occursis the main reason that a lot of gas pour into the working face. Based on the above concept, the relation is establishedamong abutment pressure during periodic weighting, permeability of coal seam and gas emission, and relation graph isdrawn. Then the loading and unloading features of coal at the moment of fracture and non-fracture of main roof are re-ealed. And finally it is presented that the process of sudden loading or unloading as periodic weighting occurs plays animportant role in rupture propagation of coal, analytical movement of gas and gas emissionKey words: mining pressure field; fracture field; gas emission featuresdesorbing movement, the laboratory experiments and1 Introductionnumerical simulation, this paper mainly discusses theBefore the working face is extracted, the coal relation among pressure field, fracture field and analytbody is in an initial stress balance state. The mining ical movement of gas characteristics of working face, inactivities destroy the balance of the original coal stress, order to further understand the basic rules of coal gaswhich inevitably leads to the deformation and destruction of coal rocks within a certain range. With the 2 Mining pressure field and its features ofworking face advancing, the mining pressure fieldevolutiongrows and evolves incessantly, causing the evolution ofthe mining fracture field. It can be said that the miningThe original rock stress is the fundamental force ofand the change of pressure field provide the necessary deformation and destruction of underground coal rockcondition for the formation and development of the min- gas emission, pressure bump as well as the occurrenceing fracture field. At the same time, the change of the and development of all other dynamic disasters in themining pressure field and the evolution of fracture field mines. Before mining, as the working face is not dis-make the desorbing movement of gas of the working turbed by engineering, the coal rock is in the originalface possible. This will cause fissures of coal seam in- stress state. When the working face is extracted, thecreasing,gas desorbing speeding up, permeability in- original state of stress field is destroyed, and the stresscreasing and gas emission speeding up within a certain of surrounding rock is redistributed. When mining pro-range in front of the working face). It is known in ceeds, with the propulsion of working face, the firstthe research 5- that if the porosity is increased by fracture of the main roof generates the first weighting50 % the gas permeability can averagely rise 2And then with the working face advancing further, the3 times, and the highest point can reach an order of roof will be fractured cyclically, called periodic weighmagnitude. Meanwhile, the permeability of the coal ting. In the whole process of pressure formation, forsurface increases several orders of magnitude. Taking the coal body ahead of working face, the process fromcoal bed of Kuznetsk mining area as an example, its the original stress to abutment pressure is a process ofnatural gas permeability is 9. 6 x 10"m, while the ex- stress changing gradually. The abutment pressurepal bed is 9. 3 x 10"m, nearly 1 000 times within the face-rid also experiences amore. Based on the law of pressure-fracture and gas ging process which can be described as from smaller to中國(guó)煤化工Received 10 August 2010CNMHGvol.10No.2,Apr.201249he maximum, and then reducing. The coal body cer-tainly will experience the process of loading or unloaCurve 1ding periodically. In the non-cracking moment of themain roof, the loading process in the coal body changesslowly with mining, which can be called slow-loadingIn this loading process, due to the impact of nonsupport roofs subsidence, the coal wall suffers the0clamping force, and the longer non-support roof is, thestronger the stress from roof to the coal wall is. alongCurve 2with the direction of mining, the closer the distancefrom the face-rid to the working face is, the bigger thesubsidence of the roof is and the higher the force isotherwise. the smalleyHowever, in the period of working facstress, the strata of the roof is in the continuing adjust-ment state of instability and balanced. Research hasCurve 3own 12) that in the moment of thefractured roof shakes and gives the face-rid a suddenHloading force. At the same time, the fractured roof re-leases energy to the face-rid, which causes coalcing in a certain scope ahead of the working faceprocess of rebound and compression appearing. If theenergy from the roof fractures releases slowly, the rockFig 1 Stress loading of coalbeam does not shake, and the roof will transit slowlyfrom the location of complete to fractured. If the roof coal rock is lessened the friction coefficient of them iscracks suddenly, the elastic beam will elastically vi- also reduced. And then static friction coefficient debrate in the new balance position and give the face-ridan additional unloading effects. The No. 272 workicreases to dynamic friction coefficient therefore, fric-tion is rapidly decreased. Then, the coal bodycanbeface of Taozhuang coal mine of Zaozhuang Coal Mining easily moved; as a consequence, its ability to impactGroup Co. Ltd. adopts borehole hydraulic pillow to re- and destroy outwardly will be strengthened. Suchord the stress chaangesof coal automatically, coopeting with circle diagram pressure machine( Fig. 1) 13)process is an important stage for the pore and fracturein coal body extending and floor heaving. MeanwhileIn Fig. 1, o is the vertical stress; t is the time; curve 1 it is also the initial force of rib spalling during the periis the characteristic curve of large scale time variation odic pressure, Coal mass outburst and being out-and slow increasing stress, in which a,, a, and a,arepressure cycles, and b,, b 2 and b, are pressure processqueezed in coal mine production. Accordingly, stresschanging, loading and especially unloading withises;curve 2 and curve 3 show the small scale time var- body can inevitably bring about fractures and their exiation and the transient loading( unloading action tension in a certain area of coal and rocksduring the period of the pressure.It can be seen that the stress changes of the coal 3 Mining-induced fracture and its featuresare in two forms, loading slowly and suddenly. The in-of evolutionitial direction is mainly upward in the loading way, andMining-induced fractures are formed in a certainen uownwaarea of coal and rock strata after coal mining. Accord-When the roof cracks, roof weighting loads on thecoal body with a quite high ratio, which will reach theing to different positions, the fracture can be classifiedas follows coal seam mining-induced fracture, roofhighest strength in a short time under such strong pres-strata mining-induced fracture, floor strata mining-sure.And then, it will become relatively soft, de- induced fracture and surface mining-induced fracture.stroyed, extended, and produce an outward deformabased on the features, the fracttion. In the following period of time, when suddenly be classified ashd。 anarated fractureunloading, the roof vibrates and rebounds, which cau-and activation of中國(guó)煤化工 subsidenceseries of mutation. The clamping force to the coalCNMHGais reduced; the cohesion between interfaces of bed separated fracture, which, together with intermit-o Engineering Sciencestent vertical fracture, forms main entries leading meth- within a certain scope ahead of the coal wall. Whenculatingsomeacesthe stress overruns the ultimate intensity of the coalsuch as working face, gob area, etc. Obviously, body, the coal will fracture and move to depth far bemining-induced fracture's distribution of coal and rock low gradually. And then it will form the abutment pres-strata has close relationship with mining-induced gas sure affected zone, namely the so-called stress- reducedflowing after unloading in coal seam, coal mine gaszone. stress-increased zone and initial stress zone incident as well as high-efficiency production of coal-bed front of the working face. The abutment pressure car-methane( CBM)ries forward continuously with the advance of the work-During mining in working face, fracture space will ing face, the cracking process and scope together withinevitably evolve due to variation of working-space the stress-change process and scope of the coal rocksshape. When immediate roof collapses, between refuseThe above mentioned is about the situation of theand main roof, there will be a gap, which is the main working face in non-weighting state. However, whenarea of high-gas accumulation. Meanwhile, in gob the working face experiences the first weighting or periarea, among refuse, because of coal and rock broken, odic weighting, both the stress state and size will haveextending, packing, gap also occurs, which becomes great changes in front of and behind the working face.the reservoir of high-gas, too. With the process of min- Consequently, the crack field is also greatly changedng, the falling immediate roof packs in gob area be- inevitably.hind the working face in a loose form, and the main 4 The relationship between mining crackedroof cracks and forms fractures on the falling refuseThree structural states existb area at this time,field and gas emissionnamely gob area not having been filled up, gob areaMining is the basic condition for gas emissionhaving been filled up and fracture area. Under the sup- The cracked field caused by mining is the channel andport of hanging roof in mine border, gob area not hav- site of the gas flowing and accumulating. The crackeding been filled up is generally located in the border of field of coal working face with different locations andentire gob; goob area haaving been filled up lies middle- different characters has disparate modes and characterlower, and fracture area is above the gob area. Among istics of gas flowing. Meanwhile, the emission amountthese areas, the fractures in the mid-upper of the gob and effect are also different for the safety of the workingarea develop mostlyThe upper strata will slowly subside after roof colThe flowing modes of gas mainly are as follows:lapsing. During this period of time, bed-separated diffusion, seepage, rising and so on. Diffusion is thestrata, under the lateral tension as well as shear stress, migratory process of gas molecules from high concentrawill generate vertical fissures inside, which wont stop tion to low concentration under the influence of itselfdeveloping until the overburden subsidence ceases. In concentration( or density )gradient. Obviously, themining-induced fracture area, the strata which have gas in the fractured coal rocks ahead of working facegenerated bed separated fissures and vertical fissures and leading fractured zones behind the working facebecome strong fractuured zone. andd the strata above that belongs to the form of diffusion. Seepage is mainly thehavent generated vertical fissures become weak frac- flowing form of gas in the entity coal ahead of the coaltured zone. In this paper, it is regarded that very weak working face, and rising is the flowing characteristic offissures that exist in some weak structural plane in coal goblarger space. The source of gas emission in-and strata play an important role in the formation of sideing face mainly has self coafractured zone. Therefore, it's easy to understand that gas, adjacent coal seam stress-relieve gas and so onthe formation of fractured zone is influenced by initial The emission of the self coal seam gas is divided intofissures and advanced mining. As a consequence, the coal wall gas emission ahead of the working face andfracture always doesnt begin from the lowest part of gob gas emission behind the working face; the adjacentstrata, but from advanced mining position, where coal seam stress-relieve gas mainly enters the workinginitial fissures exist, and then extends outwards room of the working seam through the mining fracturedgradually. At last, these fractures form a field, called field of the gob. To sum up, to discuss the gas emis-fractured zone(field)sion problem of the working face is mainly to analyzeAhead of the working face, when it is changed the problem of the cnal wall pas emission ahead of thefrom three-dimensional stress state to two-dimensional working face and中國(guó)煤化工 her articlpartial area is uniaxial )stress state nearby the coal to discuss)CNMHGwall after opening, a stress-increased zone occursAccording to the relationship between stress fieldvol.10No.2,Apr.201251and fractured field, nearby the coal wall, when mining Meanwhile, because the abutment pressure increasesstress overruns the ultimate intensity of the coal, the part gas in the coal seam converts into adsorption statecoal wall will fracture. In a certain range of depth of from the free state. Consequently, the amount of gascoal wall, the fracture form of coal body is associated emission is relatively smaller in the coal seam and in-with the intensity of the coal. When the coal body is versely proportional to the amount of the abutmentrelatively soft <1, f is the hardness factor of coal), pressure. At the corresponding point of the maximumthe fracture of the coal body is mainly the horizontal abutment pressure, air permeability of the coal bodydisplacement of the compressed coal wall from the top reduces to the minimum value that is a little smallerand middle to the working face accompanying with the than the initial permeability coefficient Ao,astension fractures, supplemented by the shear fractures shown in the test.of the bevel. When the coal body is relatively hardThe gas intensity nearby the coal wall is relatively(f>2), on the influence of the high stress in coal, the lower than that in the coal body, and it gradually be-produced fractures mainly are the almost vertical frac- comes lower from inside to outside, from which it cantures with the coal seam and the heterotrophic frac- be easily concluded that the lowest position is the planetures, and these fractures are affected by the accompa- of working face. As a consequence, gas inside the coalnied horizontal stress nearby the coal wall. The closer body ahead of working face inevitably flows towards theit approaches the coal wall, the wider the extensional working place. Meanwhile, when the abutment pres-fracture is; the deeper it reaches in the coal, the smal- sure gets higher (especially when first weighting or peler the fracture horizontal expansion is. That is to say, riodical weighting occurs, the abutment pressureowing to the open and extension of the original fractures ways is 1. 3-2 times higher than that as usual),theof the coal body in the stress-reduce zone, and the for- fracture-scope of the coal body increases; the fracturemation of the new fractures, the air permeability can degree is higher; the permeability coefficient also getssharply increase by thousands of times. As a result,. higher. As a result, in the same situation, the gradientdischarging pressure and increasing flow effects"oc- of gas intensity becomes larger, and the emission of gascurs. Meanwhile, the change of the abutment pressure also inevitably increasesleads to the change of the coal surface tension, so doesGas seepage in the working face is a one-way typethe existing state of the gas within the coal body. When of flowing, whose pressure in seepage field displays pathe coal body is unloaded, the gas will be parsed in the rabola distribution. And in the beginning of the flowingcoal body, namely, coal fracturing firstly, then stress of gas, the pressure gradient is relatively high in thereducing, then coal seam air permeability increasing, field. The permeability coefficient of gas(k)is greatlythen gas flowing speeding up, and then migrationnfluenced by the variation of abutment pressure. Thechannel being through. In this area, the gas permea- cracked degree of certain scope of coal nearby the coallity of the coal is the maximum, while it reduces wall gets higher, and the value of K greatly increases.gradually with the depth of the coal body. It is shown Meanwhile, in the same situation, the pressure gradin the test of F group of coal in Pingdingshan mining ent of methane gas becomes higher( that is, from inte-area. Generally, fissure and fracture ratio in this group rior of working face to the direction of coal wall)increases by 10 %-50 related to original coal therefore, the amount of gas emission due to seepageseam. Thus, stress-reduced zone fractured zone) will inevitably increaseahead of the working face is the main area, in whichStress field and fracture field experiencethe gas desorbs and flows through the fissures and getsdynamic evolution process with the continually pushinginto the working face. This area is called unloaded gas of the working face. Thus, the corresponding flowincreasing flow zone. Hence, gas emitting from unloa- characteristics of the gas ahead of the working face in-ded-zone is the main part of the gas emission ahead of evitably experience a dynamic change process with thethe working face. The gas flowing in this area is a type pushing of the working face. The permeability coeffi-of mixed unstable flowing called heterogeneity of poroscient of the coal seam ahead of the working face can beity, double medium fissures seepage and diffusion.expressed by the formula belowAfter entering the stress-concentrated area, thek=k0(1+B1e)increase of the abutment pressureIn Eq. (1), k is the coal seam permeability coeffision of the coal seam pore fissure and the closure of the cient in abutment pressure affected zone, md; ko is thejoint fracture, leading to air permeability reducing. initial permeal中國(guó)煤化工seam,md;The higher the abutment pressure is, the worse theis the verticalMPa; BCNMHGermeability is, and the gas stays in a closed state. B2areEngineering SciencesAccording to the actual measurement of the pore is directly related to coal seam strength, lithology ofgas emission in the unload zone within the F15 coal overlying roof and mining depthseam in Pingdingshan mining area, if the permeabilityThe dynamic loading function results in increasingcoefficient of the coal surface increases 1 000 timesand decreasing of coal hall stress in a second. The inthe distribution of the air permeability in the unload creasing stress enlarges the fractured area of coal hallzone ahead of the working face can be shown in Fig. 2. and the ultimate equilibrium area Xo during the weighKrH(1-cos)+2(CI+mC+mfo,)x2(C1150(2In Eg. (2), Xo is the width of the plastic zoneDistance to coal wall/mthe width of the limit equilibrium zone ), m; K is theFig 2 Curve of changing permeability coefficientstress concentration coefficient; r is the averageahead of the working facevolume-weight of the overburden strata, N/m; H isthe mining depth, m; a is the angle of coal seamTaking the research of coal rock permeability test, (); m is the mining thickness, m; C, is thee cohesionstress field of working face and crack field test into of the interface of coal seam and the roof and floorconsideration, the relationship among the pressure Pa; C, is the cohesion of the coal body, Pa: f, is theahead of the working face, coal seam permeability and internal friction coefficient of the interface: f, is the ingas pressure at the un-weighting moment can be shown ternal friction coefficient of the coal body; o. is thein Fig 3. In Fig3, I is the area of coal cracked, uniaxial compressive strength of the coal, Pa; a, is thepressure discharging and permeability and flow increas- uniaxial tensile strength of the coal, Pa: a is the vertiing, gas pressure decreasing; II is the area of dam- cal stress in limit equilibrium zone, Pa: Po is the reac-aged coal body where plastic, stress and permeability tion from the supporter to the coal body,Paincrease; II is the area of stress recovering, permea-The dynamic loading effect can cause larger areability decreasing and stabile gas pressure; I is the of the coal rock ahead of working face to reach and ex-area of coal in virgin state: I is the curve of abutment ceed seepage mutation critical strength and mutationpressure of face anterior; 2 is the curve of coal seamstarting penetration rate. Therefore, the relationshipermeability of face: 3 is the curve of gas pressure of among the abutment pressure ahead of the workingface anteriorface, permeability of the coal seam, and gas pressureduring the period of the weighting can be seen in1gthe basis of fig 3In Fig 4, I'is the area of gas pressure decrea-sing, pressure discharging and permeability and flowweighting; I'is the area of gas pressure increasing區(qū)就1-+-Ⅱ-+—Ⅱ+—Ⅳcracked coal mass, stress increasing, and permeabilityPositionincreasing during weighting; Ill 'is the area of stabilegas pressure, stress recovering, and permeability de-Fig 3 Curves of abutment pressure ahead ofcreasing during periodic weighting; l'is theworking face, coal seam permeability andwith positicoal body in initial state during weighting; 1'is thedistribution curve of abutment pressure ahead of theface at theduringHowever, during the first weighting and periodicighting; 2 is the distribution curve of coal seamweighting process, if the stress increases and decreasespermeability coefficient of face at the impacting anduddenly, it may lead to the sudden uploading andloading moment during weighting; 3 is the distribudownloading function to the coal halltion curve ofahead of the face duringTheoretically, the maximum dynamic load factorweighing; a is中國(guó)煤化工 ked iis 2. However, according to the actual test and simulacreased by impaCN MH Geighting; bion. it can reach 3-4 in some parts, even higher. It is the range of coal body plastic zone increased byvdl.10No.2,Apr.201253mpacting and loading during weightingHence, gas emits less and presents a reverse-ratio relationship with abutment pressure. Namely, in the corresponding point of the maximum abutment pressure, thepermeability of the coal decreases to the lowest, evenlower than the initial permeability coefficient Ao. Whenfailure stressthe coal fractures and extends; even the rib spalling ofthe coal wall in working face will be caused, andrange of coal volume strain and the porosity increaseThe gas migration channels are smooth; the range ofPositioncoal which emits gas easily enlarges and the gas emis-Fig4 Curves of abutment pressure ahead of working face, sion amount from the working face inevitably increasescoal seam permeability and gas pressureThe dynamic loading of the coal ahead of the faceduring periodic weighting with positionduring periodic weighting makes a wider range of coalAs seen in Eq.(2)and Fig 4, the increasing of to reach and surpass the critical strength of seepagemutation and the intensity of the initiative permeablethe fully-cracked area(I ')and plastic zone( XoI’+Ⅱ’) in coal body certainly leads to the increas-么國(guó)o, and leads the working face up to and more thane causal relationing of area I',I and great extension of the fractures among abutment pressure of face anterior, coal seamthe area of expansion is AXod =6),and even rip permeability and gas pressure during weighting mustspalling, roof caving and so on in the face. Thus coalncrease the amount of seepage and gas emissionenlarged, and gas desorbed-speed is quickened. As a Referencesresult, it will certainly lead to gas desorbed vastly, [1]Liu Zegong, Yuan Liang, Dai Guanglong. Research of mining coalpermeability increasing and the gas flowing unblockedseam annular fissure circle in the roof toward long drilling methodThe amount of gas emission into face will inevitably inof gas drainage [J]. Engineering Sciences, 2004, 6(5): 32-38crease with the increasing of gas-coal body scope to the [2] Li Shugang, Liu Zhiyun. Research of monitoring undergroundworking face. So it is clear that the moment of the firstpressure and gas emission in fully mechanized top-coal caving faceweighting and periodic weighting must be the period of[J]. Underground Pressure and Strata Control, 2002, 19(1)in the coal wall ahead of the face bursting into[3 Li Huamin, Wang Wen, Xiong Zuqiang. 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Beijing: China University of Mining and Tech-on breaking and vibration type pressure bump of hard roofBeijing: China University of Mining and Technology, 2005AuthorLi Huamin, male, born in 1957, graduated from China University of Mining and Technology. Now he is aprofessor and his current research is mining teaching and research. He has published over 60 papers and obtained8patents.HecanbereachedbyE-mail:lihm@hpu.edu.cnoundation item: Natural Science Foundation of China( No. 50974054 ) Doctoral Program Foundation of theMinistry of Education( No. 20070460001) National Key Basic Research and Development Program(No2012CB723103)(cont. from p. 44)[3]Wang Renkun. The initial study on the entire safety margin andDam design and safety analysis[ J]. Technology of Concrete Damontrol standards of arch dam[J]. Design of Hydroelectric Power2007(1):23-291991,7(3):4246[8] Wang R K, Lin P, Zhou W Y. et al. Experimental study on[4] Wang Renkun. Analysis and assessment on optimized design ofcracking and integrity stability of Xiluodu Arch Dam[J]. Key En-foundation base depth of arch dam at extreme height [D].Beigineering Material, 2007, 353-358: 2565-2568jing: Tsinghua University, 2007.[9 Wang Renkun, Lin Peng. Analysis and assessment on optimized[5] Wang Renkun. An overview of Xiluodu Arch Dam design [J]esign of foundation base depth of Xiluodu extremely high archHydroelectric Power, 2003(11): 17-19[6] Wang Renkun, Lin Peng, Zhou Weiyuan. Cracking and stability2008,27(10):2010-2018 ock Mechanics and Engineeringstudy on high arch dam on complicated foundation[ J ] Chinese [10] Wang Renkun. Study on optimizing design for exeavated founda-Journal of Rock Mechanics and Engineering, 2007, 26(10): 1951tion surface of Xiluodu Arch Dam[ C]//76th Annual Meeting ofICOLD[7] Wang Renkun, Zhao Wenguan, Yang Jianhong. Xiluodu ArchAuthorWang Renkun, male, born in 1962, graduated from Tsinghua University. Now he is the chief engineer ofChengdu Hydroelectric Investigation and Design Institute, China Hydropower Engineering Consulting Group Corporation. His major working scopes include hydraulic structure, geotechnical design and research. He has publishedover30papersHecanbereachedbyE-mail:wrenkun@chidi.com.cn中國(guó)煤化工CNMHGVol 10 No. 2.A