Mechanism of energy limit equilibrium of rock burst in coal mine

Mining Science and Technology(china)21(2011)197-200Contents lists available at Science DirectMining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcMechanism of energy limit equilibrium of rock burst in coal mineWang jiong .b, Yan Yubiao.b, Jiang Zhengjun a Qi Ping. b, Chen ChenabSchool of Mechanics 6 Civil Engineering China University of Mining G Technolog, Beying 100083 ChinaState Key Laboratory of Deep Rock Mechanic and Underground Engineering China Univers of Mining 6 Technolog Beving 100083, ChinaARTICLE INFOABSTRACTWith the increase of mining depth, the effect of rock burst on coal mining is becoming more and moreobvious and the rock burst mechanism becomes more and more complicated. Scholars from man22 Septembercountries had put forward different mechanisms, but no one gave a reasonable explanation to thmechanism of roc burst. In this paper, based on the energy theories, we studied the energy limitonium(Ele)of coal mine rock burst. The coal seam with rock burst is divided into energy limitjuilibrium zone(ELEz) nd foor of coal seams: in addition, we derived the limit width of ELEZ and theand elastic zone( B): we also determined the position where the rock burstoccurs, including the roolmathematic relationship between the limit width and occurrence mechanism of rock burst: the energ二a monotonically decreasing function. The graph of the energy difference function was also determined.equilibrium(ELE)hrough which we analysed the occurrence mechanism of rock burst.Copyright e 2011, China University of Mining Technology. All rights reserved.the relative movement between roof and floor of roadway(Fig. 1). InFig. 1, for the left part of area a, the rock is micro-fractured and itsRock burst is a typical dynamic disaster in mining activity. The intensity has not decreased. However, it is still compacted firmly bysudden occurrence and the severe destruction are a great threat to the roof and floor of roadway. also in ig. 1, n the deep of ELEz, iemine safety. Particularly, with the increasing depth of coal mining. area B, the coal seam is still in elastic state. The ELEZ and elasticrock burst is becoming more and more obvious[1-3). Rock burst is zone are not fixed, the stress concentration of the compacted seama dynamic phenomenon of mine pressure whose occurrence is increased due to the relative movement between the roof andechanism is very complex. Foreign scholars had put up a series of noor of the roadway. The width of zone B is moving towards themportant theories from different aspects on the basis of field deep rock, until the limit energy of rock burst and theinvestigation and laboratory studies, but these theories did not energy of ELEZ reaches balance, then the width of ELEZsolve the problems of occurrence mechanism of rock burst.change. If there is some other energy acted on the area.burst will occur2 Coal seam partitionThe width of elez has a direct relation with the relativemovement between the roof and floor of coal seam, so the energy ofBefore excavation and mining, underground rock is in a 3D stress rock burst is mainly from ELEZtate under which the coal and rock will accumulate a lot of elasticenergy. after the roadway is built and coal is explored from the 3. Coal seam ultimate stressworking face, the tunnel and coal seam between roof and floor foma system. Because the ultimate strength of coal seam is lower than If the coal seam does not slip, that is, the strength of the coalseam (4-6) During the tunnel excavation and coal mining, the the coal seam is in the compacted state [7L. Suppose that the roofseam stress will be redistributed, forming the ElEz and the elastic and floor of the coal seam are rigid and parallel to each other, whereone in the seam between the roof and floor( Fig. 1). In area A, forgrip resistance is the same and the friction force and adhesive forcethe right part, the coal seam has broken, but it is compacted during are constant, ignoring the weight of coal, then the limit stressequilibrium state of coal seam is a plane strain state(Fig. 2). Fora coal seam unit with the width dx. in order to make the coal seamponding author. TeL: +86 13810192中国煤化工 rst condition is theE-mail addressbalande stress ox. we have1674-52645-see front matter Copyright o 20IL, China University of Mining Technology AllCNMHGdoi:0o16mstc201102007w Jiong et al Mining Science and Technology(China)21 (2011)197-200RoadArea AFg 1 Schematic of rock burst partition.Mdox-2gdx≤0Fl 2 Limit stress state in coal seam.where tg= or 4go+G,o and c are the friction angle and cohesiveforce at the seam and surrounding rock junction. At the same time, where x is the width of Elez when rock burst is in the state of ELE; hto satisfy the condition that the stress is less than coal seam the distance of face roof under control in excavation space: and eostrength oc that isthe swell increment of the rock in elezOz Ox+OcSo the potential elastic energy in the rock around the coal pillarof ELEZ is,So the limit stress under the coal seam limit stress balanceWn=0.20z(S51+S2)(x+h)a2=(0+cgy(2)-1]+nd the total potential energy reserves in rock burst is,=(+cg(2(3)W=221+0202(1+)/x+h)a04. Coal seam ultimate energywhere M is the thickness depth.When the value of o is unknown, it can be determined by thest energy is the sum of elastic deformfollowing formulastored in the coal seam and in the surrounding rock.potential energy of rock burst is [8-12Substitute Eq ( 9)into Eq (8), we caw= Wy+ wn(x+h)(s1+S2)where Wy is the potential energy reserved in seam; Wn character-51+izes potential energy reserved in surrounding rock.The value of wbe roughly calculated by the followingformula.where E, the elastic modulus of rock.In essence, the occurrence of rock burst must satisfy three0v/2E2conditions. The first one is the strength condition: the stressapplied to the seam exceeds the coal strength, that is, the seamwhere ox is the limit stress acting on the coal seam; E2 the elastic shall be destroyed when the stress applied to the seam reaches themodulus of the coal seam; and Vy the volume of the coal seam in limit stress of coal seam; the second one is the energy conditionELEZ when the rock burst energy reaches ELE.the rock burst system consisting of "roof-seam-floor", then theThe reserves of rock elastic compression energy during rock system shall gather energy and can suddenly release on thelative displacement between roof and foor of the rock is 6o. so the the outsid工mwork done during the process is,tendency, with a brittle failure capacity. the first one is acondition and the latter two are sufficient conditions that isA=20(51+2)(6) if the stress added on coal seam does not exceed the seam strength,the"roof-coal-floor system does not reach limit energy state, evenwhere s, is the basal area of coal pillar in ELEZ when rock burst is in if there is a disturbing energy, the rock mass shall not be destroyed:the state of ELE: S,,, the area of face roof under control in excavation in other words, there will be no rock burst. In other conditions, sueas the " roof-coal-floor"rock burst system, although the searWe get coefficientbecause the force changes from gathers energy, if the rate of energy dissipation exceeds gatheringoz(s1+S2)to zero when roof and floor of rock moves close to lo.speed, rock burst shall not happen; the same condition is that if theThe displacement may be calculated as followseam is not destroyed suddenly, there shall be no rock burst. Baseon the above, substituting(2)and (3)into(10) so the limit energylo=0.4(x+h)Eo(7)(w)of rock burst is as follows+cgx()-1M02{0+cg)xmH中国煤化工CNMHG(12)w jiong et aL/ Mining Saence and Technology(China)21(2011)197-200(16)H(O)=020h2wss+cc唧4gqeM)-expexp( M) (oc+cctg)(exp( M3. Graph of EDF.+2|(σc+ccgq9)(exation of limit width in area aFrom the dimensional analysis, the first derivative of EDFThe energy in ELEZ during rock burst is composed of the energy w(x)<0, so the EDF is monotonic and decrease function.to form a large number of fractures and the friction and movement Let x =0, substitute it into( 15)energy between coal and rock fragments [13-17 the followingexistsw(O)=0.20ch2Through the analysis above, the graph of EDF is shown in Fig 3.Rgo is the energy absorbed from per unit cross-sectionalFrom the graph of EDF, we get some conclusions:during the formation of ELEZ, the value of 2go is(0.3-1)x 1)For the limit width in coal seam before the formation of ELEZA big value for 2go can be explained by its physical meaningthe smaller the width of ELEZ is, the greater the EDF is and it isCracks in coal seam increase duringmation of Elez eacheasier for the occurrence of rock burst.fracture will absorb lots of energy and coal fragments consume lots 2)When reaching the limit width in Elez the disturbing energyof energy during slow moving. Additional energy is produced byads to rock burst.iction among the fragments. Energy consumed on the cracks3)When the width of ELEZ is wider than the limit width, thencreases with the decrease of rock modulusenergy formed during the elez is greater than the limit energyWhen limit energy(wj)of rock burstof coal seam, rock burst would not happen.forms ELEZ in the coal seam reach balance(w=wof ELEZ will not change any longer. Taking unit leock burst seam, from(11) (12)and( 12a), we get7. CondusionsFor the"roof-coal-floorrock burst system, ELEZ and elast(oc +cctgp)expl1 Mone is formed in the seam. the top and bottom of the seam is mainenergy source of rock burst.When the rock burst occurs, before the width of EleZ is less than02{0+c(x2-1+}2k+2limit width, the smaller the width of the ELEZ is, the bigger the valueof EDF will be and it is more easily for the occurrence of rock burst.When the limit energy of rock burst and the damaged energy of(13) ELEZ reach balance, under the effect of the disturbing energy. therock burst will occur.We can get the limit width(b)of ELEZ of rock burst, which is,When the width of ELEZ is bigger than its limit width, themergy needed to form the ELeZ is greater than the limit energy of880E2+4g中coal seam, rock burst will not happen.(14)Financial support for this project, proviResearch Program of China6. Relation between Himit energy and the width of ELEZ in Major Project of Mirf Education(No. 304005)arod burstUniversity of China(No. IRTO656), is gratefully acknowledgedProgram for Changjiang Scholars and Innovative Research teanThere is a certain relationship between the limit energy of rockburst and energy during the formation of EleZ before the two reach Referencesalance for"roof-coal-floor"system.Taking the length of coal pillar as unit length, making EDF be as11] He MC Sun XM, Su YH. Jing HH, Wu x The model fitting method inring Ground Pressurex)=wj-Wp and subtracting (12)and(12a) from(11), we get[2] Dou中国煤化工 d forecasting of rock burst-d Technology 2009: 19(5):(15)luCNMHGn fault induced rock bursts.urnal of China University of Mining Technology 2008: 18(3): 321-6Jiong et al/ Mining Science14] Yan YB, L FY, Wang ww. Discussion on G coal seam partition in coal mine. [12] Lippmann H. Mechanics of "bumps" in coal mines: a discussion of violentg Zo et al. th133-43.13will. 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