Characteristics of stress distribution in floor strata and control of roadway stability under coal p

Mining oence and Technology( china)21 (2011)243-247Contents lists available at ScienceDirectMining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcCharacteristics of stress distribution in floor strata and controlof roadway stability under coal pillarsTongqiang Xiao, b, Bai Jianbiao b, Xu Leia, b, Zhang XuebinKey Laboratory of Coal Resources and Safe Mining China Universiry of Mining g Technology. Xuzhou 221008, Chinaof Mines, China Universiry of Mining G Technology. Xuzhou 221008, Chinandustry Jinan Design Institute Ltd inan 250031, ChinaARTICLE INFOABSTRACTGiven the difficulties encountered in roadway support under coal pillars, we studied the characteristicsof stress distribution and their effect on roadway stability, using theoretical analysis and numericalsimulation. The results show that, under a coal pillar, vertical stress in a floor stratum increases whileoctober 2010horizontal stress decreases, We conclude that the increased difference between vertical and horizontalstress is an important reason for deformation of the surrounding rock and failures of roadways underKeywordscoal pillars. Based on this, we propose control technologies of the surrounding rock of a roadway underClose-distance seamscoal pillar, such as high strength and high pre-stressed bolt support cable reinforcement support.single hydraulic prop with beam support and reinforcement by grouting of the surrounding rock, whichhave been successfully applied in a stability control project of a roadway under a coal pillarCopyright o 2011, China University of Mining Technology. All rights reserved.Surrounding rock control1. Introductionis the keyClose-distance coal seams are neighboring seams which haa considerable effect on each other in coal mining processes [11Many mining areas in China, such as Yanzhou, Datong, Pingding-shan and Huainan are faced with mining close-distance coal seams11] Pressure by coal pillar abutment of overlying coal seams is 2 characteristics of stress distribution of fioor strata undertransferred to floor strata during mining, causing floor strata stress coal pillarsto be redistributed, worsening the stress environment. This is animportant reason why the roadway under a coal pillar is so difficult 2. 1. Theoretical analysisto support and maintain.In China, stress transfer rules of coal pillar abutment pressure inAfter mining of coal seams, abutment pressure is formed in coalfloor strata have been studied, using theoretical analysis, numericallars and stresses in floor strata under coal pillars become redissimulation and other methods 1-7] The results show that there is tributed. when considering the supporting load of a coal pillar asan increased stress zone in floor strata under coal pillars, while a uniform load, the stress computation of floor strata can be shownecreased stress zones under goafs are far removed from coal as Fig. 1, where o is in the center of the coal pillar, x the distance topillars. If roadways are laid out in increased stress zones, they will point O, y the depth of the floor stratum, i the width of the coalbe very difficult to support and maintain. However, current pillar and p the degree of uniform load collection.esearch emphasizes particularly increased vertical stress, caused From Fig. 1. it is seen that vertical stress in a floor stratum.by coal pillar abutment pressure, to be analysed in the deformation caused by a uniform load can be obtained as:Corresponding author. State Key Laboratory of Coal Resources and Safe Mining.China University of Mining Technology, Xuzhou 221008, China. TE-mail address: xtq20009163, com (T Xiao)中国煤化工21674-5264s-see front matter Copyright o 201l, China University of Mining TechnologyCNMHGmstc20102016. Xiao et aL/ Mining Science and Technology(China)21(2011)243-2474th scamworking faciceAccording to Eq (1), when the coal pillar width is 5, 15 and 25 m.the stress distribution curves at 5, 10 and 15 m depths in a floorFig 3, Relative position between roadway and coal pillar.stratum are shown in Fig. 2As seen in Fig. 2, under the function of abutment pressure of thecoal pillar, the characteristics of vertical stress distribution of the 6.8 MPa. After the working face beside the coal pillar in the 3d coalfloor stratum are as follows: when the width of the coal pillar is seam was mined the stress distribution of the 4th coal seam issmall, the scope of the effect is small in both the direction of widthshown in Fig. 5.and depth. with increasing width of the coal pillar, its effect isAs seen from Fig. 5, the characteristics of the stress distributionincreasing. Given the same depth, vertical stress in the center line of of the 4th coal seam under the coal pillar are as followsa coal pillar is largest, while further away from the center, verticalstress decreases. The rate of decrease is largest near the edge of the 1)vertical stress increases greatly under the coal pillar, withcoal pillaia maximum of 8.9 MPa appearing under the middle of the coalTherefore, we conclude that when close-distance seamspillar, which has a stress concentration factor of 1.5. The farthermined, the mining roadway of the upper coal seam should be laidway from the coal pillar and entering the goaf, the verticalout with narrow coal pillars or entirely without any coal pillar instress decreases. Beyond 20 m from the center line of the coalorder to reduce the effect of this stress and its scope, caused by coalpillar abutment pressure. Given the situation of narrow coal pillars,pillar, vertical stress reduces to a value below 6 MPa (initialabout 3-5 m in China, after the working face beside the coal pillar 2)Horizontal stress is reduced to a value below 6.8 MPa(initialis mined the coal pillar becomes fractured or achieves a plasticstate, its abutment pressure is greatly reduced which is advanta-horizontal stress); its distribution is such that under the coapillar its value is larger. Again, farther away from the center linegouadway layout and maintenance under coal pillars.of the coal pillar, the horizontal stress gradually reduces, and itsrate of reduction becomes also gradually slower.2-2. Numerical simulation analysis3)Compared with the initial difference between vertical andhorizontal stress, this difference increases under the coal pillaiThe 4th coal seam in Jinjiazhuang Coal Mine in China is nearly from 0.8 MPa to 2.4 MPa in the center line of the coal pillar, i. e,horizontal, 2.8 m thick with a bearing depth of 250 m. Its imme-a three time increase. farther away from the center line of thediate roof is mudstone and sandy mudstone with a sandstone floor.coal pillar the difference between vertical and horizontal stressThe 3d coal seam is 15 m above the 4 th coal seam. The first workingdecreasesface of the 4th coal seam is under the goaf of the 3 d coal seam.Thedirection of their strikes is vertical to each other. a part of themining roadway of the 4th coal seam passes through the coal pillar 3. 2Effect of state of stress in fioor stratum on roadwayof the 3d coal seam, as shown in Fig 3. Given its production and stability under a coal pillargeological condition, a numerical model was established usingUDEC software, as shown in Fig 4.The changes in the state of stress of floor strata under coal pillaWhen the numerical model was balanced, the initial vertical are important factors affecting roadway stability. Increased differstress of the 4th coal seam was 6 MPa with a horizontal stress of ences between vertical and horizontal stress will intensifyabFloor stratum depth 5 mFloor stratum depth I中国煤化工Distance from coal pillar middle(m)CNMHGFig. 2. Vertical stress distribution in floor strata of different coal pillar width (a) Coal pillar width 5 m (b)Coal pillar width 15 m()Coal pillar width 25 m.. Xiao et aL/ Mining Science and Technology(ahina)21(2011)243-247tress circle under coal pillarInitial stress circleata c1-dLFig 6. Mohr-Coulomb strength condition and state of stress in a floor stratum.Flg 4. Numerical simulation modeL.1)High strength and high pre-stressed bolt supportdeformation and failure in the surrounding rock. In engineeringapplications, the Mohr-Coulomb strengthn can be writtenThe increased difference in stress causes the surrounding rock ofas[8, 9:a roadway under a coal pillar to become easily prey to shear failuresimilar to uniaxial compression. therefore, even in shallow road01=03tan28+ac(2) ways, high strength bolts should be used to avoid breaking of thewhere b is the shear failure angle, 6=(/4)+(9/2), ac the uniaxial bolts At the same time after a roadway has been excavated, thecompressive strength when confining pressure 03= 0. confining pressure in the surrounding rock of the roadwayac 2c(cos /1-sin ) c is rock cohesion and p the internal decreases immediately, so that initially a high pre-stressed forcefriction angleshould be used to improve the stress condition of the surroundingand residual strengths through the effect of bolt anchoring[10-15].Fig. 6 shows the Mohr-Coulomb strength condition and the In order to balance bolt stresses and prevent gangue caving, steelstate of stress of the floor stratum under the coal pillar. The strips and steel mesh should be used to combine bolts.increased difference between vertical and horizontal stress forcesthe stress circle of the floor stratum to approach its limiting stress 2)Cable reinforcement supportcircle. Moreover, after excavation of the roadway, the shallowsurrounding rock experiences a low confining pressure and highThe size of fractured and plastic zones of a roadway under a coalstress difference. This state of stress promotes deformation of the pillar is larger than the range of bolt anchoring. Therefore cablesrock mass, destroys its structural surface and causes a plastic area to should be used to reinforce the stability of the surrounding rock. Onform and develop toward the deeper rock mass. In short, the the one hand, cable support resistance is large, so that it canincreased difference is an important reason for deformation and enhance its strengthening effect on the shallow surrounding rockfailure of the roadway under a coal pillar.considerably. On the other hand, because the cable is long. it canreduce the amount of deformation and separation of the4. Stability control technology of a roadway under a coalsurrounding rock outside the range of bolt anchoring and act inpillar an applicationhanging and pulling the shallow surrounding rock when needed. Tobe sure. once the effect of a cable playing the role of hanging and4. 1. Stability principle and control technology of roadway underpulling is felt, roof caving becomes a distinct possibility, so wea coal pillarshould avoid its occurrenceBecause of high stresses acting on roadways under coal pillar3)Single hydraulic prop with beam supportfractured and plastic zones are large, where deformation andfailure of roadways become serious problems. In order to controlThe roof of a roadway under a coal pillar can easily be formed asactively the surrounding rock of a roadway the support system of a caving arch, hlld with a loose rock mass and a bolt arching zonea roadway under a coal pillar should be able to reduce roadwaydeformation and to adapt the deformation of the roadway Based onthis, we propose the following stability control technologies of289909009a roadway under a coal pillar.202000mm中国煤化工Distance from coal middle line(m)CNMHGYig.. Stress distribution of the 4t coal seam under a coal pillar.郎7. Bolt support layout.. Xiao et aL/ Mining Science and Technology(China)21(2011)243-is easy to cave. In order to avoid the loose rock mass from caving velocity high. During the period of 25th-40th day, referred to asand simultaneously avoid the effect of the cable playing the role of the alleviation period, the speed of the roadway deformationhanging and pulling, single hydraulwith beams support slowed down, followed by a stabilization period. After this stabili-should be used. the prop and beamstructure not only can zation period, roadway roof subsidence was 60-90 mm, whileprovide initial support resistance forut can also provide the convergence of the two sides was 100-130 mm. The roadwayability of shrinkage in order to adapt to large deformations of deformation was small. Fig 8 indicates that the integrity of theroadways under coal pillars. Moreover, props and beams can be surrounding rock and the reliability of the support structure arecycled, reducing support expenses.high. Because it achieved better control of the surrounding rock.grouting reinforcement measures of the surrounding rock did not4)Grouting reinforcement of surrounding rock.needed to be implementedAlthough peak and residual strengths of the surrounding rod5. Conclusionsare increased by bolt and cable support, this does not provida stable state. Deformation and failure continue unabatedly,causing the anchoring performance of bolt and cable to weaken1)When close-distance seams are mined, the mining roadway ofthe uppercoal seam should Ilaid out with narrow coal pillarsgradually, developing a dangerous condition in the surroundingall in order to reduce the effect ofrock. therefore, when the service life of a roadway is quite long andthe surrounding rock is crushed, grouting reinforcement measuresstress and scope caused by the abutment pressure of the coalire needed. The main function is as follows: first, it can enhance the 2)Under a coal pillar, the vertical stress in the floor stratumincreased while horizontal stress decreased, with the resultfull-size grouting of bolt and cable, fully displaying their supporthat their difference increased. This increased difference is anefficiency third, it can enhance the integrity of the support stiture and the surrounding rock grouting reinforcement measuresimportant reason for deformation and failure of roadways.are able to realize the stability of the surrounding rock during later 3)The support system of a roadway under a coal pillar should beable to reduce roadway deformation and, as well, adapt to thedeformation of the roadway. High strength and high prestressed bolt support, cable reinforcement support, single4. 2. Engineering applicationhydraulic prop with beam support and surrounding rockgrouting reinforcement can achieve improved support forAn engineering project to control the surrounding rock ofa roadway under a coal pillarroadway under a coal pillar was carried on in a mining roadwayof the 1st working face of the 4th coal seam of Jinjiazhuang CoaMine. Because of high stress and soft roof strata, a high strength Acknowledgementsbolt and cable support system was used As well, a single hydraulicprop with beam support was applied to enhance the stability of theFinancial support for this work.Natural Science Foundation of China(no77), the State KeyLaboratory of Coal Resources and SafeFig. 7 shows the bolt support layout. Five high strength levo- Subiect Foundation of China( No. SKLCRSM08X04), the Nationalutonomous Studystrength levo-rotatory 20 x 2000 mm bolts in both sides.Basic Research Program of China, the National excellenRow x spacing distances of roof and roadway sides were Degree Dissertation Special Foundation of China(No 200900 x 750 mm and 800 x 750 mm, Cable specification was New Century Talent Support Program of the Ministry of l917. x 6300 mm, with a row x spacing distance of of China(No NCET-06-0475)and the Youth Scientific Researchwere used in each bolt, which ensured an anchoring force of the 2008A002) is gratefully acknowledged.roof bolt not to be smaller than 100 kN while an anchoring force ofbolts in the roadway sides were not smaller than 80 kN. One ReferencesCK2340 and two 22360 resin anchor agents were used in each cablewith a pre-tightening stress of 100 kN.I1 Zhang BS, Yang SS, Kang LX, Zhai YD. Discussion on method for determiningAfter using these support parameters in the construction, the微m=Chinese Journal ofthe 20 days after roadway excavation, referred to as the effective by irregular pillar. Journal of china Coalperiod, the roadway deformation was large and the deformation 13) Xie WB, Shi ZF, Chen Xx, Lu SL Stability analyses of roadway surrounding rockmining. Joumal of China University of Mining4 I xiao Y凵MRoof subsidenceam. Mining Safety and Environment05:32(5:28-31 lin Ch[5] Wu Z u RG Layoutgy2002:7(41[6] Wang BT, Lu HT. Analysis on roof falsmall-interval coal seam. Ground Pressure and Strata Control 2004: 2: 41-3 in[71 Zhang BS. Study on the surrounding rock control theory and technology of中国煤化工CNMHG19) Shen MR Rock mass mechanics. Shanghai: Tongji University Press: 1999(inFR 8. Roadway deformation-time curve.. Xiao et al./ Mining Science and Technology( China)21(2011)243-247nism study on strength enhancement for the rocks [13] Sun XM, Cai F. Yang J. Cao WF. 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