Determining areas in an inclined coal seam floor prone to water-inrush by micro-seismic monitoring

Mining Science and Technology(China)21(2011)165-168Contents lists available at Science DirectMining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcDetermining areas in an inclined coal seam floor prone to water-inrushby micro-seismic monitoringSun Jian, Wang Lianguo, Wang Zhansheng Hou Huaqiang, Shen Yifeng bState Key Laboratory of Geomechanics 6 Deep Underground Engineering China University of Mining 6 Technology Xuzhou 221008, ChinaSchool of Saences China University of Mining 6 Technology. Xuzhou 221116, ChinaARTICLE I NF OA BSTRACTThe failure depth of the coal seam floor is one important consideration that must be kept in mind whenReceived 15 June 2010ining is carried out above a confined aquifer. Determining the floor failure depth is the essentialprecondition for predicting the water resisting ability of the floor. We have used a high-precision microAccepted 5 September 2010seismic monitoring technique to overcome the limited amount of data available from field measure-ments. The failure depth of a coal seam floor, especially an inclined coal seam floor, may be moreaccurately estimated by monitoring the continuous, dynamic failure of the floor. The monitoring resultsindicate the failure depth of the coal seam floor near the workface conveyance roadway(the lowerned coal seamcrossheading)is deeper and that the failure range is wider here compared to the coal seam floor near thereturn airway (the upper crossheading). The results of micro-seismic monitoring show that theingerous areadangerous area for water- inrush from the coal seam floor may be identified. This provides an importanteld measurement that helps ensure safe and highly effcient mining of the inclined coal seam above theconfined aquifer at the Taoyuan Coal Mine.Copyright o 2011, China University of Mining Technology. All rights reserved.A high-precision micro-seismic monitoring technique may beused to observe the continuous, dynamic failure characteristics ofThe failure depth of a coal seam floor is one of the important the coal seam floor, to study the dynamic changes in floor failure.COnsiderations that must be kept in mind when mining is carried and to accurately determine the failure depth and failure mode ofout above a confined aquifer. Determining floor failure depth is the the floor strata[8-11]ssential precondition for predicting the water-resisting ability of We report here the monitoring of continuous, dynamic failurefloor [1-3. The determination of floor failure depth presently characteristics of an inclined coal seam floor located abovedepends on empirical formulae and practical field measurements a confined aquifer. The monitoring results indicate that the failure(4-7). Different geological conditions and mining methods may depth of the coal seam floor near to the workface conveyanceintroduce big errors between the actual floor failure depth and the roadway (the lower crossheading)is deeper, and the failure range isfailure depth calculated from an empirical formula. This is espe- wider, than for the floor near the retum airway( the upper cross-cially true for determining the failure depth of an inclined coal heading) In its entire aspect the failure mode of the inclined coalseam floor. Practical field measurement of failure depths may be seam oor has an asymmetric appearance. the results of micro-done by the drilling injecting(de-watering) test or by observing seismic monitoring can locate the dangerous areas for water-inrushchanges in displacement(stress)pre-and post-mining after setting from the coal seam floor This provides an important basis for thedisplacement sensors in drill holes. However floor failure depths safe and highly efficient mining of the inclined coal seam above thedetermined by this static field measurement have the limitation of confined aquifer at the Taoyuan Coal Minelocalized due to the limited number of bore holes that can bedrilled and the complexity of site construction conditions. The 2. Micro-seismic monitoring principledynamic failure depth and the continuous failure of the wholeworkface floor are not well predicted by these measurementsMicro-seismic phenomena occur during rock mass failure. Byprocessing and analyzing the micro-seismic signals from a moni-toring system the unstahle state and failure position, of the rockCorresponding author. TeL: +86 13852470178mass中国煤化工 layed in three-dimen-E-mailaddresssunjian007700@vip.sina.com(s.sionalprofile of micro-seismicCNMHG1674-5264/s-see front matter Copyright o 2011, China University of Mining Technology. All5.Jian et aL/ Mining Science and Technology(China)21(2011)165-168b(x3, V, Z1, h)▲1. Micro-seismic signal wave profile. typical stratum fracture(a)and schematicdiagram illustrating fracture location(b).ignal typical of a fracturing stratum. The wave profile showssmall peak with a short time delay compared to the profile froma coal seam fracture. this distinction is helpful for the analysis andlocating the source. Locating the micro-seismic incident requiresthe arrangement of several micro-seismic detectors in the moni-tored area, as shown in Fig. 1b. when the rock mass fractures, orother abnormal condition happens, in thearea thedetectors collect the micro-seismic wave datad to themFig. 2. Underground, explosion-proof micro-seismic monitoring system.distancesbetween the energy source and each detector cause time differ- schematic diagram of the buried detectors. The parameters ofences for wave arrival at each one. These time differences can be different detectors for each floor drilling are shown in Table 1.Theseused to establish the location from14 detectors can monitor the floor failure characteristics and(x1-x)2+0y1-y)2+(x1-x)2=v2(t-t2(=1,2,3n)determine dangerous areas for water-inrush, which provides animportant field observation allowing safe and highly efficientwhere(x, y z)are the source spacial coordinates, the unknown and mining of the workface above the confined aquiferxi yi, zi)are the coordinates of the i-th detector, which is a knownnumber. The starting time of the micro-seismic vibrations is the 3. 2. Results of micro-seismic monitoringvalue t, which is also an unknown number. the t are the arrivalThe field micro-seismic monitoring started on 10 October, 2009.velocity of the wave, v, may be treated as either an unknown or and ended on 10 December, 2009. During the monitoring stage 189a known value. After processing the data and solving these micro-seismic incidents of floor failure were obtained. These incithe micro-seismic incident to be precisely located [13-15). characteristics of the inclined 1066 coal seam workface duringStudying the failure depth, and failure mode, of the floor provides mining and that allow the floor failure depth and dangerous areasa new way to determine the dangerous areas prone to water- inrush prone to water-inrush to be identified. Incidents occurring in thethrough the coal seam floor.coal seam above the floor were neglected when projecting themicro-seismic incidents into the plane figure. This allows floor3. Determining dangerous areas for water-inrushfailure to be illustrated more clearly. The three-dimensional spacecoordinates of each incident detected give a way to determine the3. 1. Geologic situation and monitoring schemefloor failure depth and dangerous areas in the coal seam floorig. 4 shows the plane-projected schematic diagram of theThe 1066 workface of the Taoyuan Coal Mine(Huaibei Mining incidents detected along the advance direction of the workfaceIndustry Group) is located in the north sixth mining area. To the left Each red dot indicates an incident(neglecting those in the coalof the 1066 workface is the rail roadway and to the right is the coalpillar of the F2 fault. The upper workface is the goaf of the 1064workface. The strike length of the 1066 workface is 790 m, the▲ Detectorinclination length is 112 m, and the average buried depth of the coalseam is 500 m. the average thickness of this coal seam is 3. 4 m witha28° coal seam pitchsandstone. A Taiyuan formation limestone aquifer, 3 MPa water29△”4。2△pressure, is about 50 m distant from the coal seam floor and water-inrush from the coal seam floor may occur during the workface13#△1OH△4mining. Therefore, the continuous and dynamic failure depth, andfailure mode, of the floor must be monitored to determine thedangerous areas for water-inrush.The Comise underground, explosion-proof micro-seismic中国煤化工onitoring system shown in Fig. 2 was used to monitor the floorfailure characteristics. By drilling the roadway, 14 detectors were Fik.3.CNMHGthe location of the monitoringburied in the 1066 workface floor. fig 3 shows a plane-projected detectorsS Jian et al/ Mining Science and Technology( China)21(2011)Table 1Parameters of the detectors for each noor drilling.200DetectorDistance from Anglworkface(m)Stress concentration area and234567450 important area of preventing5678901234500550000550seam above the floor). Fig. 4 indicates that the incidents areconcentrated in the floor strata near the workface conveyanceroadway( the lower crossheading)and the workface return airway(the upper crossheading). The areas near the conveyance roadway5, Vertical stress contour of the 1066 workface after mining.return airway forms stress concentration zones after mining as asymmetric"saddle-shaped"aspect shown as the blue curve inhown by the simulation plotted in Fig. 5.Fig. 6. The maximum failure depth of the floor near the conveyanceThe floor strata in these two stress concentration zones must roadway is about 16 m while the maximum failure depth near thesuffer from the most serious failure Pressure causes the geological return airway is about 12 m. The floor strata in these two regionstructure in these zones to become active again, which may allow within the green ellipses of Fig. 6, suffer the most serious failurhe structure evolve into a conductor of groundwater. Therefore, compared to the floor in the central workface. Therefore, these twothese two stress concentration zones must be regarded as thregions must be regarded as the important zones for predictingimportant zones for predicting water-inrush from the floor. This is water-inrush from the floor. This is especially true for the floorespecially true when there are faults in these zones. The caving rockgion near the conveyance roadway (the lower crossheading).of the 1064 goaf and floor failure there is disturbed again by themining of the 1066 workface. so there have been some micro- the roof of the workface. A tension stress also appears in a shallowseismic incidents in the 1064 goaf. Otherwise, on the other side of part of the floor. Stress concentration zones appear near the workfacethe 1066 workface few incidents have occurred in the unexploited conveyance roadway and the return airway as shown in Fig 5.coal seam. Moreover the ones seen are concentrated on the edge of Therefore. the micro- seismic incidents are concentrated in the floorthe coal-pillar of the 1066 workface conveyance roadway(the strata near the roadway and the return airway as shewn in the greenlower crossheading). This indicates that the foor strata under the ellipses of Fig. 6.These areas must suffer fromthe most serious failureunexploited coal seam are in a relatively stable state.and these two regions must be regarded as the important zones foFig 6 shows a profile-projected schematic diagram of micro- predicting water-inrush from the floor. The distribution of the micro-seismic incidents along the inclined direction of the workface thseismic incidents observed in the inclined coal seam floor resemblered dots indicate the locations of the incidents(again neglecting the shape of the vertical stress isoclines in the floor obtained fromthose above the coal seam floor). Fig 4 suggests that the distribution numerical simulations (see Fig. 5). This indirectly proves the reliof the incidents within the floor strata has an asymmetric charac- ability and validity of the micro-seismic monitoring results.teristic relative to the failure of level or nearly level coal seam floorsThe failure depth of the coal seam floor nearby the conveyanceroadway(the lower crossheading )is deeper and the failure range is120160200wider compared to the coal seam floor near the return airway( theupper crossheading ). This is illustrated as the pair of green ellipses inStress failure area and important areag. 6. The failure mode of the inclined coal seam floor presents arof preventing water-inrush from floor∴∴∷300250200150中国煤化工CNMHGo-seismic incidents along theon of the workfaces Jian er al/ Mining Science and Technology(China)21(2011)165-1684. ConclusionsThe use of a high-precision micro-seismic monitoring technique IiI Peng SP Wang JA Safety mining above aquifer zone. Beijing: China Coalto monitor the continuous, dynamic failure of an inclined coal seam2Liu HQ, Coal mining above confined water. Beijing: China Coalfloor located above a confined aquifer in the taoyuan coal mine wadescribed. The data related to fracture incidents in the stratum water-inrush from coal seam flodysis of the harmfulness ofwere used to determine dangerous areas where water-inrush fromthe coal seam floor is likely. These valuable results have been [4] Zhang JC, Zhang YZ. Liu TQ Rock seepage and water inrush in coal bed.Bei-1)The failure depth of the coal seam floor near the workfaceconveyance roadway(the lower crossheading)is deeper andZhang c yu to. on depth of issured zone in seam floor resulted from coalthe failure range is wider compared to the floor near the returnairway(the upper crossheading The failure mode of the [71 Zhang wo zhang HR, Xu FL Li F. Continuous exploration for the miningnclined floor has an asymmetric aspect.2)The maximum failure depth near the conveyance roadway isabout 16 m and the maximum failure depth near the returnwall face. Chinese Joumalairway is about 12 m. The floor strata in these two regionssuffer the most serious failure, especially near the conveyance[9] Jiang FX. Luo x Application of microseismic monitoring technology of strataoal mine. Chinese Journal of Geotechnical Engi-3) The floor regions near the workface conveyance roadway and 1o jiang ex Yang sH, luo x spatial fracturing progresses of surrounding rockreturn airway must be regarded as the dangerous areas where03: 28(4):357-60 in Chirwater-inrush from the coal seam floor might occur. This=反以CmDm人provides an important basis for safe and highly efficiencymining of the inclined coal seam located above a confinedt hanics and Engineering 2008 27(9: 1932-8 inaquifer at the Taoyuan Coal Mine112] L Qx Jiang FX, Wang CW. Ye GX Location synthesis of micro-seismicmap and its application. Joumal of Mining(13 Pang hnjing Ft Zha 2 X2. Study on nonhemeThe work was supported by the National Basic Researchimage processing method. Rock and Soil Mechanics 2004; 25(Suppl. ) 60-4Program of China( No. 2010CB202210), the National Natural Science [141 L HY, Jiang FX, Yang SH, Research and application of microsFoundation of China(No. 50874103)and the Natural Scienceof strata fracturing based on Matlab. Journal ofFoundation of Jiangsu Province(No. KB2008135), as well as by the6: 31(2): 154-8 in Chinese)-5] Lu DP, Pan YS. Visualization of micro-seismic monitoring results of mine.Qinglan Project of Jiangsu ProvinceJourmal of Liaoning Technical University 2008: 27(Suppl. ) 107-9 in Chinese)中国煤化工CNMHG