Numerical Investigation On The Ground Response Of A Gob .
HindawiShock and VibrationVolume 2021, Article ID 8838505, 7 pageshttps://doi.org/10.1155/2021/8838505Research ArticleNumerical Investigation on the Ground Response of a Gob-SideEntry in an Extra-Thick Coal SeamC.W. Zang , G.C. Zhang , G.Z. Tao , H.M. Zhu, Y. Li, and H. ZuoCollege of Energy & Mining Engineering, Shandong University of Science & Technology, Qingdao 266590, ChinaCorrespondence should be addressed to G.C. Zhang; zgchao2015@163.comReceived 27 August 2020; Revised 20 October 2020; Accepted 11 March 2021; Published 20 March 2021Academic Editor: Giuseppe RutaCopyright 2021 C.W. Zang et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.This study was aimed at the large deformation phenomenon of rock mass surrounding the gob-side entry driven in a 20 m extrathick coal seam. Taking tailgate 8211 as the engineering background, a numerical investigation was employed to analyze thedeformation law of the gob-side entry. The study results are as follows. (1) Because the immediate roof was composed of weak coalmass with a thickness of 17 m, the roof coal mass was vulnerable to fail with the effect of overlying strata pressure; thus, a visualsubsidence of roof coal mass with a maximum convergence of 800 mm was observed in the field. (2) The bearing capacity of thecoal pillar was significantly less than that of the panel rib, resulting in the pillar failing more easily under the ground pressure andthen generating large-scale squeezing deformation. (3) The roof and panel rib were in a state of shear failure with a failure depth ofabout 5 m. The coal pillar was entirely in a state of plastic failure. (4) A support scheme including an asymmetric anchor beamtruss, roof angle anchor cable, and anchor cable combination structure was proposed. The field work confirmed that this supportscheme could efficiently control the deformation and failure of the rock mass surrounding the gob-side entry. This study providesthe theoretical basis and technical support for the control of rocks surrounding the gob-side entry in similar conditions.1. IntroductionThick and extra-thick coal seam resources are rich in China,and their reserves and production account for about 45% ofthe total [1]. Currently, thick and extra-thick coal seams havebecome the main coal seam for coal mining in China. Gobside entry is the most commonly used mining mode in thickand extra-thick coal seams, which mostly retains about 20 to50 m wide coal pillars between adjacent panels to performcoal mining. In recent years, gob-side entry retained with6–10 m wide coal pillar has begun to be popularized andapplied in the thick coal seams [2, 3]. However, due to thehigh abutment stress induced by mining activities, the roofand two sides of gob-side entries are prone to large-scaledeformation and failure, as well as to vicious accidents, suchas roof fall and rib spalling [4–7]. Thus, the stability controlof gob-side entry has become a key factor restricting the highyield, efficiency, and safe mining of extra-thick coal seams.In recent years, scholars have conducted a lot of beneficial research on the ground stability of gob-side entry inextra-thick coal seams. Zhang et al. presented a comprehensive field investigation of the ground response of a gateroad subjected to high stress induced by extracting a 17 mthick coal seam [8]. Yu et al. pointed out failure modes of thegob-side entry and studied the influence of the failurestructure on the stability of coal pillars [9]. Li et al. conducted a research into the balance conditions between thekey rock blocks above the gob-side entry and proposed theentry support resistance quantitatively [10]. Shen et al.pointed out that the middle part of the roof is the main partto control the surrounding rock, and three kinds of targetedcontrol technologies were put forward [11]. Feng et al.mentioned that strengthening coal pillars with high-strengthbolts is of great significance to surrounding rock stability[12]. However, their studies suffer various limitations. Theprevious studies are mainly based on specific geologicalconditions. In reality, due to the complexity and differenceof the geological conditions of various thick coal seamsbases, the corresponding deformation and failure laws aswell as its control method of the gob-side entry show great
2diversity [13–19]. In particular, there are few studies on thedeformation law and control technology of gob-side entry inan extra-thick coal seam with a thickness of nearly 20 m.The objective of this study is to obtain a better understanding of stability principle of the gob-side entry in anextra-thick coal seam. In this study, FLAC3D numericalsimulation software was used to analyze the distributioncharacteristics of the displacement, stress, and failure fieldsof a gob-side entry in an extra coal seam. Thus, the deformation and failure laws of the gob-side entry were obtained, and consequently, targeted control technologies arederived and applied in the field.2. Project OverviewThe test site was panel 8211. The combined coal seams of #5,#3, and #3 1 were mined, which is 20.86 m thick in average.The average thickness of #5 coal seam was 13.76 m, that of #3coal seam was 5.19 m, and that of #3 1 coal seam was 1.91 m.The immediate roof was a coarse sandstone with an averagethickness of 7.3 m, having a gray-white, coarse-grained, andmassive structure. The main roof was a fine sandstone, withan average thickness of 14.7 m, having a gray-white, medium-to fine-grained, and massive structure. The immediatefloor was mudstone, with an average thickness of 5 m.The test site was tailgate 8211; it was a rectangular sectionwith a height of 5 m and a width of 3 m. The coal pillarbetween the tailgate and the adjacent goaf of panel 8210 was8 m wide, as shown in Figure 1. In actual engineeringpractice, to ensure the normal mining and excavation of themine, tailgate 8211 had to be driven shortly after the panel8210 was mined out. Thus, it became an urgent task toconduct research on the deformation and failure laws of therock mass surrounding the tailgate 8211.3. Numerical Analysis of the StabilityMechanism of the Gob-Side Entry in an ExtraThick SeamIt is well known that coal and rock mass may have plenty ofdiscontinuous, which affect the stability of undergroundstructures more or less, depending on the mechanical behavior of the discontinuities. Although continuum-basedmethods are incapable of modelling discontinuities, rockmass properties estimated from the intact cores propertiesusing the Hoek–Brown criterion can be performed in themodel, which are effective properties that accounted for therock discontinuities. Therefore, in this section, FLAC3Dsimulation software was used to analyze the distributioncharacteristics of the displacement, stress, and failure fieldsduring gob-side entry driven with a narrow coal pillar[20, 21].It should be noted that, due to that those discontinuitieswere not considered in this study, the results generated byour model is expected to be slightly different with the actualvalue in reality. And future research is recommended toinclude the effects of this factor.Shock and Vibration3.1. Numerical Model Establishment and Simulation Plan.Based on the actual geological production conditions of thepanel 8211, a plane numerical simulation calculation modelwas established (see Figure 2). The model was 200 m longalong the x-direction. The z-axis height of the model was108.5 m. The velocities of the horizontal and bottomboundary were set as 0. The stress applied at the upperboundary was 7.5 MPa, representing the overburden pressure. The horizontal stress was applied in the x- and y-directions of the model, and the lateral pressure coefficient wasset to 1.2.Coal and rock mass are defined as the Mohr–Coulombmodel. The rock/coal mass properties required in the numerical model were obtained from the properties of an intactcore by using the RocLab 10.0 software program, which isbased on the generalized Hoek–Brown failure criterion (seeTable 1). The simulation process was as follows: initial stresscalculation balance panel 8210 mining excavationof the gob-side entry.3.2. Analysis of Simulation Results. The distribution ofvertical and horizontal displacement contours is shown inFigure 3. The obtained results are as follows:(1) Distribution Characteristics of Vertical Displacement:On the whole, vertical displacement of the roof coalbody above the coal pillar (about 1200 mm on average) was much larger than that of the roof coalbody of the entry (about 700 mm on average). Fromthe surface of the entry to the depth of 3.5 m, the roofcoal body exhibited an overall sinking trend, with anaverage sinking of about 800 mm, as shown inFigure 3(a). The abovementioned vertical displacement distribution characteristics can be attributed tothe following reasons. (1) The thickness of themining coal seam reached 20 m, and the entry wasdriven along the coal seam floor. As a result, theimmediate roof of the entry was a weak and fracturedcoal mass with a thickness of nearly 17 m. It wasprone to large-scale failure under strong miningactivities, which would result in insignificant damageoverall [22, 23]. (2) Compared with the panel rib, thecoal pillar rib had a smaller size and weaker bearingcapacity. Under the same overburden movement, itsplastic failure range was larger, which in turn led to agreater vertical displacement of the roof coal bodyabove the coal pillar.(2) Distribution Characteristics of Horizontal Displacement: From shallow to deep, the horizontal displacement of the panel rib gradually decreased, withthe maximum displacement of 700 mm occurring inthe middle of the panel rib. The coal pillar side alsopresented similar deformation characteristics, butthe deformation value was larger than that of thepanel rib side, reaching 900 mm, as shown inFigure 3(b). The reason for this phenomenon is thatthe bearing capacity of the coal pillar was muchsmaller than that of the panel rib. Therefore, it was
Shock and Vibration3MainsNo. 8211 mining panelTaligate 82118 m coal pillarHeadgate 8210Stop lineGoaf of panel 8210Figure 1: Diagram of the gob-side entry in section 8211.7.5 MpaOverlying strata3# Coal seamTailgate 8211MudstoneNo. 8210 mining panel5# Coal seam108.5 mFloor8m coal pillarUnderlying strata200mFigure 2: Numerical calculation model.Table 1: Physical and mechanical parameters of strata.Rock strataFloor rockFine sandstoneCoal rockMudstoneOverlying strata 3Density (g·cm )24202500141222002100Bulk modulus (GPa)8.213.471.706.470.95Shear modulus (GPa)6.022.631.005.646.50necessary to increase the supporting strength of thecoal pillar in the field.The plastic zone and vertical stress distribution of thesurrounding rock of the gob-side entry in panel 8211 areshown in Figure 4.(1) Distribution Characteristics of the Plastic Zone: It canbe seen from Figure 4(a) that the roof coal rock masswas in a state of large-scale shear failure, and theCohesion (GPa)2.01.60.62.23.0Internal friction angle (degree)3026203035failure depth was about 5 m. The panel and coal pillarribs were in shear failure models. The failure range ofthe panel side was about 5 m, and the coal pillar ribwas in a state of plastic failure. For the shallow coalbody of the roof and two sides, it was in the tensilefailure model.(2) Distribution Characteristics of the Vertical Stress:From Figure 4(b), it can be seen that the shallow coalbody of the entry was in a state of stress release, with
4Shock and VibrationZ-displacement4.2994E – 02–1.0000E – 01–3.0000E – 01–5.0000E – 01–7.0000E – 01–9.0000E – 01–1.1000E 00–1.3000E 00–1.5000E 00–1.7000E 00–1.9000E 00Panel ribCoal pillarPanel ribCoal pillar(a)X-displacement1.0000E 008.0000E – 016.0000E – 014.0000E – 012.0000E – 010.0000E 00–2.0000E – 01–4.0000E – 01–6.0000E – 01–8.0000E – 01–1.0000E 00–1.0149E 00(b)Figure 3: Displacement distribution developed around the gob-side entry in section 8211: (a) vertical displacement; (b) horizontaldisplacement.Roof coal seam5m5mPlane ribYield coal ribColorby: stateNoneShear-n shear-pShear-n shear-p tension-pShear-pShear-p tension-pTension-n shear-p tension-pTension-n tension-p(a)ZZ-stress20.0 MPa8.0 MPa7.5 MPa3.5m9m1.3368E 05–1.0000E 06–3.0000E 06–5.0000E 06–7.0000E 06–9.0000E 06–1.1000E 07–1.3000E 07–1.5000E 07–1.7000E 07–1.9000E 07–2.1000E 07(b)Figure 4: Stress and plastic zone distribution developed around the gob-side entry in section 8211: (a) plastic zone; (b) vertical stress.an average stress value of less than 1 MPa. For thepanel rib, a peak stress of about 20 MPa occurred 9 maway from the rib side, and the stress concentrationfactor was about 2.2. For the coal pillar rib, amaximum stress of 8 MPa occurred, which wasgreater than the original rock stress by 7.5 MPa.(3) Based on the distribution trend of the stress andplastic zone of the two ribs, it can be concluded that,although the coal pillar was totally in a state of shearfailure, the stress of the coal body exceeded theoriginal stress within 1.5 m of the central part of thecoal pillar. This indicates that the coal pillar still had acertain degree of bearing capacity, which could meetthe current demand for the ground control. However, it was still necessary to control the plasticdamage range of the shallow coal body to avoidexcessive horizontal displacement of the coal pillar.4. Ground Control of Surrounding Rock of theGob-Side Entry4.1. Control Principle of Surrounding Rock of the Gob-SideEntry. Based on the actual geological production conditionsand characteristics of the surrounding rock of the gob-sideentry, the process of coal mass deformation and failure of thetailgate 8211 was analyzed as follows. (1) Tailgate 8211 wasdriven along the floor line of the coal seam, and the immediate roof was 17 m thick weak coal body with developedcracks. During entry excavation, affected by mining stress,the coal body would gradually undergo a plastic failure fromshallow to deep, and consequently, the roof would sink anddeform significantly. (2) Under the overlying strata movement, the coal pillar rib was all in the plastic failure state, andthe bearing capacity of the coal pillar was smaller. Understrong mining stress, extrusion deformation occurred to thecoal pillar rib, causing significant horizontal displacement.Based on the above numerical analysis, to ensure thesafety and stability of tailgate 8211 during its service period,the ground control should start from the following aspects[24, 25]. (1) The coal pillar was totally in a state of plasticfailure; as a result, its supporting force on the roof was smalland it was easily failed. Therefore, improving the supportintensity of the coal pillar is the key to control the deformation of the surrounding rock of the gob-side entry. (2)The roof of the gob-side entry was composed of a weak coalbody. Affected by the strong mining action induced by theentry driving and the panel retreating, the coal body surrounding the entry would be severely failed and resulted in arelatively high crushing pressure. Therefore, it was necessaryto adopt support components with a larger surface area, toimprove the support strength of the surrounding rock of thesurface.4.2. Control Technology for the Gob-Side Entry with NarrowCoal Pillars in Extra-Thick Coal Seams. A support scheme,including high-strength bolts, strong roof anchors, andreinforced anchors for coal pillars, was determined, asshown in Figure 5. The specific parameters were as follows.
Shock and Vibration5Anchor cable diameter: 17.8mmLength: 8250 mmAnchor boltsdiameter: 20 mmLength: 2600mm15 15 15 95095011009501100100015 15 10001000Length: 5250 mm15 500Anchor cablediameter: 17.8 mm15 1000100011009501000110095015 15 20015 5200Figure 5: Support scheme of the gob-side entry.5. Engineering ApplicationDuring the excavation of the tailgate 8211, four measuringstations were arranged in the entry with a distance of 50 m.The JSS30 A digital display convergence meter was used formeasurements. The displacement of the entry roof and twosides during excavation period is shown in Figure 6. It can be140120Convergence (mm)The entry roof adopted a φ20 2600 mm thread steelbolt, and the row spacing was 900 900 mm. Each row wasarranged with six bolts. The bolts at the two sides wereinclined 15 degrees outward, and the rest were arrangedvertically. The bolt was connected by a reinforced ladderbeam made of φ14 mm round steel welding.Anchor cables with a diameter of 17.8 8250 mm wereselected, and the spacing between rows was 1050 1800 mm.Four anchor cables were arranged in each row. The anchorcables at the two sides were inclined 15 degrees outward, 16channel steel was used for connection, and the middleanchor cable was vertical to the roof. A 20 2600 mm threadsteel bolt was selected for the panel rib and coal pillar rib,with a row spacing of 1000 900 mm, and each row had fourbolts. The anchor rod at the roof was inclined 15 degreesupward, and the anchor rod at the floor was inclined 15degrees downward. The rest were arranged vertically on twosides and connected by steel beams welded by φ12 mmround steel. On the coal pillar rib, the prestressed anchorcables were arranged in the middle of two adjacent rows ofbolts with a distance of 900 mm. The upper anchor cableswere inclined 15 degrees upward, and the bottom anchorcables were inclined 15 degrees downward.1008060402000102030405060708090Time after development (d)RoofPanel ribCoal pillar ribFigure 6: Deformation monitoring of the surrounding rock duringroadway excavation.seen that the surface displacement of the entry showed achanging trend of “coal pillar rib solid coal rib roof.”Finally, the deformation of the coal pillar was 131 mm, thedeformation of the panel rib was 125 mm, and the subsidence of the roof was 99 mm. It can be seen that the deformation of the surrounding rock of tailgate 8211 waswithin a controllable range, which could meet the needs of
6Shock and VibrationReferencesFigure 7: Control effect of the surrounding rock.normal panel mining. The photo of site support effect isshown in Figure 7. It should be noted that the applicability ofthe proposed support scheme on other coal mines needs tobe studied because every coal mine may have differentgeological and mining conditions, which greatly affect thesupport parameters design. Further case studies are neededin order to deliver some general principles of supportscheme design.6. Conclusion(1) Because the immediate roof was composed of weakcoal mass with a thickness of 17 m, the roof coal masswas vulnerable to failure on a large scale with theeffect of overlying strata pressure; thus, a visualsubsidence of roof coal mass with a maximumconvergence of 800 mm was observed.(2) The bearing capacity of the coal pillar was significantly less than that of the panel rib, resulting in thepillar failing more easily under the ground pressureand then generating large-scale squeezing deformation along the horizontal direction.(3) On the basis of the coal mine’s geological productionconditions and the deformation and failure laws ofthe surrounding rock along the gob-side entry, asupport scheme including an asymmetric anchorbeam truss, roof angle anchor cable, and anchorcable combination structure was proposed.It should be noted that the optimal support scheme andcoal pillar size strongly depend on the geological and miningconditions. In addition, this study was only based on aspecific coal mine model. Further case studies are needed inorder to deliver some general principles of gob-side gateroad stability design. However, the modelling procedurespresented in this study are necessary in the design of yieldpillars in other coal mine.Data AvailabilityThe data used to support the findings of this study areavailable from the corresponding author upon request.Conflicts of
of a gob-side entry in an extra coal seam. us, the de-formation and failure laws of the gob-side entry were iesare derivedandappliedinthefield. 2. Project Overview etestsitewaspanel8211.ecombinedcoalseamsof#5, #3,and#3 1 weremined,whichis20.86mthickinaverage.
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