Gradient principle of horizontal stress inducing rock burst in coal mine

J Cent. South Univ (2012)19: 2926-2932DoI:10.1007/s11771-012-13603ringerGradient principle of horizontal stress inducing rock burst in coal minehE Jiang(何江)2, DOU Lin-ming(窦林名)21. State Key Laboratory of Coal Resources and Safe Mining( China University of Mining Technology),Xuzhou 221 1 16. China:2. School of Mines, China University of Mining TechnologyXuzhou 221116 ChiC Central South University Press and Springer-Verlag Berlin Heidelberg 2012Abstract: Based on the stress distribution characteristics of rock burst multiple sites, the criterion of horizontal stress inducing layerdislocation rock burst was established. Accordingly, the influencing factors were analyzed. The analysis results indicate that the stresscondition, edge of elastic zone depth, supporting strength, and the friction angle and cohesion among coal stratum, roofand floor aresensitive factors. By introducing double-couple model, the layer dislocation rock burst was explained and the energy radiationcharacteristics were analyzed. The SOS micro-seismic monitoring system was applied to observe the rock burst hazards about amining face. The results show that P- and S-wave energy radiations produced by rock burst have directional characteristics. Theenergy radiation characteristics of the 22 rock bursts occurring on 79Z6 long-wall face are basically the same as theoretical results,that is, the ratio of S-wave energy of sensor 4 to 6 is about 1.5 and that of p-wave is smaller than 0.5. The consistency of theonitored characteristics of the energy radiation theoretically increases with the total energy increasingKey words: horizontal stress; rock burst; gradient principle; micro-seismic monitoring; directional characteristic; energy radiationaspect in rock burst research1 IntroductionTraditionally, the vertical stress was mainly studiedin inducing rock burst, while the horizontal stress wasRock burst is one of the main dynamic disasters in ignored. As analyzed, coal normally gets horizontaldeep mining, in recent years, the occurrence frequencyvelocity bursting from coal body in the process of rockand intensity of which increased rapidly and severely burst, so the horizontal stress makes a major role andrestricted mining safety and efficient productioncannot be neglectedAs the rock burst mechanism determines theSince the 1950s, the micro-seismic monitoringformulation and selection of scientific and reasonable method has presented a rapid development. As the earlycontrol techniques, the research work on it has been monitoring equipment was regional earthquake networkcontinuing without interruption. Initially, the research [13], the monitoring accuracy was low. In recent years,made a slow progress. In 1965, two representative papers many countries developed small-scale seismicissued by COoK [1-2] made a breakthrough in research monitoring network, which dramatically improved theand provided the need of experimental base and monitoring accuracy and precipitated a better monitoringtheoretical analysis of rock burst. His work of using rigiid result [14-18]testing machine to study the post-failure behavior of rockThis work concentrated on the role of horizontalset the foundation of the theory of energy release rate stress in inducing rock burst, established the gradient(ERR)and the stiffness theory [3]. Afterwards, COok principle of horizontal stress inducing rock burst andet al [4] further improved ERR theory in 1966 and applied the micro-seismic monitoring system to observePETUKHOV [5] classified rock burst hazards on the and analyze the characteristics of energybase. LiNKQV [3] thought the rock burst is a stabilityproblem caused bning and rheological 2 Establishment of principlebehavior. Recently, the mechanics and mathematicsgreatly promoted the progress of rock burst research 2.1 Criterion of horizontal stress gradient inducing[6-11]. The introduction of fractal theory to the study ofrock burst by XIE and PARiSEAU [12] indicated a newAs shown in Fig. I, the coal body around roadwayFoundation item: Project(2012LWB63)supported by the Fundamental Research Funds for中国煤化工Received date: 201/ ported by the Priority Acadamic Program Development of Jiangsu HigherCNMHGCISZBF201-6-833Corresponding author; DOU Lin-ming, Professor, PhD; Tel: +8613952261972; E-mail: Imdou@cumt.eduJ.Cent. South univ.(2012)19:2926-29322927to minimal value relative to mining depth, or(r, y) can beconsidered as symmetric about y=h/2. Then, thefollowing result can be deducedRoofF=Fr smin(F. max, Fr. max)Coal bodySupposing that(L+drFc, max=min(Fr. max, Fr. maxlining spaceComparing Eqs. (4)and (5), Fc, max can be expresseFloorceL+taFig. 1 Stress analysis of coal elementwhere Fc. max means the critical horizontal stress betweenor mining face is under the plane strain stress state. In the the layers of layer dislocation; Cc, pc and ay, c(r) are thedirection parallel to the coal wall, a unit length coal mass cohesion, friction angle and vertical stress distributioncan be taken for stress analysis. The coal element, that is function of x, respectively. Fc, max could be judgeddx in width will beaccording to the roof and floors features and verticalIf the vertical stress distribution functions of roof stress distribution. Combining Eq.(3)with Eq.(9),theand floor are o r(r), a, Ax); the horizontal stress condition of coal form coal wall to the depth of Ldistribution function of coal is o, (x, v); the coal seam occurring layer dislocation can be deducedthickness is h: the horizontal sheer stresses between coaland roof and that between the coal and the floor are F[a,(L)-os]>2CcL+2tanPc.o c(x)dx (10)and Fr, respectively, the density of coal is p; and the where as refers to the support strength on the coal wallhorizontal displacement of coal is u, the dynamicsurface. Letequilibrium equation can be written as(x)dx[o (L)-0(L+dr)hh+dF +dFr=phLwherewhere o,c(L) is the vertical equivalent uniform stresson coal seam interface of section L. Then, Eq (10)cana7=()ex, eddy(2)be simplified as2(L)-a3、2Cc+c()tancIntegrally calculating on both ends of Eq (1) from 0h(12)Lto L, the following result can be obtained:The left of Eq(12) exprexactlyh[a (L)-o]+F+F=phla(3)ad iequivalent uniform horizontalwhere a is the horizontal acceleration of coal element. coal wall to L, which can be expressed by ag,(),thatSuppose the friction angle between the coal and the roofand1(L)respectively, and cohesions are C, and C, respectively. If g.(L)(13)the coal element needs to achieve equilibrium, then Frand Fr should satisfy the following conditionFinally, the coal layer dislocation criterion can beexpressed asF sF.max=.[Cr+ov. (x).tan p ]dxEx()、2c+,c(L) tan pc(14)斤5Fm」c+:()mHEquation(14) is also the criterion of horizontalwherestress gradient inducing rock burst.ov.r(x)+pgh=o,.(r)2.2 StressVariables Fr and Fr can always automatically adjusttheir values to maintain the balance of coal bodyH中国煤化工aceCNMHGdy around miningspace will"pidScunt as mattic, plasuc, elastic and theIn consideration of coal seams thickness belonging original stress zones from outside to inside under the2928J. Cent. South Univ.(2012)19: 2926-293Therefore, regard o, c(L) as a variable and draw acurve chart on the basis of Eqs. ( 18)and(19),theIntersection(x)dislocation rock burst, as shown in Fig 3,(x)Lkc=cMiningFraction PlastickFig. 2 Stress distribution of coal bodyeffect of mining concentrated stress. The width and depthCriticalconcentrated stress, strata mechanics, and the interfacefeatures between coal and roof and that between coal and Fig 3 Critical horizontal stress gradient of rock burstfloor. So, the accurate values are difficult to be expressedby analysis methodsThe occurrence of layer dislocation rock burstThe horizontal stress in the elastic zone increasesdepends on the intercept and slope of the two curverapidly before the peak stress point, The layer dislocationThese four parameters are affected by the six physicaldepth L should beparameters of A, L, Us, h, c and Cc. The influences wereanalyzed separately as belowIOE Kkc, the layer dislocation rock burstgradient inducing rock burst. The relationship between would not occur as the vertical stress increaseshorizontal and vertical stress is [19]Noteworthy, as shown in Fig. 4(a) the following situationwill cause L to decrease suddenly to induce layer(16) dislocation rock burst(1 Supporting structure losing effectivenesswhere u is Poisson ratio of coal. Then2 Mining and advancing workG、(1、ac(D)3 Coal dynamically destroyed and stri(17) from coal body,The roadway expansionAccording to Eqs. (13)and(17), the average stress3)Coal wall supporting strengthgradient can be represented asIt belongs to a minimum value compared with(1-m)Lc(L)-vertical and horizontal stresses. But it can maintain thea(L)=4(18) broken coal to be at home position with stable and makeRecording right side of Eq(14)as gc, thenwhich can中国煤化工djacent rock mass.hcy,c( l) 2CcAs shownCNMHGd k closes to kcncreasing os can obviously enhance the layer dislocationJ.cent. South univ.(2012)19:2926-29322929k>kCatastrophepolCriticalCritical o,C(L)Criticalal arc(L)point 2point 2Fig. 4 Analytical process of layer dislocation rock burst:(a) Rock burst induced by L suddenly decreasing; (b) Critical pointincreasing rapidly with as increasing;(c)Critical point decreasing with pc decreasing;(d)Critical point decreasing with Cc decreasingcriticalparameters are sensitive ones4)Coal seam thickness hIt is the inherent attribute of coal seam and cannot 3 Micro-seismic in-situ monitoringbe changed. However, according to the layer dislocationcriterion, it is possible to determine the dangerous state 3.1 Focal mechanism of layer dislocation rock burstof rock burst. According to Eq(14), kc decreases with hLayer dislocation rock burst can be explained as theincreasing, namely, the possibility of rock burst increases double-couple model [20), the moment tensor of which iswith the thickness of coal seam increasingsymmetrical and can be written as5)Interfacial friction angle pc001As shown in Fig 3, kc increases with pc increasing(20)shown in Fig. 4(c), while the stress conditionincreases to a certain degree, the asperities on theinterface between layers will change to yield stage andThe model schematic diagram is shown in Fig.5begin to fractearing, aneinterfacial friction angle c will decrease. As shown inFig. 4(c), the curve of kc presents the tendency ofgradually decreasing, hence the critical point will reduceand the occurrence of layer dislocation rock burst will be6) Cohesive force Coto Eq.(19), the changing of Cc willdirectly cause the critical stress gradient curve gc offsetAs shown in Fig. 4(d), when Cc increases the curveoffsets upper and the critical stress gradient increases.When the stress condition rises to a certain degree, theFig 5 Double-couple model of layer dislocation rock burstfracturing of asperities will lead Cc to decrease and causekc to descend, as well as the critical stress gradient curvThe far field energy radiaticbends downwards. So, the critical stress gradient will R=sin20 cosodecrease to induce the layer dislocation rock burstIn brief, rock burst has the ingredient of layerdislocation impact more or less. Besides Poisson ratio A什H中国煤化工(21)CNMHGof coal body and coal seam thickness h, the other fourThe total amplitude of s-wave is2930J.cent. South unit.(2012)19:2926-2932IRFVcos220 cos2+cos26 sin2p(22) across the high stress area formed by pillar of upper coalseam, 22 in which occurred on face and one in roadwaywhere 0 is the angle between dislocation plane and As shown in Fig 8, six sensors of the system have goodradiation aspect projected in the vertical planar layer that monitoring condition of 79Z6 face Sensors 4 and 6 areis parallel to dislocation aspect. p is the angle between near to the region of rock bursts concentration, whileradiation aspect and the vertical planar layer is parallel to sensors 8, I1, 14 and 15 are far apart, and separated fromdislocation aspect. P- and S-wave energy radiationthe region by F6 fault. In order to avoid the influence ofpatterns are shown in Fig. 6.propagation, sensors 4 and 6 are selected for analysisAs shown in Fig. 6, the energy radiation of layerSet the seismic source as the origin of coordinatedislocation rock burst has directionality. 6 of the system, the normal unit vector dislocation direction is i,predominant direction of P-wave energy transmittingthe normal unit vector is parallel to the plane of coalseam pointing to the upper end of mining face isj, and#45, and the energy decreases with the direction the other one is k, so as to establish the vector space.deviating away, while 0 of the predominant direction ofS- wave is o°or90°Supposing the vector pointing to sensor P is p, and thenthe p projection vector on i-k plane can be written as3.2 In-situ monitoring analysisP,k=(Pl+(P·kk(23)According to the focal mechanism of layerThendislocation rock burst, the theoretical processcan beconfirmed by the micro-seismic monitoring of thecOs=Pi,k'i(24)directionalities of P- and S-wave energy transmittingThe applied micro-seismic monitoring system SOSPi kcOS p=shown in Fig. 7.P,k‖PIn 79Z6 long-wall face mining process in TaoshanAs the seismometers of the seismic monitoringCoal Mine, 23 rock bursts occurred when advancing system are vertical single component ones, the transferredDislocation planeDislocation plane学t(b)Fig 6 Energy radiation patterns of layer dislocation rock burst: (a) P-wave energy;(b)S-wave energyRecorderCollector中国煤化工CNMHFig. 7 Photos of SoS micro-seismic monitoring systemJ.cent. South Univ.(2012)19:292629322931source location [21], and the waveform integrationmethod is applied to calculate the energy of source by100mconsidering the attenuation coefficient. For p-andS-waves separate inconspicuously, the duration time ofRock burstsP-and S-waves was determined approximately as thearrival time deference between p. and S-wavesAccording to Eqs. (21)to(27), the79Z6pattern values of sensors 4 and 6 can be obtainedfasAccording to Fig 10, the conclusiobe drawnF6 fault1) Energy radiation of P- and S-waves hasdirectional characteristic. The S-wave energy radiationrecorded by sensor 4 is stronger than that of sensor 6while the P-wave energy radiation recorded by sensor 6is stronger than that of sensor 4Fig. 8 Distribution of seismic sensors around rock bursts2)The stronger rock burst energy is, the moreconsistent radiation characteristics with focal mechanismenergy observed bythe system on vertical component The process of rock burst is a compound seismic sourcecan be calculated by Eqs. (26)and (27)of coal implosion at the beginning and subsequent layerR?-[(P z)/PRdislocation. As layer dislocation is the main process ofenergy release, if the energy released less in dislocation(26) the rock burst magnitude should be smaller and theR1=(1v2)RHenergy portion of implosion should be larger. ForR=√Ry2+(R)2the implosion energy release is mainly as P-wave thewhere v is the vertical unit vector I is the unit vector thatP4/P6(calculated value) 300is perpendicular to p and on the i-j plane3.0S4/S6(calculated value)P4/P6(monitored value) 7250The rock bursts waveforms are similar and one of4/S6(monitored value)which is shown in Fig. 9. The maximum P-waveTotal energy1200amplitude of sensor 4 is smaller than that of sensor 6while the maximum S-wave amplitude of sensor 4 islarger than that of sensor 61.500The P-wave arrival time is applied to solve the6(a)0.550Number of rock burst14b)-s4P430·一S6/P6Total energy2.0Time/s100g20Number of rock burst中国煤化工 S-wave energies3.5(a)Comed ratios of p. andCNMHGTime/CHos of p-to s-waveFig 9 Typical rock burst waveform:(a) Sensor 4;(b)Sensor 6measured energies of sensors 4 and 62932J.cent. South univ.(2012)19:2926-2932transferred energy is almost the same in every aspect. As1972:1-531.( in russian)shown in Fig. 10(a), when the energy released by the [6) HU Jian-hua, SU Jia-hong, ZHOU Ke-ping, ZHANG Shi-chao,GUrock burst is small, the ratio of P-wave energy of sensorsDe-sheng. Application and establishment of time-varying mechanical4-6 is nearly 1, larger than that of larger energy releasingmodel to induction caving roof [. Journal of Central SouthUniversity: Science and Technlogy, 2007, 38(6): 1212-1218(inones. The ratios of S-wave to P-wave energy of eachsensor recorded are shown in Fig. 10(b), which further [7] WANG Bin, LI Xi-bing, MA Hai-peng. 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