Top coal flows in an excavation disturbed zone of high section top coal caving of an extremely steep

Mining Science and Technology( China)21(2011)99-105Contents lists available at science DirectMining Science and Technology( china)SEVIERjournalhomepagewww.elsevier.com/locate/mstcTop coal flows in an excavation disturbed zone of high section top coal cavingof an extremely steep and thick seamMiao Shengjun " Lai Xingping Cui Feng bCivil S Emvironmental Engineering School, University of Science and Technology. Beifing 100083, chinaSchool of Energy Engineering Xi an University of Science and Technology, Xian 710054, ChinaARTICLE IN FOABSTRACTCompared with gentle dip long-wall caving, the length of a working face in fully-mechanized top-ccReceived 13 May 2010caving for extremely steep and thick seams is short while its horizontal section is high with increasingAccepted 15 July 2010production. But the caving ratio is low, which might result in some disasters, such as roof falls, inducedby local and large area collapse of the top coal in a working face and dangers induced by gas accumu-tion. After the development of cracks and weakening of the coal body. the tall, broken section of the toptremely steep and thick coal seamclear characteristics of nonlineamovement. we have thoroughly analyzed the geological environment and mining conditions of anParticle owexcavation disturbed zone Based on the results from a physical experiment of large-scale 3D modelingand coupling simulation of top coal-water-gas, we conclude that the weakened top coal canregarded as a non-continuous medium, We used a particle flow code program to compare and analyzemigration processes and the movements of a 30 m high section top coal over time before and afterweakening of an extremely steep seam in the Weihuliang coal mine. The results of our simulation,oring show that pre-injection of water and pre-splitting blasting improve cavingmigration time of noxious gases and release them from the mined out area and so achieve safety inCopyright o 2011, China University of Mining Technology. All rights reserved.caving ratios, dynamic roof collapse and gas accumulation [2,3]Given the combined actions of overburden pressure, coal bodyThe Urumchi mine area is located in northwestern China and has weight, doubly-fed dynamic effects of framework andhgr: lease nosio间ahedisturbed zone(EDZ). Roof collapse, gas, fire, flood, coal dust and through coal bodies, clearly representing characteristics of granularother disasters are seriously impacting mining safety. Hence, new medium migration [4.5). Particle Flow Code( PFC)programs can bemethods in safety techniques for mines are being presented in high applied to analyze these migration characteristics and interactionssection top-coal caving(HSTCC)for these coal seams of the Urumchi of this granular coal medium of top-coal caving of these seams (6, 7).mining area, which should be systematically and thoroughly In order to improve mining efficiency, reduce derivative disastersinvestigated, including the determination of suitable caving ratios and achieve safety in mining, a discontinuous mechanical calcula-less than 1: 3)of sub-level working faces of extremely thick seams. tion program(PFC)is used to simulate, calculate, compare andThis should help to prevent the occurrence of gas, flood, fire, falling analyze movements and the fractured results of HSTcc in theroofs, coal dust and other disasters[1 The length of working faces of extremely steep and thick seams of the Weihuliang coal mine withHSTCC for extremely steep and thick seams is short, therefore a comprehensive analysis of the geological characteristics andincreasing section heights is an effective way to increase production, mining conditions of its working face. On-site mining and moni-but will introduce some new hazards and challenges, such as low toring shows that pre-splitting blasting and pre-injection of wateran improve top-coal caving and symmetrical caving, create relievespace for extensive dynamic collapse of top coal seams, prolongCorresponding author, TeL +86 10 62332939.E-mailaddressmiaoshengjun@163.com(M.Shengjun)中国煤化工 d1674-5264/5-see front matter Copyright o 2011 China University of Mining Technology. AllCNMHGdoi:101016mstc201012006M Shengjun et al/ Mining Science and Technology(China)21(2011)99-12. EDZ geological characteristics and condition of coal seam order to avoid cracks at the top coal within the frames. The roadwaywhich weakens the coal body was arranged at sub-level +610,2. 1. EDZ geological and mining environmentadvanced blasting was carried out to pre-split the top coal and preinjection of water and pre-splitting blasting were conducted to crack(1)Geological environment. The extremely steep seam B1 +2 of the the coal body, weaken roof or top coal strength, reduce the tempersurface ) but is affected by a strong north-south stress, which is of historical mining and frequent disturbances, many dynamic andcontrolled by a fault at the 320.5 m level. The fault strikes at potentially unstable disaster areas have been formed in this EDz. a248, at a dip angle of 48 with dip separation of 3.5 m, a roof few giant V-shaped sunken pits have emerged at the surface.displacement angle of 30. a floor displacement angle over 60and a trend displacement angle of about 70.(2 )Lithological and mechanical characteristics of seam roof and 3. Simulation analysis on top coal movements of HSTCCof this seam are siltstone, fine sandstone and mudstone. fol. 3. Large-scale 3D physical similar modeling experimentlowed by carbonaceous mudstone. Tensile strength of thecarbonaceous mudstone in both the false roof and false floor isThe length of the working face of fully-mechanized top-coalless than 30 MPa and most structural planes are thin bedding caving for extremely steep and thick seams is shorter than in gentleplanes. Lithological characteristics alternate frequently with dip long-wall caving Mined out areas can be seen as new activefaults and any disturbance is considered as a finite deformation.low permeability, which might cause collapse and the shears Due to its asymmetric effect, localized deformation easily inducesslide easily. This coal seam strikes at 55, dips to 325 at a dangle 64. and hardness is measured at 3hazards in mining. A large-scale 3D physical simulation loading(3)Spontaneous combustion. Coal is liable to combust spontane- frame(4. 42 m x 2.9 mx 195 m)was built at a scale of 1: 25, riverously. This ranges over time from 3 to 6 months with the lowest sand was selected as aggregate, we opted for plaster and talcrepeat occurrence of 45 days.powder as cement and the bulk density similar ratio was 1: 1.6. Micawas chosen as the laminated interface material of rock-rock and(5)Gas grade. The gas content of the coal seam is 2.9-4.37 m /t coal-rock. In addition, broken fissures and faults were precut towith local anomalies(6)Coal dust explosivity. The explosion index is 35%-48%a rockmass. A dynamic and static coupling loading system was usedto guarantee the similarity of boundary conditions. Total stations,borehole monitors, pressure transducers, AE monitors and otherinstruments were used to monitor shaft surface displacement,2.2. Mining method and technology of working facestresses and strains of the surrounding rock and other failurecharacteristics in all directions and in real time. The experimentWorking face +579B1+ 2E2E is located at level +579-+640, in had been equipped with an integrated monitoring system of multiacoustics optics and electricity indicators. The results and thestage 30 m(+610-+640). The coal seam is 39 m thick and the conduct of the experiment are described below [1working face 39 m wide and 4 m high. Twelve supports wereinstalled in the working face, two supports at the upper end and (1)Under the extruding pressure, the roof and floor form an archone support at the lower end. Retreating and multiple sequencestructure, in temporary balance, along the working face. As thecaving are carried out from floor to roof and the top coal is brokenworking face advances and the top coal caves, the disturbanceby loose blasting( Fig 1).may directly affect the arch structure and its stability, whichTo improve the capacity of high section top coal caving, weresults in strong pressure on the strata above the working facmaintain water pressure avoid the weakening effect from the injec.Under this pressure, the roof flexes, becomes deformed, de-tion of top coal water at the working face and forbid blasting to in(2)As the working face continues to advance, the roof cracks andop coal caves again to form an upper balanced arch.(3)The asymmetric arch formed at the top strata above theworking face makes a mechanical structure with the roof-topand coal-floor as the press -shear structure so that the lowFootside of the mine at the floor cannot be drawn out, whichleaves an amount of un-quarried triquetrous coal. with therelease of the next top seam and the effect of top coal miningan rvotand frequent disturbances, this un-quarried triquetrous coalRedway forloses its backstop from the slope to the floor. This disturbspre-blasting andthe top side arch structure and brings strong pressure sharplyer infusionto the working face. Preventing the floor from collapsing andSub-level +610keeping the gas at a prescriptive density at that time isnn rounding of top coal-water-gas中国煤化工modeling experiment byusingC Gmine as our study backpung tne top codl-water-gas of the EDZ 11lM, Shengjun er al./ Miming sance and lechnology (china) 21(2011)99-105dFig 2 Process and characteristics of initial mining and caving before and aftermining:(c) and (d) initial cavingsimilarity ratio was 1: 100, so the coal seam in ouel was 39 cm thick, the working face 4 cm high and thecavin30 cm. The equipment consisted of an AE monitor.a gas chromatograph, an oxygen detector, a red water indictor.kefore weakeningler beaconsquetrous coal flows: (a) (c). (el (s)and (o) before weakening:(b)(da pressure reducing and gas transmission instrument, an air pumpand a flow meter. From acoustic emission parameters(large events.totalistics and their measure.025507510012515017520022中国煤化工 of top coal-watr-gasTime/.hree-YHaThe experiment showsthat theCN MH Failure clearly vary withFig 3. Cohesion of top coal as a function of time.differehedta couplings, with remarked localizedM. Shengjun er al/ Mining Science and Technology(China) 21(2011)99-105ghdFig 4.(characteristics, showing three instability processes. (1)With infil-tration and migration of groundwater and gas, the strength andiffness of the coal-rock decreased considerably. the pressure in thecoal body increased with mining disturbances, so that new damagepropagation continually emerged in top coal. (2)With increases ine number of mining disturbances, internal structural variation ofcoal-rock and the dynamic effect of water, gas and other media, thepressure in coal-rock increased constantly. When this pressureeeded the maximum stress peak, the bearing capacity decreasednd the coal-rock proceeded quickly to failure. (3)In stress concen-ration zones. the three coal-rock, water and gas media were ina high-energy phase, while the coal-rock was in the phase of energyaccumulation. Cracks and collapse may occur instantaneouslyThese observations provide several useful quantitative indicatorparameters to improve the forecast precision of dynamic instabilityeleading to disaster prevention. Multiphase medium coupling. Fig. 5. Upper triquetrous coal flows: (a.(c) and(e)before weakening: (b).(d) and (ocompetition and movement of top coal-water-gas accelerate after weakeninseepage, transmission and cracking of the coal body, which arehelpful to release gas overflows, Continuous and moderate waterinjection is conducive to effective caving and safety in the mining of 3 3. Top coal flowing simulation on particle flowtop coal Water injection in the working face provides passage for gasemission, especially for its escape from the ground, provides positiveeffect for gas accumulation and movement in mined out areas and for pncreases the possibility of gas accumulation after raising the section effeH中国煤化工 ytical program developed-he analysis of mechanicalCN MHGnd interfaces(or points)height of the sub-levelamong pKea coal Doaies [ 11-15 A coal body canM Shengjun et aL/ Mining Science and Technology( China) 21(2011)99-105&9 10 11 12 13 14 15$2009 5200 7Fig. 6. Support load and mining production: (a)support load of frames No 1-No, 18: (b)mining production.evaluate particle aggregation in any disturbance. The force among structure has been drawn out (exceeding the blue line ). As shown inparticles can be shown as follows.Fig 4(c)and(d), diagonal cracks are almost linked up, the uppertriquetrous top coal moves down uninterrupted when caving theF= F"ni+Ft(1) lower triquetrous coal As shownin Fig 4(e)and (f), the overburdencoal body clearly changes, the right ends of both the upper andwhere Fi is the force among particles, Fn the normal force, f the lower coal interfaces lean forward and the angle subtended with thetangential force, n a normal vector unit of interfaces and ti the horizontal plane increases gradually. Cracks generate and link uptangential vector unit.along the diagonal of the overburden coal body: the crack dip angleA calculation model was established. based on characteristics of is close to 30. the triquetrous coal under the crack flows continu-he mine and the caving order of the coal seam at level +579-+610. ously to the working face. At this point, the cumulated cavingMining and caving were divided into three phases to analyze top accounts for about 61.07% of coal reserves In Fig 4(g), cracks havelinked up completely and the lower triquetrous coal has largelybeen drawn out. Simultaneously, the right shoulder of the upper3.3.1. Process and characteristics of initial mining and cavingtriquetrous coal collapses and moves toward the working face. InIn the initial stage of working theh). the lower triquetrous coal has been drawn out completelymoves down first. As shown in Fig. 2(a)migration velocityhe collapsed right shoulder of the upper triquetrous coal alin the center of the working face is quickweakens. Fig. 2(b)the working face.hows that the migration of the central coal body and its two sidesAs shown in Fig. 4(i)and the cracks after weakeningreflect the same trend after weakening.d fter the working face had advanced evenly, top coal outflow cracks in the coal body under the 30 dip angle have almost bounted for about 20. 14% of coal reserves. Fig. 2(c)shows that drawn out and the 30 dip angle cracks are just linked up in the unregular local collapse occurs in the coal body before it weakens weakening coal, which shows that weakened coal eases cavingreduces before and after weakening. when the particle tends to 3.3.3. Upper triquetrous coal fiowsflow in a consistent manner(see Fig 3).The right shoulder coal body(position C in Fig. 4h))is drawn outafter the upper triquetrous coal has weakened. Under pressure of the3.3.2. Lower triquetrous coal flowsoverburden rock and the coal seam, the right end of the interface ofFig 4(a)and(b)shows that diagonal cracks gradually expandhe upper triquetrous coal moves down continuously and the crackupward and the coal body waits for caving to form upper and lower lines are shortened, also on a continuous basis. That is to say, when thetriquetrous structures. Some top coal from the upper triquetrous top corner moves down, position C of the upper triquetrous coal also中国煤化工CNMHGFig 7. Surface subsidence area and noxious gas escapingM Shengjun et aL/ Mining Science and Technoiogy(China)21(2011)99-105moves down, which loosens the right shoulder of the coal body. In 5. ConclusionsFig. 5. tension is obvious after weakening and the coal is loose and caneasily been drawn out In position A, the amount of coal drawn out The geological environment of the Urumchi mining area isafter weakening, is greater than that before weakening(shown in special. The effect of roof collapse, gas, fire.Fig 5(a)(b)) In Fig. 5(d)the right shoulder of the upper triquetrous mining safety is serious and presents new challenges for HSTCC ofcoal has been drawn out completely and the top corner of the upper extremely steep and thick seams. Top coal movements in HSTCC oftriquetrous coal has also largely been drawn out, while the right an extra-steep-thick seam have been comprehensively analyzedshoulder just moves to the blue line before weakening(Fig. 5(c) through our on-site survey, theoretical calculations, experimentswhich shows that coal, drawn out, flows easily and quickly after and monitoring. Our results and conclusions follow.weakening. Because position C moves down and the overburden coalbody is constantly compressed during this process, the angle sub-(1) The mined out area formed by top- coal caving of an extremelytended between the horizontal plane and the interface, the coal bodyteep and thick seam is limited in space, which has essentiallywaiting for caving and the top coal both increase gradually and cavingbecome the EDZ In a sense, it formed a new active fault(theability increases. A comparative analysis shows that displacementmined out area) HStCC of overburden rock of the mined ouafter weakening is larger than before weakening, regardless ofrea creates distinct shear slip and, in effect, causes disttwhether dealing with the right overburden rocks or the working facebances within different strata, which decreases the strength ofroof under the coal body. As shown in Fig 5(e)and(n, part of the overburden rock and induces collapse instability and othercoal-gangue mixture has been drawn out, which leaves somederivative disasterresidual triquetrous coal. Especially in Fig. 5(e), compressive stress of (2)Calculations by the PFC show that, at first, the central top coalthe top coal is well distributed after weakening, which shows that themoves down at the initial mining stage, with the working faceroof is tightly in contact with the coal body and reduces the spaceadvancing, the coal body waiting for caving to form upper andbetween the overburden rock and the coal seam. this conditionlower triquetrous struand diagonal cracks expendingreduces the probability of dynamic ground collapse.gradually upwards and finally linking up. There is a constarOur calculations show that the top coal caving ratio is clearlydownward movement of coal. After the lower triquetrous coalmarked before and after weakening. In the initial stage of caving.has largely been drawn out, the right shoulder of the uppethe coal outflow accounts for 16.67% of coal reserves withouttriquetrous coal collapses and is drawn out. In addition, preweakening. After weakening, the coal is drawn out evenly, theinjection of water and pre-splitting blasting increases thecaving height remains consistent and the coal outflow accounts forinterior cracks of the coal body, reduces shear force and20.14% of coal reserves In the lower triquetrous coal caving, coalcohesion and leads to symmetrical caving and a small amountoutflow accounts for 61.07%. Because of strong coal cohesion beforeof residual upper triquetrous coal. the top coal and roof moveweakening, the upper triquetrous coal is difficult to be drawn out,synchronously, which reduces the risk of roof disasterthe amount of residual triquetrous coal is large and the caving ratio (3)In the process of mining, the dangers of a dynamic roof collapseis only 72.73%. But coal cohesion is reduced after weakening, theand gas accumulation have been effectively controlled afteramount of residual upper triquetrous coal is smaller and the cavingtaking suitable measures, such as symmetrical caving. Ouratio has increased to 90.48%.investigation should provide technical references for futuremining activities under similar conditions4. Analysis of on-site mining experimeOur on-site mining experiment shows that the internal structure Acknowledgmentsof the coal body clearly changes with adequate blasting and waterinjection softening as mining advances. the strata arch spanning theFinancial support for this work, provided by the National Naturalframework is gradually damaged and becomes unstable, which Science Foundation of China(No 11002021), the Doctoral Subjectobviously causes periodic pressure. Fig. 6(a) shows the average load Foundation of the Ministry of Education of China( No 20070008012)per month in frameworks No 1-No. 18. Fig. 6(b)shows the distance and the National High Technology Research and Developmentthe working face has advanced and production per month. On the Program(No 2008AA062104) is gratefully acknowledgedwhole, the support load clearly fluctuated in December 2008: itincreased specially in the No 1-No.6 and remained stable inNo.7-No 18 frames. Part of the region changed, which clearly caused Referencessurface subsidence area to increase continuously and to collapseFig. 7). The caving rock of the mined out area increased the top coal [1 Shenhua-xinjlang Energy Com Ltd Technique study on safe mining ofad above the working face.In addition, detection of acoustic speed, optical monitoring of (2! Wang NB e&& ander complex geophysics environment. Joumal of coalScience and Technology: 2008, p 22-8 In Chinboreholes and an acoustic emission system were used to analyzeheight was 10.5 m, with the greatest height 17 m In order to avoid b defoRmation efects: on an ver yang ad sessure relie gas drainage andsudden gas overrun caused by collapse of the roof in the minedotective coal seam. Mining Science and Te2009:19out area, symmetrical multi-caving along the trend line was used, 14) Shen y Hu YE Chen J. Zhang P. Numerical simulationracking top coal and induced noxious gas to be released from the 5] Wang jC, LiZG, Chen Y]. The experimemined out area(see Fig. 7). Tests of the components of released gase longwall top-coal caving Journal of China Coal Society 2004: 29(3): 260-3show that the old mined out area above the working face was notlinked up with the working face, indicating that the danger ofdynamic roof collapse and the danger of gas accumulation have 7vbeen effectively controlled after these effective measures(such asEYEs中国煤化工ymxanized top-coalCNMH GIymmetrical caving)are taken.nuu, tunes Journal of Rock Mechanicsand Engineerng 2007: 26(2): 4202-7 (In Chinese.M Shengjun et al/ Mining Science and Technology(china)21(2011)99-105parameters of drawing and coal-gangue [12] Potyondy DO. Cundall PA Simulating stress corrosion with a bonded-particletop coal cavingock. International Journal of Rock Mechanics and Mining Science[9] Zhang ZF, Lai XP. Secoal seam under complex conditons. Journal of China Coal Society 2008: 33of steep and thick [13] Jeoungseok Y Application of experimental design and optimization to8185如mfon expenAE-based locali- [14] Cai M, Kaiser PK Monoka H FLAC/PFC coupied numerical simulation of AE inarea. International Joumal of Minerals, Metallurgy and Materials 2009: 16and Mining Science 2007: 44(4): 550-64[15 Holt RM, K1111 Itasca Consulting Group Inc. Theory and background of PFC2D. Minneapolis:tional Joumal of Rock Mechanics and Mining Sence 2005: 42(7): 985-95中国煤化工CNMHG