Non-harmonious deformation controlling of gob-side entry in thin coal seam under dynamic pressure

Journal of Rock Mechanics and Geotechnical Engineering 6(2014)269-274Contents lists available at science DirectJournal of rock mechanics andGeotechnical EngineeringSRMFjournalhomepagewww.rockgeotech.orgFull length articleNon-harmonious deformation controlling of gob-side entry in thinCross Markcoal seam under dynamic pressureKegong Fan,, Hongguang Liang, Chishuai Ma, Chuanwei Zanglaboratory of mine Disaster Prevention and Control, Shandong University of science and Technology, Qingdao 266590, ChinaLiyan Mining Company of Shandong Lineng Group, Jining 273517, ChinaARTICLE IN OABSTRACTArticle history:The behavior of gob-side entry under dynamic pressure is totally different from the one driven after theReceived 7 March 2014movement of overlying strata above the adjacent coalface goaf. The gob-side entry will experience severeReceived in revised form4 March 2014roof lateral structural adjustments caused by adjacent coalface mining. Thus the deformation and failurepted 28 April 2014characteristics of narrow coal pillar along the gob should be carefully considered. On the basis of the datalable online 14 May 2014of the gob-side entry obtained in a thin coal seam under dynamic pressure, the measures to reinforce thenarrow coal pillar are put forward. In addition, the non-harmonious controlling of the rock structureKeywordsnd non-equilibrium gob-side entry deformation is proposed to avoid potential failure. Field practicesob-side entry under dynamic pressurelow that the supporting problems of the gob-side entry under dynamic pressure can be well addressed,Narrow coal pillwhich could be used in other similar mining cases.Non-harmonious control2014 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byThin coal seamElsevier B v. All rights re1. Introductioncoalface mining will be affected by the strong dynamic pressureinduced by mining activities. The adjustment of lateral roof strucThe commonly observed gob-side entry is often driven after the ture caused by adjacent coalface mining will definitely deterioratemovement of the overlying strata above the adjacent coalface goaf, the stress environments and then the narrow coal pillar along thewhere stress redistribution tends to be stable with elapsed time. gob deforms, even fails(Golshani et al, 2007; Trifu et al, 2007)The gob-side entry is basically located in the lateral stress- This leads to obvious non-equilibrium deformation and failurereduction area, which is affected by adjacent coalface mining. between the coal pillar and the gob-side entry, and then implementThus it can avoid potential high-stress induced damages to sur- of supporting becomes more difficult( Hernandez Gomez and ruiz,rounding rocks. At present, the control technologies for surround- 1993). Traditional supporting methods have their limits to meet theing rocks are pretty mature(Fan and Zhai, 2004: Bai, 2006; Liu and requirements of deformation control of surrounding rocks. AcMeng, 2009; Hua et al, 2011). In recent years, however, replace- cording to the non- harmonious characteristics of rock mass struc-ment of mining and tunneling is difficult for the increasing mining ture and non-equilibrium deformation and failure in gob-side entryintensity. In order to avoid forming isolated coalfaces because of under dynamic pressure, the beneficial exploration is carried out inskip-mining, the gob-side entry has been driven while the adjacent terms of the non- harmonious controlling of surrounding rockscoalface is moving in the opposite direction at the same time deformation. It is of great significance for harmonizing the inter-(Zhang et al, 2004; Liu et al. 2010; Wang et al., 2012: Zhao et al2012). The stability of gob-side entry excavated during adjacent2. Engineering geological settingsA case in Xuzhuang coal mine was taken to illustrate the feasiCorresponding author. Tel: +86 13589290397E-mailaddress:fankeg@163.com(K,Fan).bility of proposed controlling measures in this context. The primaryPeer review under responsibility of Institute of Rock and Soil Mechanics, Chinese coal seam of the first mining district is located in the second levelAcademy of Sciences.which belongs to the 12th coal bed of Upper Carboniferous Taiyuanckness of 1.1There are0-3 I中国煤化工0-0.33 m within thecoal seam. TheProduction and hosting by ElsevierCNMHGle. The main roof is1674-7755 o 2014 Institute of Rock and Soil Mechanics, Chinese Academy of composed of sandy mudstone with the uniaxial compressiveSciences. Production and hosting by elsevier B V. All rights reserved.strength(UCS )of 59.0 MPa. The floor is made up of limestone withhttp://dx.doi.org/10.1016/j-jrmge.2014.05,001an average thickness of about 2.75 m and an average UCs ofK Fan et al. Journal of Rock Mechanics and Geotechnical Engineering 6(2014)269-274145.6 MPa. The interlayer floor between the coal seam and the floor the inner gob-side entry close to the cut side, where the sur-(limestone) is mudstone with an average thickness of 0.2 mrounding rocks show significant plastic deformation after adjacentIn the presence of coal seam, the main coalface is arranged in the coalface mining Scheme I is adopted for the outer gob-side entry inwest of the mine. Headentry of the coalface must be driven along later mining. Through field investigation, the deformation andgoaf during adjacent coalface mining in order to ensure the normal failure characteristics of surrounding rocks in the gob-side entry inoperation of main coalfaces. For instance, headentry 04 lies on the thin coal seam under dynamic pressure can be well understood.east of the tail entry 02. Its cross-sectional dimension is providing a basis on supporting scheme selection for other gob-side3. 8 m x 2.6 m(width x height), which is driven along the floor by entries under dynamic pressureeaking the coal seam and part of roof. The narrow coal pillar(5 mThe cross-section of headentry 06 is 3. 8 m x 2.6mwide) is left between the headentry and the adjacent tail entry. (width x height). It will be driven along the floor by breaking theHeadentry 04 has been already completed before coalface 02 12th coal seam and part of roof, and thus a narrow coal pillar(5 mmining. The combined support scheme with bolts, steel meshes wide)will be left. Headentry 06 is affected by coalface 04 miningand anchor cables is adopted for headentry 04 The roof bolts adopt when driven and by coalface 06 mining when used and a typicalthread steel with size of $20 mm x 2000 mm and spacing of roadway influenced by dynamic pressure. It can be noted that the900 mm x 900 mm. Also, steel meshes with 6.5 mm in diameter are deformation and failure characteristics of surrounding rocks in theadopted. The prestressed anchor cables of 15.24 mm x 5000 mm gob-side entry under dynamic pressure should be considered inwith high strength and low relaxation are considered and the 2-1supporting scheme.2 layout is used along the roadway with row spacing of 2700 mmFiber reinforced polymer(FRP)anchor bolts of d20 mm x 1800 mm 3. Deformation and failure characteristics of surroundingand plastic meshes are adopted at sidewalls with spacing of rocks in gob-side entry in thin coal seam under dynamic700 mm x 900 mm( Fig. 1). As a typical gob-side entry under dy-pressurenamic pressure, headentry 04 has experienced the dynamic pressure from mining coalfaces 02 and 04 and supporting becomes veryThe gob-side entry under dynamic pressure roughly experiencedifficult subsequentlythree stages, i.e. roadway excavation, adjacent coalface mining, andConsidering that the original support scheme of headentry 04 mining of target coalface. Based on field monitoring of groundcannot resist the dynamic pressure induced by adjacent coalface pressure in the reinforced area of headentry 04 where Scheme llmining, the roadway supplementary support scheme should be was employed, the deformation and failure characteristics of sur-ised In the supplementary support scheme, we consider high rounding rocks in the gob-side entry under dynamic pressure arestrength bolts of 18 mm x 1800 mm for coal pillar side, and thested as followsnest anchor cables of 15.24 mm x 5000 mm for the roof near thesmall pillar. Two different supporting structures are adopted for( 1) The front abutment pressure caused by adjacent coalface minheadentry 04. Anchor bolts and steel ladders are employed inScheme i with the spacing of 900 mm x 900 mm, and in Scheme llng has a minor influence on the gob-side entry under dynamicpressure. In a certain area in vicinity of roadway affected by theonly anchor bolts are used with spacing of 900 mm x 1800 mmfront abutment pressure, the maximum convergence betweenDuring the initial period of coalface 02 mining, Scheme ll is used forthe roof and floor is up to approximately 21 mm, and that ofAnchosteel meshThe entity coalThe small coalpillar sidellar side中国煤化工CNMHGFig. 1. Support profile of headentry 04.K Fan et al Journal of Rock Mechanics and Geotechnical Engineering 6(2014) 269-274sidewalls is about 118 mm, suggesting the effectiveness of thedopted support scheme (in Fig. 2)(2) The surrounding rocks in the gob-side entry under dynamicpressure show significant plastic deformation after miningadjacent coalface. In this circumstance slip surfaces betweennarrow coal pillar and roof strata are easy to occur. Sidewallsslippage in some areas with a value of 500-800 mm. In Fig 3,we can observe the coal-rock interface characterized bybvious slipping traces. However, the lateral deformation of thesolid coal is smaller(basically less than 150 mm )and sidewallslippage is reported in very few areas, showing evident non)equilibrium deformation and failure of surrounding rockshe dynamic pressure caused by adjacent coalface mining haslong-term effect on mining stability. The strong deformation ofgob-SIesure(the sidewalls convergence speed of more than 10 mm/d is20110110deemed as strong active stage)lasts for about one month withinscope of 90 m after the coalface mining, which is within 90 maround dynamic pressure. The general deformation( the side(a)Overall outward movement of sidewall in the narrow coal pillawalls convergence speed of less than 2-10 mm/d is consideredas active stage) lasts for about three months within scope of250-270 m after coalface mining(4 )Many phenomena of serious deformation and or failure occurredin the coal pillar: ( i) FRP anchor bolts were pulled out and anchor(b) Slip phenomenon of sidewall in the narrow coal pillar.Fig. 3. Horizontal movement of narrow coal pillar in headentry 04 and its effectslates were broken. (ii) The double anti-pulling mesh was cut bythe high strength anchor plates. The anchor bolts and anchor(a)Sidewall in the narrow coal pillarplates sometimes penetrated into the reinforced body. Incontrast, the FrP anchor plates were crushed or fell off only inlocal areas for the solid coal sidewall(5) The roof convergence of the gob-side entry was smaller underdynamic pressure. In this regard, mesh bag and mesh breakageere observed in some local broken roof.The deformation and failure characteristics of the gob-side entryunder dynamic pressure show that the frp bolts cannot providelarge pre-load and or clamping force and thus enough anchoringforce, suggesting their poor capacity againstynamIc pressure.Some accidents verified the tensile failure of frp bolts, crushingfailure and slippage of FRP anchor plates. In the later period, weadopted Scheme I to reinforce the outer headentry 04 in order toeffectively control the deformation of the coal pillar4. Stability anaYH中国煤化工 nder dynamicCNMHG(b) Roadway roof.The above-mentioned analysis indicates that the deformationFig. 2. Situations of headentry 04 affected by the front abutment pressure.and failure of gob-side entry under dynamic pressure are inducedK Fan et al. Journal of Rock Mechanics and Geotechnical Engineering 6(2014)269-274,r",,,,1roof of the gob-side entry below the key block B will experienceadaptive subsidence under dynamic pressure along with overallinward movement of sidewalls. After this structural adjustmentthe surrounding rocks of gob-side entry deform seriously andbecome unstableRock massaThe behavior of gob-side entry under dynamic pressure is totallB---immediate roof ----1Blockc different from the one driven after the movement of overlyingBlockc strata above the adjacent coalface goaf. The gob-side entry willexperience severe roof lateral structural adjustments caused byadjacent coalface mining. Under the lateral abutment pressurecaused by adjacent coalface mining, the narrow coal pillar in viSmall coal pillarob areacinity of adjacent coalface is damaged firstly due to intense stressconcentration, fractures, and rotating subsidence of arc-triangleblock. The stress distribution, bearing capacity and deformationFig 4. Arc-triangle block of gob-side entry under dynamic pressure(Zhu, 1987: Hou of the coal pillar are also significantly different from those of solidand Li, 2001; Zhang et al, 2004; Bai, 2006)coal in gob-side entry under dynamic pressure. Affected by thedeformation and failure of narrow coal pillar, other parts ofmainly by mining adjacent coalface. In the initial period of roadway roadway surrounding rocks may cause roadway instability for theexcavation,one side of the gob-side entry is the solid coal and the action of stress redistribution. It can be seen that the lateral fracother side is the narrow coal pillar and untapped solid coal. The turing and rotating subsidence of main roof of the adjacent coalfacenarrow coal pillar is basically believed to be almost intact and its are the major reasons for the large deformation of surroundingg capacity is not reduced when the gob-side entry is located rocks in the gob-side entry under dynamic pressure. The narrowat the front of adjacent coalface. The front abutment pressureal pillar plays an important role in controlling roadway defor-caused by adjacent coalface mining has a great influence on the mation and maintaining roadway stability, and its deformation andgob-side entry at the front of coalface, and strata behaviors are failure will directly affect the roadway stability. Therefore, thecomparatively weaker With the adjacent coalface advancing, the narrow coal pillar is the key, also a difficulty, for supportingarc-triangle block structure is formed after periodic weighting of2001: Zhang et al, 2004: Bai, 2006 ). Arc-triangle block B, adjacent 5. Support scheme and analysthe main roof behind adjacent working face(Zhu, 1987; Hou and Li,caving block C, and rock A above the coalface without caving are 5.1. Support methodarticulated together, as shown in Fig. 4. The formation of arctriangle block is a dynamic process affected by strong dynamiche site-specific conditions of headentry 06 are described aspressure caused by the rotation and subsidence of the structure, the followsAnchorcableAnchorAnchorSteel meshThe entity coalThe small coal a中国煤化工CNMHGFig. 5. Support cross-sectional drawing of headentry 06K. Fan et al Journal of rock Mechanics and Geotechnical Engineering 6(2014)269-274Time(d)va二"≌2后8可m防子乎导字界界2始3HERRoof to floor convergence -+Sidewall convergence280公29早琴符否溶总系导器Distances from working face(m)Fig. 6. Typical surface displacement of gob-side entry under dynamic pressure induced by adjacent coalface mining(1)Headentry 06 is driven along the floor by breaking the coal seam 5.3. Result analysisand part of main roof, and the thickness of coal seam is abouthalf of the roadway height.Field observation shows that the surface displacement is small(2)A narrow coal pillar(5 m wide) is left for gob-side entry driven under dynamic pressure induced by adjacent coalface mining. Afteralong tail entry 04excavation, the maximum cumulative horizontal displacement of(3) The roadway is influenced by coalface 04 mining.idewalls is 280-370 mm, significantly reduced when compared to(4)Unbalanced stress in rock mass, bearing capacity variation and that of headentry 02 Cumulative convergence between roof andnon-equilibrium deformation and or failure of gob-side entry floor is 76-130 mm It is noted that the overall outward movementunder dynamic pressure are existingof sidewalls makes the horizontal deformation become larger.while the convergence between roof and floor is relatively smallerAccording to the above-mentioned descriptions, the combined Observation shows that the convergence of coal pillar accounts forsupport scheme with high strength prestressed anchor bolt, pre- about 70% of the sidewall movement. The surface displacement ofstressed anchor cable and steel mesh is adopted, which is based on roadway tends to be stable after excavation of 70-90 m fromthe non-harmonious control concept, support principles, and the monitoring station. The typical surface displacement of roadway isbearing capacity of surrounding rocks. The narrow coal pillarshown in Fig. 6inforced in order to control the deformation of gob-side entiunder dynamic pressures. Field monitoring results show that the 6. Conclusionsdeformation of roadway matches well with that of asymmetricsupporting structure.(1)Under dynamic pressure, the bearing capacities of the coal pillarand the solid coal are totally different, especially when the gob-5.2. Support schemeside entry is in vicinity of the adjacent coalface. Accordingly, thesymmetric characteristics of surrounding rocks should be6 has the cross-sectional dimension ofconsidered3.8 m x 2.6 m(width x length). Anchor bolts are arranged at the (2)Atfected by adjacent coalface mining, slipping surfaces betweennarrow coal pillar and the solid coal, asymmetrically, as shown innarrow coal pillar and roof strata can be formed, leading to theFig. 5. Support parameters are listed as followsoverall outward movement and larger deformation, even if theoverall outward movement of solid coal is smaller. Non-(1)The high strength bolts of 20 mm x 2000 mm are adopted forequilibrium deformation and failure of the gob-side entryhe roof, with spacing of 900 mm x 900 mm. The cold hardappear.drawn steel wire for welded mesh is adopted with diameter of 3)Support measures play an important role in controlling the4.5 mm, and grid size of 100 mm x 100 mm, and dimension ofstability of gob-side entry under dynamic pressure. According2000 mm x 1000 mm. The prestressed anchor cables with highto the non-equilibrium deformation and failure of surroundingstrength and low relaxation are employed with dimension ofrocks, it is advised to adopt non-harmonious supports to reinφ1524mm×5000mm. The layout of“2-1-2” pattern with rowforce the narrow coal pillar. Hence the deformation of thespacing of 1800 mm along the roadway is utilized to improvesurrounding rocks of gob-side entry under dynamic pressurecan be effectively controlled(2)Non-harmonious bolting with metal mesh is adopted at side-walls. Bolts of 20 mm x 2000 mm are adopted at the coal pillar Conflict of interwith spacing of 700 mm x 900 mm. FrP bolts of中国煤化工020 mm x 1800 mm are adopted at the solid coal side withspacing of 900 mm x 900 mm. The metal meshes, 4.5 mm in interest associatCNMHGOknown conflicts ofthere has been nodiameter, dimension of 2500 mm x 1000 mm and grid size of significant financial support for this work that could have infl100 mm x 100 mm, are adopted at sidewalls.enced its outcomeK. Fan et al. Journal of Rock Mechanics and Geotechnical Engineering 6(2014) 269-274Acknowledgmentsrifu CI. Shumlia V Chen D Investigation of rock mass stability through theof failure components of microseismicity at Darlot gold mine, AusThis paper is supported by the National Natural Science FounChallenges in Deep and High Stress Mining. Perth, Australia: Australiafor Geomechanics: 200dation of China(grant No. 51104093)ang M, Bai J B, Wang XY. Xu Y, Guo YH, Cao JL. The surrounding rock deforTle and control technique of the roadway driven along goaf and headiReferencesdjacent advancing coal face. Journal of Mining and Safety Engi2012:292):194-202( in Chinese)Zhang N, Li XH, Gao MS Pretensioned support of roadway driven along next gobBai JB, Surrounding rock control of gob-side entry driving. Xuzhou: China Universityand heading adjacent advancing coal face and its application. Chinese journal ofof Mining and Technology Press: 2006 (in Chinese).Rock Mechanics and Engineering 2004: 23(12): 2100-5(in Chinesen KG, Zhai DY, Analysis of deformation failure and non-harmonious control Zhao Y, Wang J, Gao MS, Liu BT, Research and application of gateway driving alongmechanism of surrounding rocks of roadways with weak structures. Beijing:goaf forward to adjacent mining coal mining face in Chensilou mine Coal En-China Coal Industry Publishing House: 2004(in Chineseineering 2012: 6: 41-3(in ChineseGolshani A, Oda M, Okui Y, Takemura T, Munkhtogoo E Numerical simulation of theZhu DR Fracture character of main roof in longwall face and its application. PhDexcavation damaged zone around an opening in brittle rock, InternationalThesis. Xuzhou: China University of Mining and Technology: 1987(in Chinese).Journal of Rock Mechanics and Mining Sciences 2007: 66(4): 835-45Hernandez Gomez LH, Ruiz C. Experimental evaluation of crack propagation velocity in PMMA under dynamic pressure loading. International Journal ofFracture1993:61(2):R21-8Dr Kegong Fan, is a professor in Shandong University ofHou C], Li XH. Stability principle of big and small structures of rock surroundingScience and Technology, and his main research interestovers ground pressuChina Coal Society 2001; 26(1): 1-7(in Chinesert. To date, as a principal investigator, Dr. FanHua XZ, Liu S, Liu ZH, Zha WH, Li YE, Research on strata pressure characteristic ofrticipated in over 60 projects. He was awarded thegob-side entry driving in island mining face and its engineering applicationSecond state-level Prize of"Achievements in educatChinese Journal of Rock Mechanics and Engineering 2011: 30(8): 1646-51 (inChineseogy Progress Awards. Dr Fan is the author of The failureLiu JH, Zhao ZH, Mou GL Research on support technology for roadway under strongand Instability Analysis and Non-harmonious Control Mechdynamic pressure while mining in fully mechanized caving face and its appllism of Weak Structure of Surrounding Rocks in Roadways,cation. Mining and Metallurgical Engineering 2010: 30(2): 14-7(in Chinesed the co-author of 6 books. He has published more thaLiu QT, Meng FW. Support technology of narrow coal pillar goaf-side gateway foacademic papers. At present, he is theully mechanized top coal caving mining face in deep mine Coal Science andber of China Coal Society for Coal Mining SpecializedTechnology 2009; 37(7): 28-30(in Chinese).中国煤化工CNMHG